A method of preparing optically active ketones having the general formula ##STR1## wherein R.sup.2 and R.sup.3 indicate hydrocarbon groups having 1 to 20 carbon atoms characterized by submitting an optically active sulfonyloxy alcohol having the general formula ##STR2## wherein R.sup.1, R.sup.2 and R.sup.3 indicate hydrocarbon groups having carbon atoms of 1 to 20 and which optionally contain N, O, P and S. The symbol * indicates asymmetric carbon atoms to rearrangement in a solvent and in the presence of organoaluminum compounds given by the general formula R.sub.3-n.sup.4 AlX.sub.n, (wherein R.sup.4 indicates an alkyl group having carbon atoms of 1 to 20 and X indicates a halogen atom, an alkoxy group or a CN group. n is 1, 1.5 or 2).
Completely stereospecific 1,2-migration of alkylgroups was achieved in Et2AlCl promoted pinacol-type rearrangement of chiral β-mesyloxy alcohols to give optically pure α-alkyl ketones including both enantiomers of 4-methyl-3-hexanone, an alarm pheromone of ant.