1,3-dipolar cycloaddition reaction of 6,7-dimethoxy-3,4-dihydroisoquinoline N-oxide with diketene: A novel consecutive rearrangement to hexahydropyrrolo[2,1-a]isoquinolines

A new kind of hexahydropyrrolo[2,1-a]isoquinolines, 1,2,3,5,6,10b-hexahydro-8,9-dimethoxypyrrolo[2, 1 -a]isoquinolin-2-one-3-carboxylic acid (3) under bar, and 3-acetoacetyl-1,2,3,5,6,10b-hexahydro-8,9-dimethoxypyrrolo[2, 1-a]isoquinolin-2-one (4) under bar, were obtained by 1,3-dipolar cycloaddition reaction of 5,6-dimethoxy-3,4-dihydroisoquinoline N-oxide (1) under bar with diketene. These products could be interpreted to be derivable via an initial cycloaddition reaction of the nitrone to the exocyclic double bond of diketene followed by a novel consecutive rearrangement involving NO bond cleavage rather than elimination of carbon dioxide. The fact that the cycloaddition reaction of (1) under bar with allene afforded 1,2,3,4-tetrahydro-6,7-dimethoxy-1-(2-oxopropylidene)isoquinoline (5) under bar supported also this mechanism. These compounds would be expected to be useful alkaloid analogues. Thus, the esterification of (3) under bar with methanol afforded methyl 1,2,3,5,6,10b-hexahydro-2-hydroxy-2,8,9-trimethoxypyrrolo[2, 1-a]isoquinoline-3-carboxylate (7) under bar and methyl 1,2,3,5,6,10b-hexahydro-2,2-dihydroxy-8,9-dimethoxypyrrolo[2,1-a]isoquinoline-3-carboxylate (8) under bar, while thermal decarboxylation of (3) under bar gave a vinylogous amide derivative (5) under bar exclusively.