A mild one-pot synthesis of thieno[3,4-c]pyrazoles and their conversion into pyrazole analogs of o-quinodimethane

The reaction of alkyl- or arylhydrazine hydrochlorides with 4-​cyano-​3-​oxo-​tetrahydrothiophene in refluxing EtOH affords, in a regioselective manner, the corresponding 2-​alkyl- or 2-​aryl-​3-​aminothieno[3,​4-​c]​pyrazoles with good yields. Some of the latter compds., after std. acetylation, were efficiently converted into the corresponding sulfolenes, which readily undergo SO2 extrusion upon heating and, in the presence of dienophiles, such as N-​phenylmaleimide and fumarate, give the corresponding [4+2] cycloadducts in good yields.