Structural studies on bio-active compounds. Part 12. Tautomerism and conformation of aryl-substituted 1-(2-hydroxyphenyl)-3-phenylpropane-1,3-diones in the solid phase and in solution

Abstract

The tautomerism of a series of aryl-substituted 1-(2-hydroxyphenyl)-3-phenylpropane-1,3-diones has been studied in deuteriochloroform solution by ¹H n.m.r. techniques and, in the case of 1-(2-hydroxy-4-methoxyphenyl)-3-phenylpropane-1,3-dione and 1-(2-hydroxy-4,6-dimethoxyphenyl)-3-phenylpropane-1,3-dione, in the solid state by X-ray crystallography. Of these compounds, most exist between 80 and 95% in the enolised form in solution and 1-(2-hydroxy-4-methoxyphenyl)-3-phenylpropane-1,3-dione adopts this tautomer in the crystal. However, 1-(2-hydroxy-4,6-dimethoxyphenyl)-3-phenylpropane-1,3-dione is present as the diketone in the solid phase and enolises very slowly in solution. 1-(6-Benzoyloxy-2-hydroxyphenyl)-3-hydroxy-3-phenylprop-2-en-1-one is shown by ¹H n.m.r. spectroscopy possibly to adopt a ‘coiled’ conformation in solution in deuteriochloroform.