Simple oxidation of 3-O-silylated glycals: application in deblocking 3-O-protected glycals
A high yielding allylic oxidation of 3-O-silylated glycals 5–10 with the reagent system PhI(OAc)2–TMSN3 is presented. The iodine(III) species generated under these conditions is a lot more effective for generating carbohydrate-derived 3-trialkylsiloxy-2,3-dihydro-4H-pyran-4-ones 11–15 than is [hydroxy(tosyloxy)iodo]benzene, the Koser reagent. Even disaccharide 9 containing the oxidation-labile phenylseleno group is smoothly oxidized to the corresponding enone 15. The hypervalent azido iodine reagent is complementary to the Koser reagent, because 3-O-benzylated or -acylated glycals cannot be oxidized. When the iodine(III)-mediated oxidation of 3-O-silylated or -benzylated glycals is followed by a reduction step, the formal 3-O-deblocking of glycals is achieved. In particular, the Luche reduction of enones obtained from the oxidation of lyxo-configured glycals 24 and 26 is highly selective and exclusively affords the corresponding lyxo-configured glycals 28 and 30. In some cases, these products can be transformed under Mitsunobu conditions into glycals with inverted configuration at C-3 in moderate yield.