Synthesis of imidazo[1,2-a]pyridines from unactivated 2-alkyl-4,5-dihydroimidazoles through conjugate N-addition
作者:Raymond C. F. Jones、Paschalis Dimopoulos、Simon C. Coles、Mark E. Light、Michael B. Hursthouse
DOI:10.1039/b001830i
日期:——
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.
2-烷基-4,5-二氢咪唑与α,β-不饱和醛和酮通过N-1氮原子的共轭加成和随后的亚胺-醛醇缩合反应发生环化,形成具有与带有C(α)活化基团的二氢咪唑不同的环化反应区域选择性的咪唑[1,2-a]吡啶;与β-酮酸酯的环化也能得到咪唑[1,2-a]吡啶,但此时的区域选择性与C(α)活化的二氢咪唑一致, necessitating对早期报告的结构进行修正。双烷基乙炔二羧酸酯经历共轭N-加成,但作为1,2-而非1,3-双电离体,形成吡咯[1,2-a]咪唑。