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

Treatment of phenyl 1‐thio‐ and allyl 2,6‐di‐ O ‐benzoyl‐ D ‐galactopyranosides 3 with methyl pyruvate and BF 3 · Et 2 O 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 ‐Glc p NAc‐(1→2)‐3,4‐( S )‐pyruvate‐β‐ D ‐Gal p ‐O(CH 2 ) 5 NH 2 ( 21 ) which represents a fragment of the Escherichia coli K 47 polysaccharide.