Site-selective acylation of natural products with BINOL-derived phosphoric acids

Utopia Point Bayesian Optimization Finds Condition-Dependent Selectivity for N-Methyl Pyrazole Condensation

Utopia Point Bayesian Optimization (UPBO) was used to identify reaction conditions that are highly selective for the formation of N1 and N2-methyl-3-aryl pyrazole constitutional isomers. UPBO was used to explore a wide chemical space and identify basic reaction conditions for a typically acid-catalyzed Knorr pyrazole condensation. These studies revealed that selectivity in the reaction stems from a condition-dependent equilibrium of intermediates prior to dehydration. For the N2-methyl isomer reaction pathway, a hemiaminal intermediate was found to form reversibly under the reaction conditions, enabling a highly selective synthesis of the N2 isomer upon dehydrative workup. UPBO was able to successfully optimize conversion and selectivity simultaneously with search spaces of >1 million potential variable combinations without the need for high-performance computational resources.

Kinetic resolution of 1,1'-binaphthyl-2,2'-diamine derivatives by chiral calcium phosphate-catalyzed acylation

1,1'-Binaphthyl-2,2'-diamine (BINAM) is a useful axially chiral compound. The kinetic resolution of BINAM is one of the most crucial methods for synthesizing chiral BINAM. We have developed a chiral calcium phosphate-catalyzed kinetic resolution of BINAM by utilizing an acylation reaction to produce a mono-amide. The kinetic resolution of BINAM derivatives was achieved by using isobutyric anhydride in the presence of chiral calcium phosphate and 4-morpholinopyridine with up to s = 127. 6,6'-Substituted BINAM derivatives were also applicable for this reaction. The resulting mono-acylated BINAM could be transformed into BINAM by hydrolysis under acidic conditions.

Double asymmetric synthesis: faster reactions are more selective and a model to estimate relative rate

The catalysed reaction of an enantiopure substrate with formation of a new chirality element may result in higher diastereoselectivity with one enantiomer of a catalyst (matched pair) than with the other (mismatched pair). The hypothesis that the matched reaction is faster was investigated using literature examples of kinetic resolution procedures that result in the formation of a new stereogenic centre. With one exception from fifteen examples, the selectivity factor (s = kfast/kslow) = kmatched/kmismatched. A model to estimate the relative rate of a fast-matched reaction vs. the corresponding slow-mismatched reaction is proposed. This model also provides insight into the basis of the selectivity displayed in the kinetic resolution procedures studied.

Mechanism to model: a physical organic chemistry approach to reaction prediction

The application of mechanistic generalizations is at the core of chemical reaction development and application. These strategies are rooted in physical organic chemistry where mechanistic understandings can be derived from one reaction and applied to explain another. Over time these techniques have evolved from rationalizing observed outcomes to leading experimental design through reaction prediction. In parallel, significant progression in asymmetric organocatalysis has expanded the reach of chiral transfer to new reactions with increased efficiency. However, the complex and diverse catalyst structures applied in this arena have rendered the generalization of asymmetric catalytic processes to be exceptionally challenging. Recognizing this, a portion of our research has been focused on understanding the transferability of chemical observations between similar reactions and exploiting this phenomenon as a platform for prediction. Through these experiences, we have relied on a working knowledge of reaction mechanism to guide the development and application of our models which have been advanced from simple qualitative rules to large statistical models for quantitative predictions. In this feature article, we describe the models acquired to generalize organocatalytic reaction mechanisms and demonstrate their use as a powerful approach for accelerating enantioselective synthesis.

Data Science Enables the Development of a New Class of Chiral Phosphoric Acid Catalysts

The widespread success of BINOL-chiral phosphoric acids (CPAs) has led to the development of several high molecular weight, sterically encumbered variants. Herein, we disclose an alternative, minimalistic chiral phosphoric acid backbone incorporating only a single instance of point chirality. Data science techniques were used to select a diverse training set of catalysts, which were benchmarked against the transfer hydrogenation of an 8-aminoquinoline. Using a univariate classification algorithm and multivariate linear regression, key catalyst features necessary for high levels of selectivity were deconvoluted, revealing a simple catalyst model capable of predicting selectivity for out-of-set catalysts. This workflow enabled extrapolation to a catalyst providing higher selectivity than both reported peptide-type and BINOL-type catalysts (up to 95:5 er). These techniques were then successfully applied towards two additional transforms. Taken together, these examples illustrate the power of combining rational design with data science (ab initio) to efficiently explore reactivity during catalyst development.

Asymmetric Photochemical [2 + 2]-Cycloaddition of Acyclic Vinylpyridines through Ternary Complex Formation and an Uncontrolled Sensitization Mechanism

Stereochemical control of photochemical reactions that occur via triplet energy transfer remains a challenge. Suppressing off-catalyst stereorandom reactivity is difficult for highly reactive open-shell intermediates. Strategies for suppressing racemate-producing, off-catalyst pathways have long focused on formation of ground state, substrate-catalyst chiral complexes that are primed for triplet energy transfer via a photocatalyst in contrast to their off-catalyst counterparts. Herein, we describe a strategy where both a chiral catalyst-associated vinylpyridine and a nonassociated, free vinylpyridine substrate can be sensitized by an Ir(III) photocatalyst, yet high levels of diastereo- and enantioselectivity in a [2 + 2] photocycloaddition are achieved through a preferred, highly organized transition state. This mechanistic paradigm is distinct from, yet complementary to current approaches for achieving high levels of stereocontrol in photochemical transformations.

Catalysis Driven Six-Step Synthesis of Apratoxin A Key Polyketide Fragment

Apratoxin A is a potent anticancer natural product whose key polyketide fragment constitutes a considerable challenge for organic synthesis, with five prior syntheses requiring 12 to 20 steps for its preparation. By combining different redox-economical catalytic stereoselective transformations, the key polyketide fragment could be rapidly prepared. Followed by a site-selective protection of the diol, this strategy enables the preparation of the apratoxin A fragment in only six steps, representing the shortest route to this polyketide.

A predictive and mechanistic statistical modelling workflow for improving decision making in organic synthesis and catalysis

The application of multivariate linear regression models has been widely utilized as a strategy to streamline the reaction optimization process. While these tools likely provide relatively safe predictions, embedding a method for forecasting the probability of achieving the desired reaction outcome would be valuable for streamlining the identification of promising structures with the best chance of success. Herein, we present a workflow that predicts the probability that a reaction will be successful and is easy and quick to apply. We show that this probabilistic framework can effectively differentiate between predictions often indistinguishable by multivariate linear regression analysis. Moreover, these techniques can enhance the development of mechanistically informative correlations by producing more direct pathways for molecular development and design. Overall, we anticipate this protocol will be generally applicable and useful for accelerating successful chemical discovery.

Phenotypic screen identifies calcineurin-sparing FK506 analogs as BMP potentiators for treatment of acute kidney injury

Acute kidney injury (AKI) is a life-threatening disease with no known curative or preventive therapies. Data from multiple animal models and human studies have linked dysregulation of bone morphogenetic protein (BMP) signaling to AKI. Small molecules that potentiate endogenous BMP signaling should have a beneficial effect in AKI. We performed a high-throughput phenotypic screen and identified a series of FK506 analogs that act as potent BMP potentiators by sequestering FKBP12 from BMP type I receptors. We further showed that calcineurin inhibition was not required for this activity. We identified a calcineurin-sparing FK506 analog oxtFK through late-stage functionalization and structure-guided design. OxtFK demonstrated an improved safety profile in vivo relative to FK506. OxtFK stimulated BMP signaling in vitro and in vivo and protected the kidneys in an AKI mouse model, making it a promising candidate for future development as a first-in-class therapeutic for diseases with dysregulated BMP signaling.

Site-Selective Acylations of α- and β-Hydroxyamides in Complex Molecules: Application of Template-Driven Acylation to Disaccharides and a Glycopeptide

Site-selective acylations of α-and β-hydroxyamides in complex polyols are described. The combination of a pyridine aldoxime ester and Zn(OTf)₂ facilitates the acylation of two types of N-glycolyl disaccharides, namely, Gal-GlcNGc and Neu5Gc-Gal, both of which are partial structures of polysaccharides responsible for biological actions, with highly site-selective modifications achieved. Furthermore, biotinylation, one of the most important techniques in chemical biology, is used to site-selectively acylate the β-hydroxyl group in a glycopeptide.

Catalytic asymmetric and stereodivergent oligonucleotide synthesis

We report the catalytic stereocontrolled synthesis of dinucleotides. We have demonstrated, for the first time to our knowledge, that chiral phosphoric acid (CPA) catalysts control the formation of stereogenic phosphorous centers during phosphoramidite transfer. Unprecedented levels of diastereodivergence have also been demonstrated, enabling access to either phosphite diastereomer. Two different CPA scaffolds have proven to be essential for achieving stereodivergence: peptide-embedded phosphothreonine-derived CPAs, which reinforce and amplify the inherent substrate preference, and C2-symmetric BINOL-derived CPAs, which completely overturn this stereochemical preference. The presently reported catalytic method does not require stoichiometric activators or chiral auxiliaries and enables asymmetric catalysis with readily available phosphoramidites. The method was applied to the stereocontrolled synthesis of diastereomeric dinucleotides as well as cyclic dinucleotides, which are of broad interest in immuno-oncology as agonists of the stimulator of interferon genes (STING) pathway.

Chiral phosphoric acid catalyzed atroposelective C–H amination of arenes: mechanisms, origin and influencing factors of enantioselectivity

The amination reaction between azonaphthalene and carbazole catalyzed by chiral phosphoric acid was theoretically investigated, and the mechanism, origin and influencing factors of enantioselectivity were elaborated.

para-Selective Arylation of Arenes: A Direct Route to Biaryls by Norbornene Relay Palladation

Biaryl compounds are extremely important structural motifs in natural products, biologically active components and pharmaceuticals. Selective synthesis of biaryls by distinguishing the subtle reactivity difference of distal arene C-H bonds are significantly challenging. Herein, we describe para-selective C-H arylation, which is acheived by a unique combination of a meta-directing group and norbornene as a transient mediator. Upon direct meta-C-H palladation, one-bond relay palladation occurs in presence of norbornene and subsequently para-C-H arylation is achieved for sulfonates, phosphonates and phenols bearing 2,6-disubstitution patterns. The protocol is amenable to electron-deficient aryl iodides. Multisubstituted arenes and phenols are obtained by postsynthetic modification of the products. The protocol allows the synthesis of hexa-substituted benzene by sequential selective distal C-H functionalization.

Base- and Catalyst-Induced Orthogonal Site Selectivities in Acylation of Amphiphilic Diols

Seeking to selectively functionalize natural and synthetic amphiphiles, we explored acylation of model amphiphilic diols. The use of a nucleophilic catalyst enabled a remarkable shift of the site selectivity from the polar site, preferred in background noncatalyzed or base-promoted reactions, to the apolar site. This tendency was significantly enhanced for organocatalysts comprising an imidazole active site surrounded by long/branched tails. An explanation of these orthogonal modes of selectivity is supported by competitive experiments with monoalcohol substrates.

A Thioxanthone Sensitizer with a Chiral Phosphoric Acid Binding Site: Properties and Applications in Visible Light-Mediated Cycloadditions

A chiral phosphoric acid with a 2,2’‐binaphthol core was prepared that displays two thioxanthone moieties at the 3,3’‐position as light‐harvesting antennas. Despite its relatively low triplet energy, the phosphoric acid was found to be an efficient catalyst for the enantioselective intermolecular [2+2] photocycloaddition of β‐carboxyl‐substituted cyclic enones (e.r. up to 93:7). Binding of the carboxylic acid to the sensitizer is suggested by NMR studies and by DFT calculations to occur by means of two hydrogen bonds. The binding event not only enables an enantioface differentiation but also modulates the triplet energy of the substrates.

Experimental and Computational Studies on Regiodivergent Chiral Phosphoric Acid Catalyzed Cycloisomerization of Mupirocin Methyl Ester

This article presents a new strategy for achieving regiocontrol over the endo versus exo modes of cycloisomerizations of epoxide-containing alcohols, which leads to the formation of five- or six-membered cyclic ethers. Unlike traditional methods relying on achiral reagents or enzymes, this approach utilizes chiral phosphoric acids to catalyze the regiodivergent selective formations of either tetrahydrofuran- or tetrahydropyran-containing products. By using methyl ester of epoxide-containing antibiotic mupirocin as the substrate, it is demonstrated that catalytic chiral phosphoric acids (R)-TCYP and (S)-TIPSY could be used to achieve the selective formation of either the six-membered endo product (95:5 r.r.) or the five-membered exo product (77:23 r.r.), correspondingly. This cyclization was found to be unselective under the standard conditions involving various achiral acids, bases, or buffers. The subsequent mechanistic studies using state-of-the-art quantum chemical solutions provided the description of the potential energy surface, which is fully consistent with the experimental observations. Based on these results, highly detailed reaction paths are obtained and a concerted and highly synchronous mechanism is proposed for the formation of both exo and endo products.

Chiral phosphoric acid catalyzed asymmetric arylation of indolesvianucleophilic aromatic substitution: mechanisms and origin of enantioselectivity

Asymmetric arylation of indolesvianucleophilic aromatic substitution can be effectively achieved using chiral phosphoric acid as catalyst, where the mechanisms and origin of enantioselectivity were explored theoretically.

Monoalkylation of aniline with trichloroacetimidate catalyzed by (±)-camphorsulfonic acid through an SN1 reaction based on dual hydrogen-bonding activation modes

Herein, the monoalkylation of anilines with trichloroacetimidates catalyzed by CSA was investigated theoretically, and it was found that the reaction occurred through an S_N1 reaction involving the dual hydrogen-bonding activation modes.