The side chain benzylic methylene is a critical element for the vanilloid activity of resiniferatoxin (2a, RTX), and introduction of branching, oxygen functions, or isosteric substitution at this center proved detrimental, with a decrease of potency of 2-3 orders of magnitude compared to the natural product. Conversely, only a modest erosion of activity was observed upon alpha-methylation and alpha-methylenation of the side chain. Surprisingly, introduction of an iodine atom in the guaiacyl moiety of the oxygen isoster 2h led to an unexpected and remarkable (>1000-fold) increase of potency, affording 2i, a compound that outperforms RTX in terms of vanilloid agonism and represents the first one-digit picomolar ligand of a TRP channel discovered to date.
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