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As the reaction progressed, a white solid precipitated from the reaction medium, the product obtained was filtered, washed with water and ethanol, and characterized as phenyl 4-phenyl-1H-pyrrolyl methanone At the same time, a solvation effect occurs in the polar solvent to increase the yield. A plausible mechanism for the reaction is depicted in Scheme Elimination of lithium p-toluenesulfinate under the action of a base to produce a C-3 substituted pyrrole derivative Finally, 1,3-hydride shift occurs to afford the product In , Nair and co-workers also reported an aroyl-substituted pyrroles synthetic route from phosphonium salt as starting material.
The phosphonium salt was neutralized with aqueous NaOH and extracted with dichloromethane to afford 1-phenyl triphenyl-phosphoranylidene ethenone Upon completion of the reaction, dichloromethane was evaporated and the reaction mixture was triturated with hexane to remove triphenylphosphine oxide. This might be due to better coordination power of lithium Scheme 31 [ 50 ].
Multi-component Van Leusen pyrrole synthesis can also occur in alkenes with aromatic ketones. Alkenes with a Nitro Group In , Xiaoqi Yu and co-workers reported that 4 3 -substituted 3 4 -nitro-1H-pyrrole can be synthesized from nitroene and TosMIC 16 in the presence of the ionic liquid 1-butylmethylimidazolium bromide [bmIm]Br. This reaction can be widely applied to aromatic, aliphatic or heterocyclic substituted nitroolefins, and the recovered ionic liquid can be repeatedly used as a solvent without significantly reducing the yield Scheme 33 [ 52 ].
Alkenes with a Cyano Group In , the Yongping Yu group reported that two equivalents of a cyano-substituted trisubstituted alkene and TosMIC 16 were dissolved in anhydrous acetonitrile as a solvent and reacted in the presence of NaH at room temperature for 3 h to form disubstituted pyrrole derivatives In the experiments, researchers also found that when keto and ester groups are simultaneously present in alkenes, due to the higher reactivity, keto group can be eliminated more easily than ester groups alone.
In addition, group with larger steric hindrance can reduce the reactivity when they are present on alkenes. Alkenes with an Aryl Group In , Smith and co-workers developed a method for the one-step synthesis of 3-aryl and 3,4-diarylpyrroles with good yields by readily available aryl or diaryl alkenes with TosMIC 16 Scheme 35 [ 55 ]. They found that the stronger the electron-withdrawing ability of the aryl group attached to the alkene in the substrate, the lower the temperature required for the reaction, the shorter the reaction time, and the higher yield.
At the same time, the steric hindrance of the aryl group will act as a deterrent to the reaction. This phenomenon is particularly evident when the aryl group is ortho-substituted. Other Alkene Synthons Magnus et al. Quinone can be also used for the Van Leusen pyrrole synthesis. N-Methylation of the latter compound with iodomethane in the presence of anhydrous potassium carbonate afforded 2-methyl-2H-benz[f]isoindole-4,9-dione Scheme 37 [ 58 ].
In , Yongping Yu and co-workers reported synthesis of polysubstituted pyrroles from TosMIC 16 and vinyl azides under mild conditions in the presence of base Scheme 38 [ 59 ]. The reaction mixture was then stirred at room temperature for 24 h to give the desired product Scheme 39 [ 59 ].
Under the same optimized reaction condition, the synthesis of the 3-isoxazole trisubstituted pyrrole derivatives was achieved by using the TosMIC derivatives 16 Scheme 41 [ 60 ]. This transformation is operationally simple, high-yielding, and displays broad substrate scope. As early as in , Saikachi and co-workers found that acetylene ester 2 equiv each and TosMIC 16 can produce 2,3,4-trisubstituted pyrrole compounds in the presence of DBU.
The 1,2,3,4-tetrasubstituted pyrrole compounds can be synthesized by another one-step addition reaction Scheme 42 [ 61 ]. In , Alizadeh et al. In this reaction, the nucleophilic addition of Ph3P to the acetylenic esters further increases the reactivity of the substrate as an electron-withdrawing group Scheme 43 [ 62 ].
In , the Adib group developed a protocol that is different with respect to the common shortcomings such as long reaction time, low yield, expensive raw materials, and harsh reaction conditions. In recent years, some research progress has been made, which provides new ideas for the synthesis of polysubstituted pyrrole ring framework.Tobias Persson, Christina W. Tobias Ankner, Casey C. Synthesis of tryptophan derivatives via a palladium-catalyzed N-heteroannulation.
This transformation is operationally simple, high-yielding, and displays broad substrate scope. Christoph K. In , Mao and co-workers reported a synthesis of 4-substituted thienyl pyrrole compounds 90 via Vilsmeier-Haack formylation, aldol condensation and Van Leusen pyrrole synthesis using 2-methoxythiophene 88 as starting material. In , the Ji group reported the reaction of TosMIC 16 with Z 2-oxoethylidene indolinone derivatives 81 to give pyrrole derivatives 82, and developed a simple and convenient synthetic approach to access of 3H-pyrrolo[2,3-c]quinolin-4 5H -one derivatives 83 by the reaction of 81 with functionalized TosMIC derivatives 16 under basic conditions Scheme 24 [ 44 ].
N-Methylation of the latter compound with iodomethane in the presence of anhydrous potassium carbonate afforded 2-methyl-2H-benz[f]isoindole-4,9-dione Scheme 37 [ 58 ].
The Journal of Organic Chemistry , 75 10 , The aldehydes 19 were converted into the corresponding methyl 3-arylacrylate esters 20 using a Wadsworth-Emmons olefination procedure. European Journal of Organic Chemistry , 30 , Wenyuan Yin, M.
Tetrahedron , DOI: Sandro Cacchi and Giancarlo Fabrizi. The 1,2,3,4-tetrasubstituted pyrrole compounds can be synthesized by another one-step addition reaction Scheme 42 [ 61 ].