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.
The first metal free report on the amidation of aldehydes with aminopyridines was accomplished using simple aqueous hydrogen peroxide (aq. H2O2) as the oxidant. No catalysts or additives were needed for this transformation and the reaction proceeded in water, an environmentally benign reaction medium. Green oxidant and reaction conditions, and the ability to construct diverse N-(pyridin-2-yl)benzamide by this elegant method render it a practical alternative for the synthesis of these amides.
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