Organotransition metal modified sugars Part 22. Direct metalation of glycals: short and efficient routes to diversely protected stannylated glycals

A complete set of D-hexose-derived silyl and isopropylidene/silyl-protected glycals bearing complementary configurations at C-3 and C-4 has been synth esized in short and efficient 1-3 step sequences from standard precursors. The glycals have been applied to metalation reactions to give storable viny l lithium equivalents by subsequent transmetalation to vinyl stannanes whic h represent valuable intermediates for transition metal-catalyzed cross-cou pling reactions. A H-1-NMR-assisted conformational analysis has been carrie d out with the protected glycals and the stannylated congeners; The isoprop ylidene/silyl-protected glycals adopt the H-4(5)-conformation caused by the bicyclic system, whereas the conformations of the fully silyl-protected mo nocyclic glycals are mainly controlled by the vinylogous anomeric effect. T he discussed galactal- and allal-derivatives show dynamic behaviour on the NMR-time-scale. At low temperatures the two possible conformers (H-4(5) and H-5(4)) have been observed demonstrating competition of steric congestion and stereoelectronic interaction via the vinylogous anomeric effect (VAE). (C) 2001 Elsevier Science B.V. All rights reserved.