Richter cyclization and co-cyclization reactions of triazene-masked diazonium ions

The conventional Richter cyclization involves diazotization of 2-alkynylanilines with HX (aq) (X = Br or Cl) and NaNO2, followed by spontaneous ring closure to give a mixture of 4-halocinnoline and 4-cinnolinone products. The different products result from competing attack of X− and H2O, respectively, upon an intermediate 2-alkynylphenyl diazonium ion during the cyclization step. In order to improve the chemoselectivity of this reaction, we have utilized triazenes as masked diazonium ions. These can be unmasked using MeSO3H in anhydrous solvents and the resultant 2-alkynylphenyl diazonium ion cyclized chemoselectively by the incorporation of a specifically added nucleophile. This process has been extended to tethered nucleophiles, leading to a Richter induced co-cyclization process to give ring-fused cinnolines.