Copper-catalysed unified [3+3] annulation of cyclopropenones with 3-aminopyrazoles and 3-aminoindazoles: access to fused pyrimidine derivatives.

We report an efficient and unified copper-catalysed approach for the synthesis of highly substituted pyrazolo[1,5-a]pyrimidines and pyrimido[1,2-b]indazoles through [3+3] annulation of cyclopropenones with 3-aminopyrazoles and 3-aminoindazoles. The Lewis acid activation of the cyclopropenone facilitates the Michael addition of an amino heterocycle, thereby inducing ring opening and proton abstraction by the carbonyl group to generate a β-amino-enal intermediate. Subsequently, intramolecular cyclization and dehydration afford the desired products in moderate to good yields. DFT studies support the mechanistic proposal. Moreover, we demonstrated the postsynthetic derivatization of the obtained products.