Photooxygenation of 1-Vinylcycloalkenes. The Competition between "Ene" Reaction and Cycloaddition of Singlet Oxygen

Article abstract of DOI:10.1021/jo00205a013

The competition between the "ene" reaction and 1,4-cycloaddition reaction of ¹O2 in a series of 1-vinylcycloalkenes (n = 5-10, 12) and 1-isopropenylcycloalkenes (n = 6-8) was studied.Ring size had a profound effect.When n = 5 or 7, products of the "ene" reaction predominated, but as ring size increased cycloaddition competed closely with the "ene" process, the crossover being at ring size 10. 1-Vinylhexenes constituted an exception with 1,4-cycloaddition greatly predominant. "Ene" products were almost exclusively conjugated diene hydroperoxides or secondary products derived from them; small amounts of unconjugated diene hydroperoxides were formed only when n = 9, 10, or 12.The influence of ring size of the "ene" to cycloaddition product ratio is compared with the rates of photooxygenation and the degree of syn-regiospecificity exhibited by (Z)-1-methylcycloalkenes and attributed to the same mainly conformational factors, as is the geometry of the "ene" products.The 1,4-cycloaddition products were converted to furans by means of ferrous sulfate.