Selection of Enantioselective Acyl Transfer Catalysts from a Pooled Peptide Library through a Fluorescence-Based Activity Assay:  An Approach to Kinetic Resolution of Secondary Alcohols of Broad Structural Scope

An assay employing a fluorescently labeled split and pool peptide library has been applied to the discovery of a new class of octapeptide catalysts for the kinetic resolution of secondary alcohols. A highly diverse library of peptide-based catalysts was synthesized on solid-phase synthesis beads such that each individual bead was co-functionalized with (i) a uniform loading of a pH-sensitive fluorophore and (ii) a unique peptide-based catalyst. The library was then screened for activity in acylation reactions employing (+/-)-sec-phenylethanol as the substrate and acetic anhydride as the acylation agent. From the most active catalysts, a lead peptide (4) was identified that provides a selectivity-factor (k(rel)) of 8.2 upon resynthesis and evaluation under homogeneous conditions. A "directed" second-generation split and pool peptide library was synthesized such that the new peptide sequences in the library were biased toward the lead structure. Random samples of the second generation library were screened in single bead assays that revealed several new peptide-based catalysts that afford improved selectivities in kinetic resolutions. Peptide catalyst 13 proves effective for the kinetic resolution of sec-phenylethanol (k(rel) = 20), as well as eight other secondary alcohols of a broad substrate scope (k(rel) = 4 to >50).