Efficient construction of new chemical space by way of strategic use of tandem reactions is highly important in drug discovery. Described herein is an atom-economical [4+1+1] annulation approach to 3-(hetero)aryl-4-acyl-3,4-dihydropyrrolo[1,2- a]pyrazines, a new chemical space, via a one-pot three-component reaction under mild reaction conditions. Formation of multiple bonds (one C-C and two C-N) was achieved by a cascade reaction sequence consisting of generation of a Schiff base, a diastereoselective Mannich reaction, and intramolecular imine formation. This modular and environment-friendly process allowed rapid access to a wide range of 4-acylated 3,4-dihydropyrrolo[1,2- a]pyrazines and their analogues, opening opportunity to explore biological activity associated with this scaffold.