Facile construction of 3-hydroxyphenanthrene-1,4-diones: key intermediates to tanshinone I and its A-ring-modified analogue

A mild synthetic approach was established allowing for a convenient construction of tricyclic hydroxyphenanthraquinones, the key precursor for total synthesis of tanshinone I. This tandem process includes decarboxylative radical alkylation, intramolecular C-H arylation and one-pot O-demethylation and aromatization. Variously substituted phenylpropanoic acids were well tolerated in this approach, and synthesis of tanshinone I (1) was finally successful in six straight steps in 19% overall yields from commercially available 5-bromovanillin.