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

Abstract

Abstract 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 .