The development of a highly efficient and simple protocol for the direct amidation of carboxylic acids is described employing ynoates as novel coupling reagents. The transformation proceeds in good to excellent yields via in situ α-acyl enol ester intermediates formation under mild reaction conditions. This useful method has been demonstrated for a range of substrates to provide a succinct access to structurally diverse amides, including key intermediates of glibenclamide, tiapride hydrochloride, and nateglinide, and can be conducted on a mole scale.
Metal-free decarboxylative coupling reactions represent an alternative paradigm for carbon-carbon or carbon-heteroatom bond formation that complement more traditional metal catalysis approaches. In this review, recent advances in metal-free decarboxylation reactions of α,β-unsaturated carboxylic acids are highlighted. Two major reaction classes are covered: 1) generation of C
1,4-Diamino-2-butynes
display both chemical and physiological properties.
Here a highly efficient synthesis avenue to generate unsymmetric 1,4-diamino-2-butynes
has been developed by microwave-assisted Cu(I)-catalyzed cross-A3-coupling/decarboxylative coupling of two different amines,
formaldehyde, and propiolic acid through a domino process. This multicomponent
reaction provides a series of target products in moderate to good
yields with high chemoselectivity.
An
efficient palladium-catalyzed reaction of N-propargyl
oxazolidines for the construction of 4-substituted isoquinolines
under microwave irradiation is developed. This transformation proceeds
through a sequential palladium-catalyzed reductive cyclization/ring-opening/aromatization
cascade via C–O and C–N bond cleavages of the oxazolidine
ring. The practical value of this method has also been explored by
conducting a millimole-scale reaction, as well as by transforming
the isoquinoline into a key intermediate for the synthesis of a lamellarin
analogue.
A novel method for the concise and effective construction of the nitrogen-tethered 1,6-enynes in moderate to good yields was developed. This transformation involves a TFA-promoted Pictet-Spengler reaction/iminium formation, copper-catalyzed alkyne-iminium...
A facile, efficient and environmentally friendly methodology for the preparation of phenolic esters is realized via metal‐free coupling of carboxylic acids and arylboronic acids. This sequential one pot reaction, employing methyl propiolate as an activating reagent, proceeds through the formation of enol ester intermediate, followed by a nucleophilic attack on the C‐O bond under the oxidation of hydrogen peroxide. These studies display that enol esters, despite previously being overlooked as synthetic intermediates, would be the valuable building blocks for developing carbon–carbon and carbon–heteroatom bond‐forming reactions.
A novel method has been developed for the desymmetrization of aromatic dicarboxylic acids by employing an esterification reaction/conjugate addition of a carboxyl group to ynoates, which can trigger a coupling reaction.
A palladium-catalyzed
reaction of N-propargyl
oxazolidines with alkenes for the synthesis of indolizidines has been
developed. Through a sequential 6-exo-dig cyclization/proton
transfer/[3+2] cycloaddition/cycloreversion/aromatization process,
a series of fused polycyclic indolizines are obtained in moderate
to good yields with high functional group tolerance. Experimental
and theoretical studies suggest that the [3+2] cycloaddition/cycloreversion
of the oxazolidine ring probably involves C–C and C–O
bond cleavage, providing a new ring restructuring approach for the
synthesis of heterocycles.
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