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‐2 H ‐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.