Alkenoic acids, conjugate

The intramolecular reaction oF allcenes with various O and N functional groups offers useful synthetic methods for heterocycles[13,14,166]. The reaction of unsaturated carboxylic acids affords lactones by either exo- or endo-cyclization depending on the positions of the double bond. The reaction of sodium salts of the 3-alkenoic acid 143 and 4-alkenoic acid 144 with Li2PdCl4 affords mostly five-membcrcd lactones in 30-40% yields[167]. Both 5-hexe-noic acid (145) and 4-hexenoic acid (146) are converted to five- or six-mem-bered lactones depending on the solvents and bases[168]. Conjugated 2,4-pentadienoic acid (147) is cyclized with Li2PdCl4 to give 2-pyrone (148) in water[i69]. 

The alkanoic acids, with the exception of formic acid, undergo typical reactions of the carboxyl group. Formic acid has reducing properties and does not form an acid chloride or an anhydride. The hydrocarbon chain of alkanoic acids undergoes the usual reactions of hydrocarbons except that the carboxyl group exerts considerable influence on the site and ease of reaction. The alkenoic acids in which the double bond is not conjugated with the carboxyl group show typical reactions of internal olefins. All three types of reactions are industrially important. 

Electrochemical reduction of conjugated alkenoic acids in alkaline solution generally leads to the dihydrocompound and negligible amounts of the hydrodimer are fomied. Examples include the conversion of maleic or fumaric acid to succinic... 

Haloalkynes and 1-haloalkenes The Hunsdiecker reaction occurs when the conjugated acids are treated with these positive halogen salts. There is a stricter limitation for such a reaction with alkenoic acids as they must be a-unsubstituted, and the P-carbon supports a positive charge well (e.g., cinnamic acid and analogues). 

Conjugated dienes are co-dimerised with 3-alkenoic acids (e.g. (40)) in the presence of RhCl(PPh3)3 to give linear and branched dienoic acids. 

The formation of acyl conjugates of vanillamine (capsaicinoids, 20) or vanillic alcohol (capsinoids, 21) with various Cg/Ci3 alkenoic and alkanoic acids is a unique chemical trait of plants from the genus Capsicum. Hot peppers are characterized by the presence of vanillyl conjugates of the amide type, absent or replaced by their nonpungent ester isosters (capsinoids) in bell (sweet) peppers [19]. Indeed, the difference between the sensory properties of capsaicin (la) and its naturally occurring ester analog capsiate (22) is a remarkable example of the biological relevance of isosterism. 

A synthesis of a-substituted S-unsaturated acids exploits the facile Claisen rearrangement of allyl ester enolates, which can be generated by conjugate addition of organocopper reagents to ally 2-alkenoates. ... 

---