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 undego 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.

在二苯二硒化物的存在下，对由L-色氨酸衍生的(2S,3aR,8aS)-8-取代磺酰基-1,2-二(甲氧基羰基)-1,2,3,3a,8,8a-六氢-2H-吡咯[2,3-b]吲哚8和9进行自由基脱羧反应，可得到末端选择性的(2R,3aR,8aS)-8-取代磺酰基-1-甲氧基羰基-2-苯基硒基-1,2,3,3a,8,8a-六氢-2H-吡咯[2,3-b]吲哚10和11。这些硒化物与路易斯酸共同作用，作为相应的N-酰基亚胺正离子的前体，这些亚胺正离子通过烯丙基三丁基硅烷、烯丙基三甲基硅烷、丙炔基三甲基硅烷和三甲基硅基氰化物进行末端选择性的末端面淬灭反应。立体化学的归属基于核磁共振数据和晶体学研究。这些反应的末端选择性可通过在亲核攻击N-酰基亚胺正离子时过渡态中烯丙基应变的最低化来解释。