Y-lactone structure

C15H20O2, Mr 232.32, colorless crystals, mp. 82 °C, bp. 275 °C, [a] 3 +197° (CHCI3), almost insoluble in water, soluble in alcohol and ether. The name of this tricyclic sesquiterpene lactone is derived from the Asteraceae ]nula helenium originally indigenous to Central Asia. H. was isolated for the first time from the essential oil of the roots (Inula oil). H. is also an older synonym for helenalin as well as the name for a metabolite from Penicillium funiculosum (ribonucleoprotein). Because of its a-methylene-y-lactone structure H. is a contact allergen. 

Acaranoic acid and acarenoic acid isolated from Acarospora chloro-phana have now been assigned the six-membered 8-lactone structures (71) and (73) respectively 182) rather than the previously accepted y-lactone structures (74) and (75). 

Intramolecular cyclization of unsaturated carboxylic acid derivatives using strong electrophiles is one of the most effective and common approaches to y-lactones. Not to mention, reactions of the unsaturated carboxylic acids with halonium sources such as X2 and NXS, the so-called halolactonization, produce the synthetically useful halolactones [4], N-Halosuccinimide (NXS) reaction is also commonly utilized for the construction of y-lactone structures. These days, development of effective catalysts, especially for enantioselective cyclization of unsaturated carboxylic acids including halolactonization reaction, is a hot topic in synthetic organic chemistry. 

These results and our previous paper indicate that carboxylic acid sodium salts and neighboring hydroxyl groups in PVA undergo an intramolecular reaction to form the y-lactone structures in the presence of hydrochloric acid and reversed in the presence of sodium hydroxide. Thus peaks Ha and Hb are assigned to methylene groups in the closed form and peaks He and Hd are also assigned to those in the open form as pointed in the figure. No carboxylic acid proton was observed in the spectrum of PVA in DMSO-dg at 6012 (not shows here). These spectra revealed a pH-dependent equilibrium as follows ... 

---