Structure–activity relationships of the ultrapotent vanilloid resiniferatoxin (RTX): The homovanillyl moiety

Starting from ROPA (2), analogues of RTX (1a) modified on the acyl side chain were prepared and evaluated for vanilloid activity in HEK-293 cells over-expressing the human recombinant TRPV1. The ROPA motif provided an enhancement of potency sufficient to expand the range of vanillyl surrogates to structural elements (e.g., an unsubstituted phenyl ring) that afford inactive analogues in compounds from the capsaicin series.