Total synthesis of the natural carbazoles Murrayanine and Murrayafoline A, based on the regioselective Diels-Alder addition of exo-2-oxazolidinone dienes

A new synthesis of the natural carbazoles Murrayanine (1) and Murrayafoline A (3) is described. The key step in the synthetic route involved the regioselective cycloaddition of the diene 4,5-dimethylene-3-phenyl-1,3- oxazolidin-2-one (4) to acrolein (6) catalyzed by Lewis acids at low temperature. Direct aromatization of the substituted cyclohexene moiety of adduct 7, and further hydrolysis of the 2-oxazolidinone ring, proved to be a more efficient strategy than the opposite synthetic sequence for the preparation of the corresponding arylphenylamines 14 and 18. Palladium-promoted cyclization of the latter furnished the desired carbazoles 1 and 3 in high overall yields.