Acylethynylpyrroles as a platform for the one-pot access to 2-(pyrrol-2-yl)-3-acylpyridines via the dihydrogenative annelation with propargylamine

Recent Strides in the Transition Metal-Free Cross-Coupling of Haloacetylenes with Electron-Rich Heterocycles in Solid Media

“…Latter [79] from the reaction of NH-acylethynylpyrroles 59 with propargylamine in the presence of CuX (X = Cl, Br, I), 3-acyl-2-(pyrrol-2-yl)-5-halopyridines 60 were unexpectedly isolated in 4-14% yields along with 3-acyl-2-(pyrrol-2-yl)pyridines 61 (28-61% yields) (Scheme 33). Evidently, the cause of this difference compared to the previous cyclization [78] was the NH-functionality of the starting acylethynylpyrroles.…”

“…The one-pot reaction of N-substituted acylethynylpyrroles 55 with propargylamine in the presence of CuI selectively afforded 2-(pyrrol-2-yl)-3-acylpyridines 56 (Scheme 31) [78]. Catalyst-free heating of the reactants led to N-propargyl(pyrrolyl)aminoenones 57 which, upon keeping with CuI (equimolar amount) for 2.5 h at the same temperature, underwent the dihydrogenative ring closure to give pyrrolyl pyridines 56 (Scheme 31).…”

“…The air oxygen also did not participate in this process: the same results were obtained both under argon blanket and on air. Therefore, the Cu + cation was considered [78] as a likely oxidant.…”

“…It is supposed (Scheme 34)[79] that hydrogen halides, reversibly generated by the interaction of the NH pyrrole moiety of the intermediate N-propargyl(pyrrolyl)aminoenone 57 with CuX, add to the triple bond activated by π-complexing with other CuX molecules to give haloallyl intermediate B(Scheme 34). Afterwards, the intramolecular addition of the CH bond to the allyl moiety takes place to form the intermediate 5-halotetrahydropyridyl intermediate C. Aromatization of the latter is finalized via the reaction with CuX and further oxidation by Cu + cations as previously described for a similar process[78].…”

Synthesis of 3‐Keto Pyridines from the Conjugated Allenone – Alkynylamine Oxidative Cyclization Catalyzed by Supported Au Nanoparticles

“…In the presence of 20–40 mol% CuBr, pyridines are formed in moderate to good yields, yet under catalysis by a base, pyrroles were seen, instead . Cyclization/aromatization has been achieved using stoichiometric amounts of CuI, as the oxidation of the cyclization products (dihydropyridines) into pyridines is not catalytic, with Cu(I) being reduced into Cu(0) . If the N atom of N ‐propargyl enaminones is trisubstituted, 1,6‐dihydropyridines can be also formed in the presence of a Cu(I)‐catalyst (3–10 mol%) .…”