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

Jones, R.C.F., Dimopoulos, P., Coles, S.C., Light, M.E. and Hursthouse, M.B. (2000) Synthesis of imidazo 1,2-a pyridines from unactivated 2-alkyl-4,5-dihydroimidazoles through conjugate N-addition. Journal of the Chemical Society, Perkin Transactions 1, (15), 2331-2342.

Abstract

2-Alkyl-4,5-dihydroimidazoles undergo annulation with alpha,beta-unsaturated aldehydes and ketones via conjugate addition of the N-1 nitrogen atom and subsequent enamine-aldol condensation of C(alpha) to form imidazo[1,2-a]pyridines having reversed regiochemistry of annulation from that observed with a dihydroimidazole carrying an activating group at C(alpha); annulation with beta-ketoesters also affords imidazo[1,2-a]pyridines, but now with the same regiochemistry as found with a C(alpha)-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.

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