Singlet oxygen addition to cyclo-1,3-hexadienes from natural sources and from organocatalytic enal dimerization

The photoxygenation reactions of the natural monoterpene aldehyde safranal (1) and its reduction product safranol (4) were studied and the endoperoxide 5 from safranol was isolated and characterized by single crystal X-ray analysis. A dyade synthesis using DCC coupling of safranol with artesunic acid delivered the reactive substrate 7 for 1,3-diene photooxygenation. Singlet oxygen reaction of the substrate 7 enabled the synthesis of the trioxane-endoperoxide dyade 8. The diastereoisomeric 1,3-cyclohexadienes 10 were obtained from the proline-catalyzed Diels-Alder dimerization of the Michael aldehyde 9. From the singlet oxygen addition with the diastereoisomeric substrates 10, only the kinetically preferred cis-endoperoxide 11 was formed.