Endo-selective quenching of hexahydropyrrolo[2,3-b]indole-based N-acyliminium ions

Radical decarboxylation of L-tryptophan-derived (2S,3aR,8aS)-8-arylsulfonyl-1,2-di(methoxycarbonyl)-1,2,3,3a,8,8a-hexahydro-2H-pyrrolo[2,3-b]indoles 8 and 9 in the presence of diphenyl diselenide results in the endo-selective formation of (2R,3aR,8aS)-8-arylsulfonyl-1-methoxycarbonyl-2-phenylselenyl-1,2,3,3a,8,8a-hexahydro-2H-pyrrolo[2,3-b]indoles 10 and 11. These selenides, in conjunction with Lewis acids, serves as precursors to the corresponding N-acyl iminium ions, which undergo selective endo-face quenching by allyltributylstannane, allyltrimethylsilane, propargyltrimethylsilane, and trimethylsilylcyanide. Stereochemical assignments rest on NMR data and crystallographic studies. The endo-selective nature of these reactions is interpreted in terms of minimization of allylic strain at the transition state for nucleophilic attack on the N-acyl iminum ion.