STEREOCHEMICAL CONTROL OVER 3 CONTIGUOUS STEREOGENIC CENTERS IN THE INTRAMOLECULAR ENE REACTION OF ACTIVATED 1,6-DIENES - APPLICATION TO THE SYNTHESIS OF (+ -)-METHYL CUCURBATE AND (+/-)-METHYL EPIJASMONATE/

The influence of a protected alcohol group adjacent to the ene or enop hile component on diastereoselectivity in both thermal and Lewis acid- catalyzed 5-(3,4) ene reactions of a series of 1,6-dienes 1-7 has been studied. The results indicate that its effect can be considerable, an d in one example, with a gem-dimethyl group on the connecting chain an d a large silyl protecting group on the hydroxyl, the diastereocontrol was almost perfect, with three stereogenic centers and one double bon d geometry set up in one step, e.g., 4 --> 10. This new finding was ex ploited in a synthesis of epijasmonoid natural products, (+/-)-methyl cucurbate (19) and (+/-)-methyl epijasmonate (18) starting from aldehy de 24, where the key step was a highly diastereocontrolled 5-(3,4) ene cyclization 23 --> 22.