An Approach to Serrulatane Diterpenes via e ndo-Selective Conjugate Nucleophilic Addition to Arene-Cr(CO)3 Complexes

Starting from a nonracemic planar-chiral arene tricarbonyl chromium complex, the serrulatane diterpenoid (+)-20-methoxy-serrulat-14-en-7,8-diol was synthesized in a highly stereoselective fashion. The key step of the synthesis is an endo-selective conjugate nucleophilic addition of lithio-methylphenyl sulfone to a 1-ethylidene-tetralin-Cr(CO)3 derivative. By employing different substrates and nucleophiles it was shown that the surprising and rather general endo selectivity must result from a unique complex induced proximity effect under participation of the Cr(CO)3 moiety.