Synthesis of optically active α-(nonafluoro-tert-butoxy)carboxylic acids

Three novel α-(nonafluoro-tert-butoxy)carboxylic esters were synthesized by two methods: the Mitsunobu reaction of optically active (S)-α-hydroxycarboxylic esters and the Williamson synthesis starting from the corresponding O-mesyl derivatives. Both procedures from ethyl (S)-lactate resulted in the formation of a fluorous optically active (R)-lactic acid ester derivative, while the reactions from methyl (S)-mandelate took place with racemization. The proposed mechanism of this racemization is described. The conversion of methyl (S)-phenyllactate under Mitsunobu conditions was also stereospecific, but the reaction of its O-mesyl derivative yielded a mixture of substitution and elimination products. The optical purity of the α-(nonafluoro-tert-butoxy)carboxylic acids prepared by the hydrolysis of the above esters was determined by 1H NMR spectroscopy.