Calabanones A‒H, chromanone derivatives from the stem bark of Calophyllum calaba and their cytotoxic activities against cancer cells

The genus Calophyllum (Calophyllaceae), distributed mainly in tropical regions, is rich in chromanone derivatives with diverse molecular structures that exhibit potential antimicrobial and anticancer effects. Phytochemical investigation of the stem bark of C. calaba collected in Thailand led to the discovery of eight previously undescribed chromanones, calabanones A-H (1‒8), and two known analogs, (‒)-isocalomembranone P (9) and (‒)-calomembranone P (10). The chemical structures of undescribed compounds were elucidated using spectroscopic analyses, particularly NMR and HRESIMS, while their absolute configurations were determined through ECD and NMR calculations combined with DP4+ probability analysis. Compounds 7 and 8 were identified as chromanone-steroid hybrids linked via an ester bond, representing the first report of such structures in plants. Cytotoxic evaluation of the isolated specialized metabolites revealed that compounds 6 and 9 displayed moderate activity against KB and HeLa S3 cancer cell lines, with IC₅₀ values ranging from 12.71 to 25.50 μM, while compounds 3‒5 selectively inhibited the growth of KB cells, with IC₅₀ values in the range of 18.77‒27.06 μM.