Synthesis of β- and γ-lactam fused dihydropyrazinones from Ugi adductsviaa sequential ring construction strategy

A modular approach for the construction of beta- and gamma-lactam fused dihydropyrazinones from the readily available Ugi adducts has been described. The sequential construction of rings through base-mediated cycloisomerization followed by acid-mediated cyclization yielded beta-lactam fused dihydropyrazinones. However, the Ugi-derived dihydropyrazinones afforded gamma-lactam fused dihydropyrazinones under base-mediated cycloisomerization. The regioselectivity in the cycloisomerization reactions is explained on the basis of ring-strain. Substrate scope, limitations and mechanistic investigations through DFT-calculations have been explored.