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The thermal lactamization of 4-(carboethoxy)-methyl-tryptophan ethyl ester (5), prepared from 4-(carboethoxy)methyl-indole 3 via Gilchrist′s method, in refluxing xylene followed by saponification provided the 8-membered cyclic tryptophan derivative 1. Selective reduction of lactam 6 via the iminium ion 9 furnished the cyclic tryptophan derivative 2
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Chung et al. (1992) studied this question.
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