Spectroscopic and quantum-mechanical study of enol-enol tautomerism of 3-acetyltetramic acid derivatives

Abstract

By the method of IR spectroscopy, Raman spectroscopy, and nonempirical (basis 3-21G//3-21G(d,p)) and semiempirical (approximations AM1 and PM3) quantum-mechanical methods the tautomeric structure of a series of 3-acetyltetramic acid derivatives has been investigated. It is shown that all the investigated compounds exist in vapor, in solution, and in the crystalline state mainly in the form of exo-enols among which the prevalent form is that in which lactam carbonyl participates in intrachelate hydrogen bonding. Complete series of relative stabilities of possible tautomeric forms of the investigated compounds have been established. General conlusions on the structure of 5-membered ring β,β′-triketones have been formulated.