Heterocycles from Morita–Baylis–Hillman adducts: synthesis of 5-oxopyrazolidines, arylidene-5-oxopyrazolidines, and oxo-2,5-dihydro-pyrazols

Starting from Morita–Baylis–Hillman (MBH) adducts, an approach for the synthesis of oxopyrazolidines, arylidene-oxopyrazolidines, and oxo-2,5-dihydropyrazoles is described. The method is based on a tandem process involving a Michael addition of amino-guanidine into silylated and acetylated MBH adducts, followed by intramolecular cyclization. The use of acetylated MBH adducts led also to the synthesis of unusual pyrazoles, which is facilitated by an unexpected base-mediated equilibrium.