Preparation of cysteine adducts by regioselective ring-opening reactions of phenyloxirane

Regioselective ring-opening reactions of phenyloxirane by protected cysteine are described, which enable the synthesis of pure regioisomeric cysteine adducts required for bioanalytical studies on adducts of protein with styrene. The reaction catalyzed by tris(pentafluorophenyl)borane proceeds regioselectively and stereospecifically to give protected S-(2-hydroxy-1-phenylethyl)cysteine (alpha-adduct) with the inversion of configuration at the alpha-carbon. By contrast, when the same reaction is catalyzed by tetraalkylammonium fluorides, S-(2-hydroxy-2-phenylethyl)cysteine (beta-adduct) is formed predominantly, and a compete racemization is observed.