Our recent studies on effective halogenation and oxidation using benzyltrimethylammonium polyhalides, stable solids, are described. Those involve electrophilic halogen-substitution (bromination, iodination, and chlorination) of aromatic compounds such as phenols, aromatic amines, aromatic ethers, acetanilides, arenes, and thiophenes, α-halogenation of arenes and acetophenones, and also halogen-addition of alkenes by the use of benzyltrimethylammonium tribromide (BTMA Br3), benzyltrimethylammonium dichloroiodate (BTMA ICl2), and benzyltrimethylammonium tetrachloroiodate (BTMA ICl4). Furthermore, oxidation of alcohols, ethers, 1, 4-benzenediols, hindered phenols, primary amines and hydrazo compounds, sulfides, and thiols, haloform reaction of methylketones, N-bromination of amides, Hofmann degradation of amides, and preparation of acylureas and carbamates by the use of BTMA Br3 are presented.
Synthetic studies are reviewed on sugar analogs containing a phosphorus atom in the hemiacetal ring, e.g., analogs of D-xylo, D-gluco, 6-deoxy-L-galacto. D-mannopyranoses, as well as D-ribo and 2-deoxy-D-ribofu-ranoses. These compounds were converted into the corresponding per-O-acetyl derivatives, whose structures and conformations were established by 1H-NMR spectral analyses and partly by X-ray crystallographic analyses. Stereoselectivity of the introduction of a phosphinyl group to various types of 6-O-tosyl-hex-5-uloses and 6-nitro-hex-5-enoses are also described.
We developed to produce optically pure glycerol derivatives, such as (R) and (S)-2, 3- dichloro-1-prop-anols, and (R) and (S)-3-chloro-1, 2-propanediols, based on microbial resolution. These compounds are precursors of optically active epichlorohydrins and glycidols which are used for wide variety of organic syntheses. We describe here the new methods to obtain these chiral C3 units and their several synthetic applications.
Furans have been recognized as useful intermediates in organic synthesis. Among the synthetic utilities of furans, 1, 4-dicarbonyl equivalence is one of the characteristic features. This article describes the methodology for the synthesis of natural products, such as physiologically active steroids and lactonic antibiotics, employing the 1, 4-dicarbonyl equivalents derived from furan derivatives. The synthetic strategy is based on oxidative conversion of chiral furfuryl alcohols into the pyranones followed by their functionalization with appropriate nucleophiles and electrophiles to provide the modified pyrans possessing desired stereochemistries. These intermediates have been transformed into withanolide and brassinolide side chains, styryl lactones such as goniothalamin and goniodiol, malyngolide, and canadensolide. Chiral furfuryl alcohols have been prepared as follows. 1) Diastereoselective addition of 2-lithiofurans to chiral carbonyl compounds. 2) Kinetic resolution of secondary furfuryl alcohols using Sharpless reagent.
Asymmetric epoxidation of unfunctionalized olefins is of current interest in synthetic organic chemistry and many interesting methodologies have so far been reported for this reaction. Still, there is no sufficient methodology in terms of enantioselectivity and generality. However, optically active (salen) manganese (III) complexes were recently found to be efficient catalysts for epoxidation of this class of olefins by us and Jacobsen et al, respectively. With these (salen) manganese complexes as catalysts, high enantioselectivity of >95% ee has been realized, especially in the epoxidation of cis-olefins. For example, 2, 2-dimethylchromene derivative 14 was converted into the corresponding expoxide of 95.5% ee. Although trans-disubstituted olefins are not very good substrates for this salen complex-catalyzed epoxidation, moderate enantioselectivity of 62.5% ee was observed in the epoxidation of trans-stilbene. Quite recently, (salen) manganese (III) complexes were also found to catalyze the asymmetric aziridination of styrene. In this paper, we describe these new developments in salen chemistry mainly on the basis of our recent study.
Transition metal-catalyzed reactions involving, or probably involving, silylmetallation as the key step are reviewed. In contrast to the extensively studied hydrometallation and carbometallation, silylmetallation, which is the addition of a transition metal-silicon bond to an unsaturated bond, has not been well studied so far for both stoichiometric and catalytic organometallic reactions. The examples cited in this review are mainly the catalytic addition reactions of organosilicon reagents (Me3Si-Y; Si-Si, Si-Sn, Si-C, Si-I, Si-Se, and Si-H) to unsaturated hydrocarbons. The initial step of these catalytic reactions is the generation of a silyl transition-metal compound, a key catalytic species, by the oxidative addition of Me3Si-Y to a transition metal complex (MLn). The resulting species, Me3Si-M-Y, undergoes silylmetallation to a carbon-carbon multiple bond to give a β-silylvinyl-or β-silylethylmetal, from which reductive elimination or coupling reaction proceeds. The catalytic reactions which, we think, are likely to involve silylmetallation are included in this review irrespective of the original authors' suggestions on the mechanism.