Chiral auxiliaries for asymmetric radical cyclization reactions: Application to the enantioselective synthesis of (+)-triptocallol

[GRAPHICS] A series of epimeric 8 aryl menthyl derivatives 5a-d and 6a-I, prepared from the same chiral source (R)-pulegone, were employed as chiral auxiliaries in the asymmetric radical cyclization reactions of beta -keto esters mediated by Mn(OAc)(3). Chiral precursors 8c and 8d provided the cyclization products 10c and 10d, respectively, as single isomers (dr > 99:1), whereas the cyclization of precursor 9k gave 13k with good stereoselectivity (dr = 24:1). Diastereomer 13e was employed as the key intermediate in the enantioselective synthesis of (+)-triptocallol in 90% ee.