Organocatalytic asymmetric robinson annulation of α,β-unsaturated aldehydes:: Applications to the total synthesis of (+)-palitantin

The highly enantioselective organocatalytic Robinson annulation of alpha,beta-unsaturated aldehydes was achieved, catalyzed by L-proline and trialkylarnines and providing the formal [4 + 2] cycloaddition adducts. Additionally, in some examples in the catalysis with diarylpyrrolinol silyl ethers, the reactions afforded the [4 + 2] adducts with high enantioselectivity (>99.5% ee). The structure of the adduct, obtained from the reaction of 3-methylbut-2-enal and (E)-3-(2-nitrophenyl)acrylaldehyde, was confirmed by X-ray analysis. The absolute configurations of some [4 + 2] cycloadducts were investigated, and the methodology was applied in the synthesis of (+)-palitantin.