A new synthesis of anthraquinones using dihydro-oxazoles and Grignard reagents derived from Mg(anthracene)(THF)3

A general synthesis of anthraquinones which depends on the displacement of the methoxy group from an o-methoxyaryldihydro-oxazole by a methoxy substituted benzylmagnesium chloride, generated by using a magnesium–anthracene complex, has been developed. The masked benzylbenzoic acids which result from these reactions are deprotected and then ring-closed to anthrones which on oxidation yield anthraquinones. In this way, the following naturally occurring anthraquinones (or derivatives thereof have been synthesized): chrysophanol (9), islandicin (19), digitopurpone (21), tri-O-methylemodin (26), and di-O-methylsoranjidiol (29).