Ring opening of sugar epoxides, epimines and episulphides

The intervention of episulphonium ion intermediates in the synthesis of azidosugars has been postulated by Christensen and Goodman in order to rationalize the product (140), formed on interaction of azide ion with the furanoside (141). Direct replacement with inversion at C(2) did not occur rather, a mixture of the diazides 140 2ind 142 was obtained by nucleophilic attack of azide ion on the episulphonium ion (143). A similar episulphonium intermediate (144) has been invoked to rationalize the conversion of methyl 4,6-0-benzylidene- 

2 - benzylthio- 2 - deoxy-3 -0-p- toluenesulphonyl - a-d - altropyranoside (145) into the azide (146) in which the configuration at is retained. 

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