Internally Protected Amino Sugar Equivalents from Enantiopure1,2-Oxazines: Synthesis of Variably Configured Carbohydrates with C-Branched Amino Sugar Units
F. Pfrengle, H.-U. Reissig – 2010
A stereodivergent synthesis of differently configured C2-branched 4-amino sugar derivatives was accomplished. The Lewis acid mediated rearrangement of phenylthio-substituted 1,2-oxazines delivered glycosyl donor equivalents that can directly be employed in glycosidation reactions. Treatment with methanol provided internally protected amino sugar equivalents that have been transformed into the stereoisomeric methyl glycosides 28, ent-28, 29, ent-29 and 34 in two simple reductive steps. Reaction with natural carbohydrates or bicyclic amino sugar precursors allowed the synthesis of homo-oligomeric di- and trisaccharides 44, 46 and 47 or a hybrid trisaccharide 51 with natural carbohydrates. Access to a bivalent amino sugar derivative 54 was accomplished by reaction of rearrangement product 10 with 1,5-pentanediol. Alternatively, when a protected L-serine derivative was employed as glycosyl acceptor, the glycosylated amino acid 60 was efficiently prepared in few steps. In this report we describe the synthesis of unusual amino sugar building blocks from enantiopure 1,2-oxazines that can be attached to natural carbohydrates or natural product aglycons to produce new natural product analogues with potential applications in medicinal chemistry.