摘要 : During the synthesis of protected tetrasaccharide fragment of the repeating unit of polysaccharide ofAzospirillum brasilensesp7 we found that glycosylation with a mixture of protectedN-trifluoroacetyl (TFA)-protected glucosamineN-... 展开 During the synthesis of protected tetrasaccharide fragment of the repeating unit of polysaccharide ofAzospirillum brasilensesp7 we found that glycosylation with a mixture of protectedN-trifluoroacetyl (TFA)-protected glucosamineN-phenyltrifluoroacetimidate and the corresponding 2-trifluoromethyl-oxazoline led to 1,2-cis-linked glycoside along with the expected 1,2-trans-linked isomer. This fact looks quite unusual bearing in mind that bothN-TFA-containing glycosyl donor and 2-trifluoromethyl-oxazoline are usually expected to favor the formation of 1,2-trans-glycosides. Formation of 1,2-cis-glycoside was explained by hydrogen-bond mediated complexation ofhydrogenatom of NHTFA at C-2 of oxacarbenium ion derived from glycosyl donor andoxygenatom of hydroxy group of the incoming nucleophile. Conversely, glycosylations withN-phthaloyl protected glycosyl donors resulted in exclusive formation of 1,2-trans-isomer presumably due to a significant steric effect and the absence of additional hydrogen-bonding sites ofN-phthaloyl group. 收起