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 f...