Tetraphenylporphyrin‐Catalyzed Tandem Photooxygenation of Polyenes and 1,4‐Dienes: Multiple and Diverse Oxyfunctionalizations

Abstract

AbstractTandem singlet oxygen reactions photocatalyzed by meso‐tetraphenylporphyrin were performed with substrates designed for primary α‐gem‐selectivity or high vinylogous gem‐selectivity. The 1,3,5‐triene ester 1 was designed as a model compound for the natural compound crocin and resulted in a highly regioselective singlet oxygen ene reaction with ε‐hydrogen activation (93:7 ene vs. [4+2] cycloaddition). The initially formed hydroperoxide 2a adds another singlet oxygen molecule in a slower reaction step to furnish the endoperoxide 3a. From the hydroperoxide stage, oxygen transfer gives the epoxide 4, reduction leads to the allylic alcohol 2b and subsequent photooxygenation (to 3b) followed by reduction results in the furan derivative 5. The allylic alcohol could also be reacted with other dienophiles as shown for the PTAD adduct 6. From the 1,4‐diene 7, a gem‐regioselective singlet oxygen ene reaction generates a mixture of the hydroperoxides 8a and 9a. Product 8a adds another singlet oxygen molecule to give the endoperoxide 10a. Catalyst activity was demonstrated by analysis of the changes in the electronic absorption properties.magnified image