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• It was found that tubocapsunolide A could undergo electrophilic addition reaction with MeOH. • The absolute configuration of tubocapsunolide MAP was determined which was a new withanolide. • It is confirmed that α-β unsaturated carbonyl unit is closely related to its anti-TNBC activity. A new withanolide, tubocaapsanolide MAP (MeOH addition product) (1), as well as its known precursor tubocaapsanolide A (2) were obtained from Tubocapsicum anomalum (Solanaceae). Compound 1 was a MeOH addition product transformed from compound 2 during the process of separation using MeOH as solvent. The structures of the two withanolides including absolute configuration were elucidated by extensive spectroscopic analysis and X-ray single crystal diffraction. In the test of anti-triple negative breast cancer (TNBC) effects, tubocapsusanlide A (2) showed potent inhibitory activity against four human TNBC cell lines, while tubocapsusanlide MAP (1) exhited significantly weaker inhibitory than that of tubocapsusanlide A (2), indicating that α - β unsaturated carbonyl unit contained in 2 was closely related to its anti-TNBC activity. The potent bioactivity displayed significant developing potential of withanolides as anti-TNBC lead compounds or drug candidates. And this report may provide some useful guidances for the preparation and bioactivity research of withanolides. [ABSTRACT FROM AUTHOR]