Substrate-Controlled [5+1] Annulation of 5-Amino-1H-phenylpyrazoles with Alkenes: Divergent Synthesis of Multisubstituted 4,5-Dihydropyrazolo[1,5-a]quinazolines

A new and efficient [5+1] annulation reaction for the first synthesis of 5,5-disubstituted 4,5-dihydropyrazolo[1,5-a]quinazolines is described. This transition-metal-free tandem cyclization was performed with 5-amino-1H-phenylpyrazole and readily available electron-deficient olefinic ester. The reaction proceeds via an aza-Michael addition/Truce-Smiles rearrangement/SNAr cyclization pathway, which was verified by DFT calculations. The participation of Truce-Smiles rearrangement is substrate dependent and could prompt the process of cascade reactions. It is also the first report of alkenes acting as one-carbon synthons for [5+1] hetero-annulations.