Carbocations disaccharides

Ketene dithioacetal derivatives (2) are available from 2,3 4,5-di-0-isopropylidene-D>arabinose diethyl dithioacetal (1) by treatment with BuOK followed by an alkyl halide to cause in situ alkylation. On exposure to nucleophiles in the presence of a Lewis acid they undergo substitution at C-3 (rather than addition to the double bond) presumably via the carbocation (3) as shown in Scheme 1. This reaction has been used, with moderate success, to introduce a thio sugar residue and hence afford a sulphur-linked disaccharide derivative. ... 

Vasella has reviewed the chemistry of glycosyl carbanions, carbocations, radicals and carbenes and related work covering his own important contributions and those of others. Specific areas to have been reviewed are carba-sugars (with ring oxygen atoms replaced by carbon), the enediyne anticancer antibiotics notably calicheamycin and esperamycin (Nicolaou s synthetic work on the disaccharide component especially). ... 

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