Cinnolines and pyrazolopyridazines. — Novel synthetic and mechanistic aspects of the Richter reaction

The thermal cyclization of 2-alkynylbenzenediazonium salts 2 known as the Richter reaction and leading, as described in the literature, only to 4-hydroxycinnolines 6 has been studied. A new probable route to these compounds involving intermediate formation of the 4-chlorocinnolines 4 followed by hydrolysis of the latter to 6 is discussed. A new understanding of the reaction mechanism has induced us to change the reaction conditions in order to synthesize some 4-bromo- (5), 4-chloro- (4), and 4-hydroxycinnolines 6 as well as 4-bromo-(8b) and 4-chloropyrazolopyridazine 8a.