A Useful Synthon Approach to Bicyclic Enols: Acid-Catalyzed and Base-Catalyzed Rearrangements of Diels-Alder Adducts of 2-Methoxy-5-methyl-1,4-benzoquinone

Article abstract of DOI:10.1021/jo00211a034

Acid- and base-catalyzed reactions of Diels-Alder adducts of the title compound 1 have been investigated.Bicyclic adducts 6, 9, and 11 isomerized to the corresponding enols 8, 10, and 13 on treatment with base followed by rapid acidification.However, thermal reactions of 1 with sorbic acid esters afforded the enol products 3 directly.Similar base treatment of tricyclic adducts 15 and 17 resulted in ether cleavage to α-keto enols 16 and 18.The bicyclic enols suffered facile aerial oxidation to the cis- or trans-hydroperoxides 31 or 33, depending on the substituent at the C-5 position.The trans-hydroperoxides 33 were reduced to the alcohols 34 on treatment with silica gel; however, the cis-hydroperoxides 31 underwent a novel decarbonylation to afford β-keto esters 32.Acid treatment of adducts 6 gave α-keto enols 35, but the ester-substituted adducts 4 were stable to acid.These interesting structure-reactivity relationships are summarized in Scheme I.