Benzo[f]isoindole analogs II. Thieno[3,4-b]quinoxaline

The transient existence of thieno3,4-b]quinoxaline ( 2d ) as a product of dehydration of 1,3-dihydrothieno3,4-b]quinoxaline 2-oxide ( 5 ) was demonstrated by trapping experiments with N-phenylmaleimide and dimethyl acetylenedicarboxylate. Attempts to isolate 2d from reaction mixtures arising from dehydration of 5 and from dehydrogenation of 1,3-dihydro-thieno3,4-b]quinoxaline ( 8 ) were unsuccessful.