The synthesis of 1-alkyl- and 1-aryl-1 H -naphtho[2,3- c ]pyran-5,10-diones, bearing a C(3)−C(4) double bond, was performed by two alternative cyclization strategies of 2,3-disubstituted 1,4-naphthoquinones. Lemieux−Johnson oxidation of 2-allyl-3-hydroxymethyl-1,4-dimethoxynaphthalenes and subsequent oxidative demethylation of the intermediate 3,4-dihydro-5,10-dimethoxy-1 H -naphtho[2,3- c ]pyran-3-oles to the corresponding 3,4-dihydro-3-hydroxy-1 H -naphtho[2,3- c ]pyran-5,10-diones gave, after acid-catalyzed dehydration, the desired 1 H -naphtho[2,3- c ]pyran-5,10-diones. Alternatively, the pyranonaphthoquinone ring system was constructed by intramolecular acid-catalyzed condensation of (3-hydroxymethyl-1,4-naphthoquinone-2-yl)acetaldehyde acetals. Using this synthetic approach, the synthesis of two naturally occurring naphthoquinone antibiotics, pentalongin and psychorubrin, is reported.
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Kesteleyn et al. (1999) studied this question.