Isolation and characterisation of an unexpected byproduct in the regioselective butane diacetal protection of α-methyl galactopyranoside.
Carbohydr Res 2017 Nov 6;455:14-17. Epub 2017 Nov 6.
Chemistry, University of Southampton, Highfield, Southampton, SO17 1BJ, UK. Electronic address:
The regioselective protection of both methyl galactopyranoside anomers at the 2 and 3-positions as the butane diacetal (BDA) is well known. Here we describe the formation of an unexpected byproduct, which mainly occurs when α-methyl galactopyranoside is reacted with 2,3-butanedione under BF3•OEt2 catalysis. The structure of the byproduct, which did not arise from anomerisation to the β-anomer or from BDA formation at the galactopyranoside 3,4-positions, was elucidated by NMR and X-ray crystallographic analysis, and proved to be the expected BDA protected galactopyranoside, but in which the stereochemistry of both its BDA acetal centres are inverted. Read More