o.Formylarylazomethylenetriphenylphosphoranes: A facile thermally promoted rearrangement to 3-oxo-indazoline and 4-oxo-dihydroquinazoline derivatives

The $\text{o}$.formylarylazomethylenetriphenylphosphoranes carrying an electron withdrawing group on the ylidic carbon undergo thermal $\text{intra}$ molecular cyclization to 3-oxo-indazolin-2-yl-methylenetriphenylphosphorane derivatives. The latter compounds, and their 1-alkyl derivatives, in turn, undergo thermal and/or acid catalyzed rearrangement to 4-oxo-l,4-dihydroquinazoline derivatives and PPh₃. Some possible reaction mechanisms are discussed, and some synthetic applications of the above reactions are shown.