Compounds of the general formula:
wherein R₁, R₂, R₃, R₄ and R₅ are herein described, A is a straight or branched (C₂-C₁₂)alkyl or a phenyl moiety and B is a moiety of the formula:
The compounds are effective in potentiating the activity of chemotherapeutic anti-cancer agents by increasing the sensitivity of multi-drug resistant cells to such chemotherapeutic agents.