Carbasaccharidesvia Ring-Closing Alkene Metathesis. A Synthesis of (+)-Valienamine fromD-Glucose

(+)-Valienamine (16) was prepared in seven steps and in an overall yield of 17% from commercially available 2,3,4,6-tetra-O-benzyl-D-glucopyranose. Stereoselective addition of vinylmagnesium bromide to the 1,3,4,5-tetra-O-benzyl-6,7-dideoxy-L-xylo-hept-6-en-2-ulose (2) gave diene 3 (86%). Ring-closing alkene metathesis of 3 in the presence of 0.15 equiv. of Grubb's catalyst 1 gave the cyclohexene 4 (58%), that was converted into (+)-valienamine (16) in three steps and in 47% yield. Similarly, ring-closing alkene metathesis of the D-mannose-derived diene 20 gave the cyclohexene 21 (89%).