Proton magnetic resonance spectra and conformation of mevalonolactone and 3,3-dimethylvalerolactone

Abstract

The ring-proton spectrum at 60 MHz of mevalonolactone (3-hydroxy-3- methylvalerolactone) has been examined in a range of solvents and that of 3,3- dimethylvalerolactone in deuterochloroform a t 30°. All the spectra are complicated by long-range coupling between the 2- and 4-methylene protons, but a complete set of magnetic parameters, for the MM'XX' spectrum of the 4- and 5-methylene groups of 3,3-dimethylvalerolactone may be extracted from the spectrum of the 5-methylene (XX') protons alone. Such parameters as can be extracted for mevalonolactone suggest there is little dependence of conformation on solvent. At 100 MHz in hexa- deuteroacetone alone, or in mixtures with benzene, the 2-methylene protons show only a small internal chemical shift and are readily completely decoupled; the residual 4,5-methylene-proton four-spin spectrum gave a set of parameters used as the starting point for iterative analysis of the complete six-spin ring-proton spectrum. I t is proposed that mevalonolactone in solution is a rapidly inverting equilibrium mixture of half-chair conformers in which the conformer having the hydroxyl group axial is favoured c. 4.5 : 1 at 30°. The corresponding free-energy difference (c. 750 cal mol^-1) is close to the difference of conformational free-energies of methyl and hydroxyl groups in cyclohexane systems, which suggests that no special polar factor is operating in the mevalonolactone equilibrium.