Chiral separation, absolute configuration, and bioactivity of two pairs of flavonoid enantiomers from Morus nigra

Abstract An undescribed isoprenylated flavonol racemate, nigranol C, with an unprecedented 7/6/6 ring system, was isolated from the twigs of Morus nigra L. The structure was assigned through a comprehensive analysis of HRMS, IR, and NMR data. Chiral separation of nigranol C was successfully carried out to yield a pair of enantiomers, nigranol C-a and nigranol C-b, whose absolute configurations were determined by ECD calculation. A plausible biogenetic pathway for nigranol C was proposed. A previously isolated sanggenon-type flavonone racemate, nigragenon E, was also well resolved by chiral HPLC to offer another pair of enantiomers, nigragenon E-a and nigragenon E-b, whose stereo configurations were determined by ECD data. All of the isolated compounds showed prominent α-glucosidase inhibitory activities with IC 50 values ranging from 9.79 to 30.21 μM, while only the sanggenon-type flavonones exhibited tyrosinase inhibitory effects comparable to that of the positive control, kojic acid, the IC 50 value of which was 27.14 μM. In addition, it was found that the stereo configurations of these compounds seemed to play a negligible role in their inhibitory activities towards the two enzymes.