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-gal acto-oct-1-ynitol (beta-D-mannosyl acetylene, 1) with 5 equivalents of n-bu tyllithium at either 0 or - 78 degrees C resulted in the elimination of ben zyl alcohol to yield 3,7-anhydro-5,6,8-tri-O-benzyl-1,2,4-trideoxy-D-arabni no-oct-3-en-1-ynitol (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. Furtherm ore, substrates in which the acetylene moiety is replaced by either a methy l or phenyl group do not produce a glycal product under these conditions. F inally, treatment of 1 with phenllithium provides 3 in low yield. Deuterium labeling studies suggest that the reaction proceeds through an E2, rather than an ElcB, mechanism. (C) 2000 Elsevier Science Ltd. All rights reserved .