Mechanistic observations in the gewald syntheses of 2‐aminothiophenes

The Gewald syntheses were employed to prepare a series of 2‐amino‐3‐carboethoxythiophenes, and the syntheses of two of these, namely, the 3,4‐trimethylene (1f) and 3,4‐tetramethylene (1g) derivatives, were examined in detail. In two preparations of1f, octahydro‐6a‐(4‐morpholinyl)‐2‐thioxocyclopenta[b]pyrrole‐3‐carboxylic acid (7) was a co‐product. The structure of7was ascertained from its 300 MHz1H nmr and13C nmr spectra, and by its conversion to 1,4,5,6‐tetrahydro‐2‐mercaptocyclopenta[b]pyrrole‐3‐carboxylic acid ethyl ester (8). Isolation of7and other observations led to postulated mechanisms for three of the Gewald thiophene syntheses.