Crystallographic studies of some cyclic benzylidene acetals: Key synthons for o-glycoamino acid building blocks and solid-phase oligosaccharide synthesis

Synthesis of methyl 2-azido-2-deoxy-4,6-O-benzylidene-β-D-galactopyranoside (1), one of the key components in the synthesis of O-glycoamino acids, was undertaken in order to synthesize Tn and TF(3) antigen building blocks. In pursuit of an alternative approach, benzylidenation of the crude D-galactal (2a) afforded methyl 2-deoxy-4,6-O-benzylidene-α-D-galactopyranoside (3c) and 3,4-O-benzylidene-D-galactal (3b) besides the expected 4,6-O-benzylidene-D-galactal (3a). Formation of compound 3c was explained based on the presence of methyl 2 deoxy-α-D-galactopyranoside (2b) isomer and/or trace amount of methanol in the crude mixture of deacetylated product of 2 prior to benzylidenation. On the other hand, formation of 3b in substantial quantities appears to be a thermodynamically controlled product and its formation is found to be common during prolonged Lewis-acid catalyzed benzylidenation reaction. Crystal structures of these important and useful precursors were deduced by X-ray diffraction methods to enumerate their complete molecular structure as well as to understand the effect of the cyclic acetal on the pyranose ring conformation. Compound 1 crystallizes in the orthorhombic space group P2₁2₁2₁ with cell dimensions a = 5.058(7), b = 12.766(7), c = 22.557(7) Å; 3b crystallizes in the hexagonal space group P6₁ with cell dimensions a = 18.265(4), b = 18.265(3), c = 6.323(2) Å; 3c crystallizes in the monoclinic space group P2₁ with cell dimensions a = 10.614(3), b = 4.963(2), c = 12.730(3) Å, and β = 95.47(3)°.