Synthesis and characterization of the twelve isomeric gem-dimethylphenylvalerolactones and some related compounds

Abstract

The twelve isomeric gem-dimethylphenylvalerolactones may each in principle be obtained by one of the general methods : (i) selective reduction of the corresponding trisubstituted glutaric anhydride, (ii) selective reduction of a gem-dimethyl-5-oxo-5-phenylvaleric acid, or (iii) selective reaction of 2- or 3-phenylglutaric anhydride with 2 moles of methylmagnesium iodide.     Substituted glutaric anhydrides for method (i) were obtained by Michael addition of an appropriate saturated ester to an appropriate α,β-unsaturated ester, and reduced to the corresponding pairs of lactones. Two of the intermediates for method (ii) may be obtained by Friedel-Crafts reaction of 2,2- or 3,3-dimethyl-glutaric anhydrides with benzene, but 4,4-dimethyl-5-oxo-5-phenylvaleric acid is not produced in this way, and was obtained by an alternative method in the literature.     Some of the required lactones were obtained by variants of the general methods, and 5-t-butyl-3,3-dimethylvalerolactone was prepared for comparison by reaction of 3,3-dimethylglutaric anhydride with t- butylmagnesium chloride. The product lactones were structurally identified from chemical shifts and the general nature of coupling patterns in their p.m.r. spectra, reinforced by values of geminal coupling constants where available, and, in the case of 4,4-dimethyl-3- phenyl-valerolactone, by results of nitration. Coupling constants obtained by iterative analyses of p.m.r. spectra for all but two of the lactones are tabulated. Carbonyl-stretching frequencies for the solid state (or liquid films) and for carbon tetrachloride solutions were also measured, and are discussed.