Reaction of a brominated benzolactone/lactam with 4-methoxythiobenzamide and thiourea: an Eschenmoser coupling reaction, ring transformation, or dimerization?

Abstract

The reactions of 3-bromo-1-benzofuran-2(3H)-one (1a) and 3-bromo-1,3-dihydro-2H-indol-2-one (1b) with 4-methoxythiobenzamide and thiourea under mildly basic conditions are reported. While brominated lactone 1a gave the expected 5-(2-hydroxyphenyl)-2-(4-methoxyphenyl)-1,3-thiazol-4-ol (2) or 2-amino-5-(2-hydroxyphenyl)-1,3-thiazol-4(5H)-one (5) products, the analogous brominated lactam 1b reacted with the thioamide via an unexpected Eschenmoser coupling reaction to give (3Z)-3-[amino(4-methoxyphenyl)-methylidene]-1,3-dihydro-2H-indol-2-one (3). When lactam 1b was treated with thiourea, isoindigo (4) was the only isolated product. The reaction mechanisms, involving formation of α-thioiminium or isothiouronium salts and their base-catalyzed decomposition are also proposed.