The cell surface-bound polysaccharide sialyl-Lewis X antigen (SLe^<X>1) was synthesized. We revealed that both D-glucal and D-lactal derivatives where only the primary alcohol functions were protected underwent regiospecific L-fucosylation at the allylic alcohol. In the Dlactal series D-sialylation occurred specifically at the C3 hydroxyl in the galactosyl domain. The conciseness of our route enabled us to produce gram quantities of 9. To overcome the difficulty in extending our sunlfonamideglycosylation methodology, stannyl alkoxide as a glycosyl acceptor was investigated. We demonstrate successful application of stannyl alkoxide addition to glycal-derived iodosulfonamides, resulting in the total synthesis of sialyl-Lewis X antigen (1) and synthesis of hexasaccharide 20.
Thioglycosides, which can be used as the protecting group at the anomeric position but can be used as efficient glycosyl donors through activation with appropriate thiophilic reagents, have been widely used for the synthesis of complex carbohydrates. In the present study, we described two novel activating methods for thioglycosides. A hypervalent iodine (III) reagent prepared in situ from iodosobenzene (PhIO) and trifluoromethanesulfonic anhydride (Tf_2O) effectively activates thioglycosides. Glycosylation with benzyl-protected methyl thioglycosides as glycosyl donors ("armed" donors) proceeded smoothly under mild reaction conditions. Less reactive acylated methyl thioglycosides ("disarmed" donors) were also activated in the presence of silica gel to form 1,2-trans glycosides. The allyl group, which is susceptible to many of the strong thiophilic reagents, was stable under the present glycosylation conditions. Because of its high reactivity and chemoselectivity, [PhIOTf_2O] is expected to be versatile for the synthesis of complex natural oligosaccharides. The glycosylation using thioglycosides was also effected by use of a combination of N-bromosuccinimide (NBS) with either tributylammonium triflate (Bu_4NOTf) or diphenyliodonium triflate (Ph_2IOTf). Both "armed" and "disarmed" thioglycosides smoothly reacted with acceptors under mild and neutral reaction conditions to give corresponding glycosides in good yields. With regard to the availability and the stability of the reagents as well as the operational simplicity, this method will also be useful for the synthesis of complex oligosaccharides.
The elicitor active hexa-β-D-glucopyranosyl-D-glucitol (1), isolated from mycelial wall of Phytophthora megasperma f. sp. glycinea, stimulates the formation of antibiotic phytoalexins in soybean. Our interest associated with (1) is to elucidate the conformational differences between the elicitor active and elicitor inactive saccharides. As the oligosaccharide (1) is composed of two repeating segments 2, we plan to examine the synthesis and conformational analysis of the trisaccharide 2. Here we report a new synthetic approach of a branched oligosaccharide such as the trisaccharide 2 by spacer mediated glycosilation, and the conformational analysis of the trisaccharide 2 in water based on molecular mechanics calculations. Spacer mediated glycosilation was performed on trisaccharide 5 and 6. The regioselectivity in the intramolecular glycosilation was predicted based on AMBER calculations. These calculations suggested that the reaction of 5 and 6 should result in regioselective glycosilation at 4-position of the glycosyl acceptor. The experimental result indicated that the glycosilation was carried out to afford 4-O-glycoside 18, which could not be obtained in intermolecular glycosilation of 20 with 21. The conformational details of oligosaccharides are essential for understanding the process of chemical binding at receptor site. Thus we performed the conformational analysis of the trisaccharide 2 in water based on MM2, AMBER (United Atom), AMBER (All Atom) programs and NMR spectra. The conformation of 2 obtained from NOESY spectrum data in D_2O showed good agreement with the conformation obtained from AMBER (United Atom) program with solvation. These results indicate that we have developed a protocol for full confomational search of oligosaccharide in water based on GB/SA solvation model.
Sialic acid (N-acetylneuraminic acid) is an essential component of ganglioside (sialoglycosphingolipid) which is involved in cellular interactions, differentiation, and growth. Many syntheses of sialic acid using chemical and enzymatic methods were reported. However, the practical synthetic method has not been developed yet. In order to supply enough amount of sialic acid we exploited a convenient chemical synthesis through aldol condensation of D-glucose with oxalacetic acid in relatively short steps. Pyranose (1) was converted to 1,5-lactone to distinguish 5-hydroxyl group from other hydroxyl groups in protection and then introduced an acetylamino group at C5-position with Walden's inversion. Finally, deprotection of all of protecting groups was performed to give N-acetylneuraminic acid. Synthetic N-acetylneuraminic acid was completely identical with natural one in all respects. This synthetic method may supply enough amount of sialic acid from commonly available starting materials. The furanose isomers (2) and (3) as aldol condensation products were also converted to corresponding 1,6-lactones and then transformed into the sialic acid isomers (4) and (5) respectively in the similar way. These furanose isomers with 6-acetylamino group, especially compound (4) with the same stereochemistry at C4 as that of N-acetylneuraminic acid, exhibit remarkable inhibitory activities against neuraminidases of influenza virus A, A2, and B. Therefore, the isomer (4) or (5) is promising as a new anti-influenza drug.
Ferrier rearrangement is one of the most efficient reactions for the synthesis of optically active substituted cyclohexanones from corresponding carbohydrates. We have found that the Ferrier rearrangement proceeds in high yields with a catalytic amount of (CF_3CO_2)_2Hg in a neutral solvent system. In this paper, chiral syntheses of natural products starting from D-glucose utilizing catalytic Ferrier rearrangement as the key reaction are described. Total Synthesis of (-)-Hygromycin A (15). Hygromycin A (15) is an antibiotic produced by Streptomyces and possesses a unique structure. The aminocyclitol moiety (13) was prepared from D-glucose. Catalytic Ferrier rearrangement of 3, synthesized in 7 steps from D-glucose, followed by β-elimination afforded the cyclohexenone 5. Luche reduction of 5 and the introduction of an amino function gave 9. Glycolation with OsO_4 and a methylene acetal formation, followed by deprotection provided the aminocyclitol 13. Condensation of 13 with the sugar carboxylic acid 14, followed by deprotection completed the first total synthesis of (-)-hygromycin A. Formal Total Synthesis of Paniculide A (28). Catalytic Ferrier rearrangement of the bicyclic enopyranoside 21, prepared from D-glucose in 10 steps, followed by β-elimination gave a cyclohexenone 22. 1,4-Addition of a methyl group and successive Saegusa reaction gave the optically active enone 24, which was converted into the known synthetic intermediate 27, for highly oxygenated sesquiterpene, (±)-paniculide A 28. Stereoselective Total Synthesis of (+)-Lycoricidine (41). The phenanthridone alkaloid, (+)-lycoricidine 41 is known to show high cytotoxic activity. The 2-azido-5-enopyranoside derivative 31, prepared from D-glucose in 9 steps, was effectively converted into the enone 32 by catalytic Ferrier rearrangement and subsequent β-elimination. The compound 32 was converted into fully protected bromo-olefin 35, which was cyclized by Pdcatalyzed reaction to provide the phenanthridone 37. Inversion of the configuration of 2-hydroxy group and the removal of the protecting group completed the total synthesis (+)-lycoricidine.
Carbohydrates are important sources of chirality in organic synthesis. We became aware of possible stereocontrol elements, in which the intrinsic molecular architecture of a carbohydrate may serve as a template for chirality transcription irrespective of individual chiral carbon. Oxidation of diacetone-D-glucose affords 1,2: 5,6-di-O-isopropylidene-α-D-ribo-hexos-3-ulose 1. Each of α- and β-substituents introduced onto the C-3 carbonyl group of 1 must sterically interact with the bulky α-oriented 1,2-isopropylidene group and the β-oriented substituent at C-4, respectively. These steric interactions may dictate the conformation of the substituents, thereby controlling stereochemistry of reactions on the chiral template 1. Indeed, we were successful to develop highly enantioselective synthesis of D- and L-alanine and chiral deuterated glycines as well as 3-isopropylmalic acid. In the present studies, theoretical analysis has been carried out for the aforementioned transcription methodology based on the calculation of transition states. Transition-state structures of the allyl trichloroacetimidate rearrangement leading to chiral amino acids were calculated by the semi-empirical MNDO-PM3 method. Experimentally, kinetic parameters of the reaction were collected by variable temperature studies within a NMR tube. The calculated results were in good accordance with all of available experimental data. Apparently, an α-oriented bulky substituent at C-3 is a crucial determinant for highly enantioselective chirality transcription on the template 1. With chirally monodeuterated glycerol prepared by this chirally transcription approach, stereochemical studies of the triose-phosphate isomerase reaction were carried out with the halophilic archaebacteria Halobacterium, which appeared to show that the 3R hydrogen of dihydroxyacetone-phosphate is exchangeable with a medium.
Glutamate receptors mediate synaptic excitation in the mammalian central nervous system. The receptors have been classified into two major classes, ionotropic glutamate receptors and metabotropic glutamate receptors. Our recent work concerning the syntheses and the pharmacology of L-2-(carboxycyclopropyl)-glycines (L-CCG-1〜IV) have demonstrated that an extended conformer of L-glutamate is one of the most effective factor for activating metabotropic glutamate receptors, and a folded one for NMDA receptors. In this study, conformational requirements of glutamate receptors regarding rotamers around α-amino acid moiety have been examined by the spectroscopic studies of CCGs and by the synthesis of the 3'-substituted analogues of CCGs, L-2-(2-carboxy-3-methoxymethylcyclopropyl)glycines (cis- and trans-MCGs), which freeze CCG's possible rotamers at C2-C1'. NMR studies of CCGs under variable pHs and variable temperatures indicated that the C2-C1' bond of CCGs can not freely rotate but relatively fixes either CC or NC conformer in aqueous solution (Figure 2). This was well supported by the facts that the conformationally frozen analogues of CCGs [cis-MCG-I (NC conformer) and IV (CC conformer), Figure 4,5], synthesized from D-serinal derivative in a stereocontrolled manner, showed similar activities as that of L-CCG-I and IV, respectively. Thus, we propose that the conformational requirement of metabotropic glutamate receptors is an anti-(NC) conformer of L-glutamate and that of NMDA receptors is a gauche-(CC), as shown in Figure 6. In addition, trans-MCG-IV and CPG-IV exhibited kainate-type responses at the dorsal root C-fibre of the immature rat. These results suggest that a folded form of L-glutamate, gauche-(CC), is also responsible for activating kainate receptors.
According to Schultz's procedure, immunization of the KLH-phosphonate 1 adduct 2 afforded 34 IgG's. After purification by standard protocols, of the 34 antibodies isolated, 4A1, 5H2, and 1G2 were found to have catalytic activities for carbonate 3a with values of kcat/kuncat of 3×10^3, 450, and 440 at 37 C, pH 8.5, respectively. In the examination of substrate specificity for these abzymes in the vicinity of scissile bond, however, one of antibodies, 4A1 showed unpredicted rate acceleration for (S)-13 having 2,2-dimethyl-1,3-dioxolane group with value of kcat/kuncat of 6.4×10^4. Similar propensity was also observed in the case of the R and S isomers of 13-15 subjected to significant changes in structure around the scissile bond. When these values are compared with 3.7×10^3 for carbonate 6 most similar to the induced hapten structure, it is indicated that an accepted propensity, which demand fairly strict substrate homology to the hapten, is not always preserved in this case. In order to explain the unprecedented substrate specificity for 4A1 antibody, several kinetic parameters (km, kcat, kcat/km) and dissociation constants (kd) for various substrates and transition state analogs were determined. These analyses were strongly suggested that another binding element should exist in the vicinity of 4A1 combining site. This assumption was finaly demonstrated by product inhibition of substrate 22 containing a-acetylamino phenetyl group. Thus, we have demonstrated that 4A1 antibody has an unexpected binding element(s) in close proximity to the active site. It is probable that large catalytic efficiency for the modified substrates is due to cooperative influences between the active site and the subsite,
Neocarzinostatin (NCS) complex is a potent antitumor antibiotic, in which a nonprotein chromophore (NCS-chr, 1) is tightly and specifically bound to an apoprotein (apo-NCS, 113 amino acids). NCS-chr with an unusual structure responsible for DNA cleavage is very labile to heat, light and higher pH(>6) when isolated, but greatly stabilized through binding to apo-NCS. Their binding structure and the stabilizing interactions are very interesting problem in terms of molecular recognition and protein transport. Here we report the three-dimensional structure of NCS complex determined by the 2D-NMR analyses and the distance geometry calculations. The DADAS90 calculations of the tertiary structure of apo-NCS part based on the constraints obtained by 2D-NMR analyses revealed that it consists of three antiparallel β-sheet structural domains, i.e., the external three-strand β-sheet, the internal four-strand β-sheet, and the small two-strand β-sheet. Their foldings are similar to those reported for related protein antibiotics, actinoxanthin and auromomycin, and the internal β-sheet forms the hydrophobic pocket (12 x 9 x 9A). The DGEOM calculations by using the best DADAS90 structure and intermolecular NOEs showed that the naphthoate moiety of NCS-chr sits on the bottom of the pocket while the aminosugar and the carbonate group face outwards. It indicates that the interactions between the some amino acid residues of the pocket and the naphthoate moiety are essential for the binding, which has been confirmed by the high association constants of the model compounds with naphthoate moiety (2-5) to apo-NCS. Interestingly, when bound to the apoprotein, the aminosugar moiety turned out to have an induced conformation in which the protonated methylamino group is covering the active center C12 of the core of chr. This steric hindrance is very likely to lead to stabilization.
Phototropism is the curvature of a plant organ under the influence, and particularly also of the direction of the irradiation. Went (1928) proposed that phototropism is caused by a lateral gradient of growth-promoting auxin in the bending organ. This led to the Cholodny-Went theory of photo- and geotropic curvatures (1937). This elegant theory, is it true? Bruinsma and Hasegawa (1990) recently found that the shaded half did not contain more auxin than the illuminated one as required by the C.-W. theory. Instead it was found that the even distribution of auxin was accompanied by a lateral gradient of growth-inhibitory substances during phototropic curvature. In the radish seedlings, the inhibitors playing a role in phototropic curvature have been identified, and their role has been largely elucidated by Hasegawa's group. The activity is mainly exerted by raphanusanins and, to a smaller extent, by raphanusamide. The chemical structures of raphanusanins and raphanusamide were once reported to be piperidinethione and 1,3-oxazepin-2-one derivatives, respectively. However, their structures were recently revised by X-ray crystallographic structure analysis and chemical synthesis (Harada, Yamamura, and Hasegawa): raphanusanin A, (3R^*, 6R^*)-3-[methoxy(methylthio)methyl]-2-pyrrolidinethione; raphanusanin B, (3R^*, 6S^*) isomer, raphanusamide, (E)-3-methoxymethylene-2-pyrrolidinethione. The structure-activity studies of raphanusanins and analogues clarified that thioamide and thioacetal or thiomethylmethylene moieties were important for their growth inhibitory activities.
Human hepatoenteritis caused by toxic cyanobacterial blooms in domestic water supplies that have become eutrophic is a growing concern. In 1979 an outbreak of hepatoenteritis on Palm Island in Australia was traced to a cyanobacterium, Cylindrospermopsis raciborskii, which had never been implicated before in poisonings. We describe the isolation and structure determination of an unusual alkaloid, cylindrospermopsin, that is responsible for the hepatotoxicity of this alga. Cylindrospermopsin (1), LD_<50>=0.5mg/kg in mice by intraperitoneal injection, was obtained as white microcrystals from aqueous extract of the alga. The molecular formula, C_<15>H_<21>N_5O_7S, of 1 was determined by HRFABMS. The UV spectrum (λ_<max> 262nm) and ^<13>C NMR (δ 154.8, 168.2, 99.6, 158.1) were consistent with 1 being a substituted uracil. By means of 2D NMR spectroscopy such as HMBC partial structure (Figure 3) was deduced. Consideration of NOEs, proton-proton coupling constants, and the deuterium-induced chemical shift experiments (^13C NMR) led us to structure 1. This structure was confirmed by a C, C-COSY spectrum of 80+% ^<13>C-enriched toxin and N, C-COSY of 80+%^ <13>C, 90+% ^<15>N-enriched toxin, produced by the alga grown on ^<13>C-bicarbonate and ^<13>C-bicarbonate and^<15>N-nitrate, respectively.
Identification of a probable ciguatoxin precursor, GT4B, in the dinoflagellate Gambierdiscus toxicus collected from the wild suggested the organism as the primary source of toxins implicated in ciguatera. Nevertheless, unsuccessful results in many laboratories to prove the toxins in cultured organisms have casted a doubt about the true origin of the toxins. In view of the frequent intraspecies variation in the type of secondary metabolites of microorganisms, we tested four different strains of G. toxicus production of ciguatoxin congeners. One strain collected at Rangiroa Atoll of French Polynesia was proven for the first time to produce three ciguatoxinlike toxins coded RGT1, CTX3C, and CTX4A, respectively. HR-FABMS data of RGT1 gave a molecular formula C_<43>H_<64>O_<11>. Extensive 2D NMR experiments revealed that RGT1 has a novel skeleton consisting of eight contiguous ether rings (6/6/6/6/7/6/6/7) fused in trans (Fig. 1). The structure of CTX3C (C_<57>H_<82>O_<16>, Fig. 3c) closely resembled that of GT4B (Fig. 3b) but lacked the butadiene side chain of the latter. The oxepene ring (E) of GT4B was replaced by an oxocene ring in CTX3C. The molecular formula of CTX4A was the same as that of GT4B (C_<60>H_<84>O_<16>). Comparison of NMR data revealed that the two are the epimers at the spiroketal carbon.
Zooxanthellae are typical symbiotic dinoflagellates associated with a wide range of marine invertebrates and they play important roles such as CO_2 fixation and production of organic molecules at marine environments. While screening symbiotic microorganisms from marine invertebrates for bioactive compounds, we discovered potent vaso-constrictive polyols named Zooxanthellatoxin-A (ZT-A) and ZT-B from a cultured zooxanthella, Symbiodinium sp. isolated from the flat worm Amphiscolops sp. collected at Okinawa. Zooxanthellatoxins are sulfate esters of highly unsaturated polyols with a molecular weight of ca. 2900. Periodate oxidation of ZT-A gave seven fragments (A〜G). Among these fragments, the most charactristic fragment E (C_<47>H<76>O_<14>) was established to be an allylic alcohol-rich polyhydroxypolyene with a diepoxide unit. The structure of fragment A (C_<56>H_<91>O_<22>SNa) was shown to be a sulfate ester of a polyol having an elongated structure of fragment E. The other fragments are peracetylated and the structures of the acetates were determined by homo-spin decoupling and NOE experiments. These results revealed that Zooxanthellatoxins are a new type of marine toxins.
In our search for bioactive compounds from marine microorganisms, we have examined extracts of symbiotic bacteria associated with various marine invertebrates and isolated a new cytotoxic alkaloid, alteramide A (1), from the cultured mycelium of a bacterium Alteromonas sp. separated from the marine sponge Halichondria okadai. The molecular formula of 1 was determined to be C_<29>H_<38>N_2O_6 on the basis of HRFABMS. The ^1H and ^<13>C NMR spectra of 1 indicated the presence of a dienone and a dienoyltetramic acid. The whole structure of 1 was elucidated by extensive analyses of ^1H-^1H COSY, HMQC, and HMBC spectra and the relative stereochemistry of 1 was established by the NOESY spectrum of compound 2 derived from 1. The hexacyclic derivative 3 was generated photochemically from 1 through a unique intramolecular [4+4] cycloaddition. Alteramide A (1) exhibited cytotoxicities against murine leukemia P388 cells, murine lymphoma L1210 cells, and human epidermoid KB cells in vitro with the IC_<50> values of 0.1, 1.7, and 5.0μg/mL, respectively, while compound 3 showed no cytotoxicity.
Ohwaraitake is Japanese name of a poisonous mushroom Gymnopilus spectabilis and means "a loud laugh mushroom". Accidental ingestion of it causes hallucinosis and abnormal behavior. A hallucinogenic mushroom contains usually psilocybin or its analogues but it is reported that any psilocybin or its congenor does not found in Japanese Ohwaraitake. The toxic symptoms suggest that any neuroexcitatory substance must be contained. The isolation was carried out monitoring depolarizing activity on the new born rat spinal cord. 1. Chromatographical fractionation and bisassay showed that the neuroexcitatory active compounds were gimnopilins (G) which were known as bitter principles of this fungus. Further fractionation revealed that G (1) was inactive and activities of G (2) increased from n=7 to n=5 and besides, G (2), which was newly isolated this time, was more active than G (2). 2. The structure of the new G was determined by the chemical degradation as formula 3. 3. The Chirality of hydroxymethylglutamic acid (HMGA) part was determined as S configuration by the acquisition of (R)-mevalonaloctone through LiBH4 reduction or G (1 and 2). 4. G (2) does not work as an ion carrier probably, since it does not particularly take metal ion from aqueous solution into an organic solvent. 5. Determination of the chirality of tert-alcohols in gymnoprenol (G'), that is, G without HMGA part, was attempted. Eight isomers of the model compounds corresponding to G' (m=2, n=4) were synthesized and were not discriminated by HPLC and NMR. Since two diastereoisomers of MTP-ethoxy-methoxy derivatives of G' (m=1, n=2) were distinguished by ^1H and ^<19>FNMR, these derivatives may be applicable to identification of the synthetic and the natural G'.
The crude extract of Eurycoma longifolia Jack (Simaroubaceae) has long been used as one of traditional folk medicines in the Southeast Asia. During the survey of cytotoxic antitumor compounds from the title plant, three novel cytotoxic squalene-type triterpene ethers, named eurylene (1), 14-deacetyl eurylene (2) and longilene peroxide (3), and 14 novel quassinoids were isolated from the woods together with teurilene (4). Their structure were elucidated by spectroscopic data, chemical evidence and X-ray analysis. The absolute stereostructure of 1 was determined by an advanced Mosher's method. Molecular conformation of 1 with linear form was different from those of 3 and 4 with a turn structure. Conformational analysis of 2 showed that 2 possess the similar turn structure with an intramolecular hydrogen bonding to 3 and 4. The new skeleton C_<19>-type quassinoids, named as longilactone and 6-dehydroxylongilactone, were found to form a lactonic linkage at C-7 and many C_<20>-type quassinoids were isolated. These structures were elucidated by spectroscopic methods and X-ray analysis. The cytotoxic activities of these squalene-type triterpene ethers and quassinoids are also discussed.
Bulbs of the lily Lilium maximowiczii (Japanese name: Yurine) have been used in Japanese dishes. Basal rot caused by Fusarium oxysporum has been a serious disease of Lilium maximowiczii. Ethyl acetate extract obtained from the diseased lily bulbs was separated by repeated chromatography, guided by TLC bioassay, to give two types of antifungal compounds: yurinelide (1) and chlorine-containing compounds (6 to 12). The first lily phytoalexin yurinelide was also induced in UV-irradiated or Fusarium oxysporum-inoculated bulb scales. Its unique 3-benzylidene-1, 4- benzodioxin-2(3H)-one structure was elucidated on the basis of HMBC, HMQC, and NOE experiments, as well as methanolysis products of 1. The structures of chlorine-containing compounds were elucidated as chlorinated orcinol derivatives from spectral data and synthetic studies. Timecourse study of these compounds in UV-irradiated lily scales indicated that these are produced in the plant tissue under stressed conditions. Increase of chloroperoxidase activity in UV-irradiated tissue suggested that these organochlorine compounds are synthesized by chlorination of non-chlorinous precursors with Cl^- and stress-induced H_2O_2 and chloroperoxidase.
Cladosporol(1) was isolated from the culture filtrate of a fungus, Cladosporium cladosporioides, which showed activity promoting root elongation of lettuce seedlings. The molecular formula C_<20>H_<16>O_9 was determined from the HR-EIMS of its triacetate. Cladosporol has two acetophenone-type chromophores indicated by UV spectrum and two of a conjugated and a hydogen bonded conjugated ketone from IR spectrum. The six partial structures A to F were deduced from the spectral analysis of ^1H-NMR, ^<13>C-NMR, DEPT, HSQC and ^1H-^1H COSY of cladosporol and triacetylcladosporol. These partial structures were unambiguously combined together to construct the plain structure of cladosporol based on the HMBC analysis of triacetylcladosporol. The absolute stereochemistry of an epoxyalcohol moiety(C_2,C_3,C_4) was determined from the J H-H values of hydrogens on these carbons and positive Cotton effect at 340nm comparing with those of the known compounds, (+)-epoxydon and (+)-isoepoxydon. The strong splitting Cotton effect centering at 216nm, observed in the CD of cladosporol, should be derived from the exciton chirality of two tetralone chromophore interaction reflecting the remaining C4'. The most stable conformations of two epimers at C4' were calculated by a computer with QUANTA and the sign of splitting Cotton effect was estimated for each epimer. The CD spplitting pattern of the S epimer agreed with the observed CD. Thus, the structure of cladosporol was elusidated as shown in Fig. 5 with absolute stereochemistry.
Novel phospholipase A_2 (PLA_2) inhibitors were obtained from the culture broth of a fungus: Thielavia terricola RF-143. Bioassay guided isolation afforded twelve new compounds: thielocins A1α (1a), A1β(1b), A2α (6a), A2β (6b), A3 (fig.3), A4α (fig.4), A4β (fig.4), B1 (12a), B2 (16) and B3 (20) along with thielavins D (32) and E (33). Structrues of these novel compounds were determined on the bases of chemical degradation and spectral analysis. Method of synthesis of thielocin B3 was developed strating from thielavin B which was produced in large amount in the culture broth. Total synthesis of the tridepside thielavins B and D were also reported. Studies on their PLA_2 inhibitory activities have shown their specificity on the group II type enzymes. IC_<50> values of the most potent compound, thielocin B3, were 0.074μM for human synovial fluid PLA_2, 4.2μM for the human pancreas enzyme, respectively.
Gilvocarcin M (1) shows the key structural features of a growing class of aryl C-glycoside antibiotics which share a common aromatic nucleus, 6H-benzo[d]naphtho[1,2-b]pyran-6-one, to which various rare sugars are connected through a C-C bond. The attractiveness of these compounds as synthetic targets is due to the challenges presented by their unusual C-glycoside structures linked to the highly functionalized skeleta, and because some of the members show significant antitumor activity with exceptionally low toxicity. The reported synthetic endeavors, however, have addressed only the aglycon portion (i.e., defucogilvocarcin), and the synthesis of the full structure of natural product with the sugar moiety remains a challenging problem. In the present study, we accomplished the first total synthesis of both enantiomers of gilvocarcin M (1). The synthetic route is exceedingly concise and effective by virtue of two selective conversions. (1) O→C-Glycoside rearrangement: Coupling of the acetate 19, derived from L-fucose, with the phenol 20 proceeded regioselectively to give C-glycoside 21. Some silver perchlorate reagents or Cp_2HfCl_2-AgClO_4 effected stereoselective coupling to give the desired, contra-steric, α-L-C-glycoside 21-α. (2) Benzyne-furan cycloaddition: The benzyne, generated from ortho-haloaryl triflate 22 by halogen-lithium exchange, underwent [4+2]-cycloaddition with 2-methoxyfuran (17) to yield the 1,4,5-naphthalenetriol derivative 23 with high regioselectivity. Acylation of 23 with the acid chloride 24 followed by a Pd-catalyzed cyclization and final deprotection gave 1. The overall yield was 41% for six steps. Synthetic 1 ([α]^<23>D+209°) turned out to be the enantiomer of the natural gilvocarcin M ([α]^<20>D-209°). The natural enantioner of 1 was synthesized from D-fucose by the same scheme thereby establishing the D-absolute configuration of the natural product.
A highly enantio-selective ene reaction was developed on the bases of chirality transfer. An optically pure ene substrate (S)-3-(tert-butyldimethylsiloxy)-2-(ethylthio)-1-butene 1b was prepared in three steps from (S)-ethyl lactate. Under the influence of Me_2AlCl, the reactions of 1b with aromatic, aliphatic, or α, β-unsaturated aldehydes afforded the optically active adducts 2 in high ee. Based on these results, an efficient synthetic method of an optically pure anthracyclinone precursor was developed. The ene reaction of aldehyde 10 with 1c afforded the almost enantiomerically pure adduct 11 in high yield. Diol 12 was prepared through regioselective lithiation and hydroxymethylation. Construction of A-ring to give ketone 15 was performed by trichloro-acetylation of 12 followed by treatment with Me_2AlCl. Diastereoselective oxidation of enol silylether 18, which was obtained through reductive silylation of α-(methylthio)ketone, afforded the anthracyclinone precursor 19.
Aplysiatoxin (1a) and debromoaplysiatoxin (1b) isolated from the digestive grand of sea hare, Stylocailous longicauda, are active tumor promoter and are structurally characterized by spiroacetal, large diolide ring, and four consecutive asymmetric centers. Although many synthetic efforts have been directed toward 1, the total synthesis of 1 was achieved only by Kishi et al. Now we would like to report here a formal total synthesis of 1 that was achieved by the convergent synthesis of Kishi's intermediate 2 because 2 contained all eight asymmetric centers required for the synthesis of 1 and had been reported to be convertible to 1 by further 6 steps. In a retrosynthetic analysis, 1 could be divided into four fragments, B, C, D and E as shown in Fig. 1. These fragments were synthetically corresponding to 9 (Scheme 1), 14 (Scheme 2), 24 (Scheme 3 and 4) and 30 (Scheme 5) which were prepared by using asymmetric epoxidation as a key reaction. With these fragments in hand, the construction of 2 was commenced by a coupling of 14 and 24 (Scheme 6). The desired coupling product 31 was converted to terminal epoxide 19 which was coupled with 30 to afford 2 after esterification with 9 by using Yamaguchi method (Scheme 7).
Swinholide A (1), isolated from a sponge of the genus Theonella swinhoei, is a unique 44-membered dimeric macrolide and exhibits potent cytotoxicity against KB and L1210 tumor cells in vitro. We now report the synthetic studies of swinholide A (1) and the related compounds. 1) Synthesis of the C-11〜C-23 Segment Aldol condensation of aldehyde 12 (2 eq) and MeNO_2 (1 eq) in the presence of Et_3N under super high pressure (11Kb) produced diol 13 effectively. 13 was converted into 19 via an epimerization of ketone 14. On treatment of 19 with BH_3・Me_2S, hydroboration of exomethylene and successive reductive cleavage of p-methoxybenzylidene acetal took place stereoselectively producing diol 20. Treatment of lactone 24 with LDA, MeI gave a single α-methyl lactone 25 which was converted into aldehyde 27. Aldol condensation of 27 and 28 produced 29 as a single product which was transformed into aldehyde 31. 2) Synthesis of the C-24〜C-31 Segment Ketone 35 was synthesized starting from methyl (S)-hydroxy butyrate (32) via lactone 34. Reduction of 35 with LiAlH_4 followed by methylation afforded α-methoxy compound 36 exclusively. Treatment of 36 with allyltrimethylsilane in the presence of BF_3・Et_2O gave only β-allyl compound 37 which was converted into 41 via ester 39. 3) Synthesis of the C-11〜C-31 Segment Aldol condensation of aldehyde 31 and 41 gave a single product 42 which was converted into alcohol 44. Synthesis of 1 and 2 from 44 is now in progress.
Among a number of attempts to improve the biological properties of erythromycin A (1) and B (2). we have reported that 6-O-methylated erythromycin A possesses high acid-stability and pertains strong antibacterial activity. In the present work, the reported contrast about the regioselectivity in the O-methylation of erythromycin derivatives, namely preferential reactions at C11-hydroxy group for erythromycin A derivative 3, and at C6 hydroxy for erythromycin B derivative 4 and 9-oxime of erythromycin A derivative 5, was studied by generating and examining significantly populating conformers using a conformational space search algorithm (CONFLEX3). Global energy minima of model compounds 6-8 have almost the same ring structure which agrees with the X-ray structure of 1. We observe that, in this conformer, C11-OH is less reactive in the O-methylation than C6-OH because the C11-hydroxy proton forms strong intramolecular hydrogen bonds with carbonyl oxygens at C1 and C9, and also with C6-hydroxy oxygen. A new conformation having C11-hydroxy group in sterically accessible position appears only in 6 in significant population. On the other hand, C6 and C12-OH are held in the ring by formation of a hydrogen bond with C9-carbonyl oxygen. Analyzing of predominantly distributed conformers of model compounds, we conclude that the origin of determining the regioselectivity between C6 and C11-hydroxy groups in the O-methylations of erythromycin derivatives lies in the formation and deformation of intramolecular hydrogen bondings in significantly populating conformers.
The polyether antibiotics, well known as the ionophore, have provided a widespread field for organic chemists because of their complex structures and biologically important properties. In connection with our interest in establishing a common synthetic methodology of the polyether antibiotics, we chose four representative polyether antibiotics, lysocellin (1), salinomycin (2), lasalocid A (3), and isolasalocid A (4), as the synthetic targets. Syntheses of three compounds, 2, 3, and 4 have already finished. We describe herein the stereocontrolled total synthesis of lysocellin (1), remained final target molecule. Lysocellin (1), isolated from Streptomyces cacaoi var asoensis in 1975, is the first and representative member of the lysocellin family. The relative and absolute configuration was determined by X-ray crystallography of the silver salt. Lysocellin (1) was retrosynthetically divided into three convenient fragments, C1〜C9 fragment (6), C11〜C15 fragment (9) and C16〜C23 fragment (10) respectively. Two fragments 6 and 9 were stereoselectively prepared from D-glucose. On the other hand, 10 was also stereoselectively synthesized from the β-ketophosphonate (21) and the aldehyde (22), derived from D-mannitol, using the stereocontrolled Michael reaction with Me_2CuLi and the Grignard reaction to the α, β-unsaturated ketone (23). After coupling 9 onto 10, 28 was converted into the triol (37) via 34 and 35. PCC oxidation of 37 and the, subsequent acid treatment of 38 stereoselectively afforded the γ-lactol derivative (38a). Finally, the zinc enolate of the ethylketone (7) was coupled with the aldehyde (6) to give the lysocellin (1), whose sodium salt was identical, in terms of its melting point, optical rotation and spectral data (^1H NMR and IR), with the natural product.
(+)-Asperlin (3) of antibiotic and antitumor activities is recognized structurally as the most fundamental and smallest representative of biologically active 5-oxygenated 5,6-dihydro-2-pyrones with the oxygen-functionalized sidechain at the 6-position. We have developed a short-step transformation of tartaric acid (6) as a chiral source effectively utilizing the C_2 symmetry to optically active 6,8-dioxabicyclo[3.2.1]octane derivative (14) inaccessible from common 6-carbon sugars, and applied the method to natural product synthesis. Now we report a stereoselective total synthesis of (3) starting from (S,S)-tartaric acid (6) featuring regioselective olefination of the bicyclic system (14), the method of which was utilized to a short-step synthesis of optically active 7-exoethyl-5-methyl-6,8-dioxabicyclo[3.2.1]oct-3-ene (8), one of the components of the pheromone of the male house mouse Mus musculus, and regio- and stereoselective introdduction of the chiral oxygen functionalities at the C(2)- and C(7')-positions of the bicyclic olefin (16) (the latent 5- and 8-positions of 3, respectively) under favor of the architecture of 16. Furthermore, a mild and efficient method for construction of unstable oxygenated 5,6-dihydro-2-pyrone skeleton through partial ring opening of the bicyclic system providing a pyranoid (25) followed by MnO_2-oxidation of unsaturated cyclic hemiacetal (30) was developed. The strategy described is believed to be potential for wide applicability to syntheses of biologically important oxygenated 6-substituted-2-pyrones.
A water-soluble basic antibiotic bellenamine produced by Streptomyces nashvillensis MD743-GF4 has a unique open-chain aldoaminal structure and D-β-lysine which has been first found as the natural amino acid. Bellenamine inhibits weakly the growth of some Gram-positive bacteria and has strong activity against human immunodeficiency virus (HIV). The absolute structure, (R)-N-aminomethyl-3,6-diaminohexanamide was confirmed by the total synthesis which was achieved by a modified Curtius procedure starting with D-β-lysine. Bis(N-Cbz)D-β-lysylglycine was converted to tris(N-Cbz)bellenamine through the acid azide. Removal of the protecting groups gave bellenamine. D-β-Lysine was synthesized from D-ornithine by the Arndt-Eistert homologation sequence. L-β-Lysyl, D- and L-lysyl congeners synthesized by a similar method, showed no antibacterial activities. In an acidic solution at 75℃, bellenamine was converted into D-β-lysinamide and a new cyclized compound, (R)-6-(3-aminopropyl)-1,3-diazacyclohexan-4-one. Bellenamine was produced in a synthetic medium containing D-galactose, dextrin, (NH_4)_2SO_4 and CaCO_3. The biosynthesis was studied by feeding ^<13>C and ^<15>N labeled precursors to the synthetic medium. The high incorporation of D-[1-^<13>C]β-lysine indicated that it is a direct intermediate. While, supplement of L-β-lysine repressed the productivity of bellenamine. [2-^<13>C]Glycine was well incorporated into CH_2 of the aldoaminal. All four nitrogens were derived from [^<15>NH_4]SO_4 in the synthetic medium, and the presence of L-lysine and glycine showed high incorporation of ^<15>N into CONH. The feeding experiments of ^<13>C labeled acetates suggested that the D-β-lysine moiety was derived from L-lysine by catalysis of a new 2,3-aminomutase, and L-lysine was biosynthesized from acetates via the TCA cycle and diaminopimelic acid pathway.
The most important function of carotenoid pigments is thought to protect organisms against photooxidative damage. carotenoids also function as a precursor of vitamin A in mammals. The enzymes and genes which mediate the biosynthesis of carotenoids are virtually unknown. We have elucidated for the first time the pathway for their biosynthesis at the level of enzyme-catalyzed reactions, using bacterial carotenoid biosynthesis genes. These genes were cloned from a phytopathogenic bacterium, Erwinia uredovora, in Escherichia coli and located on a 6,918-bp fragment whose nucleotide sequence was determined. Six open reading frames were found and designated the crtE, crtX, crtY, crtl, crtB, and crtZ genes. the carotenoid biosynthetic pathway in Erwinia uredovora was clarified by analyzing carotenoids accumulated in E. coli transformants in which some of these six genes were expressed, as follows: FPP crtE GGPP crtB phytoene crtl lycopene crtY β-carotene crtZ zeaxanthin crtX zeaxanthin-β-D-diglucoside. β-Carotene was produced in some microorganisms such as Saccharomyces cerevisiae which can produce no carotenoid, by introducing the four crt genes required for β-carotene biosynthesis. Some carotenoid biosynthesis genes of the other organisms such as higher plants were also functionally assigned.
The anthelmintic antibiotic avermectin complex produced by Streptomyces avermitilis is a family of eight closely related components. Our interests are focused breeding mutants which produce new components and selective producer of a useful component(s) alone. To resolve the above subjects, we investigated the biosynthesis of avermectin by genetic and molecular genetic approaches. Several kinds of biosynthetically blocked mutants and one multiple-blocked mutant were isolated. The latter mutant produced four new components (I, II, III and IV). All mutations of blocked mutants tested were mapped as a cluster on the chromosome. This was also supported by the result of cloning of genes for avermectin biosynthesis. Designing the selective producer which produces a useful component(s) alone was performed by in vivo recombination. One of recombinant strains produced only two components of avermectins. In addition, a toxic compound oligomycin produced by S. avermitilis is difficult to be separated from avermectin components then S. avermitilis which did not produce oligomycin were obtained by disruption of the oligomycin biosynthetic gene by transposon(Tn4560) insertion mutagenesis.
(4S)-(-)- and (4R)-(+)-Limonenes (2a and 2b) are biosynthesized from geranyl diphosphate (GPP: 1a) with limonene synthase in Mentha spicata and Citrus unshiu, respectively. We have investigated the stereocontrol in the biosynthesis of the limonenes, and now describe evidence involving the role of the divalent metal ion, the mechanistic interpretation of the biospecific generation of the chirality at C-4 from non-chiral 1a and the regioselective formation of the double bond at C-8. Limonene synthase was activated only in the presence of divalent metal ions. The metal ion, as studied with Mg^<2+>, was found to form a 1 to 1 chelate at an equivalent distance from both P_α and P_β of 1a and to facilitate the elimination of the diphosphate moiety by weakening the C-OP bond, and the subsequent formation of a cationic intermediate that can readily cycling into limonene. Studies using (1S)-[1-^2H] and (1R)-[1-^2H]GPPs and (3S)-(+)-, (3R)-(-)-, and (3RS)-LPPs as substrate indicated that the cationic intermediate is generated from 1a by the limonene synthase of M. spicata by elimination of the diphosphate moiety at si-si face of the C-2 double bond and is in the anti-endo spatial arrangement leading to the formation of 2a. In the case of C. unshiu, in contrast, the elimination of the diphosphate moiety took place at the re-re face of the C-2 double bond and to give the anti-endo cationic intermediate which is enantiomeric to the intermediate in the case of M. spicata. The chiral generation at C-4 in the biosynthesis of limonene was found to be determined by the facial direction of elimination of the diphosphate moiety from 1a. The C-8 double bond of both 2a and 2b was proved to be formed by regioselective elimination of the hydrogen atom from the C-10 methyl group of 1a, on the basis studies using [8,8,8-^2H3] and [10,10,10-^2H3]GPP as substrate.
Aculeximycin (ACM) was isolated from a culture of Streptosporangium albidum, which exhibits an inhibitory against Gram-positive bacteria, fungi and yeast. It also shows a potent uncoupler of oxidative phosphorylation with rat-liver mitochondria. N-Diacetyl derivative (2) of ACM was degraded by the following sequence, 1) DBU-MeOH, 2) O_3 / NaBH_3CN and 3) NaIO_4/NaBH_4. Treatment of 2 with 5% DBU-MeOH gave an N-Ac-aculexitriose (3) and an epimeric pair of counterparts named N-Ac-pseudoaglycones I (4) and II (5). Ozonolysis of 4 followed by reduction with NaBH_3CN yielded three components, AOP-I (6), AOP-IIs (7) and AOP-IIIs (8). Further oxidation of 8 with NaIO_4, followed by NaBH4 reduction afforded APP-I (12) and APP-II (13). The structures of these degradation products and their derivatives have been determined by spectroscopic analyses. The relative and absolute configurations of them were tried to determine by NOE, Rychnovsky's, advanced Mosher's and/or exciton chirality methods. The structures of 8 were established by deuterium labeled experiment and preparation of partial periodate oxidative products, APP-I+C1 and DM APP-II+C2. A construction of 6, 7 and 8 moieties leading to 4 was accomplished by application of HMBC and HOHAHA to 4. The location of the remaining glycosidic linkage proved to be at C-11 in consideration of a mechanism of a specific glycosidic bond cleavage with DBU and the resulting epimeric mixture of 4 and 5. Finally, the total structure of ACM can be proposed as shown in Fig. 1.
Xanthoquinodins A, B, C and D produced by Humicola sp. FO-888 were found to exhibit anticoccidial activity in an in vitro assay using Eimeria tenella as a parasite and BHK-21 cells as a host. The deduced structures of xanthoquinodins were confirmed by analysing ^<13>C-labeled xanthoquinodins prepared biosynthetically in ^<13>C-labeled precursor feeding experments on the basis of extensive spectroscopic techniques including ^1H-^1H COSY, ^<13>C-^1H COSY and HMBC. Xanthoquinodins are heterodimers of a xanthone and an anthraquinone moiety, both of which are biosynthesized via octaketide. The stereochemistry of xanthoquinodin B were determined by NMR including NOE experiment. Accordingly, the stereochemistry of xanthoquinodins A, C and D were also deduced by studing the mechanism of nonenzymatic interconversion among xanthoquinodins.
Enacyloxins (RNXs) are antibiotic metabolites produced by Frateuria sp. W-315 cultured in the spent medium of N. crassa, and were shown to be active against Gram-positive and Gram-negative bacteria. The chemical structure of ENX IIa, the main component and the most active congener among ENXs, was studied via spectroscopic and degradative works. It was elucidated as a novel structure of (1S,3R,4S,2'E,4'E,6'E,8'E,10'Z,20'E)-3-(19'-carbamoyloxy-11',18'-dichloro-6',12'-dimethyl-15'-oxo-13',14',17'-trihydroxytricosa-2',4',6',8',10',20'-hexaenoyloxy)-4-hydroxy-1-cyclohexanecarboxylic acid. ENX IVa, isolated as a biosynthetic intermediate of ENX IIa, was determined be 15'-hydroxy analog of ENX IIa. Conversion of ENX IVa to ENX IIa was catalyzed by the soluble enzyme present in the culture medium of Frateuria. The enzyme exhibited the oxidative activity even in a refrigerator. At room temperature, ENX IVa in the culture supernatant encountered the enzymatic hydrolysis of carbamoyl moiety as well as the above mentioned oxidation. Factors concerning the production of ENXs in the spent medium of N. crassa were also investigated.
In our continuing search for physiologically active substances, we recently found two novel DNA topoisomerase inhibitors, BE-13793C (1) and BE-22179 (2) from a Streptoverticillium sp. and a Streptomyces sp., respectively. BE-13793C (1) was the novel indrocarbazole antibiotic having hydroxyl groups, which exhibited remarkable DNA topoisomerase-inhibiting activity and also antitumor activity. However, the lack of solubility in water hampered further evaluation as anticancer agents. Therefore, ED-110 (3), a glycosyl derivative of 1 was synthesized by the reaction sequencing as shown in scheme 2. The anticancer candidate (3) showed improved solubility and superior cytotoxicity than the aglycone (1). Interestingly, the desired compound (3) only exhibited DNA topoisomerase I-inhibiting activity. BE-22179 (2) is one of the novel cyclic peptide antibiotic which has interesting functional groups; S-S bridge, quinoline rings and thioester, and exhibits significant DNA topoisomerase II inhibition. Although the NMR spectrum of the depsipeptide (2) in CDCl_3 suggests the presence of three conformers, the diacetate of 2 exists as one predominant conformer, and consequently structure elucidation has been achieved fortunately. Furthermore, the structure-activity relationship is also described.
The chemical structure of Cyclothiazomycin with renin inhibitory activity has been given as a bicyclic structure including non-common amino acids by conventional NMR spectroscopy. To confirm the stereochemistries, a program called DADAS90, which is served to generate three-dimensional structures of molecules satisfying the NMR-obtained distance constraints, was applied. DADAS90 implemented in the MolSkop system (JEOL Ltd.) can analyze the conformations of natural organic compounds with cyclic structures of non-common amino acids as well as linear peptides of common amino acids.
A number of complicated polycyclic compouds have been found in nature. A novel approach to the polycyclic natural products was developed using Intramolecular Tandem Reactions by the treatment with trialkylsilyl triflate or trialkylsilyl halide in the presence of amine. (A) Construction of Bicyclic Compounds Having an Angular Nitrogen -Asymmetric Synthesis of (-)-Tylophorine- It was found that double Michael reaction of the α, β-unsaturated enamides (1) is taken place by treatment with TBSOTf in the presence of Et_3N giving the bicyclic compounds (2). Appling the above reaction to 3 having a chiral auxilialy, readily available from D-glucose, the first asymmetric total synthesis of (-)-tylophorine (5) was achieved. (B) Synthesis of trans-Hydroindanes Construction of trans-hydroindane (7) was accomplished by the tandem Michael reactions of α, β-unsaturated ketone (6) having a sulfide group at the β-position using TBSOTf in the presence of Et_3N. It indicates this type of reaction to be a stepwise process for the annulation via the zwitterion intermediate (10) that the same product (7) was obtained as a major product from the four possible geometrical isomers of 6. (C) Syntheses of Polycyclic Systems Fused to Cyclobutane by Tandem Intramolecular Michael-Aldol reaction Polycyclic systems (17-19) fused to cyclobutane were produced by the treatments of the α, β-unsaturated esters (14-16) with TBSOTf in the presence of Et_3N. The treatment of keto ester (23) under the same reaction conditions provided only the silyl enol ether (24). The Michael-aldol reaction of 23 was achieved by the action of TMSI in the presence of (TMS)_2NH in dichloroethane. (D) Development of New Type of Intramolecular Michael Reaction and Michael-Claisen Reaction The reaction of bis-ester (31) with TBSOTf in the presence of Et_3N produced monocyclic 32 and bicyclic compounds 33.
We have developed an efficient preparation of both enantiomers of the ketodicyclopentadiene (KDCP) (1) by asymmetric kinetic resolution of the hydroxydicyclopentadiene (3) and its acetate (4) using a lipase. Enantiomerization of KDCP (1) has also been developed in three steps by employing the Wharton reaction as the key step. Utilizing the optically active KDCP (1), we first describe the enantiocontrolled synthesis of (+)-equilenin (10) by conventional convex-face selective nucleophilic addition and electrophilic double alkylation reaction on the substrate ketone. We next describe two new methodologies of convex-face selective modification of the KDCP molecule, the Fischer indolization reaction and the Diels-Alder cycloaddition reaction. The Fischer indolization of the methyl-ketone (19) obtained from KDCP (1) furnishes the corresponding carbinolamines, (23), (41), and (42), each as a single epimer at the benzylic center by introduction of an aryl group selectively from the convex face of the substrate ketone. This leads to enantiocontrolled syntheses of the Calabar bean alkaloids, (-)-physovenine (33) and (-)-physostigmine (38), and the cuparane sesquiterpenes, (-)-herbertene (48), (+)-β-cuparenone (49), (-)-debromoaplysin (58), and (-)-aplysin (59). The Diels-Alder reaction of (-)-KDCP (1) and the Dane's diene (60) proceeds regio- and diastereoselectively from the convex face of the dienophile to yield the "exo"-adduct (61) as a single epimer. Stereoselective methylation from the convex face allows the construction of trans-C/D ring-juncture to give 63 that is transformed into (+)-estrone (65) without difficulty. Overall yield of (+)-estrone (65) from (-)-KDCP (1) is 28% in six steps.
We now describe totally stereocontrolled method for the synthesis of both trans- and cis-2,5-substituted tetrahydrofurans 3 based on the stereocontrolled hydride reduction of cyclic hemiketals 1 followed by the stereospecific one-step cyclodehydration of the resulting anti- and syn-1,4-diols 2. Highly stereoselective hydride reduction of the cyclic hemiketal 1 which provides either anti- or syn-1,4-diols (anti-2 or syn-2) depending on hydride reagents employed has been developed. Thus, high degree of and selectivity (5: 1〜>99: 1) is obtained in a series of reactions with LiAlH4 in THE at -78 ℃. In striking contrast, a series of reactions with NaBH_4 in ethanol at 0 ℃ displays good syn selectivity (3: 1〜5: 1). The one-step cyclodehydration of anti-2 and syn-2 with p-TsCl in pyridine proceeds effectively at room temperature to produce trans and cis tetrahydrofurans (trans-3 and cis-3) in complete stereospecificity, respectively. Based on these methods, we have demonstrated the stereoselective asymmetric synthesis of the marine natural product isolated from the brown alga, Notheia anomala, by starting from the optically active epoxide 9. We have also performed the stereocontrolled synthesis of the multifunctional 2,5-substituted tetrahydrofurans 28-31 by starting from the optically active epoxides 18 and 19, which are our key intermediates for the synthesis of a series of polyketide-derived fatty acid derivatives, Annonacious acetogenines, isolated from tropical and subtropical trees.
We have shown that the optically active 1,2: 5,6-diepoxyhexane (2) can be used as a versatile C_2 symmetric chiral building block in the enantiodivergent synthesis of natural products. For the preparation of (R,R)- and (S,S)-diepoxides 2, the methods via di-O-pivaloyl intermediate 11 and via the bisbenzoyloxy bromide 14, both derived from 3,4-dideoxy-D-threo-hexitol (1), provided the best results in terms of the selectivity, yield, and optical purity. With the chiral building block 2 in hand, we initially attempted to synthesize both enantiomers of dendrobatid alkaloids, indolizidines 195B, 223AB, 239AB, and 239CD, belonging to the 3,5-disubstituted indolizidine subclass. Thus, the syntheses of the (-)- and (+)-enantiomers of these alkaloids began with (R,R)-2 and (S,S)-2, respectively, and were carried out by way of pyrrolidine formation via the cyclic sulfates leading to the (2R,5R)- and (2S,5S)-trans-2,5-dialkylated pyrrolidines, which were converted to the (-)- and (+)-enantiomers, respectively, of the indolizidine alkaloids 195B, 223AB, 239AB, and 239CD. These syntheses involve the first chiral preparations of indolizidines 239AB and 239CD both in natural (-)- and unnatural (+)- enantiomeric forms, which confirm the absolute configurations of natural 239AB and 239CD as 3R,5S,8aR and 3R,5R,8aR, respectively. To demonstrate further the general utility of the diepoxide chiral building block 2 in natural product synthesis, we next synthesized macrocyclic diolide antibiotics (-)-pyrenophorin and (-)-pyrenophorol from (R,R)-2. These appraoches involve double lactonization of the hydroxy seco acids 38 and 46 under Mitsunobu conditions.
Recent achievements in our laboratory on the stereodivergent approaches to either diastereomer of the addition products of organometallics to chiral carbonyl and imino compounds have prompted us to explore a new synthetic method for β-lactam skeletons in a highly stereodivergent manner. Optically active β-lactams are an important class of compounds for the synthesis of a series of β-lactam antibiotics and β-lactamase inhibitors. In this presentation diastereoselective addition reaction of ester enolates to a chiral imine 1 possessing (4S,5S)-4,5-dimethoxymethyl-2-methyl-1,3-dioxane ring as a chiral auxiliary is reported, in which varying enolate metals resulted in the selective formation of either (4S)- or (4R)-β-lactam from a single chiral imine 1. Addition reaction of the enolates of ethyl α,α-disubstituted acetate to the imine 1 gave (4S)- or (4R)-β-lactam 6 selectively depending on the metal enolate species used. The reaction with the lithium enolate gave (4S)-β-lactams, whereas (4R)-isomers were obtained from the triisopropoxytitanium enolate. For the preparation of β-lactam without a substituent at the 3-position, by using the triisopropoxytitanium or lithium enolate derived from t-butyl acetate, (3S)-β-amino ester 7 was initially obtained predominantly, whereas the chlorozinc enolate gave (3R)-β-amino ester 7 predominantly. Those β-amino esters were readily converted without racemization into (4S)- and (4R)-β-lactams 9, respectively in good overall yields. The reaction of the metal enolates of α-mono-alkylacetic acid t-butyl esters realized the stereodivergent synthesis of four possible diastereomers of 3,4-disubstituted β-lactams. The metal enolates of t-butyl propionate or butyrate underwent diastereoselective addition reaction to the imine 1 followed by the facile transformation into β-lactams as in the case with t-butyl acetate to give (3R,4S)-isomers 12 (R = Me or Et) from the triisopropoxytitanium enolates, (3S,4S)-isomers 12 (R = Me or Et) from the lithium enolates, and (3R,4R)-isomers 12 (R = Me or Et) from the chlorozinc enolates, respectively, with excellent selectivity. Upon treatment with potassium t-butoxide cis-3R,4R)-β-lactams 12 (R = Me or Et) were readily converted into trans-(3S,4R)-isomers 12 (R = Me or Et). In particular, (3R,4S)-isomer 12 (R = Et) was shown to be a good intermediate for the synthesis of an antibiotic PS-5. Thus, a new method for the stereodivergent construction of β-lactam skeleton possessing substituents at 3- and/or 4-positions from a single chiral imine has been developed by taking advantage of different coordination states of the lithium, titanium, and zinc enolates. Almost complete reversal of the diastereoface-discrimination with respect to the C-4 of the β-lactam skeleton attained in the present system coupled with flexibility in the selection of the enolates and ready removal of the chiral auxiliary offers a useful and practical addition to the existing methodologies
Alternaric acid (1) which was isolated from Alternaria solani, causal fungus of early blight disease on potato, has phytotoxic and antifungal activities. Though the stereochemistry at C-12 and C-17 position was elucidated to be 12-S and 17-R by the degradation products of 1 during structural studies, the stereochemistry at C-10 and C-11 has been unknown. We here present the synthetic studies and the determination of complete stereochemistry of 1. Determination of the stereochemistry of 1 was achieved by the synthesis of four possible diastereomers of degradation product 12 of 1. Aldehyde 4 was prepared by the procedure leading to 12. Phenylsufone 5 was prepared from 21 through 11 steps, which included retro Diels-Alder reaction of 22 and palladium-catalyzed allylic alkylation of 23 as key reaction. β-Keto-δ-valerolactone 6 was prepared via Claisen condensation of β-hydroxyester 27 with t-butyl acetate. The coupling reaction of each segment was performed by Julia-Lythgoe olefination of aldehyde 4 and phenylsulfone 5. Construction of 3-acyl-4-hydroxy-5,6-dihydropyrone 2 via Fries type rearrangement of enol acyl group of β-keto-δ-valerolactone toward α-position of the δ-lactone was newly developed by utilizing DCC and DMAP. Synthesis of alternaric acid (1) from 2 is now in progress.
In 1971, Jizba et. al. reported the isolation and the determination of planar structure of osladin, a sweet principle in rhizome of Polypodium vulgare (Polypodiaceae). Although the stereochemistry of the aglycone was proposed by Havel and Cerny in 1975 based on the chemical correlation with solasodin, that of glycosidic part was assumed to be α-rhamnoside without any firm evidence. Thus, structure 2 has generally been accepted as the structure of osladin. As we have been interested in synthesis of sweet glycosides, we have undertaken the total synthesis of this intensely sweet substance. However the compound 2 synthesized to this end, exhibited no sweet taste at all. Since the structure 2 has been widely accepted by reviews, books, and dictionaries, and many saponins seem to follow this sort of casual assignment, we decided to carry out the structural reinvestigation of osladin. As neither the sample of osladin or the spectra were available from original investigators, we have isolated it from the rhizome collected in southern part of Germany by courtesy of Professors Asakawa and Becker, and found that the structure should be revised to 1 on the basis of single crystal X-ray diffraction study. Thus the stereochemical assignments at C-22, 25 and 26 are corrected from S, R, S to R, S, R. Total synthesis of the sweet osladin (1) was accomplished from aldehyde 7 as follows. Inversion of the C-22S stereochemistry of the major alkylation product 8 was achieved by SN2 type replacement with KO_2 via mesylate 10 to give 9. The alcohol 9 was transformed to lactone 13 by ozonolysis, oxidation of hemiacetal, methylation, and solvolysis sequence. 2'-Discriminated and (3-selective glucosylation of 13 with 14 was achieved by using catalytic amount of TfOH to give 15. Rhamnose residue was introduced by our original thermal glycosylation procedure in α-specific manner affording 17. Although the C-25 methyl group of lactone 17 did not equilibrated under basic condition, but the hemiacetal obtained by DIBALH reduction was easily isomerized to give single equatorial isomer 18. The second rhamnosylation into hemiacetal hydroxyl group was accomplished by Koenigs-Knorr procedure using AgOTf to yield trisaccharide 19. Hydroboration, PDC oxidation, and Pd(OH)_2 catalyzed deprotection gave osladin (1). It was sweet.
A general method based on difference circular dichroic spectroscopy for assigning multiple chiral centers in acyclic 1,3-polyols is presented. The validity of this method was demonstrated using 1,2,4,6-tetrols, 1,2,4,6,8-pentols, and 1,2,4,6,8,10,12-heptols as model systems. Sixteen pairs of hydroxy benzoates with a terminal hydroxyl group and the corresponding allylic benzoates were prepared. Subtraction of a CD spectrum of a hydroxy benzoate from that of the paired allylic benzoate gave a difference CD (DIF CD) spectrum, where benzoate/benzoate exciton interactions were canceled. The Cotton effect of the DIF CD spectrum reflects only the exciton interaction of a terminal_allylic benzoate system. A positive sign of the DIF CD is correlated to the (S)-configuration of the allylic position, whereas a negative sign correlated to (R)-configuration. Combination of this DIF CD method and reiterative chemical degradation enables one to determine the absolute configuration of 1,3-polyols, even if the relative stereochemistry is unknown.
An efficient synthesis of 1α, 25- (OH)_2 vitamin D_3 (1) was accomplished based on new synthetic methods for A-ring and CD-ring. Reductive cleavage of 5-methylnorbornan-6-one-2-carboxylates with lithium naphthalenide gave D-ring synthon 11 selectively, which was transformed to CD-ring 2 via Robinson annulation, 1,2-enone transposition, reduction to 8α-hydroxy compounds and hydrogenation of the olefin to the trans-fused indanonl 17 with Willkinson's catalyst. A-ring synthon 27 was prepared by palladium-catalyzed cyclization of 8-bromo-2,8-nonadienoates 31 stereoselectively. After the ester 27 was transformed to the phosphine oxide 3, coupling reaction of 2 and 3 followed by deprotection afforded 1α,25-(OH)_2 vitamin D_3 (1).