Ring-closure reactions, electrophile-mediate

By comparison, the reaction of L-tryptophan with peroxyacetic acid21 or photosensitized oxygenation22, involving ring closure and introduction of hydroxy and hydroperoxy groups at the 3a-position, proceeded with poor stereoselectivity. The related cyclization reactions of tryptophan and tryptamine derivatives mediated by electrophiles are described in Section 7.2.6.2.1. 

The observed excellent stereoselectivities (dr=91 9 to >95 5, 94 to >99% ee) could be ascribed to the steric hindrance created by the employed catalyst in each step of the catalytic cycle reported below (Scheme 2.56). Once the chiral amine (S)-70 activates the acrolein 131 as electrophile by generating the vinylogous iminium ion A, the indole 171 performs an intermolecular Friedel-Crafts-type reaction. The resulting enamine B acts as nucleophile in the Michael addition of the nitroalkene 140 leading to the iminium ion D, which upon hydrolysis liberates the catalyst and yields the intermediate aldehyde 173. The latter compound enters in the second cycle by reacting with the iminium ion A, previously formed by the free catalyst. The subsequent intramolecular enamine-mediated aldol reaction of E completes the ring closure generating the intermediate F, which after dehydration and hydrolysis is transformed in the desired indole 172. 

Electrophilic Cyclization of Trichloroacetimidates. Trichloroacetimidates derived from allylic " and homoallylic alcohols undergo electrocyclic ring closure when treated with a source of I" " (eq 5). Cyclization can also be triggered via the Lewis acid-mediated opening of epoxides. In at least one case the imidate proved more reactive than related reactions using carbamates. These are useful methods for the stereoselective introduction of nitrogen into cyclic and acyclic systems. 

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