Preparation of Diastereomeric 3,4‐O‐Pyruvate Acetal‐Containing D‐Galactopyranose Derivatives, Structural Assignment and Use for Oligosaccharide Synthesis

Abstract

Treatment of phenyl 1‐thio‐ and allyl 2,6‐di‐O‐benzoyl‐D‐galactopyranosides 3 with methyl pyruvate and BF₃ · Et₂O in various solvents gave 1,6‐anhydro‐3,4‐di‐O‐benzoyl‐2‐deoxy‐2‐phenylthio‐β‐D‐idopyranose (4) and the corresponding diastereomers of 3,4‐O‐pyruvate acetal‐containing galactosides 5. The phenyl 1‐thio‐β‐galactoside R‐5a and the allyl a‐galactoside R‐5b were both converted into methyl 3,4‐O‐[1‐(R)‐(methoxycarbonyl)ethylidene]‐β‐D‐galactopyranoside (9), the structure of which as well as that of 4 was confirmed by X‐ray crystallography. Compound R‐5a was converted into 5‐[(benzyloxycarbonyl)amino]pentyl 6‐O‐benzoyl‐3,4‐O‐[1‐(R)‐(methoxycarbonyl)ethylidene]‐β‐D‐galactopyranoside (18) by using the (2‐chloroacetoxymethyl)benzoyl (CAMB) group for the temporary protection of position 2. Glucosamination of 18 and subsequent deblocking of the intermediate disaccharide gave β‐D‐GlcpNAc‐(1→2)‐3,4‐(S)‐pyruvate‐β‐D‐Galp‐O(CH₂)₅NH₂ (21) which represents a fragment of the Escherichia coli K 47 polysaccharide.