Asymmetric Diels–Alder reactions. A route to chiral carbocycles via bicyclic lactams

Abstract

The L-valinol-derived unsaturated lactam methyl 3-isopropyl-7a-methyl-5-oxo-2,3-dihydro-7aH-pyrrolo[2,1-b]oxazole-6-carboxylate functions as an excellent chiral dienophile in asymmetric Diets-Alder reactions. Diene approach occurs exclusively from the α-(endo) face to provide optically pure tricyclic lactams. In the presence of Lewis acids, increases in rate, regiospecificity, and ‘Alder endo’ stereoselectivity are observed. Elaboration of the cycloadducts provided an unexpected entry into cyclopropane-containing polycyclic systems via an N-acyliminium ion-enamide rearrangement. In addition, a number of optically pure carbocycles were readily prepared via this methodology.