A study of the photochemical reactions of methoxy cinnamic acid esters

Cinnamic acid esters are commercially available as sunscreen agents. Although, like other sunscreens, very little is known about their photochemical properties. Thus, this research was established to investigate the properties of these compounds and to develop new ones. In this study, a number of cinnamic acid esters were synthesized and evaluated as potential sunscreen agents with emphasis on their photochemistry, the time required to achieve a photostationary state and the products produced from irradiation by a medium pressure lamp. The Z and E ortho- and para-methoxy cinnamic acid esters of, 2-ethylhexyl-$\beta$-methyl-p-methoxy cinnamate; ethyl-$\beta$-methyl-p-methoxy cinnamate; 2-ethylhexyl-$\beta$-methyl-o-methoxy cinnamate; and, ethyl-$\beta$-methyl-o-methoxy cinnamate were synthesized and their photochemical reactions investigated. The E compounds were synthesized by the Reformatsky Reaction, and the Z isomers were photochemically prepared. Also produced were Z and E ethyl-$\beta$-methyl-2,4-and ethyl-$\beta$-methyl-2,6-dimethoxy cinnamates as well as Z and E ethyl-$\beta$-methyl-2,4,6-trimethoxy cinnamate. These compounds were prepared by the Horner-Emmons Reaction in better yields than had been previously reported for similar syntheses. All the compounds produced in this research, with the exception of two, were new and had not been reported in literature. In addition, Correlated Spectroscopy (COSY) and Nuclear Overhauser Enhancement Spectroscopy (NOESY) NMRs were performed on the Z and E di- and trimethoxy cinnamates. These spectra confirmed the correct stereochemistry for both isomers of 2,4-,2,6-dimethoxy and 2,4,6-trimethoxy cinnamates. From this research it was found that photoisomerization was the major photochemical reaction observed with the cinnamates. The compounds studied were prepared in a pure inert solvent with no other substances present and the only products obtained were the respective isomers. The photochemical conversion of the E-cinnamate (or Z) to the Z isomer (or E isomer), in equal or nearly so ratios, occurred after hours of irradiation in an inert solvent. The structure-activity relationships among the cinnamates for promoting or inhibiting photoisomerization and degradations were found to be the same for all the compounds.