Richter-type cyclizations

Dialkyltriazenes ortho to an acetylene substituent on an aryl ring as in 184, undergo ring closure in modified Richter-type cyclizations, forming substituted cinnolines 185 and isoindazoles 186 in fair to very good yields . 

Vinogradova OV, Sorokoumov VN, Balova lA (2009) A short route to 3-alkynyl-4-bromo (chloro)cinnolines by Richter-type cyclization of OTtho-(dodeca-l,3-diynyl)aryltriaz-l-enes. Tetrahedron Lett 50 6358-6360... 

This approach is important for the synthesis of cinnolines. CHEC(1984) <1984CHEC(2)1> already covered several examples. The methodology used starts with an aniline ortho substituted with an alkene (Widman-Stoermer type), alkyne (Richter type), or enolisable ketone (Borsche type) which is diazotisized, delivering the sixth atom, allowing cyclization. CHEC-II(1996) <1996CHEC-II(6)1> gave more examples. 

A combination of the preceding type of synthesis and of cyclization of 4-amino-5-arylazopyrimidine can be seen in the novel procedure of Richter and Taylor. Proceeding from phenylazomalonamide-amidine hydrochloride (180), they actually close both rings in this synthesis. The pyrimidine ring (183) is closed by formamide, the triazole (181) one by oxidative cyclization in the presence of cupric sulfate. Both possible sequences of cyclization were used. The synthetic possibilities of this procedure follow from the combination of the two parts. The synthesis was used for 7-substituted 2-phenyl-l,2,3-triazolo[4,5-d]-pyrimidines (184, 185). An analogous procedure was employed to prepare the 7-amino derivatives (188) from phenylazomalondiamidine (186). 

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