Alkynes alkynoic acids

Coumarins are formed by the intramolecular hydroarylation of an activated alkyne. The basic starting materials for this fast, efficient and general Pd-catalysed reaction are phenols and alkynoic acids (Scheme 42), An intermolecular version in which electron-rich phenols react with 4-methoxycinnamic acid leads to high yields of 3,4-dihydrocoumarins... 

Isomerization of triple bonds.1 This base is easier to prepare than potassium 3-aminopropylamide, but is less effective for isomerization of triple bonds to the terminal position. However when used together with potassium l-butoxide, isomerization of triple bonds occurs in 1-4 hours at 25°. Isomerization of acetylenic acids results in terminal alkynoic acids and 3,5-dienoic acids. However, acetylenic alcohols (primary or secondary) can be isomerized to w-hydroxy-l-alkynes in high yield. 

A wide variety of functionally substituted alkynes react with HCI to produce vinylic chlorides (equations 61-65).29 95 96 Both syri19 and antP7,98 adducts have been reported to predominate in the reaction of alkynoic acids. 

Alkynoic acids in which the C—C triple bond is far removed from the acid moiety behave like simple alkynes. 63,166 Propiolic acid, on the other hand, adds HBr to form almost exclusively (Z)-3-bromoprope-noic acid (equation 115).98... 

Mercury salts, such as mercury(II) acetate,521-525 mercury(II) oxide,524,526-528 metcury(II) trifluoroace-tate,529,530 mercury(II) sulfate524,531 and mercury(II) phosphate531 catalyze the addition of carboxylic acids to alkynes. Acetic anhydride in the presence of boron trifluoride etherate can also be effectively used in this reaction (equation 292).521,522 Alkynoic acids undergo mercury-catalyzed cyclization to lactones (equation 293).523,532,533... 

Alkyne-vinylcyclopropanes, metal-catalyzed [5+23-cycloadditions, 10, 605 Alkynides, with tungsten carbonyls, 5, 666 Alkynoic acids, stannylmetallation, 9, 374 Alkynoles, hydration-dehydration reactions, 6, 841... 

With the exception of (trimethylsilyl)acetylene (equation26)79, BF3-etherate methodology has not been applied to typical terminal alkynes [RC=CH]. However, when various alkynoic acids are subjected to the action of iodosylbenzene and BF3-etherate, intramolecular participation of the carboxyl group occurs, and lactonic vinyliodonium fluorobo-rates are obtained135. Some examples are presented in equation 176. H NMR studies of these compounds showed no appreciable NOE enhancement between the vinylic and allylic protons 135, consistent with the -configuration about the carbon-carbon double bonds. 

Isomerizations. A convenient method for the conversion of alkynes to conjugated dienes is by treatment with Ph P. The synthetic application is shown in the preparation of 2,4-alkadienols from 2-alkynoic acids involving esterification with pentafluorophenol, isomerization with PhjP (PhMe, 50°), and reduction with DIBAL-H. Even propargyl bromide can be isomerized to give 1-bromopropadiene, albeit in 29% yield. Conjugated alkadienoic esters and amides are obtained in one step from the pentafluorophenyl alkynoates on reaction with alcohols and amines, respectively, after treatment with catalytic amount of PhjP. 

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