Enol esters, from alkynes with carboxylic acids

A synthetically useful virtue of enol triflates is that they are amenable to palladium-catalyzed carbon-carbon bond-forming reactions under mild conditions. When a solution of enol triflate 21 and tetrakis(triphenylphosphine)palladium(o) in benzene is treated with a mixture of terminal alkyne 17, n-propylamine, and cuprous iodide,17 intermediate 22 is formed in 76-84% yield. Although a partial hydrogenation of the alkyne in 22 could conceivably secure the formation of the cis C1-C2 olefin, a chemoselective hydrobora-tion/protonation sequence was found to be a much more reliable and suitable alternative. Thus, sequential hydroboration of the alkyne 22 with dicyclohexylborane, protonolysis, oxidative workup, and hydrolysis of the oxabicyclo[2.2.2]octyl ester protecting group gives dienic carboxylic acid 15 in a yield of 86% from 22. 

Enol and vinyl esters were successfully obtained starting from aliphatic and aromatic carboxylic acids with terminal alkynes, 1-hexyne or phenylacetylene. The addition reaction took place principally in the Markovnikov fashion to give 1-alkenyl esters, and it was facilitated by the use of catalytic amounts of [lr(cod)Cl]2 combined with small amounts of P(OMe)3 and Na2C03 [48, 58] (Scheme 35). 

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