Nucleophilic substitution at the anomeric positions of tetrahydropyranyl (THP) and related carbohydrate-derived esters that proceeded through pyridinium-type salt intermediates have been developed. Treatment of the 6-substituted α-acetoxy-tetrahydropyrans with TMSOTf (TMS=trimethylsilyl) and 2-substitutited pyridines, such as 2-p-tolylpyridine and 2-methoxypyridine, led to the efficient generation of cis-pyridinium-type salts. These salts reacted with various nucleophiles, such as alcohols, azides, and organozinc reagents, to form nucleophilic-substitution products. A characteristic feature of these processes was that they took place under mild conditions, which did not affect acid-labile protecting groups. Furthermore, the reactions that employed azides and C-nucleophiles generated 2,6-trans products with high degrees of stereoselectivity.

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http://dx.doi.org/10.1002/asia.201200234DOI Listing

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