Cyclic tautomers of tryptophans and tryptamines—4: Synthesis of cyclic tautomers of tryptophans and tryptamines

N_b-Methoxycarbonyltryptophan methyl ester (dl- and l-13) was cyclized to the corresponding rans cyclic tautomer (14) in excellent yield in various acids such as 85% phosphoric acid or trifluoroacetic acid. The cis cyclic tautomer (15) was formed as the less stable and kinetically controlled product and converted to the more stable trans isomer (14) under the reaction condition. The trans isomer (14) was reverted to 13 on treatment with 10% sulfuric acid in methanol. Other tryptophan and tryptamine derivatives (6 and 19a) also cyclized to the corresponding cyclic tautomers in similar acidic media.