It is highly important to find a novel chiral building unit for the synthesis of enantiopure natural products of biological interest and also for developing novel asymmetric reactions.
L-Quebrachitol (1L- (-) -2-O-methyl-chiro-inositol) is a naturally occurring optically active cyclitol obtained from an exudate of the rubber tree. By developing novel method for the selective demethylation of methyl ether in the presence of cyclohexylidene acetal in L-quebrachitol moieties, total syntheses of myo-inositol 1-phosphate, (+) -conduritol B, and cyclophellitol were achieved.
It was found that chiral cyclitols derived from L-quebrachitol were excellent chiral auxiliaries. For instance, diastereoselective reduction and addition to α-keto ester attached to chiral cyclitol derivatives took place highly stereoselectively. Diels-Alder reactions and [3+2] cycloaddition of nitrile oxide to acryloyl ester derived from the chiral cyclitol also proceeded with high diastereoselectivity.

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Several workers have synthesized chiral and achiral N, N'-ethylene-bridged dipeptides (piperazin-2-one derivatives), and used them mainly as units of biologically active pseudopeptides such i : as enkephalin analogs because these structurally constrained and lipophilic dipeptides afford the character resisting enzymatic hydrolysis to biologically active peptides. However, their synthetic methods have some problems (nonstereoselective, multi-steps, troublesome techniques, expensive reagents etc.) Therefore; we have developed more convenient procedures for preparing N, N'-ethylene-bridged dipeptides. This article reports four kinds of synthetic methods of chiral N, N'-ethylene-bridged dipeptides, in which various (R) or (S) -α-amino acids are used as starting materials. Moreover, a series of studies on the structures and the functionalities (enantioselective inclusion and transport of (R) - and (S) -ammonium salts, complexing of metal ions) of macrocyclic pseudopeptides and on the enkephalin- and dermorphin-like biological activities of linear pseudopeptides containing these dipeptides as their units are introduced and discussed. Also, the synthesis of a macrocyclic polyamine obtained by the BH₃ reduction of macrocyclic pseudopeptide and the structure of its metal complex ion are described.

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The thermolysis of 3, 4-benzo-1, 1, 2, 2-tetraethyl-1, 2-disilacyclobut-3-ene (2) at 250°C gave o-quinodisilane (1, 2-bis (diethylsilylene) cyclohexa-3, 5-diene) as a reactive species, while the photolysis produced a reactive silene, 1-ethyl-1- (2-diethylsilylphenyl) -1-silaprop-1-ene. The reaction of 2 with a catalytic amount of tetrakis (triethylphosphine) nickel (O) in benzene afforded a benzene adduct. 1-diethylphenylsilyl-2- (di-ethyl) silylbenzene in quantitative yield. Similar reaction of 2 with a catalytic amount of tetrakis (triphenylphosphine) palladium (O) in benzene yielded a dimerization product in high yield. With (η²-ethylene) bis (triphenylphosphine) platinum (O), 2 afforded an isomerization product of 2 as a major product. The nickel, palladium, and platinum-catalyzed reactions of 2 with various organic substrates have been described.

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Inclusion behavior and crystal structures of inclusion compounds of bile acids and their derivatives are described. Molecular recognition properties of the steroidal hosts are dependent on molecular structure. Especially, cholic acid, deoxycholic acid and their amides are found to be versatile hosts to include a wide variety of organic compounds. However, Some derivatives of cholic acid provide only guest-free crystals, and they can not form inclusion crystals with common organic solvents at all. X-ray crystallographic studies reveal that the crystal structures consist of stacked bilayers except that of lithocholic acid. Facially amphiphilic molecular structure of the bile acids gives the multibilayer structure in crystalline state. Side chain of the steroidal skeleton controls the structural variety of multibilayer structure. On the basis of molecular recognition and dynamic properties of inclusion crystals of the steroidal bile acid, we can say that bile acids have similar structure and function to proteins through molecular assembly.

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A variety of optically active fluorinated molecules have been prepared by diastereoselective reactions of chiral N-acyloxazolidinones. The trifluoromethyl radical generated from iodotrifluoromethane with a radical initiator, triethylborane, reacts on the si-face of the lithium enolates of the oxazolidinones with good diastereomeric excess. The perfluoroalkylation and ethoxycarbonyldifluoromethylation of the lithium enolates mediated by triethylborane also proceeds diastereoselectively via a radical mechanism. The bromodifluoromethylation of the lithium enolates proceeds not via a radical mechanism involving bromodifluoromethyl radical but via an ionic chain mechanism involving the insertion of difluorocarbene. The Evans aldol reaction of hexafluoroacetone and trifluoroacetaldehyde causes complete reversal of diastereofacial selectivity. The boron enolate derived from chiral N-acyloxazolidinones reacts with trifluoroacetaldehyde to give anti and “non-Evans” syn aldols with stereoselectivity in the range of 7 : 317 : 3. This unexpected and usual reversal of π-face selectivity was reproduced by semiempirical molecular orbital calculations.

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Our recent studies concerning to the ozonolysis of various azoles are reviewed in this paper. While azoles are remarkably stable under the acidic, basic, oxidative and/or reductive conditions due to their aromatic characters, azoles are specifically labile toward ozone. These characters of azoles satisfy the requirement for the protection-deprotection of the functional group. Pyrroles are easily prepared from primary amines, and the side chains of these pyrroles are functionalized by base catalyzed alkylation, hydride reduction, and aminolysis. By the ozonolysis, pyrroles are converted into the corresponding amides. Through this series of the reactions, pyrroles are demonstrated to be useful as the protecting group of amino functions for the synthesis of α-aminoalcohols, α-aminoketones, α-aminoaldehydes, and some peptides. Oxazole ring is also useful as the protecting group of carboxyl group in peptide synthesis, and the ring opening is carried out by ozonolysis. In this deprotection, oxazoles are converted into acid anhydrides, which are the reactive forms of carboxyl group and are directly available for the further peptide formation. This is the most remarkable example that the protecting group takes a role of the precursor of the active form of the functional group.

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