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One-pot synthesis of polycyclic nucleosides with unusual molecular skeletons.

An alpha hydroxy pyrrolidine tricyclic nucleoside 3 and its spontaneous reaction with acetone is described. In this transformation highly functionalized polycyclic nucleosides with rather unusual molecular skeletons are formed in a complete regio- and stereoselective way. The reaction involves the formation of three new bonds, two of them novel carbon-carbon bonds, in a one-pot way. An enamine-iminium mechanism with participation of carbinolamine, iminium ion, and enamine intermediates is proposed as a plausible explanation for this transformation. The scope of the reaction is briefly studied concluding that the nature of the ketone (R(1)COR(2)) is critical for the initial attack of the NH to the carbonyl group.