Abstract
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.
Highlights
Multicomponent reactions (MCRs) represent a potent and multipurpose synthetic concept, where high efficiency and abundant diversity are elegantly combined for rapidly accessing compound libraries
In sequential MCR all components are added in a defined order and the reaction conditions are maintained constant
Employing oxalyl chloride in copper-catalyzed alkynylations alkynediones become accessible for subsequent multicomponent reactions
Summary
Multicomponent reactions (MCRs) represent a potent and multipurpose synthetic concept, where high efficiency and abundant diversity are elegantly combined for rapidly accessing compound libraries. Omitting copper as a cocatalyst circumvented the inherent alkyne dimerization effectively upon employing an electron-rich palladium complex in variant II [31] This method allows concatenating single reaction steps to one-pot sequences since the solvent can be readily varied. 1,4-dioxane, acetonitrile and dichloromethane work well as solvents, and in particular, acetonitrile and dichloromethane are very favorable for subsequent Michael additions Another possibility to significantly broaden the substrate scope was established by employing oxalyl chloride for en route activation of carboxylic acids III [32], tolerating N-heterocyclic substituents and electronrich π-nucleophiles IV [33]. Zeng and coauthors dispensed a different approach utilizing Sonogashira coupling of N-acylsaccharins 4 and alkynes 5 followed by a selective triethylamine mediated C–N bond cleavage This palladium-catalyzed sequence retains from high catalyst loadings and from copper complexes enabling a broad substituent pattern for 29 examples embedding electron-rich and electron-poor (hetero)aryls, allyl and alkyl groups in good to excellent yields (Scheme 3) [38]. Copper-catalyzed oxidative coupling of α-carbonyl aldehydes 8 and alkynes 5 to ynediones 3 under aerobic conditions
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