Asymmetric Protonations of Enol Derivatives

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Abstract
Described herein are two methods of asymmetric protonation of enol derivatives under acidic and basic conditions. The process under basic conditions is protonation of prochiral simple enolates with (S,S)-imide 1 which possesses an asymmetric 2-oxazoline ring. Various lithium enolates derived from the corresponding α-monoalkylated cycloalkanones have been protonated by 1 and (R,R)-imide 2 with 97-68% ee. The catalytic enantioselective protonation of lithium enolate has been realized using a catalytic amount of (S,S)-imide 1 and stoichiometric amount of an achiral proton source. Diastereoselective protonation of lithium enolate of (-)-menthone has also been achieved stoichiometrically and catalytically. The process under acidic conditions is protonation of silyl enol ethers and ketene bis(trialkylsilyl) acetals with Lewis acid assisted chiral Brønsted acid (LBA) which is prepared from optically active binaphthol and tin tetrachloride. The catalytic process has been realized using a catalytic amount of optically active 2-hydroxy-2'-methoxy-1,1'-binaphthyl and a stoichiometric amount of 2,6-dimethylphenol in the presence of tin tetrachloride.

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