Unexpected formation of glycals upon attempted C-alkylation of protected glycosylacetylenes.

Treatment of 3,7-anhydro-4,5,6,8-tetra-O-benzyl-1,2-dideoxy-D-glycero-D-galacto-oct-1 -ynitol (beta-D-mannosyl acetylene, 1) with 5 equivalents of n-butyllithium at either 0 or -78 degrees C resulted in the elimination of benzyl alcohol to yield 3,7-anhydro-5,6,8-tri-O-benzyl-1,2,4-trideoxy-D-arabino-oct-3-en-1-yn itol (glycal acetylene, 3) as the major product. Additional studies showed that 3 is also produced from two isomers of 1 with alpha-D-mannosyl and beta-D-glucosyl stereochemistry, but in lower yields. Furthermore, substrates in which the acetylene moiety is replaced by either a methyl or phenyl group do not produce a glycal product under these conditions. Finally, treatment of 1 with phenyllithium provides 3 in low yield. Deuterium labeling studies suggest that the reaction proceeds through an E2, rather than an E1cB, mechanism.