The impact of Lewis acids on photochemical reactions of conjugated carbonyl compounds was studied in two projects. The first project used model complexes and a combination of experiments, theory and spectroscopy to evaluate mechanistic hypotheses and find novel reactivity. The second project investigated the photoinduced E-Z isomerization of cycloheptene derivatives, including cyclohept-2-en-1-one and cyclohept-1-ene-1-carbaldehyde. Thermal background reactions with Lewis acids made them unsuitable for photoinduced Diels-Alder reactions.