Total Synthesis of (10R)- and (10S)-Corossolin: Determination of the Stereochemistry at C-10 of the Natural Corossolin and the Differential Toxicity toward Cancer Cells Caused by the Configuration at C-10

(10 R ) and (10 S )-corossolin have been synthesized by coupling of the alkyne intermediate 18 with epoxide 27 or 29, respectively. Comparison of the physical data of both synthetic corossolins with those reported for the natural one shows that the configuration at C-10 of the natural corossolin is highly likely to be R . The R isomer is found to be 18 times as active as the S isomer in the preliminary in vitro tests against the B16BL6 cell line.