The asymmetric [2, 3] sigmatropic rearrangements of 2-alkylamino-3-phenyl-2-pentenyl-methylsulfonium methylides were accomplished by the reaction of ethylphenylketenimines 3 having a chiral carbon next to the nitrogen atom with trimethylsulfonium ylide, and acidic hydrolysis of the imines 4 thus obtained led to optically active 3-methylthiomethyl-3-phenyl-2-pentanone (5). The reaction of ethylphenylketene (-)-menthylimine (3g) with trimethylsulfonium ylide at -78°C resulted in the highest optical yield of (R)-(-)-5 in the above sequence.