Isoquinolinium N-arylimides and acetylenic dipolarophiles; cycloadducts and their rearrangements

Article abstract of DOI:10.1016/S0040-4020(98)00430-X

Dimethyl acetylenedicarboxylate, methyl propiolate, and ethyl phenylpropiolate surpass the corresponding ethylenic carboxylic esters in dipolarophilic activity vs. isoquinolinium N-arylimides, a class of azomethine imines. The cycloadducts contain a N(β)-vinylphenylhydrazine system and enter into a Fischer indole synthesis which stops one step short of the indole. The [3.3]-sigmatropic rearrangement involved is likewise faster for the cycloadducts of acetylenic dipolarophiles than for ethylenic ones and does not require acid catalysis; in some cases the initial adduct escapes ¹H NMR observation. The products 11-17, obtained with ethyl phenylpropiolate, provide beautiful NMR models for steric interaction of ben ring E and the 12-phenyl group. On treatment with strong acid, the pentacyclic rearrangement products suffer fragmentation; e.g., 11 furnishes 4-(o-aminophenyl)-isoquinoline and methyl benzoylacetate in methanolic HCl.