EFFICIENT SYNTHESES OF VINYL ETHERS OF SPIROQUINOL KETALS AND THEIR HIGH-YIELD PHOTOCHEMICAL OXYGEN-TO-CARBON [1,3]-SHIFT TO SPIRO-FUSED 2,5-CYCLOHEXADIENONES

An efficient route to spiroquinol vinyl ethers involves addition of a 1-lithio-2-(trimethylsilyl)acetylene-substituted benzene to the monoethylene ketal of benzoquinone followed by desilylation/cyclization of the resulting product to give vinyl ethers of spiroquinol ketals. A high-yield photochemical conversion of these vinyl ethers of spiroquinol ketals to ketals of spiro-fused 2,5-cyclohexadienones has been developed. A complication in some of these photochemical reactions is formation of secondary products from light absorbed by the product, spiro dienone ketals. This has been solved by conducting the reaction in the presence of piperylene, which quenches the triplet-state chemistry of the product spiro dienone ketal without altering the singlet excited-state chemistry of the quinol ketal vinyl ether. The quantum yield for the photochemical [1,3]-shift reaction in a methyl-substituted vinyl ether is 0.4. Finally, irradiation of quinol spiro vinyl ethers was also observed to give spiro dienones in good yields. Although the spiro dienone is absorbing light in competition with starting quinol vinyl ether in this system, a high yield of product was obtained. The unexpected photochemical stability of these spiro dienones is discussed. The chemistry reported herein establishes an efficient high-yield route to spiro-fused 2,5-cyclohexadienones and their ketals under very mild conditions.