The isolation of secondary metabolites from Rheum ribes L. and the synthesis of new semi-synthetic anthraquinones: Isolation, synthesis and biological activity

•Six isolated known compounds and novel semi-synthetic anthraquinones were obtained.•Novel bioactive semi-synthetic anthraquinones were obtained from emodin and aloe-emodin.•Antioxidant, antiproliferative and human serum albumin activities were assayed.•Two semi-synthetic derivatives exhibited excellent antiproliferative activities.•The most activesemi-synthetic compoundwasAE-1 on HeLa and PC-3 cell lines. Rheum ribes L. (Rhubarb) is one of the most important edible medicinal plants in the Eastern Anatolia region and is called “Işkın” by local people. Resveratrol and 6-O-methylalaternin were isolated from the Rhubarb for the first time in addition to well-known secondary metabolites including emodin, aloe-emodin, β-sitosterol and rutin. The new semi-synthetic anthraquinone derivatives with the NαFmoc-l-Lys and ethynyl group were synthesized from the isolated anthraquinones emodin and aloe-emodin of Rhubarb to increase the bioactivities. Aloe-emodin derivative with NαFmoc-l-Lys shows the highest inhibition values by 94.11 ± 0.12 and 82.38 ± 0.00% against HT-29 and HeLa cell lines, respectively, at 25 µg/mL. Further, modification of the aloe-emodin with both the ethynyl and the NαFmoc-l-Lys groups showed an antioxidant activity-enhancing effect. From molecular docking studies, the relative binding energies of the emodin and aloe-emodin derivatives to human serum albumin ranged from −7.30 and −10.62 kcal/mol.