Propynoate esters

Because the enophiles are normally the electrophilic reagent, their reactivity increases with addition of EWG substituents. Ene reactions between unsubstituted alkenes have high-energy barriers, but compounds such as acrylate or propynoate esters... 

Similarly, propynoic esters react with phenols in the presence of a palladium catalyst to afford coumarins in moderate to high yield (Equation 276) <1996JA6305, 2000JOC7516, 2002CL380, 2003BCJ1889, 2003JA4518>. 

Silver(l) tetrafluoroborate and indium(m) chloride can also promote the reaction of propynoic esters with phenols to afford coumarins. However, substitution of the alkyne is not tolerated for the silver(l) promoted reaction <2003JA4518, 2004SC1909>. 

In the presence of Ni(dppe)Br2 the oxabicyclic alkenes 693 react with propynoic esters to yield 2H-benzo[ ]coumarins 696 (Scheme 172) <2001AGE1286>. The reaction mechanism involves cyclometallation of the propynoic ester and oxabicyclic alkene 693 to form the nickelacyclopentene intermediate 694. (3-Oxy elimination then forms the intermediate 695, which undergoes protonation and isomerization of the double bond followed by intramolecular lactonization to afford the desired 2/7-benzo[ ]coumarins 696 (Scheme 172) <2001AGE1286>. 

Cuprates of the type Li CuR + i, where > 1, react with the carbonyl group of propynoate esters rather than give the conjugate addition product 61). Extension of the reaction of propynoate esters to vinylcopper compounds and allylcopper compounds affords a stereospecific synthesis of 1,3- and 1,4- dienes, respectively 64, 205). 

The regioselectivity of 1,3-dipolar cyclization is affected by steric and electronic factors. Addition of cyclic A -oxides to propynoic esters shows regioselectivity as only the 1-carboxylic ester is obtained when the temperature is kept at 60°C. When the phosphorane (105.2) is heated in methanol, the isoxazole is formed instead of the expected diarylmethane. 

Disubstituted benzoic esters arise from an Ir-catalyzed [2+2+2]cycloaddition involving propynoic esters and two equivalents of 1-alkynes. Aryl eth3myl suUdnes also react similarly. ... 

Addition reactions. As an alternative to the Baylis-Hillman approach (Z)-2-hydroxy-aUcyl-2-alkenoic esters are assembled from propynoic esters via a Cu(I)-catalyzed Grignard reaction and trapping with RCHO. ... 

Potassium fluoride. 13, 2S6-2S7 15. i As base. In the absence of solvent silylacetonitrile with aldehydes to give 1 wave as energy source the reaction is cot 18-crown-6 successfully effects the Mic esters and propynoic esters also. 

Coumarins. A new synthesis of coumarins is directly from phenols and propynoic esters (8 examples, 46-79%). 

In the past, carbanions of propynoate esters have been difficult to obtain cleanly owing to nucleophilic attack of the base used (e.g. Bu"Li) on the ester function. It has now been found that such anions are readily available using the ubiquitous base lithium di-isopropylamide. Furthermore, subsequent reactions of such anions with a trialkyborane followed by decomposition of the borane complex with iodine afford high yields (70—80%) of substituted alk-2-ynoates. ... 

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