Acylethynylpyrroles undergo facile (rt, MeCN or MeOH,
24–72
h) catalyst-free annulation with 1-pyrrolines to afford acylmethylenetetrahydrodipyrrolo[1,2-a:1′,2′-c]imidazoles in up
to 93% yield and 90% E-stereoselectivity of the olefin
moiety.
The ring-opening/functionalization of 1-pyrrolines by cyanoacetylenes or acetylenic ketones (20-80 oC, MeCN, H2O) affords δ-keto aminoacrylonitriles and δ-keto aminoenones, mostly as the Z-isomers, in up to 85% yields. The synthesis involves the C(2)-N-bond cleavage in the intermediate hemiaminal resulted from the intermediate 1,3(4)-dipolar 1-pyrroline/acetylene complexes and water.
Electron‐deficient propargylic alcohols (EWG = CN, C(O)Ph, CO2Me, C(O)NH2) under catalyst‐free mild conditions are annulated with Δ1‐pyrrolines to afford functionalized hexahydropyrrolo[2,1‐b]oxazoles, mostly in 19–93 % yield. The synthesis is stereoselective with respect to disposition of functionalized ethenyl group relative to the condensed heterocyclic system thus formed in most cases. This methodology provides a simple one‐pot access to a novel family of synthetically and pharmaceutically prospective.
Available pyrrolylalkynones with tetrahydroindolyl, cycloalkanopyrrolyl, and dihydrobenzo[g]indolyl moieties, acylethynylcycloalka[b]pyrroles, are readily annulated with Δ1-pyrrolines (MeCN/THF, 70 °C, 8 h) to afford a series of novel pyrrolo[1′,2′:2,3]imidazo[1,5-a]indoles and cyclohepta[4,5]pyrrolo[1,2-c]pyrrolo[1,2-a]imidazoles functionalized with an acylethenyl group in up to an 81% yield. This original synthetic approach contributes to the arsenal of chemical methods promoting drug discovery. Photophysical studies show that some of the synthesized compounds, e.g., benzo[g]pyrroloimidazoindoles, are prospective candidates for TADF emitters of OLED.
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