Domino synthesis of functionalized pyridine carboxylates under gallium catalysis: Unravelling the reaction pathway and the role of the nitrogen source counter anion

The catalytic potential of various metal Lewis acid catalysts have been assessed to derive a high-yielding, multi-component domino synthesis of functionalized pyridines from (E)-3-(dimethylamino)-1-aryl/heteroaryl-prop-2-en-1-ones, 1,3-dicarbonyl compounds, and an ammonium salt (as the nitrogen precursor). Amongst the various metal halides, tetrafluoroborates, perchlorates, and triflates used as the catalyst GaI3 proved to be the most effective. The mechanistic course of the most plausible pathway has been outlined as the intermediate formation of imine/enamine by the reaction of the 1,3-dicarbonyl compound with ammonia (liberated in situ from the ammonium salt used as the nitrogen source), which participates in the domino nucleophilic Michael reaction to the (2E)-3-(dimethylamino)-1-aryl/hetroarylprop-2-en-1-one by its active methylene carbon through its enamine form followed by intramolecular cyclization and aromatization. The effect of different ammonium salts as the nitrogen source has been investigated and NH4OAc was found to be best. The influence of the acetate counter anion of NH4OAc on the progress of the reaction was studied and its specific role in the cyclization and subsequent aromatization has been revealed. This work offers distinct advantages compared to the literature reported methodologies on the count of several green index parameters.

Heterocycles by Consecutive Multicomponent Syntheses via Catalytically Generated Alkynoyl Intermediates

Multicomponent processes are beneficial tools for the synthesis of heterocycles. As densely substituted bifunctional electrophiles, ynones are essential intermediates by applying cyclocondensations or cycloadditions in numerous heterocycle syntheses. The respective alkynoyl intermediates are generally accessible by palladium-, copper- and palladium/copper-catalyzed alkynylation. In turn, the mild reaction conditions allow for a fast and versatile entry to functional heterocycles in the sense of consecutive multicomponent processes. This review collates and presents recent advances in accessing thirteen heterocycle classes and their applications by virtue of catalytic alkynoyl generation in diversity-oriented multicomponent syntheses in a one-pot fashion.

Acyl Sonogashira Cross-Coupling: State of the Art and Application to the Synthesis of Heterocyclic Compounds

The acyl Sonogashira reaction represents an extension of Sonogashira cross-coupling to acid chlorides which replace aryl or vinyl halides, while terminal acetylenes are used as coupling partners in both reactions. The introduction of a carbonyl functional group on the alkyne backbone determines a radical change in the reactivity of the products. Indeed, α,β-alkynyl ketones can be easily converted into different heterocyclic compounds depending on the experimental conditions employed. Due to its potential, the acyl Sonogashira reaction has been deeply studied with particular attention to the nature of the catalysts and to the structures of both coupling compounds. Considering these two aspects, in this review, a detailed analysis of the literature data regarding the acyl Sonogashira reaction and its role in the synthesis of several heterocyclic derivatives is reported.