Tandem Oxidative Acetalization−Intramolecular Diels−Alder Reactions of 2-Methoxyphenols. Simple Synthesis of Bicyclo[2.2.2]octenone Derivatives

Intramolecular Diels−Alder reactions of in situ generated masked o -benzoquinones are described. Oxidation of methyl vanillate ( 2 ) in the presence of allyl alcohol ( 1a ), trans -crotyl alcohol ( 1b ), cinnamyl alcohol ( 1c ), and homoallyl alcohol ( 1d ) resulted in the formation of masked o -benzoquinones 8a − d that underwent intramolecular Diels−Alder reactions under reaction conditions to furnish adducts 14a − d in 53−75% yields. This tandem oxidative acetalization−intramolecular Diels−Alder process was extended to other 2-methoxyphenols such as 2-methoxy-4-methylphenol ( 3 ), guaicol ( 4 ), methyl isovanillate ( 5 ), methyl syringate ( 6 ), and 2,6-dimethoxy-4-methylphenol ( 7 ) to obtain adducts 15a − d, 16a − c, 17a − d, 18a − d, and 19a − d, respectively. While intramolecular Diels−Alder reactions of the masked o -benzoquinones 10a, 12d, and 13d were found to be less efficient, the masked o -benzoquinones 9a − d, 10b, 10c, 11a − d, 12a − c, and 13a − c furnished the desired products in 38−80% yields. Masked o -benzoquinones 21a − d generated from 2 and substituted acrylic acids 20a − d underwent intramolecular Diels−Alder reactions to provide the tricyclic lactones 22a − d in 32−40% yields.