Domino routes to substituted benzoindolizines: tandem reorganization of 1,3-dipolar cycloadducts of nitrones with allenic esters/ketones and alternative cycloaddition–palladium catalyzed cyclization pathway

Reactions of C-(4-oxo-4H[1]benzopyran-3-yl)-N-phenyl nitrones (7) with allenic esters (8a–c) and allenic ketones (18a–d) furnish benzoindolizines (9a–k, 19a–d) in good yields. The formation of benzoindolizines is postulated to involve regioselective addition of 1,3-dipole to C2–C3 π bond of allenic esters/ketones followed by domino transformation of the cycloadducts, which involve an intramolecular aza Diels–Alder reaction in the intermediate C. DFT calculations of various parameters for diene and dienophile components in the proposed intermediate C have revealed that conformational constraints imposed by the alkyl groups (R=Me, Et) favor intramolecular aza-Diels–Alder cycloaddition. An alternative domino route to benzoindolizines (9a,d,g) involving sequential one-pot cycloaddition of azadienes (22a–c) with silyl-enol ether (23) followed by palladium(0)-catalyzed Heck coupling reaction has also been developed. Both these approaches represent novel domino routes for the synthesis of benzoindolizines.