A Biomimetic Synthesis of Racemic Dihydroanthracen‐1(2H)‐ones Using Sodium Borohydride in Water

Herein, we report a single step synthesis of racemic dihydroanthracenones by the reduction of anthraquinones using NaBH4 and Na2S2O4 in water. The racemic mixture is utilized to determine the enantiomeric excess for the chiral dihydroanthracenones reportedly synthesized by the chemoenzymatic reduction of anthraquinones. The results show > 99% ee for the putative biosynthetic intermediates, (R)‐configured dihydroanthracenones obtained through chemoenzymatic reduction of emodin, lunatin, and citreorosein using MdpC of Aspergillus nidulans or PHAR of Cochliobolus lunatus. This also implied for the confirmation about the optical purity of the natural products which are synthesized using chiral dihydroanthracenones. We report a first synthesis of racemic dihydroanthracenones through regioselective reduction of hydroanthraquinones using NaBH4 in water. Their use confirms the enantiopurity of chiral dihydroanthracenones synthesized chemoenzymatically and of the bisanthraquinone analogues.