Binaphthyl-Modified Quaternary Phosphonium Salts as Chiral Phase- Transfer Catalysts: Asymmetric Animation of β-Keto Esters

摘要

In contrast to the broad synthetic utility of chiral quaternary tetraalkylammonium salts in asymmetric phase-transfer catalysis, chiral quaternary tetraalkylphosphonium salts have not been regarded as reliable phase-transfer catalysts due to facile formation of the corresponding ylides (Wittig reagents) under basic conditions. Indeed, catalytic asymmetric synthesis utilizing chiral quaternary tetraalkylphosphonium salts as phase-transfer catalysts remains poorly studied, and only a few special examples have been reported so far with limited success. In this context, we are interested in using certain chiral quaternary phosphonium salts in asymmetric phase-transfer catalysis. Here we report their application in asymmetric amination of β-keto esters. Such an asymmetric transformation of cyclic five-membered β-keto ester 1 is valuable for preparing key intermediate 2 for asymmetric synthesis of aldose reductase inhibitor AS-3201 (Ranirestat), as shown in Scheme 1.