General Installation of (4H)-Imidazolone cis-Amide Bioisosteres Along the Peptide Backbone

Abstract

Imidazolones represent an important class of heterocycles present in a wide-range of pharmaceuticals, metabolites, bioactive natural products, and serve as the active chromophore in green fluorescent protein (GFP). Recently imidazolones have received attention for their ability to act as a non-aromatic amide bond bioisotere which improves pharmacological properties. Herein we present a tandem amidine installation and cyclization with an adjacent ester to yield (4H)-imidazolone products. By using amino acid building blocks, we can access the first examples of α-chiral imidazolones which have been previously inaccessible. Additionally, our method is amenable to on-resin installation which can be seamlessly integrated into existing solid-phase peptide synthesis (SPPS) protocols. Finally, we show that peptide imidazolones are potent cis-amide bond surrogates which pre-organize linear peptides for head-to-tail macrocyclization. This work represents the first general approach to backbone and side-chain insertion of imidazolone bioisosteres at various positions in linear and cyclic peptides.

Graphical Abstract

Incorporation of heterocyclic motifs along the peptide backbone represents a critical tool to address issues in peptide drug metabolism and cell permeability.^1–4 Nature has evolved its own biological machinery to include heterocycles and expand the complexity of peptides beyond the standard suite of canonical amino acids.^5–8 The late biochemist Christopher T. Walsh credited these heterocycles as “a recurring motif in Nature’s medicinal chemistry toolbox”.⁹ In nature, backbone heterocycles are installed via cyclization from an adjacent side-chain onto the backbone amide linkage,⁸ leading to oxazole-type (from Ser or Thr), and thiazole-type (from Cys) heterocycles (Figure 1A left).^5–8 Synthetic chemists have mimicked this approach by using on-resin chemical activation of the amide bond, or by incorporating non-natural side-chains into linear peptides to access aromatic backbone heterocycles such as oxazoles,¹⁰ thiazoles,¹⁰ imidazoles,¹⁰ pyrazoles,¹¹ oxadiazoles,¹² 4-imidazolidinones,¹³ 2-imidazolidines,¹⁴ 1,2,4-triazoles,¹⁵ 1,2,3-triazoles,^16,17 and iminohydantoins.¹⁸ The substitution pattern imparted by all of these heterocycles generates a 1,3-trans-amide-like conformation along the peptide backbone (Figure 1A left) which mimics the native trans-amide conformation.

Figure 1.

(A) Heterocycles can geometrically restrict the peptide backbone to either cis-amide-like or trans-amide-like motifs. (B) (4H)-Imidazolones can act as non-aromatic amide bond isosteres. (C) Thioimidate dipeptides enable facile entry to imidazolone cyclization products by use of an amidine intermediate.

One can imagine a second option for backbone heterocyclic installation, derived solely from the amide linkage. These heterocycles would yield a 1,2-cis-amide-like motif along the peptide backbone (Figure 1A right), geometrically constraining the amide bond to a non-native cis-amide conformation. Because the amide bond of peptides only exists in the cis-conformation 0.1–0.2% of the time (at room temperature),¹⁹ cis-amide bond surrogates are of exceeding interest to synthetic and medicinal chemists for their ability to initiate turn motifs in peptides,^20,21 reduce proteolysis,¹¹ and facilitate peptide macrocyclization.^22,23

Current methods to insert heterocycles which lock the cis-amide conformation, however, are not ‘plug-and-play’ and suffer from significant challenges regarding implementation and compatibility with solid-phase peptide synthesis (SPPS)—the work-horse method for peptide synthesis. Previous methods have focused exclusively on cis-substituted triazoles and tetrazoles.^24–28 Many of the reported methods are not compatible or provide unreliable results on solid phase, instead requiring cumbersome pre-synthesis of the heterocyclic precursors prior to installation into tripeptide fragments ahead of coupling to solid-support.^24,25,29,30 The synthesis of these tripeptides in solution is often not trivial because of the protecting group management required for formation (i.e. the compatibility and orthogonality of both the reactive N- and C-termini, and all side-chain functional group protection must be considered when designing the tripeptide fragment, which highlights why SPPS is so desirable for peptide synthesis).²⁴ Dipeptide fragments are often simpler to assemble because of fewer protecting group variables, however in the context of cis-substituted heterocycles, such designs typically lead to diketopiperazine side products that compromise yields.³¹

Here we report a method to form (4H)-imidazolone heterocycles within the peptide backbone. Imidazolones have never before been incorporated into peptides and as such have unexplored effects on the conformational land-scape of peptides, as well as unknown biological properties. Indeed, imidazolones have recently been shown to be a non-aromatic bioisostere of the amide bond with favorable pharmacological properties, creating a compelling need for further exploration (Figure 1B).³² Additionally, (4H)-imidazolones are high-value heterocycles appreciated for their anti-hypertensive,³³ anti-cancer,³⁴ anti-psychotic,³⁵ anti-viral,³⁶ and cytotoxic³⁷ effects; they are also known to the agrochemical field as potent broad spectrum herbicides³⁸ (Figure 2, 1-6). Thus, new methods to form imidazolones under gentle conditions will enable access to amino-acid derived imidazolone natural products that have yet to be synthesized.³⁹

Figure 2.

Selected examples of (4H)-imidazolones present in pharmaceuticals,^33–36 herbicides,³⁸ and natural products^37,39

The new method reported here also provides the first access to highly-functionalized and peptide-based (4H)-imidazolones with α-C chiral groups. Further, these imidazolones are substituted to give an all-cis-amide conformation. Unlike other cis-amide locked heterocycles, we can access imidazolone precursors in 2–3 steps from commercially-available building blocks, and install them on-resin at the N- and C-termini of the peptide, the middle of the peptide, and on the side-chain (enabling branched structures) (Figure 1C). Finally, we show that our imidazolone performs better than other cis-amide surrogates in the pre-organization of a head-to-tail macrocyclization of natural product Mahafacyclin B. Notably, we do not observe any diketopiperazine side-products which have been observed quantitatively from the installation of other cis-amide bond surrogates.³¹

Previously, our group utilized thioimidates to install amidines site-selectively within the backbone of peptides.⁴⁰ We sought to further exploit the reactivity of the thioimidates for the construction of other amide bond bioisosteres. Yamada and co-workers had previously shown that if an ester was present as the C-terminal chemistry of the n+1 residue, then installation of the amidine would engender down-stream 5-exo-trig cyclization to (4H)-imidazolone. Additionally, Houghten and co-workers had shown that under harsh HF acidiolytic resin cleavage, guanidines could cyclize onto the adjacent n+1 amide residue. We therefore hypothesized that reaction of an elongating peptide on solid-support with a thioimidate precursor could form a transient amidine species capable of producing an imidazolone cyclization product on solid support.^41,42

We sought to develop optimal conditions to form imidazolone products in hopes that we could install imidazolone moieties during the course of SPPS. The Cbz-protected thioimidate dipeptide (7) was selected as a model substrate. The α, α-substitution of 2-aminoisobutyric acid (Aib) was chosen to mimic the majority of natural products and industrially-relevant imidazolones (Figure 2) which contain α, α-substitution. Mono-substition leads to rapid racemization of the stereocenter at this position due to the thermodynamically-stable hydroxyimidazole tautomerization.³⁹

Initial testing with our previous conditions for the formation of amidines from thioimidates yielded moderate yields (Table 1, Entry 1). Unfortunately 10 equivalents of the amine nucleophile complicates purification and restricts the scale of this chemistry. Therefore in an effort to reduce the equivalents of amine required for this transformation, we employed Design of Experiments (DoE) methodology to locate optimal reaction conditions. DoE (compared to standard one-variable at a time optimization) enables us to understand not only the effects of our variables (amine and acid stoichiometries) on yield, but also how these variables affect each other.⁴³ From this DoE optimization we observed two major trends: (1) amidine formation is driven primarily by the amount of amine nucleophile, which can be enhanced by concentration of the solution or stoichiometry, and (2) acid equivalency has a minor affect on yield (Table 1, Entries 5–7), but is required for the transformation (See Supplementary Information).

Table 1.

Optimized Conditions for Imidazolone Cyclization

entry	equiv. amine	equiv. AcOH	solvent (v/v)	temp. (°C)	time (h)	yielda (%)
1	10	1	DMF:TFE	23	24	73
2	5	2	DMF:TFE	23	24	93
3	5	0.5	DMF:TFE	23	24	50
4	3	1.25	DMF:TFE	23	24	64
5	1	2	DMF:TFE	23	24	31
6	1	0.5	DMF:TFE	23	24	25
7	1	5	DMF:TFE	23	24	27
8	3	1.25	DMF	23	24	7
9	3	1.25	DCM:TFE	23	24	71
10	3	1.25	THF:TFE	23	24	98
11	3	1.25	MeCN:TFE	23	24	>99
12	1	0.5	MeCN:TFE	23	24	60
13	1	0.5	MeCN:TFE	70	8	90

^aYield determined by crude NMR analysis with dimethyl terepthalate internal standard.

Rows 2–6 (gray) were part of the full-factorial Design of Experiments (DOE) methodology (See Supplementary Information for more details).

We found that binary solvent mixtures containing 2,2,2-trifluroethanol (TFE) accelerate amidine installation which we attribute to solvent-enhanced hydrogen-bonding properties that may facilitate proton-transfer steps (Table 1, Entry 8).⁴⁰ Solvent mixtures of THF:TFE and MeCN:TFE (1:1 %v/v) were found to be the most optimal for this transformation, however DCM:TFE and DMF:TFE furnished the imidazolone product in moderate yield (Table 1, Entries 7 & 9–11). Heating the reaction (Table 1, Entry 13) enabled us to form imidazolone products with stoichiometric amine quantities which is critical for the application of this chemistry in the context of SPPS where reaction rates are greatly reduced.

With the optimized conditions from Table 1, we explored the types of amine nucleophiles that were compatible with this reaction (Figure 3). We found that ammonium acetate (10a) and sterically-unencumbered primary amines (10b-10c) formed imidazolone products in very-high yields. Ani-lines (10d-10f) produced the corresponding imidazolones in moderate yield, attributed to the decreased nucleophilicity of these amines due to delocalization of the nucleophilic lone-pair into the aromatic π-system. Imidazolone formation also tolerated an array of pharmaceutically-valuable heterocyclic functionalities including an α-nucleophile (10g), pyridines (10h and 10j), thiophene (10i) and indole (10k).

Figure 3.

Scope of amine nucleophiles tolerated in imidazolone formation. Isolated yields shown. Concentration of all reactions was 0.25 M in 7. ^aNo AcOH added. ^b40 °C. ^cAmmonium salt neutralized with TEA (2 eq.) additive. ^dDMF:TFE (v/v) as solvent. ^e3 eq. of amine used. Ammonium salt neutralized with TEA (3 eq.) additive.

Pharmaceutical and bioderived amines (10j-10l) with multiple functional groups were also well-tolerated. Diminished yield for primaquine derivative (10j) was attributed to the poor solublity observed for primaquine bisphosphate in organic solvents. Strong acid ammonium salts (i.e. HCl, H₃PO₄) were neutralized in-situ via addition of stoichiometric amount of triethylamine (TEA). We found that the strong acid salts alone were not able to catalyze the formation of imidazolones without addition of a weak acid. Amino acid nucleophiles were compatible with this chemistry and the reaction conditions did not racemize the stereogenic center of 10l despite the use of heat and acid (see Supplementary Information). We found that more sterically-hindered amines or electron-deficient amines produced only trace imidazolone products which were not isolated (See Supplementary Information).

Originally reported as a trapping product, the benzylidene imidazolone adduct is recognized as the primary fluorophore in GFP.^41,44 We therefore envisioned that in-situ imidazolone cylization of 11, a glycine methyl ester thioimidate derivative, would enable us to form the benzylidene structure through an acid-catalyzed aldol reaction. A one-pot tandem cyclization-condensation with benzaldehyde afforded the benzylidene product 12 in high yield with excellent selectivity (Scheme 1).

Scheme 1.

One-pot imidazolone cyclization-aldol condensation to access benzylidene imidazolones which mimic GFP hexapeptide.

With an understanding of the scope of amine nucleophiles, we sought to explore the tolerance of our reaction on peptidic substrates. Notably, methods to generate chiral-enriched variants of imidazolones are largely underdeveloped.³⁹ While our initial attempt to synthesize unsubstituted imidazolone 8a (Figure 4) provided good yield, a subsequent crystal structure displayed a non-centrosymmetric space group (see Supplementary Information), indicative of a mixture of both R- and S- imidazolones in the crystal (Figure 4). The formation of thioimidates and amidines on peptidic substrates has been well-studied and the conditions employed do not racemize the α-position of peptides.^40,45 Previously, Drabina and co-workers had reported trifluoroacetic acid (TFA) was capable of racemizing Boc-L-proline-imidazolone during the course of Boc-deprotection.⁴⁶ Thus, we hypothesized that the reaction conditions led to the racemization of the α-stereochemistry after formation of the imidazolone, presumably through protonation of the imidazolone nitrogen, leading to an increase in acidity of the α-CH. We therefore sought conditions to minimize racemization of the final product.

Figure 4.

Stereoretentive imidazolone formation with primary amine nucleophiles. Isolated yields shown. Non-stereogenic H-atoms of 8a were ommitted for clarity.

To identify if the pK_a of the acid used for the transformation impacted the racemization, we explored pyridinium p-toluenesulfonate (PPTS, pK_a = 5.2) and hexafluoroiso-propanol (HFIP, pK_a = 8.3) compared to acetic acid (AcOH, pK_a = 4.8). Intuitively, weaker acids such as PPTS and HFIP should protonate the imidazolone to a lesser extent and reduce racemization. Unfortunately, protonation of the thioimidate is also necessary for initial formation of the amidine ahead of cyclization (Figure 1C). Accordingly, weak acids like HFIP did not furnish imidazolone in sufficient yield (Table 2, entry 2–4). We then attempted the reaction at lower temperatures, resulting in an enantiomeric ratio (er) of 90:10 using AcOH, however attempts to further improve the er using AcOH were unsuccessful (Table 2, entry 6 & 7). Thus, a combination of a weaker acid, PPTS, and room temperature reaction conditions provided the best compromise between isolated yield and enantiomeric ratio (Table 2, Entry 8).

Table 2.

Stereoretention of the α-position of imidazolones.

entry	eq. amine	acid	eq. acid	temp. (°C)	isolated yield (%)	er
1	1	AcOH	1	70	75	50:50
2	3	AcOH	1	50	89	77:23
3	3	PPTS	1	50	83	83:17
4	3	HFIP	1	50	34	87.5:12.5
5	3	AcOH	1	23	71	91:9
6a	3	AcOH	1	23	79	85.5:14.5
7	3	AcOH	0.5	23	40	94.5:5.5
8	3	PPTS	1	23	58	97:3
9	5	PPTS	1	23	71	73:27
10	10	PPTS	1	23	78	77:23

^aReaction concentration 0.25 M.

With these optimized conditions, we prepared the first examples of enantiopure α-chiral imidazolones (8b–8d). We found that nucleophilic alkyl amines performed best for this chemistry to yield imidazolones in an overall >92:8 er. Our attempts to use aniline derivatives or hindered amines did not yield imidazolone products due to their poor room temperature reactivity (see Supplementary Information). Interestingly, despite the development of stereoretentive conditions, attempts to generate enantiopure variants of unsubstituted imidazolone 8a were not successful.

With an efficient method to form imidazolones established, we sought to incorporate these heterocycles in to peptides through conventional solid-phase peptide synthesis (SPPS) procedures.

We selected Apidaecin Ib (1–7, H-GNNRPVY-NH₂) as our model peptide to test imidazolone cyclization using the free −NH₂ group on the resin as our amine nucleophile. Apidaecin Ib and synthetic derivatives are currently being evaluated for their potential to treat multi-drug resistant gramnegative pathogens.^47,48 Recently, Moore and co-workers showed that N-terminal guanidinylation of Apidaecin Ib peptide enhanced the anti-microbial activity and proteolytic stability.⁴⁹ We hypothesized that imidazolones, a basic heterocyclic motif, might impart similar activity to the guanidinylated form by introducing a site for protonation. Positive charge has also been demonstrated as a useful tool to enhance the accumulation and uptake of anti-bacterial peptides and small molecules.^50–52 However, on-resin installation of imidazolones required new considerations for our reaction conditions. THF was chosen as a co-solvent instead of MeCN because it swells polystyrene-based SPPS resins similarly to DMF and DCM, but provided the highest yields in our solution-phase solvent screening (Entries 4 & 9–11, Table 1).⁵³ Since the amine nucleophile is immobilized on resin, we utilize an excess of thioimidate, Fmoc-Gly^SMe-Aib-OMe (13a, Scheme 2) for the reaction. We found that the typical concentration employed for amide coupling on resin (0.1 M) was sufficient to effect complete installation of the imidazolones when treated with 0.1 M 13a and 0.1 M AcOH under gentle heating conditions at 55 °C (Scheme 2). Final resin cleavage and purification afforded 14, the N-terminal imidazolone analogue of Apidaecin Ib (1–7) (9% yield over all synthetic steps and HPLC purification).

Scheme 2.

On-resin N-terminal imidazolone formation on Apidaecin Ib (1–7).

To form C-terminal imidazolones we hypothesized that reaction of thioimidate with the amine of an amide resin itself (e.g. Rink amide) could yield N-unsubstituted imidazolones, enabling facile access to C-terminal imidazolone analogues in which the imidazolone is the direct link to the solid support (Scheme 3). We selected C-terminal imidazolone 15 which mimics the molecular shape of the proline found at the C-terminus of the thyrotropin-releasing hormone (TRH). The clinically-used synthetic analogue of TRH, taltirelin, contains an unnatural dihydropyrimidine heterocycle.⁵⁴ We envisioned that the imidazolone synthesis platform, which enables rapid diversification of the imidazolone core through SPPS, could be used in the drug discovery of new TRH analogues. After iterative cycles of SPPS coupling and Fmoc-deprotection the imidazolone remained intact and no discernible by-products related to scaf-fold degradation were isolated upon HPLC purification of peptide 15 (54% yield over all synthetic steps and purification).

Scheme 3.

Imidazolone cyclization on Rink amide resin linker and subsequent elognation toward C-terminal imidazolone TRH analogue (15).

(4H)-Imidazolones are also a well-known class of biological metabolites known as advanced glycation end products (AGEs), which are non-enzymatic metabolites formed by the reaction of protein side-chains and sugars.⁵⁵ AGEs are a product of normal metabolism, however high levels of AGEs have been linked to oxidative stress and inflammation; which are indicative of metabolic disorders, namely diabetes.^56,57 AGEs react with the side-chains of cell surface receptor proteins, and alter their structure and function.⁵⁸ For this reason we chose the peptide H-IKVAV-NH₂ which is a functional component of the laminin α1 chain, an extracellular matrix (ECM) protein responsible for cell adhesion and appreciated for its use as a hydrogel.^59,60 By selectively deprotecting the ϵ-NH₂ of the Lys residue (using a commercially-available 4-methoxytrityl protecting group), we could install a side-chain imidazolone modification to form 16a which mimics AGE formation common to Arg and Lys side chains (Scheme 4).⁶¹ Branched peptides have been appreciated by the medicinal chemistry community for their enhanced proteolytic stability.^62,63 Accordingly, subsequent elongation from the side-chain imidazolone afforded a branched peptide structure 16b (14% yield over all synthetic steps and purification), another demonstration of the stability of the imidazolone core to SPPS conditions.

Scheme 4.

Imidazolone formation on Lys side-chain to yield AGE-related modification (16a) and subsequent elongation to the Lys branched peptide (16b).

With an effective method to install imidazolones directly on to solid-support, we sought to evaluate whether the stereochemistry of α-chiral imidazolones could withstand the basic (Fmoc-deprotection) and acidic (global deprotection and cleavage) conditions required for SPPS. We subjected a sample of 8c to 75% TFA in DCM (v/v) to mimic the conditions required for resin cleavage and global deprotection. To our delight, short exposure times of 1 hour showed <1% epimerization of the α-position (see Supplementary Information).

To evaluate the stability of the stereochemistry during the longer exposure times needed for on-resin installation of the imidazolone, we synthesized authentic L- and D-thioimidate precursors (13b) and reacted them with loaded resin (H-AK-NH₂) to quantify the degree of epimerization of the α-C stereochemistry. We found that our standard conditions of 0.1 M thioimidate and AcOH with heating led to 25% racemization of the stereochemistry. By increasing the concentration of the thioimidate dimer in solution to 0.2 M (with 0.1 M AcOH) we were able to effect installation at room temperature without the need for heating. Room temperature conditions led to <10% epimerization of the stereochemistry according to HPLC-MS which we validated by co-injection of the L- and D-Phe variants (Scheme 5). Subsequent deprotection, elongation and cleavage led to α-chiral imidazolone (17) (7% over all synthetic steps and HPLC purification).

Scheme 5.

Modified installation conditions enable insertion of α-chiral imidazolones with minimal epimerization.

Finally, with effective conditions to install imidazolones on-resin, we sought to explore their utility as cis-amide bond surrogates. Head-to-tail macrocylization of peptides is assisted by the introduction of at least one cis-amide bond which helps to pre-organize the ends of cyclic peptides.^6,64 Mahafacyclin B (cyclo-TFFGFFG), a cyclic peptide natural product appreciated for its anti-malarial properties, has been used as a benchmark structure to test head-to-tail cyclization strategies.⁶⁵ Native cyclization of the linear Mahafacyclin B yields only 30% of the cyclic peptide and requires extremely long reaction times of up to 3 days.^23,65 Jolliffe and coworkers, however, found that pseudoproline, a cis-amide surrogate derived from condensation with acetone at the Thr site, could pre-organize the peptide for cyclization. Their strategy yielded 50% of the target cyclic peptide in less than 3 hours, underscoring the importance of cis-amide linkages in assisting the cyclization of Mahafacyclin B.²³ Robinson and coworkers used a non-canonical amino acid with alkene side chains and a ring-closing metathesis catalyst to form a tethered linkage to pre-organize the peptide.⁶⁶ Their tethered structure yielded 60% of the cyclic peptide in 4 hours which could be later deprotected using ring-opening metathesis to yield a Mahafacyclin B analogue.⁶⁶ We opted to use Mahafacyclin B as a benchmark cyclization for our cis-amide inducing imidazolone.

To a Gly-loaded Wang resin we synthesized and installed our imidazolone at the central glycine residue. Cleavage from Wang resin afforded the linear peptide precursor (H-TFFG^ImiFFG-OH, 18a, 12% yield over all synthetic steps and HPLC purification) which was allowed to react under the same macrocyclization conditions employed by Jollife and Robinson using a pentafluorophenyl diphenylphosphinate (FDPP) coupling reagent (Scheme 6).^23,66 Gratifyingly, the isolated yield of peptide 18b (71% yield over cyclization and HPLC purification) was higher than either previously reported macrocyclization of the Mahafacyclin B sequence. Additionally, since imidazolones have recently been employed as bioisosteric replacements for the amide bond, the geometric constraints imparted by imidazolones on bioactive linear peptides may be a useful strategy to retain bioactivity in cyclic form.³²

Scheme 6.

Imidazolones as cis-amide surrogates to pre-organize head-to-tail macrocyclization of Mahafacyclin B analogue with bioisosteric replacement.

(4H)-imidazolones are important heterocycles found in natural products,^37,39 pharmaceuticals,^33–36 agrochemicals,³⁸ fluorescent probes,^44,67 and biological metabolites.⁶¹ Our method enables access to novel imidazolone scaf-folds which can be synthesized rapidly from commercially-available starting materials. We have shown that imidazolone cyclization is tolerated by a wide-scope of primary amine nucleophiles, with excellent functional group tolerance. Previous methods to form imidazolones required high temperatures and strong acid or base to form.³⁹ The mild conditions employed enable us to access imidazolone products with α-stereochemistry—derived from amino acid starting materials—the first examples of stereochemical retention at this postion.

Additionally, we show that imidazolones can be easily incorporated onto the N-terminus or side-chain of an elongating peptide chain during SPPS by reaction with a thioimidate dipeptide. C-terminal installation can be achieved by direct reaction with amide resins, which cleave to the free N–H imidazolone. Imidazolones also tolerated peptide elongation after installation, enabling them to be installed in to the center of peptides and on to side-chains. Side-chain imidazolones provided access to branched peptides and AGE-related products with potential for the study of receptors for AGEs (RAGE).⁵⁸ As a newly discovered bioisostere of the amide bond,³² our general method for insertion and functionalization of imidazolone motifs will assist in the discovery of other small-molecule and peptide-based therapeutics.

Finally, the geometric constraints imparted by the imidazolones heterocycle leads to cis-amide surrogates which are especially useful in pre-organizing linear peptides for head-to-tail macrocyclization.^6,64 We show that the imidazolone surrogate performs better than existing methods for the head-to-tail cyclization of the Mahafacyclin B sequence.^23,65,66 We anticipate that this work will assist in the synthesis of currently inaccessible imidazolone natural products,³⁹ assist in the design of cyclic peptides and peptide-based therapeutics, and provide a new conformational tool for peptide chemists to interrogate the effect of a cis-amide conformation on peptide interactions.

Data Availability Statement

The data underlying this study are available in the published article and its Supporting Information.