Epithio alcohols

Evidence has been presented591 that favours the involvement of thiirane intermediates in the formation of ( , )-divinyl sulfides (455) from the reaction of thiocarbonyl ylides (454) with activated alkynes, while the reaction of l-alkynyl-2,3-epithio alcohols (456) with a catalytic amount of Hg(II) has been shown to afford592 substituted thiophenes (457) (see Scheme 117). 

Epithio alcohols. Thiourea is known to convert epoxides into episulfides. In the presence of Ti(0-<-Pr) , traits 2,3-epoxy alcohols (1) also react with thiourea to provide epithio alcohols (2) without loss of enantioselectivity (equation I). 

It would, in principle, be possible to convert the polyol 115 into the 6"-deoxy sugar 118, a direct precursor of /3-acarbose. However, we decided that it would be better to reassess the whole synthesis and reasoned that a 1,6-epithio sugar such as 119 would be an ideal pivot for a synthesis of acarbose the amine 81 could easily be introduced at C-4, the sulfur at C-1 allows for activation to produce a glycosyl donor ready for attachment to the disaccharide alcohol 113 and, finally, desulfurization at C-6 leads to the necessary methyl group. 

Cassipourine has been isolated from Cassipourea species (Rhizophoraceae).8 Wrobel and Glinski have devised a synthesis of racemic material (18).9 The starting material was the / -epoxide (13), which was converted into the three steps (see Scheme 4). Thiobenzyl alcohol reacted regio- and stereo-specifically with this material (14) to give the thiol (15), which oxidized in air to give dimeric products. The racemate (16) and the meso-ioxm were separated by column chromatography. Reduction of the racemate (16) with sodium in liquid ammonia yielded the dithiol (17), which afforded ( )-cassipourine (18) on aerial oxidation. 

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