Reactivity studies of [1]benzothieno[3,2-b][1]benzofuran

Electrophilic substitution and metallation reactions of the title compound 1 were studied. Bromination, acetylation, benzoylation, formylation, and nitration usually afforded nonseparable mixtures of 2- and 7-substituted derivatives as the main product. Disubstitution reactions preferably led to 2,7-disubstituted derivatives. [1]Benzothieno[3,2-b][1]benzofuran 10,10-dioxide (17) and [1]benzothieno[3,2-b][1]benzofuran 10-oxide (18) can be selectively obtained by oxidation of 1. Mononitration of 17 and 18 led selectively to corresponding 7-nitro derivatives 19 and 21, respectively. Only sulfoxides 19 and 20 were successfully reduced. Reduction of the 5-oxide moieties was successful only at the sulfoxide stage. Metallation of 1 with butyllithium preferably proceeds in positions 1 and 6; subsequent reaction with methyl iodide or carbon dioxide led to the corresponding dimethyl derivative 25 or esters 26 and 27, respectively. An unusual addition of butyllithium to the central double bond of 1 was also observed in a small extent producing 9b-butyl-4b,9b-dihydro[1]benzothieno[3, 2-b][1]benzofuran-4b-ol (29). The structures of all products were elucidated by NMR.