THE PHOTOOXYGENATION OF DIHYDROPYRANS - FORMATION OF 1,2-DIOXETANES AND 1,2,4-TRIOXANES

3,4-Dihydro-2H-pyran (1), 5,6,7,8-tetrahydrochroman (9), and 2-oxabicy clo[4.6.0]dodec-1(6)-ene (13) on dye-sensitized photo-oxygenation give the corresponding 1,2-dioxetanes (3), (10), and (14) in yields of 35- 53% as isolable, stable entities with the exception of 10. 2-Hydropero xy-5,6-dihydro-2H-pyran (5) is also obtained from 1 in 15% yield. No h ydroperoxide is formed from 9, whereas 13 affords 3,4,6,7,8,9-hexahydr o-2H-cycloocta[b]pyran-4a-yl hydroperoxide (15) in variable, but minor yields depending on the choice of solvent, temperature and sensitizer . Further [2 + 2] addition to 15 produces the 1,2-dioxetane(16). Repet ition of the photo-oxygenation of 1, 9 and 13 in the presence of aceta ldehyde affords the same products as before. In the case of 1, the cis -fused epimeric 1,2,4-trioxanes (7) are additionally formed in 2% yiel d. TMSOTf-catalyzed treatment of 3, 10, and 14 with acetaldehyde affor ds the corresponding 1,2,4-trioxanes in yields of 30, 30 and 43%. Ther molysis of 3, 10, and 14 breaks the dioxetane ring to produce the mono cyclic keto lactones.