Carboxylate Anions Accelerate Pyrrolidinopyridine (PPy)-Catalyzed Acylation: Catalytic Site-Selective Acylation of a Carbohydrate by in Situ Counteranion Exchange.

Acylpyridinium ions have been known as catalytically active species in acylation reactions catalyzed by 4-dimethylaminopyridine and its analogues. Acylpyridinium carboxylates were found to be 800-1300 times more reactive than the corresponding acylpyridinium chlorides. A catalytic cycle was developed, in which acylpyridinium carboxylates were generated by in situ counteranion exchange from the acylpyridinium chlorides. A catalyst loading as low as 0.01 mol % and catalyst turnover number of up to 6700 were achieved for site-selective acylation of a carbohydrate.