Synthesis of Methyl 5a′‐Carba‐β‐lactoside and N ‐Acetyl‐5a′‐carba‐β‐lactosaminides, and Related 5a′‐Carbadisaccharides

Construction of the ether‐linked methyl 5a′‐carba‐β‐lactoside ( 3 ) and N ‐acetyl‐5a′‐carba‐β‐lactosaminide ( 4 ) were carried out starting from the coupling products 15 and 16 , readily obtained by coupling between 1,2‐anhydro‐4,6‐ O ‐benzylidene‐5a‐carba‐D‐mannopyranose ( 7 ) and the oxide anions generated from methyl 2,3,6‐tri‐ O ‐benzyl‐β‐D‐glucopyranoside ( 8 ) and methyl 2‐acetamido‐3,6‐di‐ O ‐benzyl‐2‐deoxy‐β‐D‐glucopyranoside ( 10 ), respectively. Their 5a‐carba‐α‐D‐mannopyranose moieties were transformed into those of 5a‐carba‐β‐D‐galactopyranose by a sequence of reaction: Oxidation of the 2′‐OH group, epimerization of the C‐1′ with DBU, selective reduction of the carbonyl group, and epimerization of the C‐4′ via oxidation and then reduction of 4′‐OH or S N 2 reaction of the 4′‐mesylate with an acetate anion. Reaction of 1,2‐anhydro‐6‐ O ‐benzyl‐3,4‐ O ‐isopropylidene‐5a‐carba‐α‐D‐galactopyranose ( 6 ), initially expected as the potential donor, with these oxide anions did not give any ether‐linked products, rather resulting in elimination reaction of 7 . However, coupling of the epoxide 6 with methyl 2‐acetamido‐4‐amino‐2,4‐dideoxy‐β‐D‐glucopyranosides ( 19 ) easily gave rise to imino‐linked 5a′‐carbadisaccharide derivative 38 , which, after deprotection, gave the imino‐linked congener 5 . On the other hand, two biologically interkesting carbadisaccharides including methyl N , N ′‐diacetyl‐5a′‐carbachitobioside ( 45 ) were obtained from the versatile intermediate 24 .