Tricyclic nucleosides derived from D-glucose. Synthesis and conformationalbehaviour

Two anomeric nucleosides with tricyclic carbohydrate moieties, 3 and 14, ar e synthesised in 11 steps from diacetone-D-glucose, taking advantage of a s tereoselective Grignard reaction, a stereoselective dihydroxylation and a r egioselective tandem ring-closing procedure. The configuration of 3 is conf irmed by measuring the (3)J(HH) coupling constants in connection with molec ular modelling and ab initio calculations, as these exclude alternative tri cyclic nucleoside structures. The conformational preference for 3 is descri bed. The furanose ring is found to be in an O-4'-endo conformation and the gamma torsion angle in the +ap range.