Complex peptide macrocycle optimization: combining NMR restraints with conformational analysis to guide structure-based and ligand-based design.

Systematic optimization of large macrocyclic peptide ligands is a serious challenge. Here, we describe an approach for lead-optimization using the PD-1/PD-L1 system as a retrospective example of moving from initial lead compound to clinical candidate. We show how conformational restraints can be derived by exploiting NMR data to identify low-energy solution ensembles of a lead compound. Such restraints can be used to focus conformational search for analogs in order to accurately predict bound ligand poses through molecular docking and thereby estimate ligand strain and protein-ligand intermolecular binding energy. We also describe an analogous ligand-based approach that employs molecular similarity optimization to predict bound poses. Both approaches are shown to be effective for prioritizing lead-compound analogs. Surprisingly, relatively small ligand modifications, which may have minimal effects on predicted bound pose or intermolecular interactions, often lead to large changes in estimated strain that have dominating effects on overall binding energy estimates. Effective macrocyclic conformational search is crucial, whether in the context of NMR-based restraints, X-ray ligand refinement, partial torsional restraint for docking/ligand-similarity calculations or agnostic search for nominal global minima. Lead optimization for peptidic macrocycles can be made more productive using a multi-disciplinary approach that combines biophysical data with practical and efficient computational methods.

Isothiourea-Catalyzed Atropselective Acylation of Biaryl Phenols via Sequential Desymmetrization/Kinetic Resolution.

Axially chiral phenols are attractive targets in organic synthesis. This motif is central to many natural products and widely used as precursors to, or directly, as chiral ligands and catalysts. Despite their utility few simple catalytic methods are available for their synthesis in high enantiopurity. Herein the atropselective acylation of a range of symmetric biaryl diols is investigated using isothiourea catalysis. Studies on a model biaryl diol substrate shows that the high product er observed in the process is a result of two successive enantioselective reactions consisting of an initial enantioselective desymmetrization coupled with a second chiroablative kinetic resolution. Extension of this process to a range of substrates, including a challenging tetraorthosubstituted biaryl diol, led to highly enantioenriched products (14 examples, up to 98:2 er), with either HyperBTM or BTM identified as the optimal catalyst depending upon the substitution pattern within the substrate. Computation has been used to understand the factors that lead to high enantiocontrol in this process, with maintenance of planarity to maximize a 1,5-S⋅⋅⋅O interaction within the key acyl ammonium intermediate identified as the major feature that determines atropselective acylation and thus product enantioselectivity.

Second-Generation Synthesis of the Northern Fragment of Mandelalide A: Role of π-Stacking on Sharpless Dihydroxylation of cis-Enynes.

The development of a π-stacking-based approach for increased stereoselectivity in Sharpless asymmetric and diastereomeric dihydroxylation of cis-enynes is disclosed. The use of neighboring, electron-rich benzoate esters proved key to the success of this process. Density functional theory study suggests that the substrate benzoate ester group rigidifies the dihydroxylation transition states by forming a favorable π-stacking interaction in both Major-TS and Minor-TS. The energetic preference for the Major-TS was found in part because of the favorable eclipsing conformation of the alkene substituent as opposed to the disfavored bisecting conformation found in the Minor-TS. The application to a second-generation synthesis of the C15-C24 northern portion of mandelalide A is demonstrated.

Carbene-Catalyzed Enantioselective Decarboxylative Annulations to Access Dihydrobenzoxazinones and Quinolones.

A direct decarboxylative strategy for the generation of aza-o-quinone methides (aza-o-QMs) by N-heterocyclic carbene (NHC) catalysis has been discovered and explored. This process requires no stoichiometric additives in contrast with current approaches. Aza-o-QMs react with trifluoromethyl ketones through a formal [4+2] manifold to access highly enantioenriched dihydrobenzoxazin-4-one products, which can be converted to dihydroquinolones through an interesting stereoretentive aza-Petasis-Ferrier rearrangement sequence. Complementary dispersion-corrected density functional theory (DFT) studies provided an accurate prediction of the reaction enantioselectivity and lend further insight to the origins of stereocontrol. Additionally, a computed potential energy surface around the major transition structure suggests a concerted asynchronous mechanism for the formal annulation.

Enantioselective Synthesis of α-Amidoboronates Catalyzed by Planar-Chiral NHC-Cu(I) Complexes.

The first highly selective catalytic hydroboration of alkyl-substituted aldimines to provide medicinally relevant α-amidoboronates is disclosed. The Cu(I)-catalyzed borylation proceeds with excellent facial selectivity when a set of planar-chiral N-heterocyclic carbenes (NHCs) were employed as ligands. Density functional theory computations suggest that interactions between BPin and the planar-chiral catalyst are responsible for the observed stereoselectivity. Important pharmacophores, such as the boronate analogue of isoleucine, can be prepared using a chromatography-free protocol starting from commercially available reagents. The application of these NHC ligands in these Cu(I)-catalyzed processes offers a significant contribution to existing strategies for laboratory-scale preparation of enantioenriched α-amidoboronates.

Computational and Experimental Evidence of Emergent Equilibrium Isotope Effects in Anion Receptor Complexes.

The measurement of a deuterium equilibrium isotope effect (EIE) for the aryl CH···Cl- interaction of anion receptor 1H/1D is reported. Computations corroborate the results of solution measurements for a small, normal EIE in the full complex (KaH/KaD = 1.019 ± 0.010). Interestingly, isotope effects involving fragments of the anion receptor (urea, aryl ring, etc.) are predicted to produce an inverse effect. This points to an unusual and subtle structural organization effect of the anion receptor complex that changes the nature of the combined interactions to a normal isotope effect. The reversal of EIE values suggests that overall architecture of the anion receptor can dramatically impact the nature of bonding in these complexes.

Catalytic Enantioselective [2,3]-Rearrangements of Allylic Ammonium Ylides: A Mechanistic and Computational Study.

A mechanistic study of the isothiourea-catalyzed enantioselective [2,3]-rearrangement of allylic ammonium ylides is described. Reaction kinetic analyses using 19F NMR and density functional theory computations have elucidated a reaction profile and allowed identification of the catalyst resting state and turnover-rate limiting step. A catalytically relevant catalyst-substrate adduct has been observed, and its constitution elucidated unambiguously by 13C and 15N isotopic labeling. Isotopic entrainment has shown the observed catalyst-substrate adduct to be a genuine intermediate on the productive cycle toward catalysis. The influence of HOBt as an additive upon the reaction, catalyst resting state, and turnover-rate limiting step has been examined. Crossover experiments have probed the reversibility of each of the proposed steps of the catalytic cycle. Computations were also used to elucidate the origins of stereocontrol, with a 1,5-S···O interaction and the catalyst stereodirecting group providing transition structure rigidification and enantioselectivity, while preference for cation-π interactions over C-H···π is responsible for diastereoselectivity.

Visible-light-mediated, nitrogen-centered radical amination of tertiary alkyl halides under metal-free conditions to form α-tertiary amines.

A mild and operationally convenient amino-functionalization of a range of tertiary alkyl halides by reaction with iminoiodinanes (PhI[double bond, length as m-dash]NNs) and I2 has been developed. According to the mechanistic experiments described within, the reaction is speculated to proceed through a light-promoted, N-centered radical pathway involving a N,N-diiodosulfonamide reactive species. This method of direct N-incorporation offers an attractive alternative to the production of α-tertiary amines, a synthetically challenging structural class found in a variety of bioactive molecules.

A Distributional Model of Bound Ligand Conformational Strain: From Small Molecules up to Large Peptidic Macrocycles.

The internal conformational strain incurred by ligands upon binding a target site has a critical impact on binding affinity, and expectations about the magnitude of ligand strain guide conformational search protocols. Estimates for bound ligand strain begin with modeled ligand atomic coordinates from X-ray co-crystal structures. By deriving low-energy conformational ensembles to fit X-ray diffraction data, calculated strain energies are substantially reduced compared with prior approaches. We show that the distribution of expected global strain energy values is dependent on molecular size in a superlinear manner. The distribution of strain energy follows a rectified normal distribution whose mean and variance are related to conformational complexity. The modeled strain distribution closely matches calculated strain values from experimental data comprising over 3000 protein-ligand complexes. The distributional model has direct implications for conformational search protocols as well as for directions in molecular design.

Discovery and Structure-Based Design of Macrocyclic Peptides Targeting STUB1.

Recent evidence suggests that deletion of STUB1─a pivotal negative regulator of interferon-γ sensing─may potentially clear malignant cells. However, current studies rely primarily on genetic approaches, as pharmacological inhibitors of STUB1 are lacking. Identifying a tool compound will be a step toward validating the target in a broader therapeutic sense. Herein, screening more than a billion macrocyclic peptides resulted in STUB1 binders, which were further optimized by a structure-enabled in silico design. The strategy to replace the macrocyclic peptides' hydrophilic and solvent-exposed region with a hydrophobic scaffold improved cellular permeability while maintaining the binding conformation. Further substitution of the permeability-limiting terminal aspartic acid with a tetrazole bioisostere retained the binding to a certain extent while improving permeability, suggesting a path forward. Although not optimal for cellular study, the current lead provides a valuable template for further development into selective tool compounds for STUB1 to enable target validation.

Conformational Strain of Macrocyclic Peptides in Ligand-Receptor Complexes Based on Advanced Refinement of Bound-State Conformers.

Macrocyclic peptides are an important modality in drug discovery, but molecular design is limited due to the complexity of their conformational landscape. To better understand conformational propensities, global strain energies were estimated for 156 protein-macrocyclic peptide cocrystal structures. Unexpectedly large strain energies were observed when the bound-state conformations were modeled with positional restraints. Instead, low-energy conformer ensembles were generated using xGen that fit experimental X-ray electron density maps and gave reasonable strain energy estimates. The ensembles featured significant conformational adjustments while still fitting the electron density as well or better than the original coordinates. Strain estimates suggest the interaction energy in protein-ligand complexes can offset a greater amount of strain for macrocyclic peptides than for small molecules and non-peptidic macrocycles. Across all molecular classes, the approximate upper bound on global strain energies had the same relationship with molecular size, and bound-state ensembles from xGen yielded favorable binding energy estimates.

XGen: Real-Space Fitting of Complex Ligand Conformational Ensembles to X-ray Electron Density Maps.

We report a new method for X-ray density ligand fitting and refinement that is suitable for a wide variety of small-molecule ligands, including macrocycles. The approach (called "xGen") augments a force field energy calculation with an electron density fitting restraint that yields an energy reward during the restrained conformational search. The resulting conformer pools balance goodness-of-fit with ligand strain. Real-space refinement from pre-existing ligand coordinates of 150 macrocycles resulted in occupancy-weighted conformational ensembles that exhibited low strain energy. The xGen ensembles improved upon electron density fit compared with the PDB reference coordinates without making use of atom-specific B-factors. Similarly, on nonmacrocycles, de novo fitting produced occupancy-weighted ensembles of many conformers that were generally better-quality density fits than the deposited primary/alternate conformational pairs. The results suggest ubiquitous low-energy ligand conformational ensembles in X-ray diffraction data and provide an alternative to using B-factors as model parameters.

Mechanism and origins of selectivity in the enantioselective oxa-Pictet-Spengler reaction: a cooperative catalytic complex from a hydrogen bond donor and chiral phosphoric acid.

Enantioselective additions to oxocarbenium ions are high-value synthetic transformations but have proven challenging to achieve. In particular, the oxa-Pictet-Spengler reaction has only recently been rendered enantioselective. We report experimental and computational studies on the mechanism of this unusual transformation. Herein we reveal that this reaction is hypothesized to proceed through a self-assembled ternary hydrogen bonding complex involving the substrate, chiral phosphate ion, and a urea hydrogen-bond donor. The computed transition state reveals C2-symmetric grooves in the chiral phosphate that are occupied by the urea and substrate. Occupation of one of these grooves by the urea co-catalyst tunes the available reactive volume and enhances the stereoselectivity of the chiral phosphate catalyst.

Connecting remote C-H bond functionalization and decarboxylative coupling using simple amines.

Transition metal-catalysed C-H functionalization and decarboxylative coupling are two of the most notable synthetic strategies developed in the past 30 years. Here, we connect these two reaction pathways using bases and a simple Pd-based catalyst system to promote a para-selective C-H functionalization reaction from benzylic electrophiles. Experimental and computational mechanistic studies suggest a pathway that involves an uncommon Pd-catalysed dearomatization of the benzyl moiety followed by a base-enabled rearomatization through a formal 1,5-hydrogen migration. This reaction complements 'C-H activation' strategies that convert inert C-H bonds into C-metal bonds prior to C-C bond formation. Instead, this reaction exploits an inverted sequence and promotes C-C bond formation prior to deprotonation. These studies provide an opportunity to develop general para-selective C-H functionalization reactions from benzylic electrophiles and show how new reactive modalities may be accessed with careful control of the reaction conditions.

Catalytic enantioselective synthesis of perfluoroalkyl-substituted ß-lactones via a concerted asynchronous [2 + 2] cycloaddition: a synthetic and computational study.

The enantioselective preparation of a range of perfluoroalkyl-substituted ß-lactones through an isothiourea (HyperBTM) catalysed reaction using symmetric anhydrides as ammonium enolate precursors and perfluoroalkylketones (RF = CF3, C2F5, C4F9) is reported. Following optimisation, high diastereo- and enantioselectivity was observed for ß-lactone formation using C2F5- and C4F9-substituted ketones at room temperature (26 examples, up to >95 : 5 dr and >99 : 1 er), whilst -78 °C was necessary for optimal dr and er with CF3-substituted ketones (11 examples, up to >95 : 5 dr and >99 : 1 er). Derivatisation of the ß-lactones through ring-opening, as well as a two-step conversion to give perfluoroalkyl-substituted oxetanes, is demonstrated without loss of stereochemical integrity. Density functional theory computations, alongside 13C natural abundance KIE studies, have been used to probe the reaction mechanism with a concerted asynchronous [2 + 2]-cycloaddition pathway favoured over a stepwise aldol-lactonisation process.

Conformationally flexible arylethynyl bis-urea receptors bind disparate oxoanions with similar, high affinities.

Conformationally flexible hosts with relatively small binding pockets are seldom shown to bind oxoanions preferentially over other guests. Herein, we disclose the binding of diprotic, monoprotic, and aprotic tetrahedral oxoanions with three different pyridylethynyl bis-urea scaffolds. In less polar solvent, the trend in association constants appears to be heavily influenced by solvation and entropic effects. However, in a more polar solvent, the trend in association constants matches that of the pKa of the conjugate acid of the anionic guest, as expected for H-bond donating hosts.