A Concise and Convenient Synthesis of dl-proto-Quercitol and dl-gala-Quercitol via Ene Reaction of Singlet Oxygen Combined with [2 + 4] Cycloaddition to Cyclohexadiene

Photooxygenation of 1,4-cyclohexadiene afforded hydroperoxy endoperoxides 3 and 4 in a ratio of 88:12. Reduction of 3 with LiAlH4 or thiourea followed by acetylation of the hydroxyl group and KMnO4 oxidation of the double bond gave proto-quercitol 10b. Application of the same reaction sequences to 4 resulted in the formation of gala-quercitol 14. Quercitols were easily obtained by ammonolysis of acetate derivatives in MeOH. The outcome of dihydroxylation reactions were supported by conformational analysis.