Attempted synthesis of a tenidap isomer and formation of an unexpected stable water adduct

An attempt to synthesize the Tenidap isomer 13 was performed starting from 2‐ethoxy‐5‐chloroindole (9). The acylation and carbamoylation reactions of 9 led to the expected intermediate 12, however the hydrolysis of 12 in acidic or alkaline media did not yielded the expected 13. Instead an unusually stable water adduct 18 or the cleavage product 16 was formed. The formation of the unexpectedly stable water adduct 18 was caused by the steric and electronic effect of the thienoyl group in position 1 as proved by the hydrolysis of 10 and 16.