Oxidative N‐heterocyclic carbene catalyst mediated cyclization proved to be an efficient strategy for the construction of various N‐ and O‐ heterocycles. Among several metal‐free methods available for the construction of heterocyclic compounds, oxidative NHC catalysis proved to be an elegant and efficient strategy as several enantioenriched compounds with excellent enantioselectivities can be synthesized in good yields. The most common method of the generation of these heterocycles proceeds via the commonly encountered intermediate in oxidative NHC catalysis (i. e.) acyl azolium intermediate. This intermediate proves to be the active species as it can undergo reaction with suitable substrates leading to the formation of numerous N‐ and O‐ heterocycles. Herein, recent reports on different strategies that enable formal [2+2], [3+2], [3+3], [4+2] or [4+3] annulations between the incoming substrates and acyl azolium species or other active intermediates which can add to suitable substrate leading to various heterocycles were summarized.
The efficiency of elemental sulfur for the synthesis of 2H‐pyrido[1,2‐a]pyrimidin‐2‐ones has been demonstrated. This strategy involves coupling of 2‐aminopyridines and β‐oxo esters under neat condition in the absence of external oxidant. The reaction does not require pre‐functionalization of the substrates, thus making it an alternate approach for the synthesis of 2H‐pyrido[1,2‐a]pyrimidin‐2‐ones. The reaction was tolerant to several substituted 2‐aminopyridines and β‐oxo esters.
A facile, catalyst‐ and solvent‐ free, three‐component reaction was developed for the synthesis of tetrasubstituted pyrroles at room temperature and under white light. The reaction proceeds via enamine formation between primary amines with 1,3‐dicarbonyl compounds followed by Michael addition with β‐nitro styrene and subsequent intramolecular cyclization and aromatization to yield 1,2,3,4‐tetrasubstituted pyrroles in good to excellent yields. Further, the present strategy is highly atom‐economical and environmentally benign. The preliminary in vitro cytotoxic studies on a set of compounds were performed against Hepatocellular carcinoma cells (HepG2) and one of the compounds 4 ac showed significant activity with an IC50 value of 17.82 μM.
A tandem formal [3+3] cycloaddition/oxidation between chalcones and β-enamine esters for the construction of various substituted m-terphenyls employing Iodine as catalyst was developed. A wide range of chalcones and β-enamine esters were tested under metal- and solvent-free condition for the synthesis of substituted m-terphenyls in good to excellent yields in the presence of sulfur as oxidant. This reaction proceeds with the formation of four new bonds and a ring with high atom economy.
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