Behaviour of 5,6-dihydrothieno[2,3-h]cinnolin-3(2H)-one and 5,6-dihydrothieno[3,2-h]cinnolin-3(2H)-one towards hydrazine. Synthesis of thienocinnolinones and of 4-aminothienocinnolinones

The isomeric compounds 5,6-dihydrothieno[2,3-h]cinnolin-3(2H)-one (7a) and 5,6-dihydrothieno[3,2-h]cinnolin-3(2H)-one (7b) rapidly tautomerise to the corresponding 1,4-dihydrothienocinnolinones 8a,b when kept in refluxing hyd razine hydrate. With longer reaction times the initially formed 8a,b dehydr ogenate to the thienocinnolinones 9a,b which eventually are aminated to 4-a minothienocinnolinones 10a,b. This behaviour recalls that reported for the related 5,6-dihydrobenzocinnolin-3(2H)-one (1) which under the same conditi ons undergoes dehydrogenation to bento[h]cinnolin-3 (2H)-one (2) followed b y 4-amination to 3, but differs for the stability of the intermediates, for the mechanism of the final amination, and for the higher reaction rate. Al l these differences can be rationalised in terms of the heats of formation of the intermediates and products of the two series of transformations.