Synthesis of thieno[3,4-b]quinoxalime and derivatives

The highly reactive o-quinonoid heterocycles thieno[3,4-b]quinoxaline (1a) and its 6,7-dimethyl derivative (1b) have been synthesized by a base-catalyzed Pummerer reaction and isolated in crystalline form. In contrast, the 1,3-dibromothieno[3,4-b]quinoxaline (8) could be characterized in solution but not isolated in pure farm. The readily prepared 1,3-dihydrothieno[3,4-b]quinoxaline (5a) underwent a direct Knoevenagel condensation with thiophene-2-carboxaldehyde to give the stilbenoid 14a. Sulfide 5a was also converted in a two-step process to the stable 1,3-diformylthieno[3,4-b]quinoxaline 11a.