2.4- Pentadieno-4-lactone

Acylcobalt tetracarbonyl complexes, formed from Na[Co(CO)4] and alkyl or acyl halides, react with alkynes to give 2,4-pentadieno-4-lactones (Scheme 20). The reaction is catalytic in cobalt and yields are around 60% for a variety of substituted alkynes and alkyl halides. 

Proton-activating groups such as nitrile, ester, or p-nitrophenyl, on carbon atom 5 of the 7r-(penteno-4-lactonyl)cobalt tricarbonyl complexes make the compounds reactive toward bases. Bases cause the elimination of cobalt tricarbonyl anion and 5-substituted 2,4-pentadieno-4-lactones are produced 17). 

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]. 

When the acylcobalt species is derived from a compound containing halogen on an activated carbon (e.g. an a-halo ester or nitrile) an elimination may occur to introduce an exocyclic double bond in the final product. This sequence, leading to lactones of pentadienoic acids, is general for both terminal and internal alkynes in the presence of amine bases (equation 17). ... 

Protoanemonin. S-Methylene-2(SH)-furanone 4-hydroxy-2,4-pentadienoic acid y -lactone 5-methylene-2-... 

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