Studies on chromium(0)-promoted higher-order cycloaddition-based benzannulation. Total synthesis of (+)-estradiol

A benzannulation sequence featuring [6 pi + 4 pi] cycloaddition of (eta(6)-thiepin 1,1-dioxide)-tricarbonylchromium(0) complexes with highly substituted dienes followed by Ramberg-Backlund rearrangement has been developed. Enantiomerically pure (+)-estradiol (estra-1,3,5(10)-triene-3,17 beta-diol) has been synthesized by employing a higher-order cycloaddition between an appropriately substituted triene dioxide chromium(0) complex and a diene partner derived from an enantiomerically pure indandione precursor as the key ring construction event. Subsequent Ramberg-Backlund rearrangement of this cycloadduct and routine functional group interchanges afforded the steroid target.