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The angucyclinones with aromatic ring B (1a-c and 2a-c) are prepared in a biomimetic-type synthesis by two successive aldol cyclizations starting from the substituted naphthoquinones 12a-c. In both cyclization steps the C-H acidity of the potential nucleophilic centers determines the mode of cyclization under kinetically controlled conditions. The tetrahydroanthraquinones 13a-c/14a-c are hydroxylated at C-4 to the phenolic anthraquinones 16a-c upon treatment with excess NMO.
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Krohn et al. (1997) studied this question.
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