RESUMO
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 (EtAlCl2 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 ß-santalol (1) and 10(E)-ß-santalic acid (13).