Functional Analysis of an Aspartate‐Based Epoxidation Catalyst with Amide‐to‐Alkene Peptidomimetic Catalyst Analogues

Abstract

Keywordsepoxidation; peptide; asymmetric catalysis; olefin isostere; fluorine; peptidomimetic The biosynthesis of natural products that contain epoxides represents a powerful stimulus for the study of "epoxidase" enzymes. [i] Likewise, these processes have inspired a generation of science focused on small molecule catalysts that mediate selective epoxidations through a variety of mechanisms. [ii] With respect to the naturally occurring epoxidases, the mechanistic basis of O-atom transfer is often associated with the chemistry of either flavinoid cofactors, P450 enzymes containing a heme group, or chloroperoxidases that lead to stepwise ring formation. [iii] In thinking about the known biosynthetic apparatus for epoxide formation, we became curious about an alternative mode for O-atom transferone based on functional groups available in proteins, but perhaps not well-documented in the biosynthesis of epoxides. In particular, we speculated and recently showed that asparticacid-containing peptides (e.g., 1; Figure 1a) might shuttle between the side-chain carboxylic acid and the corresponding peracid (e.g., 2) creating a catalytic cycle competent for asymmetric epoxidation with turnover of the aspartate-derived catalyst. Such an approach is orthogonal to the Julia-Colonna epoxidation, a complementary peptide-based epoxidation based on a nucleophilic mechanism. [iv] Indeed, as shown in Figure 1b, this new electrophilic epoxidation catalytic cycle mediates the asymmetric epoxidation of substrates like 3 to give products like 4 with up to 92% ee. [v] Mechanistic questions abound in this catalytic system. To date, we have identified a number of relevant aspects. For example, we observed off-catalytic cycle intermediates, including catalytically inactive diacyl peroxides (6). [vi] We also showed that these off-cycle intermediates could be reinserted into the productive pathway through the action of nucleophiles such as DMAP or DMAP-N-oxide (7). On the other hand, the basis of stereochemical information transfer was not immediately clear. Indeed, the high precision delineation of the stereochemical mode of action of chiral catalysts is a critical frontier in the discipline of asymmetric catalysis, whether the catalysts are enzymes or small molecules. With this back-drop, we began a detailed study of the mode of action for catalyst 5.The conversion of 3 to 4 was originally undertaken with the hypothesis that substratecatalyst hydrogen bonding might contribute to transition state organization. [vii] Indeed, a substrate lacking obvious H-bonding capability (phenylcyclohexene) was found to undergo epoxidation with catalyst 5 with low enantioselectivity (~10% ee). Thus, we envisioned several potential loci for contacts between 3 and 5 (Figure 2a). Shown in blue is the site that

Function-oriented investigations of a peptide-based catalyst that mediates enantioselective allylic alcohol epoxidation.

“…8d As shown in Scheme 1, we found a peptide-based catalyst 6 that is highly selective in forming the 2,3-epoxide of farnesol ( 8 ) over the 6,7-epoxide ( 9 ) or the 10,11-epoxide ( 10 ) with 1:1:>100 site selectivity ( 10:9:8 ) and 86% ee in forming 8 . In addition, we found another catalyst ( 7 ) that exhibits highly unique site selectivity for the 6,7-position (1:8:1, 10:9:8 ) of farnesol, albeit with low enantioselectivity (10% ee).…”

“…Previously reported results for prenol and nerol, which were epoxidized using previously reported conditions (a), indicated that ( Z )-olefins were best suited for the system. 8d Epoxynerol ( 11 ) can be synthesized with 93% ee and 79% yield and epoxyprenol ( 12 ) is formed in 92% ee and 75% gas chromatography (GC) yield (Table 1, entries 1 and 2, respectively). The optimal pattern for the substitution, in terms of steric bulk of substituents, remained an intriguing avenue for investigation.…”

“…8e,23 While we had examined truncated versions of the catalyst 6 on-bead, 8d we endeavored to perform this type of analysis on farnesol with peptides in solution (Table 2). While catalyst 6 delivers excellent selectivity in solution (1:1:>100, 10:9:8 ; 86% ee for 8 ), each of the further truncated analogues of 6 delivers incrementally worse selectivity.…”

“…Studies of the most site-selective library for farnesol epoxidation suggest that the identity of the i + 4 side chain (a DPhe in catalyst 6 ) may not be important, as a number of catalysts were identified with variability in this position. Given the broad sequence tolerance 8d we decided to investigate the conformation of catalyst 6 in solution using 2D NMR techniques.…”

“…While this role is not known for aspartate in enzymatic catalysis, which instead most often employs co-factors for biosynthetic epoxidations, 14 we have been intrigued about the possible intermediacy of aspartyl peracids ( 2 ) – co-factor-free intermediates – for peptide-based epoxidation catalysis. Our initial studies into the epoxidation of allylic carbamates 8a,b were based on a catalytic cycle employing an aspartyl peptide catalyst, and hydrogen peroxide as the oxidant (Figure 1). The plausible catalytic cycle involves carboxyl activation with a carbodiimide reagent and capture of the derived active ester either as the carbodiimide adduct ( 3 ) or its nucleophile-derived adduct (e.g., 4 ) with hydrogen peroxide.…”