2-Ynoic acid

Hydrogenations with Lindlar catalysts frequently slow down or stop after absorption of 1 mol of hydrogen. Reduction of 4-hydroxydodec-2-ynoic acid stopped spontaneously when about 95% of the theoretical hydrogen had been absorbed 39). 

Acyl azides 268, derived from furan, thiophene and selenophene, add slowly at room temperature to the strained double bond of 5-methylenebicyclo[2.2.1]hept-2-ene. Two regioisomeric triazolines, 269 and 270, which form in the first step, are unstable and decompose with elimination of nitrogen to provide aziridine derivatives 271. Products 271 are isolated in good yield (73-85%). It is worthy to note that not only the terminal, unstrained double bond in the starting material, 5-methylenebicyclo[2.2.1]hept-2-ene, is unaffected, but also the typical dipolarophiles like esters of crotonic, propiolic and byt-2-ynoic acids do not react with azides 268 under these conditions (Scheme 39) <2002J(P1)1420>. 

Isolated instances of 1,4-addition reactions of other hetero-nucleophiles to 4-en-2-ynoic acids and derivatives have been reported172-174. Thus, treatment of methyl 4-methyl-4-penten-2-ynoate with phenolate provided the 3-phenoxy-substituted conjugated dienoate (equation 71)172, and the 1,4-addition of water-soluble phosphines to 4-octen-2-ynoic acid afforded dienylphosphonium salts which were transformed into the corresponding phosphine oxides (equation 72)174. 

Scheme 3.13. Reagent regioselectivity in the stannylcupration of 2-ynoic acids [63].

Dimethoxyphenyl)prop-2-ynoic acid reacted with sulfur monochloride in the presence of pyridine to give low yields of substituted 1-benzothiophenes 17 and 18 (1979AJC833 Scheme 9). 

Trifluoromethyl-substituted pyrazoles are easily obtained using trifluoromethyl-alkynes as dipolarophiles (Table 8.2, entry 9). Thus, treatment of 4,4,4-trifluorobut-2-ynoic acid with excess diazomethane gave methyl 4-(trifluoromethyl)pyrazole-4-carboxylate (45%) accompanied by its N - (32%) and -methylated (6.5%) derivatives (267). Another convenient route to CF3-substituted pyrazoles involves dipolar cycloaddition of appropriately CF3-substituted alkenes followed by eliminative aromatization (76,77,268). For example, the reaction of alkenes such as (CF3)2C=C(H)COAr with ethyl diazoacetate gave 4-aroyl-5-trifluoromethylpyra-zole-3-carboxylates (268). 

A variant of this method utilizes the ring closure of 4,5,5-trichloropent-4-en-2-ynoic acid prepared from a hexachloropentadienol (62CB1245). 

The dianion derived from but-2-ynoic acid reacts with aldehydes to give 5-hydroxyalk-2-ynoates (539). Partial reduction over a Lindlar catalyst and acid-catalyzed cyclization of the resulting enoate gives the dihydropyran-2-one (78LA337). The route is exemplified by the synthesis of the naturally occurring massoia lactone (Scheme 200). In previous work (46JCS954) the hydroxyalkynoic acids themselves, obtained from epoxides and acetylene, were used. 

Under these conditions isomerisation to the more stable ( ) isomer is not apparent. The retention times for (Z)-but-2-enoic acid, ( )-but-2-enoic acid and but-2-ynoic acid are 6.8, 10.0 and 19.8 minutes respectively. 

A useful route to a,/(-acetylenic acids, illustrated by the preparation of but-2-ynoic acid (Expt 5.217), involves the base-induced decomposition of the dibromopyrazolone which is obtained by brominating pyrazol-5-one, which is itself prepared by the reaction of a / -keto ester with hydrazine. 

Allyl alk-2-ynoates can be readily converted into alk-2-ynoic acids by reaction with morpholine as the nucleophilic scavenger and Pd2(dba>3 as catalyst together with l, 3-diphenylphosphtnopropane as associated ligand, thereby enabling deprotection of allyl esters of 2,2,3,3-tetradehydro-PGE derivatives [Scheme 6.64]. 

Polyfluoroalk-2-ynoic acids such as 20 readily undergo tandem interniolecular-intramolecular Michael addition with ethane-1,2-dithiol to provide the 2-pol)dluoroalkylated 1,3-dithiolanes 22 m good to excellent yields. The method can be extended to other binucleophiles including propane-1,3-dithiol.2-sulfanylethanol, and 2-aminoethanethiol. ... 

Addition of Ethane-I.2-dithiol to Polyfluoroalk-2-ynoic Acids 20 General Procedure ... 

Polyfluoroalkyl-substituted 4//-[l]benzothiopyran-4-ones 26 can be obtained from the addition of arenethiols 24 to polyfluoroalk-2-ynoic acids 20 (giving the intermediates 25), followed by an intramolecular Friedel-Crafts acylation reaction. ... 

Polyfluoroalk-2-ynoic acids 7 readily undergo an intermolecular-intrarnolecular Michael addition reaction with a variety of bifunctional azanucleophiles and o-phenylenediamine. to give the corresponding carboxylated and/or decarboxylated 2-(polyfluoroalkyl)imidazolidine (e. g, 8 and 9. respectively), thiazolidine, and oxazolidine derivatives in moderate to good yields. The products formed depend strongly on the reaction conditions employed. For example, the decarboxylated derivative 9 is only formed under reflux conditions. ... 

Most catalysts also bring about the cis addition of hydrogen to alkynes. For example, [RhCl(PPh3)3] catalyzes the initial formation of (Z)-but-2-enoic acid from but-2-ynoic acid. 

The reaction of adenine, as well as of its riboside, with 4-chlorobut-2-ynoic acid esters, C1CH2C = CC0,R, has been studied in neutral and slightly acidic media (phi 4-5). Adenosine reacts at pH 4 5 with formation of the lactam 15 while at neutral pH the isomeric, slightly fluorescent compound 16 is obtained. 

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