Selective reactivity of 2-mercaptoethanol with 5β,6β-epoxide in steroids from Withania somnifera

2-Mercaptoethanol reacts selectively with the 5β,6β-epoxy steroids isolated from Withania somnifera substituting the epoxide by a six-membered oxyethylene-2′-thio ring whereas it failed to show such reactivity on 6α,7α-epoxy withasteroids. The structure of the product has been elucidated by spectroscopic methods, especially applying extensive 2D NMR methods. The anticancer activity of withaferin A was lost in the reaction product indicating that its activity is also linked to the free 5β,6β-epoxide functional group.