Regioselectivity in the Multicomponent Reaction of 5-aminopyrazoles, Meldrum’s Acid and Triethyl Orthoformate

A regioselective multicomponent synthesis of pyrazolo[1,5-a]pyrimdines-7(4H)-ones 5 is reported. The reaction proceeded regioselectively via initial Michael addition of exocyclic amino function to the intermediately formed 5-ethoxymethylene Meldrum’s acid followed by a ring opening of Meldrum’s acid and subsequent ring closure with the ring nitrogen. The regioselectivity of the reaction was confirmed by the isolation and characterization of the Michael adduct.