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Volume 58, Issue 8
Displaying 1-9 of 9 articles from this issue
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Synthetic Developments and Mechanistic StudiesAtsuyuki Ashimori, Larry E. Overman2000 Volume 58 Issue 8 Pages 718-727
Published: August 01, 2000
Released on J-STAGE: April 23, 2010
JOURNAL FREE ACCESSWithin the last decade, the enormous synthetic potential of the Heck reaction has been revealed. When employed intramolecularly, Heck reactions succeed in even forming quaternary carbon centers. The ability of intramolecular Heck reactions to reliably fashion carbon-carbon bonds in polyfunctional molecules has led to wide application of this reaction at the strategy level for the synthesis of complex natural products. One of the most important developments in this area is the increasing success being realized in effecting catalytic asymmetric Heck reactions using enantiopure ligands. Although only modest enantiocontrol was achieved in early stages, many asymmetric Heck reactions have now been demonstrated that proceed with >90% ee. Another development of the Heck reaction has led to greater understanding of its mechanistic details with two distinct pathways, the neutral and cationic pathways, being recognized. This review summarizes the synthetic developments of the intramolecular asymmetric Heck reaction for forming quaternary carbon centers and the mechanistic factors recently revealed to effect enantioselectiv- ity.View full abstractDownload PDF (1554K) -
Akio Fujii, Emiko Hagiwara, Mikiko Sodeoka2000 Volume 58 Issue 8 Pages 728-735
Published: August 01, 2000
Released on J-STAGE: November 13, 2009
JOURNAL FREE ACCESSCarbon-carbon bond-forming reactions that involve the addition of resonance-stabilized nudeophiles, such as enols and enolates, to iminium salts and imines, the so-called Mannich-type reactions, comprise one of the most important classes of reaction in organic synthesis. A number of methods for the diastereoselective reaction of imines with enolates of carboxylic acid derivatives or silyl ketene acetals have been reported, but examples of enantioselective variants of this type of reaction are quite limited. This article briefly reviews recent progress of enantioselective Mannich-type reactions, and describes our studies on the enantioselective addition of enol silyl ethers to imines catalyzed by optically active palladium complexes including its mechanistic aspects.View full abstractDownload PDF (1273K) -
Akiharu Satake2000 Volume 58 Issue 8 Pages 736-744
Published: August 01, 2000
Released on J-STAGE: January 22, 2010
JOURNAL FREE ACCESSPyridinyl azole ligands, such as pyridinylpyrazoles, pyridinylimidazoles, and pyridinylpyrrole, were used as a new type of bisnitrogen ligands for π-allylpalladium complexes. Reaction of π-allylpalladium chloride dimer with pyridinyl azole ligands formed cationic π-allylpalladium complexes, which could be converted into neutral forms with bases. The neutral π-allylpalladium complexes worked as effective catalysts for cyclopropanation of ketene silyl acetals with allylic acetates whereas the cationic π-allylpalladium complex having bipyridyl ligand was almost unreactive. Reaction of cinnamyl acetate with ketene silyl acetal of ethyl 2-methylpropionate in the presence of the palladium-pyridinylimidazole catalyst and sodium acetate in DMSO gave a cyclopropane derivative selectively in a 83% yield. This reaction was also applicable for asymmetric cyclopropanation. When chiral pyrazole-palladium catalysts were used, chiral cyclopropane derivatives were obtained up to 54% ee. To examine the difference between cationic and neutral palladium complexes, X-ray diffraction analysis and temperature variable NMR studies were performed. Bond lengths between palladium and nitrogens of the neutral pyridinylimidazole complex were shorter than those of the cationic complex in the crystal structures. Further, kinetic constants of syn-syn and anti-anti exchange of the allyl moiety in the neutral complex was smaller than that in the case of cationic complex in DMF-d7 and DMSO-d6. These results shows that palladium-nitrogen bonds of the neutral complex are stronger than that of the cationic complex, and these difference affects the reactivity for catalytic cyclopropanation.View full abstractDownload PDF (1347K) -
Ikuro Abe2000 Volume 58 Issue 8 Pages 745-755
Published: August 01, 2000
Released on J-STAGE: November 13, 2009
JOURNAL FREE ACCESSEnzymatic cyclizations of squalene and oxidosqualene lead to various skeletal types of sterols and triterpenes. The cyclases for these reactions catalyze formation and stabilization of polycyclic carbocations and direct the enzyme-specific, templated formation of new carbon-carbon bonds in regio- and stereochemically defined manner. Studies of these enzymes are now progressing rapidly and promise to reveal the intimate three-dimensional structural details of the enzyme-catalyzed processes. The development of mechanism-based irreversible inhibitors, photoactivatable inhibitors, and numerous substrate analogs have helped to unravel the stepwise events occurring in the catalytic sites of these enzymes by covalent modification of specific amino acid residues.View full abstractDownload PDF (1653K) -
Shigeki Sano, Yoshimitsu Nagao2000 Volume 58 Issue 8 Pages 756-765
Published: August 01, 2000
Released on J-STAGE: November 13, 2009
JOURNAL FREE ACCESSNovel methodologies for the preparation of a-substituted serines are described. A newly designed bislactim ether, ethyl (5S) - or (5R) -3, 6-diethoxy-2, 5-dihydro-5-isopropyl-2-pyrazinecarboxylate, was treated with base or Lewis acid-tertiary amine to generate an enolate or enaminate (imine anion, metalloenamine). Alkylation or aldol-type reaction with the resultant enolate or enaminate proceeded in a highly diastereoselective manner to give the corresponding products. Reduction of these diastereomers followed by hydrolysis afforded the desirable α-substituted serines. The highly enantioselective aldol-type reaction of an achiral bislactim ether, ethyl 3, 6-diethoxy-2, 5-dihydro-2-pyrazinecarboxylate, was also achieved by employing Sn (OSO2CF3) 2-triethylamine in the presence of a catalytic amount of (-) -sparteine. Interestingly, the stereoselective outcome of the Sn (II) -mediated reaction differed from that of the Mg (II) -mediated one in the aldol-type reaction of the bislactim ethers with aliphatic aldehydes.
On the other hand, chemoenzymatic synthesis of enantiomerically pure α-substituted serines must also be a convenient and useful procedure. Porcine liver esterase (PLE) or rabbit liver esterase (RLE) catalyzed hydrolysis of the pro-S ester group of diethyl α-alkyl-α- (benzyloxycarbonylamino) malonates to afford (R) -ethyl α-alkyl-α- (benzyloxycarbonylamino) malonates each in excellent enantiomeric excess. Enantiodivergent reductions of these acid esters readily furnished both the corresponding enantiomeric α-substituted serines.
Finally, the application of these methodologies mentioned above to a total synthesis of ISP-I (a potent immunosuppressant) and (+) -conagenin (a low molecular weight immunomodulator) is described.View full abstractDownload PDF (1410K) -
Yoshinari Satoh2000 Volume 58 Issue 8 Pages 766-775
Published: August 01, 2000
Released on J-STAGE: November 13, 2009
JOURNAL FREE ACCESSCholecystokinin (CCK) is a major gastrointestinal hormone with an important role in regulating various target organs such as pancreas, gallbladder and gut. CCK is also one of the most widely distributed neuropeptides in the brain and is considered as a putative central nervous system transmitter. The CCK receptors are divided into two CCK-A and CCK-B sub-type receptors. We have researched these respective receptor antagonists in the 1, 4-benzodiazepine derivatives and succeeded in discoveries of a selective CCK-A receptor antagonist, FK 480, a selective CCK-B receptor antagonist, FR 175985, and a dual CCK-A and -B receptor antagonist, FR 208419.
This manuscript describes the syntheses and the structure-activity relationships of these respective CCK receptor antagonists.View full abstractDownload PDF (1377K) -
An Approach from Monomer SynthesisToshihiko Matsumoto2000 Volume 58 Issue 8 Pages 776-786
Published: August 01, 2000
Released on J-STAGE: January 22, 2010
JOURNAL FREE ACCESSPolyimides obtained from dianhydrides and diamines by the polycondensation are classified into four distinct categories according to the chemical structure of the monomers. Aliphatic polyimides derived from aliphatic monomers are soluble in organic solvents and form the colorless and flexible films, whereas most of fully aromatic polyimides such as Kapton® are insoluble and absorb visible light. Especially, alicyclic monomer based polyimides have attracted much attention in recent years because of their potential applications including use as liquid crystal orientation layers or low dielectric materials. In this review, a historical aspect and recent advances in the polyimide synthesis are described with particular focus and emphasis on the monomer synthesis using the Diels-Alder reaction, the Pd-catalyzed methoxycarbonylation and so on.View full abstractDownload PDF (1653K) -
Yoshinori Hara, Kazunari Takahashi2000 Volume 58 Issue 8 Pages 787-794
Published: August 01, 2000
Released on J-STAGE: November 13, 2009
JOURNAL FREE ACCESSγ-Butyrolactone (GBL) is produced by a two-stage hydrogenation of maleic anhydride (MAH) in the liquid phase : a hydrogenation of MAH to succinic anhydride (SAH) in the first stage and a subsequent hydrogenation of SAH to GBL in the second stage. A novel ruthenium catalyst system consisting of Ru salts, trialkylphosphine and p-toluenesulfonic acid (p-TsOH) was found very effective for the hydrogenation of SAH affording GBL in selectivities not less than 95%. Acid promoters like p-TsOH induce structural change in the Ru complexes, leading to the cationic complexes, which show higher catalytic activity and selectivity than the neutral ones. A new process to produce GBL was also developed, in which the features are external preparation of Ru complexes, the reaction protocol which combine over-flow reaction with stripping and the catalyst recovery system by the extraction.View full abstractDownload PDF (1190K) -
[in Japanese], [in Japanese], [in Japanese], [in Japanese]2000 Volume 58 Issue 8 Pages 795
Published: August 01, 2000
Released on J-STAGE: November 13, 2009
JOURNAL FREE ACCESSDownload PDF (207K)
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