The genus Stereospermum (family Bignoniaceae) has been documented to possess several pharmacological potentials, including antidiabetic and antibacterial activities, as well as the identification of numerous bioactive compounds. However, Stereospermum neuranthum Kurz, a Thai edible plant, remains unexplored in terms of its bioactivity and phytochemical composition. Thus, the present study was carried out to isolate and characterize the phytochemical constituents from the bark of S. neuranthum and to evaluate their antibacterial activity and α‐glucosidase inhibitory potential. Chromatographic separation of the ethyl acetate (EtOAc) and methanol (MeOH) extracts from S. neuranthum bark afforded a new natural naphthoquinone, 2‐hydroxy‐3‐geranyl‐1,4‐naphthoquinone, and fourteen known compounds, including sterekunthal B, pinnatal, sterequinone B, sterequinone H, paulownin, wodeshiol, p ‐hydroxybenzoic acid, p ‐coumaric acid, 1‐O‐ p ‐coumaroyl‐β‐D‐glucopyranose, cinnamic acid, β‐sitosterol, β‐sitosterol glucoside, β‐sitostenone, and lupeol. Their structures were elucidated using IR, 1D NMR ( 1 H, 13 C, and DEPT), and 2D NMR techniques (HSQC, COSY, and HMBC). Eleven isolated compounds were evaluated for antibacterial activity against three Gram‐positive and three Gram‐negative bacteria using the broth microdilution method. Moreover, their α‐glucosidase inhibitory activity was determined using a spectrophotometric method with p ‐NPG as a substrate, and molecular docking was also studied. Sterekunthal‐B and pinnatal exhibited moderate antibacterial activity against Bacillus cereus , Bacillus subtilis , and Staphylococcus aureus with the same MIC value of 64 μg/mL. In addition, paulownin showed moderate inhibitory effects against both B. cereus and Pseudomonas aeruginosa . Notably, the new natural naphthoquinone showed slightly stronger α‐glucosidase inhibitory activity (IC 50 = 309.27 μM) than the standard acarbose (IC 50 = 323.67 μM). Molecular docking analysis validated using root mean square deviation (RMSD), revealed a favorable binding free energy (ΔG = −7.56 kcal/mol) at the α‐glucosidase active site, and demonstrated key hydrogen bond interactions of the carbonyl group with the Arg‐315 residue and the hydroxyl group with the Tyr‐158 and Asn‐415 residues, which supported the in vitro experimental results. Based on these findings, the new naphthoquinone shows potential for further study as a candidate for antidiabetic drug discovery.
Suchaichit et al. (Thu,) studied this question.