Experimental lineage and functional analysis of a remotely directed peptide epoxidation catalyst

Abstract

We describe mechanistic investigations of a catalyst (1) that leads to selective epoxidation of farnesol at the 6,7-position, remote from the hydroxyl directing group. The experimental lineage of peptide 1 and a number of resin-bound peptide analogues were examined to reveal the importance of four N-terminal residues. We examined the selectivity of truncated analogues to find that a trimer is sufficient to furnish the remote selectivity. Both 1D and 2D (1)H NMR studies were used to determine possible catalyst conformations, culminating in proposed models showing possible interactions of farnesol with a protected Thr side chain and backbone NH. The models were used to rationalize the selectivity of a modified catalyst (17) for the 6,7-position relative to an ether moiety in two related substrates.

Scheme 1. Previously Reported 6,7-Oxidation and Proposed Peptide-Catalyzed Epoxidaton Catalytic Cycle

DIU = N,N′-diisopropylurea.

Figure 1

(A–C) Ternary plots showing an overlay of peptide selectivities from successive generations of 6,7-selective peptides, where each axis represents the fraction of the total monoepoxide: (A) the first library demonstrating 6,7-selectivity (first generation, red); (B) the first biased library for 6,7-selectivity (second generation, green); (C) second biased library (third generation, blue). Points that are higher (further away from the triangle base) are more 6,7-selective. The highlighted generation from each plot is in the solid color in the foreground atop the other library generations in the background. Solid markers indicate peptides that were sequenced. (D) List of a portion of sequenced peptides from each library generation (shown with solid markers in A–C). Residues shown in blue indicate were picked from the pool of variable residues from their particular library. (E) Selectivity with amino acids at the i + 2 position, which can be compared to the library with i + 2 Thr(Bn) directly to the left (C, third generation, blue). Product ratios were measured by GC. In all of the on-bead screening studies, the formation of diepoxides was intentionally limited through the use of 0.3 equiv of DIC, resulting in analysis of low-conversion reaction mixtures.

Figure 2

(A) Selectivities observed with truncated peptides. (B) Abbreviated 1D ¹H NMR spectra of 1 and its truncated analogues acquired at −20 °C at 20 mM.

Figure 3

(A) Selected ¹H–¹H ROESY correlations found in 1 at 25 °C and truncated peptide 7 at −20 °C. (B) Two structural ensembles from the 20 CNS-generated structures of 1 computed using 25 °C ROESY data. (C) One structure chosen from the 20 structures shown with side chains.

Figure 4

Hypothesized models for selectivity toward 5.

Figure 5

Hypothesized interactions of 13 with i + 3 side-chain variants of 1 and 17. One of the several scenarios based on Figure 4 is depicted.

Scheme 2. Comparison of Reactions with Difarnesyl Ether