Abstract
X-ray structure analysis of the more laevorotatory isomers of 2′-O-tetrahydropyranyl-4N-benzoylcytidine (4b) and of 2′-O-tetrahydropyranyluridine (5b) confirmed their chirality at the satellite anomeric centre C2″ to be S. The other diastereomers (4a resp. 5a) exhibited an unexpected reversal of 3′/5′-regio-selectivity when treated with 9-fluorenylmethoxycarbonyl chloride in pyridine. The X-ray crystallographic results form the basis for a mechanistic proposal.