Sayyed Faramarz Tayyari - Chemistry Department, Ferdowsi University of Mashhad, Mashhad ۹۱۷۷۵-۱۴۳۶, Iran
Soheila Holakoei - Chemistry Department Islamic Azad University, Shahroud Branch, Shahroud, Iran
Farnaz Naghavi - Chemistry Department Islamic Azad University, Shahroud Branch, Shahroud, Iran
Behzad Chahkandi - Chemistry Department Islamic Azad University, Shahroud Branch, Shahroud, Iran

Conjugated dicarbonyl compounds, such as β-dicarbonyls and diacyl-cyclopentadienes, are capable to form very strong intramolecular hydrogen bond. The strength of hydrogen bond in these systems increases by π−electron conjugation with the chelated rings [1].In spite of extensive works on structure and properties of hydrogen bonds in the enol form of β-carbonyl compounds, only a few reports considering the hydrogen bonding in diacyl-cyclopentadienes are appeared in the literatures. These compounds solely exist in the enol form, characterized by a seven-membered ring [2,3], and their proton chemical shifts are consistent with a very strong hydrogen bond [4,5].