Fast Attrition-Enhanced Deracemization of Naproxen by a Gradual In Situ Feed

Crystallisation Based Deracemisation and Chiral Resolution of Small Molecules

Enantiomeric separation of chiral molecules is pivotal for exploring fundamental questions about life's origin and various other scientific domains. Crystallisation is an important platform for the separation of chiral molecules, elegantly applied to many systems, for instance, the formation of conglomerates, where the enantiomers crystallise as separate phases. Many approaches have been proposed to explore crystallisation-driven enantiomeric separation with fewer insights into the complex pathways associated with the separation processes. Controlling derecemisation or chiral resolution through crystallisation by applying mechanical forces, magnetic substrates, and seeds have attracted significant attention due to better outcomes. Despite these advancements, the mechanisms behind the separation of enantiomers using crystallisation as a tool, especially the delicate balance between kinetically and thermodynamically controlled pathways, are unclear, particularly for small organic molecules critical to pharmaceutical applications. This review aims to address many of these challenges, providing synthetic chemists with a deeper understanding of the role of crystallisation for innovative enantiomeric separation strategies.

Mechanochemical Deracemization: A Sustainable Approach to Enantiopurity

We introduce mechanochemical deracemization (MCDR) as a novel strategy for obtaining enantiopure compounds. This study demonstrates the successful transposition of six archetypical deracemization reactions from a solvent-based to a solvent-minimized ball milling environment. The scope includes a ketone, isoindolinones, imines, an ester, and an inorganic compound, all of which deracemized successfully. Key parameters such as milling material, ball number and size, the use of a bulk material and liquid-assisted grinding (LAG) were systematically investigated, revealing their crucial role. Quantitative enantiomeric excesses (ee) were achieved, while reaction times were reduced by up to 97 % and solvent consumption by as much as 100 %. This work establishes MCDR as a versatile, sustainable pathway to enantiopure compounds. By highlighting the generalizability of this approach and its huge potential for minimizing waste, this study provides the foundation for future advancements in mechanochemical deracemization.

A Complex Reaction Network Model for Spontaneous Mirror Symmetry Breaking in Viedma Deracemizations

Attrition-enhanced chiral symmetry breaking in crystals, known as Viedma deracemization, is a promising method for converting racemic solid phases into enantiomerically pure ones under non-equilibrium conditions. However, many aspects of this process remain unclear. In this study, we present a new investigation into Viedma deracemization using a comprehensive kinetic rate equation continuous model based on classical primary nucleation theory, crystal growth, and Ostwald ripening. Our approach employs a fully microreversible kinetic scheme with a size-dependent solubility following the Gibbs-Thomson rule. To validate our model, we use data from a real NaClO3 deracemization experiment. After parametrization, the model shows spontaneous mirror symmetry breaking (SMSB) under grinding. Additionally, we identify a bifurcation scenario with a lower and upper limit of the grinding intensity that leads to deracemization, including a minimum deracemization time within this window. Furthermore, this model uncovers that SMSB is caused by multiple instances of concealed high-order autocatalysis. Our findings provide new insights into attrition-enhanced deracemization and its potential applications in chiral molecule synthesis and understanding biological homochirality.

Rapid deracemization through solvent cycling: proof-of-concept using a racemizable conglomerate clopidogrel precursor

We demonstrate that a conglomerate-forming clopidogrel precursor undergoing solution phase racemization can be deracemized through cyclic solvent removal and re-addition. We establish that the combination of slow growth and fast dissolution of crystals is ideal for rapid deracemization, which we achieve by repurposing a Soxhlet apparatus to realize the slow removal and fast re-addition of solvent autonomously.

Identifying a Hidden Conglomerate Chiral Pool in the CSD

Conglomerate crystallization is the spontaneous generation of individually enantioenriched crystals from a nonenantioenriched material. This behavior is responsible for spontaneous resolution and the discovery of molecular chirality by Pasteur. The phenomenon of conglomerate crystallization of chiral organic molecules has been left largely undocumented, with no actively curated list available in the literature. While other crystallographic behaviors can be interrogated by automated searching, conglomerate crystallizations are not identified within the Cambridge Structural Database (CSD) and are therefore not accessible by conventional automated searching. By conducting a manual search of the CSD and literature, a list of over 1800 chiral species capable of conglomerate crystallization was curated by inspection of the racemic synthetic routes described in each publication. The majority of chiral conglomerate crystals are produced and published by synthetic chemists who seldom note and rarely exploit the implications this phenomenon can have on the enantiopurity of their crystalline materials. With their structures revealed, we propose that this list of compounds represents a new chiral pool which is not tied to biological sources of chirality.

Functional Chirality: From Small Molecules to Supramolecular Assemblies

Many structures in nature look symmetric, but this is not completely accurate, because absolute symmetry is close to death. Chirality (handedness) is one form of living asymmetry. Chirality has been extensively investigated at different levels. Many rules were coined in attempts made for many decades to have control over the selection of handedness that seems to easily occur in nature. It is certain that if good control is realized on chirality, the roads will be ultimately open towards numerous developments in pharmaceutical, technological, and industrial applications. This tutorial review presents a report on chirality from single molecules to supramolecular assemblies. The realized functions are still in their infancy and have been scarcely converted into actual applications. This review provides an overview for starters in the chirality field of research on concepts, common methodologies, and outstanding accomplishments. It starts with an introductory section on the definitions and classifications of chirality at the different levels of molecular complexity, followed by highlighting the importance of chirality in biological systems and the different means of realizing chirality and its inversion in solid and solution-based systems at molecular and supramolecular levels. Chirality-relevant important findings and (bio-)technological applications are also reported accordingly.

Spontaneous mirror symmetry breaking in reaction–diffusion systems: ambivalent role of the achiral precursor

Reaction–diffusion simulations reveal that the achiral substrate concentration may play an ambivalent role in spontaneous mirror symmetry breaking.

Combining Incompatible Processes for Deracemization of a Praziquantel Derivative under Flow Conditions

An efficient deracemization method for conversion of the racemate to the desirable (R)-enantiomer of Praziquantel has been developed by coupling incompatible racemization and crystallization processes. By a library approach, a derivative that crystallizes as a conglomerate has been identified. Racemization occurs via reversible hydrogenation over a palladium on carbon (Pd/C) packed column at 130 °C, whereas deracemization is achieved by alternating crystal growth/dissolution steps with temperature cycling between 5-15 °C. These incompatible processes are combined by means of a flow system resulting in complete deracemization of the solid phase to the desired (R)-enantiomer (98 % ee). Such an unprecedented deracemization by a decoupled crystallization/racemization approach can readily be turned into a practical process and opens new opportunities for the development of essential enantiomerically pure building blocks that require harsh methods for racemization.

Sequential Suzuki-Miyaura Coupling/Lewis Acid-Catalyzed Cyclization: An Entry to Functionalized Cycloalkane-Fused Naphthalenes

Functionalized angular cycloalkane-fused naphthalenes were prepared using a two-step process involving a Pd-catalyzed Suzuki-Miyaura coupling of aryl pinacol boronates and vinyl triflates followed by a boron trifluoride etherate-catalyzed cycloaromatization.

Photoracemization-Based Viedma Ripening of a BINOL Derivative

Viedma ripening is a deracemization process that has been used to deracemize a range of chiral molecules. The method has two major requirements: the compound needs to crystallize as a conglomerate and it needs to be racemizable under the crystallization conditions. Although conglomerate formation can be induced in different ways, the number of racemization methods is still rather limited. To extend the scope of Viedma ripening, in the present research we applied UV-light-induced racemization in a Viedma ripening process, and report the successful deracemization of a BINOL derivative crystallizing as a conglomerate. Irradiation by UV light activates the target compound in combination with an organic base, required to promote the excited-state proton transfer (ESPT), leading thereafter to racemization. This offers a new tool towards the development of Viedma ripening processes, by using a cheap and "green" catalytic source like UV light to racemize suitable chiral compounds.

A chiral switch: balancing between equilibrium and non-equilibrium states

Herein we introduce a "chiral switch" - a sequence of operations that alternate between equilibrium and non-equilibrium conditions to switch the absolute configuration of a chiral center. The generality and practical potential of the technique are demonstrated with three unnatural α-amino acid precursors.

Applications of ultrasound to chiral crystallization, resolution and deracemization

Industrial synthesis of enantiopure compounds is nowadays heavily based on the separation of racemates through crystallization processes. Although the application of ultrasound in solution crystallization processes (sonocrystallization) has become a promising emerging technology, offering several benefits (e.g. reduction of the induction time and narrowing of the metastable zone width, control over the product size, shape and polymorphic modification), little attention has been paid so far to the effects of ultrasound on chiral crystallization processes. Several recent studies have reported on the application of acoustic energy to crystallization processes that separate enantiomers, ranging from classical (diastereomeric) resolution and preferential crystallization to new and emerging processes such as attrition-enhanced deracemization (Viedma ripening). A variety of interesting effects have been observed, which include among others, enhanced crystallization yield with higher enantiomeric purity crystals, spontaneous mirror symmetry breaking crystallization, formation of metastable conglomerate crystals and enhanced deracemization rates. The objective of this review is to provide an overview of the effects of ultrasound on chiral crystallization and outline several aspects of interest in this emerging field.

Intensified deracemization via rapid microwave-assisted temperature cycling

Rapid cooling and microwave heating substantially speed up temperature cycling-enhanced deracemization, while limiting the concomitant side reactions. During fast cooling, secondary nucleation is shown to enable deracemization.

Solid state deracemisation of two imine-derivatives of phenylglycine derivatives via high-pressure homogenisation and temperature cycles

After reviewing several techniques for solid-state deracemisation, we study the deracemisation of two drug precursors with two different techniques.

Practical Stereochemistry

The relationship between fundamental and applied is often uneasy, particularly in modern political climates. A familiar political view, aimed negatively at the scientific community, is that the former is a waste of money whereas the latter gives value for investment. The answer that fundamental is required as the basis for practical suffers from the fact that the timelines between fundamental and practical are often long and the routes contorted and unexpected. This has been my experience. In this Account, examples are given from the research in which I have been involved wherein quite fundamental considerations have led to various applications. The longer the time, the clearer and broader the relationship. Fundamental can and does lead to application. They need and depend on each other. I have seen this both from the side of academia and from small companies. In the course of the past 40 plus years, I have been involved in various aspects of stereochemistry and, in particular, chirality. It has been rewarding to see that several of the developments, most originally grounded in fundamental research considerations, have been used in the chemical community and given new dimensions and often practical applications by others. In this Account, a path-not planned deliberately by me-from orbital symmetry and Woodward-Hoffmann rules through crown ethers to conformational analysis to diastereomeric resolutions to deracemizations powered by Ostwald ripening and the Gibbs-Thomson effect to nucleation to helicenes is described. In order of discussion, the orbital symmetry aspects have via an unusual and unpredicted path has resulted in, among other things, a synthesis of hindered alkenes useful for the production of molecular motors. The crown ether aspects led to discovery of the utility of cesium salts particularly for racemization sensitive nucleophilic substitutions. Work on diastereomeric resolutions has concentrated on the mechanistic as well as practical/commercial aspects of the use of multiple resolving agents (Dutch resolution). During this work the complex relationship between nucleation and chirality in diastereomeric resolutions began to reveal itself. In general, nucleation, especially with involvement of chirality, is a topical challenge that has attracted the attention of many groups. The contribution of this knowledge to the development of attrition driven deracemizations of racemizable conglomerates is described. This remarkable technology allows, without intervention of chiral aids, conversion of certain racemates in quantitative yield and absolute enantiomeric excess to a single enantiomer. From a practical standpoint, this methodology has been used for the production in enantiomerically pure form of commercially interesting compounds like naproxen and clopidogrel (Plavix). Finally an STM investigation of the nucleation behavior of a helicene, prepared via a remarkably short and efficient route, on a metal surface is described.

Speeding up Viedma ripening

Viedma ripening allows the conversion of a solid state racemate into a single enantiomer. Using the gradual conversion of a metastable racemic compound into the conglomerate, the speed of deracemization for two amino acid derivatives could be considerably increased from several days to a few hours.

The RNA World as a Model System to Study the Origin of Life

Understanding how life arose is a fundamental problem of biology. Much progress has been made by adopting a synthetic and mechanistic perspective on originating life. We present a current view of the biochemistry of the origin of life, focusing on issues surrounding the emergence of an RNA World in which RNA dominated informational and functional roles. There is cause for optimism on this difficult problem: the prebiotic chemical inventory may not have been as nightmarishly complex as previously thought; the catalytic repertoire of ribozymes continues to expand, approaching the goal of self-replicating RNA; encapsulation in protocells provides evolutionary and biophysical advantages. Nevertheless, major issues remain unsolved, such as the origin of a genetic code. Attention to this field is particularly timely given the accelerating discovery and characterization of exoplanets.

Mechanism of chirality conversion by periodic change of temperature: Role of chiral clusters

By grinding crystals in a solution, the chirality of crystal structure (and the molecular chirality for the case of chiral molecules as well) can be converted, and the cause of the phenomenon is attributed to crystal growth with chiral clusters. We show that the recently found chirality conversion with a periodic change of temperature can also be explained by crystal growth with chiral clusters. With the use of a generalized Becker-Döring model, which includes enantio-selective incorporation of small chiral clusters to large solid clusters, the change of cluster distribution and the mass flow between clusters are studied. The chiral clusters act as a reservoir to pump out the minority species to the majority, and the exponential amplification of the enantiomeric excess found in the experiment is reproduced in the numerical calculation.

Construction of highly functionalized naphthalenes using an in situ ene–allene strategy

Construction of highly functionalized naphthalene derivatives remains a challenging task for organic chemists because of the effect of the substituent.

Viedma ripening: a reliable crystallisation method to reach single chirality

Crystallisation processes have evolved to practical methods that allow isolation of an enantiopure product in high yield. Viedma ripening in particular enables access to enantiopure products in a reliable way, simply through grinding of crystals in a solution. This tutorial review covers the basic principles behind asymmetric crystallisation processes, with an emphasis on Viedma ripening, and shows that to date many novel organic molecules can be obtained in enantiopure solid form.

Absolute Asymmetric Synthesis: Viedma Ripening of [Co(bpy)3](2+) and Solvent-Free Oxidation to [Co(bpy)3](3.)

Syntheses of [Co(bpy)3](2+) yield racemic solutions because the Δ- and Λ-enantiomers are stereochemically labile. However, crystallization and attrition-enhanced deracemization can give homochiral crystal batches of either handedness in quantitative yield. Subsequently, solvent-free oxidation with bromine vapour fixes the chirality because [Co(bipy)3](3+) does not enantiomerize in solution at ambient temperature. This combination of Viedma ripening and the labile/inert Co(II)/Co(III) couple constitutes a convenient method of absolute asymmetric synthesis.

Absolute asymmetric synthesis: protected substrate oxidation

Three new conglomerates incorporating bidentate sulfide ligands coordinated by Ru(II) centers have been prepared. Total spontaneous resolution by slow crystallization gives highly enantioenriched crystal batches, which are used in enantioselective oxidation of the sulfide ligands to give chiral sulfoxide complexes with >98 % ee. All relevant stereoisomers have been characterized by single-crystal X-ray diffraction, CD spectroscopy, and chiral HPLC. If the ligand range can be extended to monodentate sulfides, a large-scale and recyclable process for enantioselective oxidation of sulfides can be designed.

On the concomitant crystallization of amino acid crystals upon dissolution of some amino acid salt crystals

Amino acid crystals form spontaneously upon dissolution of their chloride salts.

The racemate-to-homochiral approach to crystal engineering via chiral symmetry breaking

Racemate-to-homochiral crystallization was highlighted for symmetry breaking phenomena by showing clear pictures of the mechanism and development history.

Enantiopure isoindolinones through Viedma ripening

Here we demonstrate that deracemization of isoindolinones using Viedma ripening is possible starting from a racemic mixture of conglomerate crystals. Crystals of the enantiopure isoindolinones lose their chiral identity upon dissolution even without the need for a catalyst. This enabled complete deracemization of the reported isoindolinones without a catalyst.