Novel skeletal transformations of bicyclo [4.2.0] octan-2-ones via 8-membered-ring carbocation intermediates have been developed. The acid-catalyzed rearrangement of 5, 6-disubstituted bicyclooctanones is a highly-efficient method for synthesis of polycyclopentanoids such as diquinanes, angularly-fused triquinanes, spiro-annulated triquinanes, and tetraquinanes. The reaction of 5-6-4 fused ring ketones with acid in acid anhydride is a unique method for construction of 8-membered-ring compounds such as bicyclo [6.3.0] undecanes. The acid-catalyzed rearrangement of 6-substituted bicyclooctanones provides an excellent method for synthesis of bicyclo [4.2.1] nonanes. Utility of our novel method has been demonstrated by synthetic studies of terpenoids such as 5-oxosilphiperfol-6-ene, silphiperfol-6-ene, 3-oxosilphinene, and mediterraneol B.
This article deals with synthetically useful reactions based on organotin (IV) triflates [R2Sn(OTf)2]. Topics included are (1) development of R2Sn(OTf)2, (2) reaction of carbonyl compounds or their acetals with silylated nucleophiles or thiostannane nucleophiles, (3) Mukaiyama-Michael addition reaction of α, β-unsaturated ketones and enol silyl ethers and its application to new Robinson annulation reaction, and (4) practical method for constructing contiguous quaternary carbon centers.
Recent study in this laboratory is reviewed. Combination of group 4 metallocene dichloride (Cp2MCl2 : M= Zr, Hf) and silver salt (AgX : X = ClO4, OTf etc.) is a particularly effective reagent for activating glycosyl fluoride, which offers a new and efficient method for O-glycoside synthesis, which made us to succeed in the first total synthesis of mycinamicin macrolide antibiotics. Mechanistic consideration of this activation led us to find that the reagent combination in 1 : 2-ratio, rather than the original 1 : 1-ratio, of Cp2MCl2 and AgX engendered further enhanced reactivity. Some of the recent application in oligosaccharide synthesis is listed to show the broad scope of this new glycosidation reaction in natural product synthesis. Furthermore, this combinational reagent offers a powerful basis for aryl C-glycoside synthesis and the total syntheses of three prototypical compounds of this class, vineomycinone B2 methyl ester and gilvocarcin M and V, were successfully carried out by this glycoside activation method.
Rh (I) -catalyzed cyclization of substituted 4-pentenals into cyclopentanones found first by our laboratory was developed into the highly diastereoselective and enantioselective asymmetric cyclization by using the Rh (I) -complex with chiral ligand such as BINAP, and (+) -DIPMC. Cationic Rh (I) with BINAP afforded the best asymmetric cyclization in chemical yields and enantioselectivity as well as the diastereoselectivity. This cyclization method could be successfully applied for the synthesis of iridomyrmecin and isoiridomyrmecin from (-) -i-monene. In addition, Rh (I) -catalyzed cyclization of oct-6-enal with the cis-cyclohexane-1, 2-dioxy function at C 3-position afforded two conformational diastereomers due to a cis-cyclohexane-1, 2-diol moiety. The interesting findings found by our group are summarized as main topics in this paper.
This review mentions the total sytheses of the highly functionalized natural products, pederine, aplasmomycin, and virantmycin. The total synthesis of pederine was accomplished by employing the new method for stereocontrolled formation of N- (α-methoxyalkyl) amide. Based on the novel remote controlled asymmetric redutions of some chiral ketones, the formal total synthesis of aplasmomycin was achieved. The stereospecific total synthesis of virantmycin and its diastereomer were performed by using intramolecular addition reaction of nitrene to olefin, and relative stereochemistry of virantmycin was established unambiguouly.
(η6-Arene) chromium complexes have some characteristic properties due to the strong electron withdrawing ability and steric bulkiness of tricarbonyl chromium group. Their signficant applications have been developed utilizing following effects; 1) nucleophilic addition to the arene ring, 2) lithiation of the aromatic hydrogen, 3) stabilization of benzylic carbanion, 4) stabilization of benzylic carbocation, and 5) steric hindrance of Cr (CO)3 group. Also, (arene) chromium complexes can exist in two enantiomeric forms, when the phenyl ring is substituted at ortho- or meta-position by different substituents. This review focuses on our investigations and the related works of (η6-arene) chromium complexes in organic synthesis.
New synthetic method of benzofuranol compound (2, 3-dihydro-2, 2-dimethyl-7-hydroxybenzofuran) that is used as an important intermediate of low toxic carbamate insecticides, has been developed and established by using the strategy of aromatic compounds from aliphatic precursors. New process comprises following 5 steps; (1) cross aldol condensation of cyclohexanone with isobutyl aldehyde, (2) chlorination of 2-isobuty-lidene-cyclohexanone, (3) dehydrochlorination of 2, 2-dichloro-6-chloro-6- (1-chloroisobutyl) cyclohexanone (aromatization), (4) hydrolysis of 2-chloro-6-isobutylphenol, (5) cyclization of 3-isobutenyl-catechol.