Studies on quinones. XII. Cycloketalization of michael adducts from hydroxyquinones

The reaction of 2-hydroxy-3-(1-phenyl-3-oxobutyl)-1,4-naphthoquinone (1) with either acidic methanol or a mixture of trimethyl orthoformate, methanol and ammonium chloride resulted in the formation of the p-quinonic cycloketals: trans- and cis-4-phenyl-2-methyl-2-methoxy-3,4-dihydro-2H-naphtho[2,3-b]pyran-5,10-dione (2a,2b). Cyclization of the Michael adducts 6, 10 and 11, which are structurally related to 1, with trimethyl orthoformate-methanol-ammonium chloride gave the corresponding p-quinonic cycloketals 7, 12 and 13. The structures of the regioisomers 2a and 2b are proposed based on the spectral properties of compound 7 and by analysis of its proton nmr spectra.