Abslrart: The synthesis of a new, conformationally restrained a~log of tryptoghan, 1,2,3,4ktrahydtv-2-ami-2-mrboxy_cyflopentfbJindole (5) is descrhd. The key step involzm the BF3-Et,0 catalyzed intramolecular cycl&atian of adinoketone 2b into the lt3,4tetrahydro_~~ (3). Conformationally restricted aminoacids are actively sought. They can be incorporated in peptides in place of the corresponding aminoacid thus becoming stereochemical probes for bioactive conformations or selective interactions. They can confer improved metabolic stability, increase the binding stability to receptors or exhibit enzyme inhibiting properties. As a part of a program directed to the preparation and biological evaluation of conformationally restrained analogs of aminoacids 1 we become interested in the synthesis of cyclic analogs of tryptophan having the aminoacidic moiety relatively fixed in space with respect to the planar indole nucleus. A derivative fulfilling this requisite, 3-amino-1,2,3,4tetrahydrocarbazole-3carboxylic acid was reported previously and shown to be a competitive inhibitor of achimotrypsin.2 We report now the first synthesis of the structurally related, conformationally more restrained 1,2,3,4tetrahydro-2-amino-2-carboxy-cyclopentlblindole (5). A survey of the literature revealed that while a variety of methods have been developed for the synthesis of 1-0x0-3 and 3oxo-1,2,3,4tetrahydro-cyclopentlblindole derivatives,43 1,2,3,4tetrahydro-cyclopentJb]indol-2-one (31, key to our synthesis, was still unknown.