Four new stilbenes and one new flavonoid with potential antibacterial and anti-SARS-CoV-2 activity from Cajanus cajan.

Four new stilbenes (1-4) and one new flavonoid (5), named cajanines D-H, together with three known stilbenes (6-8) were isolated from the leaves of Cajanus cajan (L.) Millsp. (pigeon pea). The structures of these compounds were elucidated unambiguously on the basis of IR, 1D, and 2D NMR, as well as HRESIMS data. Structurally, stilbenes 1-4 bore an isopentyl side chain, and further hydroxylation of compounds 1-3 generated a greater variety of structural forms. Compound 5 was a flavonoid owning an isopentyl side chain. Besides, antibacterial activity of the isolated compounds against Staphylococcus aureus, Bacillus cereus, and Escherichia coli was studied in vitro. Compounds 1-8 were endowed with profound antibacterial activity. Among them, the MIC values of compounds 4, 6, and 7 against S. aureus were 1.56, 0.78, and 0.78 µg/mL, respectively, among which 6 and 7 had better antibacterial activity than the positive control Vancomycin with the MIC values of 1.56 µg/mL. Additionally, the anti-SARS-CoV-2 main protease activity of all the isolated compounds was evaluated, and it was worth mentioning that the IC values of compounds 5-7 were 8.27, 4.65, and 8.30 µM, respectively, being comparable to the positive control Ebselen. Our findings may provide valuable guidance for the application of stilbenes as lead compounds in the future for the development of drugs with antibacterial or anti-COVID-19 activity.