Highly Selective Formal [3+3] Cyclohexenone Annulation

Significance

Chen and co-workers report an organocatalytic formal [3+3] annulation of unsaturated aldehydes 1 and phosphorus ylides 2 to produce cyclohexenones 4. The reaction proceeds via iminium-catalyzed Michael addition followed by a ring-closing Wittig reaction. To effect high enantioselectivity, the authors designed a bulky variant of the Jørgensen-Hayashi catalyst 3a and found that additional activation of the nucleophile by a coordinating LiClO₄ salt as well as catalytic base (DABCO) was required. The adducts 4 were shown to undergo further transformations with high diastereoselectivity.