Synthesis of heterocyclic compounds using amidines as their ene-1,1-diamine tautomers. I. Synthesis of 4,5-dihydro-3H-pyridin-2-one, 3,4-dihydropyrrol-2-one and 1,3-dihydropyrrol-2-one derivatives by the reaction of amidines with α,β-unsaturated esters