This review represents a survey of stereochemical control through sigmatropic rearrangements with emphases on their mechanistic grounds and their applicabilities (and potentials) as tools for acyclic stereocontrol. The rearrangements concerned are thermally-allowed [3, 3]- and [2, 3]- sigmatropic rearrangements. The 1st chapter deals with an introductory aspect of the stereochemistries of the sigmatropic rearrangements in general and their synthetic potentials for achieving diastereoselective synthesis (internal asymmetric induction), chirality transfer (self-immolative asymmetric synthesis), and a combination thereof. The 2nd chapter is concerned with stereocontrol via [3, 3]-sigmatropic rearrangements, mainly the Claisen variants. First, the general stereochemistry of the Claisen rearrangement is discussed on mechanistic grounds. Second, a variety of highly diastereoselective Claisen modifications and their applications to natural product syntheses are described. Third, the synthetic potentialities of the 1, 3-transfer of chirality via the Claisen rearrangement are illustrated in the contexts of recent stereocontrolled syntheses of complex natural products. Fourth, the possibility of net remote asymmetric induction via the Claisen variant is discussed. The 3rd chapter deals mainly with stereocontrol via the [2, 3]-Wittig rearrangement with some emphasis on recent works in our laboratory. First, the transition-state models for the [2, 3]- Wittig variants are discussed in details. Second, a variety of highly diastereoselective [2, 3]-Wittig variants and their applications to natural product syntheses are described. both the 1, 3-and 1, 4-transfers of chirality via the [2, 3]-Wittig rearrangement are described. Fourth, the possibility of net 1, 5-asymmetric induction via the sequential [2, 3]-Wittig-Claisen and the [2, 3]- Wittig-[2, 3]-Wittig rearrangements are discussed.
The natural products synthesis, in a way, represents a reaction sequence which almost unavoidably involves a chemical process of creating one or more additional chiral centers on a chiral substance with proper relative stereochemistry. This task demands, particularly in the construction of acylic systems, a combination of both an enantiomerically pure substance and reagent, as the diastereofacial selectivity of the reacting prochiral species constitutes a major factor in controlling the stereochemical course of reactions. This article elaborates on this fundamental problem of organic synthesis for which solutions are provided in the case of the aldol reaction, Several important aspects of aldol chemistry are covered : (1) the mechanism of the aldol reaction, (2) enolate geometry, (3) boron chiral enolates, and (4) coordination of a metal cation with the oxygen lone pair, with the central theme of diastereofacial stereoselection. Many fundamental findings made in this pursuit bring about an extremely powerful aldol strategy. The application of this strategy to the synthesis of 6-deoxyerythronolide B, the ansa chain of rifamycin S, and tylonolide, representing major macrolide metabolites of medium structural complexity, is also presented.
A system mimicking vitamin B6-dependent transaminases capable of enantio-face differentiation has been developed through the synthesis and the utilization of a couple of pyridoxamine analogs with planar chirality. Diverse α-keto acids were transformed into the corresponding α-amino acids with high enantiomeric excesses through the reaction with one of the chiral pyridoxamine analogs in the presence of Zn+2. The molar ratio of Zn+2vs. the chiral pyridoxamine analog was an important factor which greatly influenced the enantiomeric excess of the product. The observed strict correlation between the absolute configuration of the pyridoxamine analog used and that of the amino acid produced was well rationalized.
In recent years polymers containing transition metals have attracted much interest. In this article the recent studies on the synthesis and property of organotransition metal polymers are reviewed. Recent synthetic aspects on polymerization of organotransition metal monomers including capture of transition metal complexes in layered compounds and zeolites are treated, and, in addition, a discussion of potential applications is presented on the basis of the catalytic and electronic properties.
Dimers and trimers of 1, 3-dienes can be readily produced using various transition metal complexes as the catalyst. This oligomerization affords linear and cyclic isomers, each of which is further divided into cis or trans isomers concerning the position of the remaining double bonds. This paper deals with the effect of the transition metal catalyst particularly, the electronic and steric effects of the ligand combined with the transition metal on the oligomer distribution. In this article we describe a trial to predict the selectivity and reactivity of 1, 3-dienes according to the LFER method.
Molecular dynamical aspects of triptycene molecules are discussed. After a short introduction to many interesting facets of triptycene chemistry, correlated internal rotation in bis (9-triptycyl) -X type molecules is described. Strict coupling of the rotation around the two C-X bonds was realized for the first time in these molecules in the shape of a bevel gear. In spite of the rapid geared rotation, the disrotatory motion gave birth to new stereoisomerism due to different phase relationship between the labeled cogs on the two wheels. The number of possible isomers is counted. Potential energy barriers associated with the gear meshing and gear clashing processes are discussed. Photoexcitation of triptycenes produces the molecules in the excitonic state from which unique chemistry of direct bonding between two of the three benzene rings with expulsion of the bridgehead atom as the monocentric diradical species, namely, carbenes and nitrenes, emerges. Mechanistic details of the reaction are given for 1-azatriptycene.
The current status of prostaglandin research is reviewed mainly on the primary prostaglandins, widely distributed in the living body, and on prostaglandin analogues having more stable structure and higher selective biological activities, which were developed by reference to the structures and activities of the primary PGs. Especially, recent aspect on their clinical application and development is explained. Moreover, the new field is viewed mainly on the future commercialization of PGI2 and TXA2 synthesis inhibitor, which will be applied mainly on circulatory system, from Yin-Yang theory. Further, recent rapid progress in leukotriene research on lipoxygenase system in arachidonic acid cascade will also been mentioned.
High molecular weight polyetherimines having trans-configuration were synthesized by low temperature solution polymerization in m-cresol. Water resistant yellow and flexible films were cast from the reaction solutions.