Synthesis of imidazo[1,2-a]pyridines from unactivated 2-alkyl-4,5-dihydroimidazoles through conjugate N-addition

Abstract

2-Alkyl-4,5-dihydroimidazoles undergo annulation with α,β-unsaturated aldehydes and ketones via conjugate addition of the N-1 nitrogen atom and subsequent enamine–aldol condensation of C(α) to form imidazo[1,2-a]pyridines having reversed regiochemistry of annulation from that observed with a dihydroimidazole carrying an activating group at C(α); annulation with β-ketoesters also affords imidazo[1,2-a]pyridines, but now with the same regiochemistry as found with a C(α)-activated dihydroimidazole and necessitating a revision of earlier reported structures. Dialkyl acetylenedicarboxylates undergo conjugate N-addition but act as 1,2- rather than 1,3-bis-electrophiles to form pyrrolo[1,2-a]imidazoles.