Relative Reactivities of Activated Carboxylic Acids in Amide-Forming Reactions Employed for the Synthesis of Tetraphenylporphyrin Libraries

Abstract: Presented here is a method for rapidly testing the reactivity of carboxylic acids in amide-forming reactions. For this, a mixture of two acids, one a reference compound, and one acid whose reactivity is unknown, are coupled to an aminoacylated tetrakis(paminophenyl) porphyrin under typical peptide coupling conditions. The product distribution in the resulting library is analyzed via MALDI-TOF mass spectrometry to reveal the relative reactivity. This rapid reactivity test requires sub-nanomole quantities of acids, does not involve cleavage from a support or any potentially biasing work-up, and is automatable. Thus, it is well suited for testing building blocks for combinatorial syntheses. Further, it is demonstrated that step-wise coupling can produce near-statistically distributed libraries of porphyrins when acids of very different reactivity are employed.