It is desirable and practical to produce both enantiomers of a target from the same chiral starting material by stereodifferentiation of prochiral compounds, for instance utilizing a chiral ligand derived from a natural (L) amino acid. During the past 7 years, excellent results have been achieved in several cases by multiple stereodifferentiation of chiral ligands derived from (S)-indoline-2-carboxylic acid: highly diastereo- and enantioselective pinacol coupling reactions of chiral alpha-ketoamides gave both (S,S)- and (R,R)-quaternary tartaric acid for the first time; asymmetric Diels-Alder cyclization of chiral acrylamides in the presence of Lewis acid afforded extremely high diastereoselectivities of both opposite configurations of the cyclized diastereomers depending upon the structures of chiral ligands and Lewis acids; and asymmetric alkylation of aldehydes to both enantiomers of secondary alcohols and asymmetric hydrogenation of ketones to both enantiomers of chiral secondary alcohols have been achieved using catalysts derived from (S)-indoline-2-carboxylic acid.
Several new sulfonyl or sulfonyl derivativives, thionitrates (RSNO2), sulfonyl nitrites (RSO2NO), were successfully isolated by treating corresponding thiols and sulfinic acids with dinitrogen tetraoxide (N2O4). Spectroscopic data of both stable and many rather unstable compounds were determined and compared with those of corresponding alkyl nitrites (RONO) or alkyl nitrates (RONO2). Chemical reactivities of these uncommon, novel S-nitroso and S-nitro compounds were investigated.
Oxidation of an unsymmetrical disulfide, methyl phenyl disulfide, with peroxy acids takes place mainly at more electron-rich sulfur atom to form thiosulfinic S-ester at the first stage, while oxidation of unsymmetrical thiosulfinic S-ester with peroxy acids gives usually four thiosulfonic S-esters, together with sulfinic and sulfonic acids, further oxidation products. Oxidation of S-methyl benzenethiosulfinate 6 affords, as one of the products, S-phenyl methanethiosulfonate 11, in which the original sulfinyl oxygen is brought into methanesulfonyl group. A small amount of S-phenyl benzenethiosulfonate 9 (or benzenesulfonic acid 15) is obtained in the oxidation of S-phenyl methanethiosulfinate 7. 18O-Label of the starting thiosulfinic S-ester 6 was found to be incorporated to some extents into all four products, i.e. 8–11, while a part of 18O-label of S-phenyl methanethiosulfinate 7 was also observed in S-phenyl benzenethiosulfonate 9 and benzenesulfonic acid 15, although most of 18O-label of 7 was incorporated into S-phenyl methanethiosulfonate 11. Thiosulfinic S-esters are stable and do not undergo disproportionation and 18O-exchange under the conditions. These observations suggest the formation of α-disulfoxide as an intermediate in the oxidation of thiosulfinic S-ester to thiosulfonic S-ester, especially in the oxidation of 6, although no peak corresponding to α-disulfoxide was observed directly in the NMR spectra taken during the oxidations of a few thiosulfinic S-esters with peroxy acids. Mechanism of the oxidation is discussed.
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