Methylene blue-sensitized photo-oxygenation of Z-2-(3′-oxobutyl)methoxymethylidenecyclohexane (7)in CH2Cl2 at −78°, followed by catalysis with Amberlyst-15, gave an epimeric pair of methoxy-1,2,4-trioxanes (9 and 10) and an isomeric peroxide (11) in 67 and 30% yields. The E-isomer (8) under the same conditions also gave 9–11 (48%) and 2-formyl-3-(3′-oxobutyl)cyclohexene (30%). Trioxanes are formed by cyclization of an initially formed 1,2-dioxetane. The E and Z enol ethers 7 and 8 react with singlet oxygen to give dioxetanes which on acid catalysis rearrange to 1,2,4-trioxanes (9 and 10) and the peroxide 11.