Diastereoselective, Lewis acid-mediated Diels-Alder reactions of allenoic acid derivatives and 1,3-cyclopentadienes.

Allenes with different substituents at their terminal carbon atom display axial chirality and can be obtained in enantiopure form by a photochemical deracemization protocol. It has now been studied under which conditions allenoic acid derivatives undergo a Diels-Alder reaction with 1,3-cyclopentadienes and which products result. Cyclic derivatives (lactams, lactones) underwent an exo -selective reaction catalyzed by the Lewis acid Eu(fod) 3 , while acyclic derivatives yielded with high preference the endo -products (EtAlCl 2 as the preferred Lewis acid). The exocyclic double bond forms with exquisite diastereoselectivity and the chirality transfer is close to perfect. The method was applied to the synthesis of the sesquiterpenes β-santalol (1) and 10( E )-β-santalic acid (13).