2- Alkynoic acids

Methyl 2 hexynoate has been prepared by the esterification of 2-hexynoic acid, which wras prepared by the carboxylation of sodium hexynylide.4 a,/5-Alkynoic acids have generally been obtained by either carboxylation of metal alkynylides or by eliminationreactions.5In particular, they have been prepared by the elimination of enol brosylatcs and... 

The iridium phosphine complex [IrC PEt,),] 39b can also activate O-H bonds of carboxylic acids. The stoichiometric reaction with a,(o-alkynoic acids RC=C(CH2)2 CO2H (R = Me, Ph) gave cis-hydrido(carboxylato)iridium(III) complexes 92 (Eq. 6.26), and the molecular structure of 92a was determined crystallographically [59]. 

Schemes 6-26 A proposed reaction pathway for Rh(l)-catalyzed cyclization of a,co-alkynoic acids to alkylidene lactones...

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... 

Alkylidene lactones. In the presence of this catalyst, alkynoic acids (2) can cyclize to exocyclic enol lactones (3) with marked preference for the Z-isomer. The reaction in CH,C12 proceeds at room temperature. Cyclization to five-mem-... 

In a very recent study, trialkylated 1,2,4-triazoles were shown to produce the novel bridging diylidynes with metals coordinated at C3 and C5. For example, with (cod)M (M = Rh and Ir) as the metal fragment they proved active in transfer hydrogenation, although it is possible that their activity in the cyclization of alkynoic acids may be more relevant [39]. 

The a-enynyl complex Ru(Tp)[PhC=C(Ph)C=CPh](PMe Pr2) 10 efEciently catalyzed the regioselective cyclization of a,(D-alkynoic acids involving an anti-Markovnikov intramolecular addition to give unsaturated lactones [29] (Equation 10.5). 

Yamanaka and co-workers (74) effected the 1,3-dipolar cycloaddition between miinchnones 134 and polyfluoro-2-alkynoic acid esters to afford the corresponding 4-(polyfluoroalkyl)pyrrole-3-carboxylates (135). The reaction proceeds rapidly at low temperature and the various miinchnones (134) were generated from 1,3-oxazolium perchlorates 133 using the method of Boyd and Wright (6,7). Under... 

Reaction of 2-alkynoic acids with an excess of sodamide in hquid ammonia followed by treatment with mineral acid (after evaporation of the ammonia) gives 3-alkynoiC acids in excellent yields [183], Presumably, the 2-alkynoic acid is convened into the disodium compound of the 3-alkynoic acid via a process of proton removal and proton donation ... 

The same 3-alkynoic acids can also be obtained (in lower yields) by heating 2-alkynoic acids with an excess of aqueous potassium hydroxide [184]. 

Addition of a 2-alkynoic acid to alkali amide in liquid ammonia initially gives a solution of the alkali salt of the carboxylic acid. If an excess of alkali amide is present, the weakly basic salt is further deprotonated at a position next to the triple bond 183], This double deprotonation which may be compared with the formation of di-anions from 1,3-diketones and alkali amides [71], is essentially complete. The high kinetic stability of the alkynoic acid-dianion may be explained on the basis of resonance stabilization ... 

The protolysis with ammonium chloride is apparently highly regioselective, since the product obtained after acidic work-up is almost pure 3-alkynoic acid. 

Dichloropyrazolinones with alkali give alkynoic acids (Scheme 66) (58JA599). 

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. 

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