Diels‐Alder Reactions of 6,9‐Dipropyl‐1, 4‐Dioxaspiro[4.5]Deca‐6, 8‐Diene‐2, 10‐Dione with Substituted Alkenes. An Entry to Bicyclo[2.2.2]Oct‐5‐Ene‐2, 3‐Diones and 1, 3‐Cyclohexadienes

Abstract

Abstract The cycloadditions of the title compound, 1, a masked 3, 6‐dipropyl‐ o ‐benzoquinone to various disubstituted (diethyl maleate, diethyl fumarate, trans ‐ and cis ‐stilbene, and cycloheptene) and monosubstituted alkenes (methyl acrylate, methyl vinyl ketone, styrene, ethyl vinyl ether, and 1‐hexene) have been studied; the yields of the Diels‐Alder adducts, 2, are generally high (> 75%) except for stilbenes. These adducts are effectively transformed into the corresponding bicyclo[2.2.2]oct‐5‐ene‐2, 3‐diones, 3, whose stereochemistries are determined from the ring current effect of the respective quinoxalines, 4, on the chemical shifts of the vinyl and the methine protons and/or the lanthanide (Eu(fod) 3 ) induced shift on the pertinent protons of 3. It is interesting to note that the quinoxaline ring current makes the vinyl protons down‐field shifted by 0.15–0.27 ppm whereas the methine protons (R 3 =H) syn to the quinoxaline moiety up‐field shifted by 0.18–0.39 ppm as compared to the chemical shifts of the corresponding protons in 3. The transformations of α‐diketones 3 into the corresponding 1, 3‐cyclohexadienes, 5, by irradition with a Hanovia medium pressure lamp through a uranium glass filter are almost quantitative. The present study provides facile and effective methods for the preparation of bicyclo[2.2.2]oct‐5‐ene‐2, 3‐diones and 1, 3‐cyclohexadienes from catechols via masked o ‐benzoquinones.