Synthesis of methylene isosteres of α- and β-d-galactopyranosyl-l-serine

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The BF3·Et2O promoted coupling of tetra-O-benzyl-α-D-galactopyranosyl trichloroacetimidate with a silyl enol ether carrying an oxazolidine ring leads to the α-C-glycosylmethyl ketone (32%, 95% ds) that is converted into the title α-C-glycosyl amino acid by carbonyl oxygen removal (Barton–McCombie) and oxidative cleavage (Jones) of the heterocyclic ring, and into the β-isomer by anomerization and the same two-step sequence.