Synthesis, conformation and glycosylation reaction of substituted 2,6-dioxabicyclo[3.1.1]heptanes:1,3-anhydro-2,4-di-O-benzyl-α-l-arabinopyranose

The title 1,3-anhydro sugar (10) was obtained as crystals by the ring closure of 3-O-acetyl-2,4-di-O-benzyl-α,β-l-arabinopyranosyl chloride (9) which was prepared from methyl β-l-arabinopyranoside via a sequence of reactions involving initial selective 3-O-allylation. Methanolysis of 10 gave 1,3-cis arranged methyl 2,4-di-O-benzyl-α-l-arabinopyranoside as the major product. Glycosylation of 10 with 1,2:3,4-di-O-isopropylidene-α-d-galactopyranose or with serine derivatives resulted in the same stereochemical outcome, while glycosylation of the 1,3-anhydro galacto- (19) and manno-pyranose (24) analogues afforded predominantly 1,3-trans ring opening products. Conformational analysis of 10, 19 and 24 revealed that the C-5 head of 10 is flexible. The conformation of 10 in solution is close to E2 while 19 and 24 prefer the B2,5 form.