The absolute configuration of the various asymmetric centres in a-santonin was well established,except For C₁₁ which has been in controversy.By application of the Prelog's asymmetric induction method to the hydroxy compound(ⅩⅩ),we were able to obtain a mixture of atrolactic acid which was rich in the dextrorotatory isomer(ⅩⅩⅩ),and t9}us confirmed the 11α-CH₃ configuration of a-santonin as indicated in(Ⅰ).