Synthesis of 5,6-dihydropyrrolo[2,1-a]isoquinolines by rearrangement of 5-methyleneisoxazolidines generated from 1,3-dipolar cycloaddition of 3,4-dihydroisoquinoline N-oxides with allenes: a novel consecutive rearrangement to fused-ring pyrrole derivatives

Abstract

5-Methyleneisoxazolidines 3, obtained by 1,3-dipolar cycloaddition of 1-methyl- and 1-(substituted methyl)isoquinoline N-oxides 1 with electron-deficient allenes 2, undergo thermal rearrangement when heated at 130–150 °C to afford two isomers, 4 and 5, of 5,6-dihydropyrrolo[2,1-a]isoquinoline derivatives. The formation of these fused-ring pyrroles can be rationalized on the basis of two competitive consecutive rearrangements occurring, one of which, as the minor route, involves an initial 1,3-hydrogen shift to give 4-isoxazolines followed by known rearrangement via acylaziridine intermediates, while the other, the major one, involves transient formation of pyrrolidin-3-ones followed by their novel rearrangement via 3,4 bond scission and cyclocondensation.