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Volume 60, Issue 3
Displaying 1-7 of 7 articles from this issue
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Tatsuya Nabeshima, Toshiyuki Saiki, Shigehisa Akine2002 Volume 60 Issue 3 Pages 184-194
Published: March 01, 2002
Released on J-STAGE: November 13, 2009
JOURNAL FREE ACCESSConstruction of macrocyclic and macrobicyclic compounds by utilizing coordinate bonds is one of the most fascinating current topics in supramolecular chemistry. A variety of valency of the metal ions and direction of the bonds are available according to combination of the ligands and the metals. Many metal complexes are known to catalyze chemical reactions. Thus, various molecules which work not only as a host but also as a catalyst can be designed on the basis of the strategy described above.
Linear or branched molecules containing a suitable ligand at ends of the chains are converted to the corresponding pseudocrown ethers, pseudocryptands and pseudocyclophanes by complexation with a metal ion to result in an allosteric effect on the guest binding. Molecular assembling by coordinate bonding is another excellent way to yield metallo-macrocycles. The structure and function of the complexes can be modulated by reaction conditions, additional ligands, etc.View full abstractDownload PDF (1589K) -
Masaaki Shibuya, Yutaka Ebizuka2002 Volume 60 Issue 3 Pages 195-205
Published: March 01, 2002
Released on J-STAGE: November 13, 2009
JOURNAL FREE ACCESSCyclization of oxidosqualene into tetra- and pentacyclic carbon skeleton of sterols and triterpenes, catalyzed by oxidosqualene cyclases (OSCs), is one of the most complex and fascinating reactions found in nature. OSCs generate multiple stereogenic centers in a single reaction, and are responsible for the diverse triterpene skeletons. In order to investigate the origin of structural diversity of triterpene skeletons, cDNA cloning of OSCs and analysis of their product specificity were carried out. From triterpene producing plants, over twenty-five OSC clones were obtained, and their enzyme function established by expression in yeast. They included cycloartenol, cucurbitadienol, lupeol, β-amyrin, isomultiflorenol and mixed amyrin synthases. Studies of chimeric proteins between β-amyrin synthase and lupeol synthase, and mutant proeins constructed by site directed mutagenesis identified the amino acid residues responsible for their product specificity. Trp 259 of β-amyrin synthase (PNY) was identified to be the critical residue controlling β-amyrin formation. In further mutation studies, PNY Y 261 H mutant produced dammara-18, 21-dien-3β-ol (as a 3 : 5 mixture of E/Z isomer at Δ18) together with minor amount of dammara-18 (28), 21-dien-3β-ol. These triterpenes have not been reported from nature, and therefore, could be categorized as “unnatural” natural products. The results of this study opened up the possibility of generating new triterpene synthases with additional novel functions through point mutations.View full abstractDownload PDF (1751K) -
Hiro Ohtake, Xiaoliu Li, Hideyo Takahashi, Shiro Ikegami2002 Volume 60 Issue 3 Pages 206-217
Published: March 01, 2002
Released on J-STAGE: November 13, 2009
JOURNAL FREE ACCESSSpiro orthoesters between sugar molecules have been found in the orthosomycin family of antibiotics. This article summarizes our recent works on orthoester sugars, which focus on preparation, structure determination, and reactivities to anions. A series of orthoesters were prepared from monosccharides with TMSOMe and TMSOTf. The configurations of the spiro carbons of these orthoesters were determined or estimated by X-ray crystallographic analysis and molecular modeling studies, which revealed the significance of anomeric effect for these molecules. The reductive cleavage of orthoester sugars afforded the corresponding glycosides with exclusive β-selectivity even in the cases of mannosides. Treatment of AlMe3 as a methyl anion source to orthoesters caused sequencial ring opening to afford enol ethers, by which a novel shortcut procedure for cyclitol synthesis was developed. Spiro keto-disaccharides, structurally related to orthoester sugars, were also prepared, and their structures were determined or estimated by X-ray crystallographic analysis and NMR studies.View full abstractDownload PDF (1749K) -
Tsugio Kitamura2002 Volume 60 Issue 3 Pages 218-231
Published: March 01, 2002
Released on J-STAGE: November 13, 2009
JOURNAL FREE ACCESSReaction of [ο- (arylthio) phenyl] ethenes with bromine or ICl undergoes intramolecular cyclization to give 1-aryl-1-benzothiophenium salts. Electrophilic addition of [ο- (phenylthio) phenyl] alkynes with electrophiles such as HClO4, HBF4, Br2, and PhSCl also affords 1-phenyl-1-benzothiophenium salts. Copper-catalyzed reaction of 1-benzothiophenes with diphenyliodonium triflate gives 1-phenyl-1-benzothiophenium triflates directly. This method is especially effective for the synthesis of parent 1-phenyl-1-benzothiophenium salt. Reaction of 1-phenyl-1-benzothiophenium salts with halide ions leads to the formation of 1-benzothiophenes and (Z) -haloethenes, while the reaction with alkoxide ions provides only (Z) -alkoxyethenes. In the cases of 2-alkyl-subsituted and 3-unsubstituted benzothiophenium salts, allenes and alkynes are formed, respectively. Cycloaddition of 1-phenyl-1-benzothiophenium salts with cyclopentadiene or 1, 3-diphenylisobenzofuran proceeds efficiently in the cases of less substituted 1-phenyl-1-benzothiophenium salts. Phenyl migration is observed in the thermolysis of 1-phenyl-1-benzothiiophenium triflates, while the phenyl migration competes with the dephenylation in the photolysis. The reactions observed in this study are attributed to the loss of aromaticity of the thiophene ring.View full abstractDownload PDF (1957K) -
Toshiyuki Itoh, Shohei Shirakami2002 Volume 60 Issue 3 Pages 232-239
Published: March 01, 2002
Released on J-STAGE: January 22, 2010
JOURNAL FREE ACCESSReaction of racemic biaryldicarboxylic dichloride with methyl 4, 6-O-benzylidene-α-D-glucopyranoside gave methyl 4, 6-O-benzylidene-2, 3-O- [(R) -biaryldicarbonyl] -α-D-glucopyranoside with perfect diastereoselection. This reaction provided a new method of optical resolution of axially- and planar chiral dicarboxylates. Using the methodology, several types of novel ferrocenyl sugars which showed anti-malaria parasite (P. falciparum) activity have been prepared. Ten types of chiral azacrown ethers were synthesized from α-D-glucose and their catalytic property for the asymmetric Michael addition has been investigated; enantioselectivity switching which is dependent on the azacrown ether catalysts has been achieved.View full abstractDownload PDF (1382K) -
Masaru Takayanagi2002 Volume 60 Issue 3 Pages 240-249
Published: March 01, 2002
Released on J-STAGE: November 13, 2009
JOURNAL FREE ACCESSA number of valuable new synthetic strategies, such as the triazene-driven biaryl ether synthesis, have been developed during the total synthesis of vancomycin. Modern catalytic asymmetric reactions were employed for the construction of the required amino acid building blocks, which were then assembled to the appropriate peptide fragments, whose cyclization in the order A-B/C-O-D/D-O-E led to the framework of vancomycin aglycon. Sequential attachment of the required sugar moieties onto a suitably protected aglycon derivative, followed by deprotection, allowed the stereoselective total synthesis of the glycopeptide antibiotic vancomycin.View full abstractDownload PDF (1589K) -
[in Japanese], [in Japanese], [in Japanese], [in Japanese]2002 Volume 60 Issue 3 Pages 250
Published: March 01, 2002
Released on J-STAGE: November 13, 2009
JOURNAL FREE ACCESSDownload PDF (200K)
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