Developing Processes for Crystallization-Induced Asymmetric Transformation

Switchable Enantio- and Diastereoselective Michael Additions of β-Keto Amides to Nitroolefins: Crystallization-Based Inversion of Kinetic Stereocontrol

Asymmetric catalytic reactions rely on chiral catalysts that induce highly ordered transition states capable of imparting stereoselectivity in the bond-forming step(s). Productive deviations from this paradigm are rare yet hold the potential for accessing different stereoisomers using the same catalyst. Here, we present an enantio- and diastereoselective Michael addition of β-keto amides to nitroolefin electrophiles proceeding via an unusual scenario where the kinetic diastereocontrol imparted by the catalyst may be overridden by crystallization to provide the complementary stereoisomer of the product.

Recent advances in the total synthesis of galantamine, a natural medicine for Alzheimer's disease

Covering: 2006 to 2023(-)-Galantamine is a natural product with distinctive structural features and potent inhibitory activity against acetylcholine esterase (AChE). It is clinically approved for the treatment of Alzheimer's disease. The clinical significance and scarcity of this natural product have prompted extensive and ongoing efforts towards the chemical synthesis of this challenging tetracyclic structure. The objective of this review is to summarize and discuss recent progress in the total synthesis of galantamine from 2006 to 2023. The contents are organized according to the synthetic strategies for the construction of the quaternary center. Key features of each synthesis have been highlighted, followed by a summary and outlook at the end.

Enantioselective Synthesis of Phosphine Boranes via Crystallization-Induced Dynamic Resolution of Lithiated Intermediate by Understanding the Underlying Epimerization Process

Described herein is the successful crystallization-induced dynamic resolution (CIDR) of an α-lithiated phosphine borane utilizing the easily accessible and inexpensive ligand (R,R)-TMCDA. Starting from the essential P-prochiral building block dimethyl phenyl phosphine borane we were able to obtain phosphine boranes in yields up to 80 % and e.r. up to 98 : 2 by crystallization of the lithiated intermediate prior to the trapping reaction. NMR-based deuterium labeling experiments indicate that the epimerization in solution is based on the intermolecular proton transfer between nonlithiated phosphine borane and the corresponding lithiated intermediate, rendering the presence of the remaining starting compound in an optimized solvent mixture the main factor for successful enantioselective synthesis. Quantum chemical calculations using different model systems based on solid state structures confirm these experimental results. By gaining insights into the epimerization mechanism, essential principles for CIDR of lithiated phosphine boranes are elucidated that may be expanded to other important P-stereogenic compounds and simple chiral amines.

On Solute Recovery and Productivity in Chiral Resolution through Solid-State Deracemization by Temperature Cycling

Temperature cycling represents an effective means for the deracemization of chiral compounds that crystallize as conglomerates and racemize in solution. In such a process, a suspension enriched in the desired enantiomer is converted into an enantiopure one through periodic cycles of crystal dissolution and crystal growth. We show that performing temperature cycling at higher temperatures leads to faster deracemization and, consequently, higher productivity. However, this comes at the cost of lower recovery, as the solution contains potentially relevant amounts of solute due to the higher solubility at an elevated temperature. In this work, we introduce and compare two process variants that mitigate this issue. The first involves temperature cycling, followed by linear cooling, whereas the second is based on merging the temperature cycles and cooling crystallization. Experiments carried out with the chiral compound N-(2-methylbenzylidene)-phenylglycine amide show that the former variant is faster than the latter, and it is easier to design and implement. In this process, the choice of an appropriate cooling rate is essential to avoid nucleation of the undesired enantiomer.

Crystallization-Enabled Stereoconvergent Michael Additions of β-Keto Esters to Nitroolefins

Asymmetric Michael additions are powerful tools to meet the growing need for stereochemically complex products. While 1,3-dicarbonyls are common nucleophiles, the successful use of configurationally unstable β-keto esters in diastereoselective variants remains understudied. In this Letter, crystalline β-keto esters were leveraged in a two-phase, one-pot merger of an asymmetric Michael addition with a crystallization-induced diastereomer transformation. Tuning the crystallinity of β-keto ester adducts enabled stereoconvergence of the products, which were isolated by filtration.

Development of a Crystallization-Induced Diastereomer Transformation of Oxime Isomers for the Asymmetric Synthesis of (1S,6R)-3,9-Diazabicyclo[4.2.1]nonane

Herein we report a practical crystallization-induced diastereomer transformation (CIDT) of oxime isomers for the scalable asymmetric synthesis of the bicyclic diamine (1S,6R)-3,9-diazabicyclo[4.2.1]nonane derivative that serves as a valuable building block in medicinal chemistry. The developed approach utilizes (S)-phenylethylamine as a chiral auxiliary handle for CIDT, and the starting nortropinone derivative is prepared in one step from commercially available materials. The resulting E-oxime is subjected to a stereospecific Beckmann rearrangement, followed by reduction of the resulting lactam with LiAlH4 to afford the monoprotected (1S,6R)-3,9-diazabicyclo[4.2.1]nonane derivative. The development of the CIDT and understanding of the mechanistic implications leading to the high selectivity are reported.

Development of organic reactions that productively leverage physical properties

Crystallization-induced stereoconvergent Michael additions demonstrate the potential of using physical properties to reveal unique reactivity.

Stereoselective formation of boron-stereogenic organoboron derivatives

Four-coordinate organoboron derivatives present interesting chemical, physical, biological, electronical, and optical properties. Given the increasing demand for the synthesis of smart functional materials based on chiral organoboron compounds, the exploration of stereoselective synthesis of boron-stereogenic organo-derivatives is highly desirable. However, the stereoselective construction of organoboron compounds stereogenic at boron has been far less studied than other elements of the main group due to configurational stability concerns. Nowadays, these species are no longer elusive and configurationally stable compounds have been highlighted. The idea is to show the potential of the stereoselective building of the four-coordinate boron centre and encourage future endeavors and developments in the field.

Enantio- and Diastereoselective Mannich Reactions of ß-Dicarbonyls by Second Stage Diastereoconvergent Crystallization

The synthesis of chiral α-monosubstituted-ß-dicarbonyls is a challenging task in asymmetric catalysis due to the rapid, typically uncontrolled, product racemization or epimerization under most reaction conditions. For this reason, diastereoselective additions of unsubstituted ß-dicarbonyls to π-electrophiles are unusual. Herein, we disclose a simple catalytic crystallization-driven enantio- and diastereoselective Mannich reaction for the synthesis of stereodefined α-monosubstituted-ß-keto esters, dissymmetric ß-diesters, dissymmetric ß-diketones, and ß-keto amides that productively leverages product epimerization in solution. Mechanistic studies suggest a scenario where the initial enantioselective, diastereodivergent skeletal assembly is catalyzed by a chiral tertiary amine organocatalyst, which then facilitates second stage crystallization-induced diastereoconvergence to provide the challenging α-stereocenter in excellent stereoselectivity.

Stereoselective Synthesis of the IDO Inhibitor Navoximod

A highly efficient asymmetric synthesis of the IDO inhibitor navoximod, featuring the stereoselective installation of two relative and two absolute stereocenters from an advanced racemic intermediate, is described. The stereocenters were set via a crystallization-induced dynamic resolution along with two selective ketone reductions: one via a biocatalytic ketoreductase transformation and one via substrate-controlled hydride delivery from LiAlH(Ot-Bu)₃. Following this strategy, navoximod was synthesized in 10 steps from 2-fluorobenzaldehyde and isolated in 23% overall yield with 99.7% ee and high purity.

Crystallization-Induced Deracemization: Experiments and Modeling

Inspired by deracemization via temperature cycles, which enables the collection of crystals of the desired enantiomer from an initially racemic mixture, we focus in this work on an alternative batch process, namely crystallization-induced deracemization. This process starts with a suspension of enantiomerically pure crystals, which undergoes a simple cooling crystallization, coupled with liquid-phase racemization. The experimental and model-based analysis of such a process, carried out here, revealed that: (i) deracemization via temperature cycles is a safe choice to operate with high enantiomeric purity, although its throughput is limited by the suspension density; (ii) if the distomer is less prone to nucleation, crystallization-induced deracemization is a simple process; however, its performance is strongly limited by the solubility; (iii) the purity achieved with crystallization-induced deracemization can be increased by utilizing large seed mass and by optimizing the cooling profile or catalyst concentration. Alternatively, the purity increases via partial dissolution of the seeds, which resembles the heating part of the deracemization process via temperature cycles.

Continuous Flow Chiral Amine Racemization Applied to Continuously Recirculating Dynamic Diastereomeric Crystallizations

A new, dynamic diastereomeric crystallization method has been developed, in which the mother liquors are continuously separated, racemized over a fixed-bed catalyst, and recirculated to the crystallizer in a resolution-racemization-recycle (R³) process. Separating the racemization from crystallization overcomes problems of using catalysts in situ, that suffer conflicting sets of conditions, inhibition, and separation. Continuous racemization has been achieved through the covalent attachment of [IrCp*I₂]₂ SCRAM catalyst to Wang resin solid support to give a fixed-bed catalyst. One tertiary and a variety of secondary optically enriched amines have been racemized efficiently, with residence times compatible with the crystallization (2.25-30 min). The catalyst demonstrates lower turnover (TOF) than the homogeneous analogue but with reuse shows a long lifetime (e.g., 40 recycles, 190 h) giving acceptable turnover number (TON) (up to 4907). The slow release of methylamine during racemization of N-methyl amines was found to inactivate the catalyst, which could be partially reactivated using hydroiodic acid. Dynamic crystallization is achieved in the R³ process through the continual removal of the more soluble diastereomer and supply of the less soluble one. The solubility of the diastereomers was determined, and the difference correlates to the rate of resolution but is also affected by the rates of racemization, crystal growth, and dissolution. A variety of cyclic and acyclic amine salts were resolved using mandelic acid (MA) and ditoluoyl tartaric acid (DTTA) with higher resolvability (S = yield × d.e.) than the simple diastereomeric crystallization alone. Comparing resolvabilities, resolutions were 1.6-44 times more effective with the R³ process than batch, though one case was worse. Further investigation of this revealed an unusual thermodynamic switching behavior: rac-N-methylphenethylamine was initially resolved as an (S,S)-bis-alkylammonium tartrate crystal but over time became the equivalent (R,S) salt. Thermal, mixing, concentration, stoichiometry, and seeding conditions were all found to affect the onset of the switching behavior which is only associated with difunctional resolving reagents.

Interrogation of fractional crystallization behavior of a newly exploited chiral resolution method for racemic 1-(pyridin-2-yl)ethylamine via DFT-D3 calculations of cohesive energy

A novel chiral separation method for 1-(pyridin-2-yl)ethylamine is developed and the underlying energetics is investigated by DFT-D3.

Highly diastereoselective crystallization-induced asymmetric transformation of 1,3-disubstituted-tetrahydro-β-carbolines in water

A green and highly stereoselective method for the synthesis of cis or trans-1,3-disubstituted-tetrahydro-β-carbolines has been developed using water as the solvent.

Stereoselective synthesis of the Halaven C14-C26 fragment from D-quinic acid: crystallization-induced diastereoselective transformation of an α-methyl nitrile

Crystallization-induced diastereoselective transformation (CIDT) of an α-methyl nitrile completes an entirely non-chromatographic synthesis of the halichondrin B C14-C26 stereochemical array. The requisite α-methyl nitrile substrate is derived from D-quinic acid through a series of substrate-controlled stereoselective reactions via a number of crystalline intermediates that benefit from a rigid polycyclic template. Therefore, all four stereogenic centers in the Halaven C14-C26 fragment were derived from the single chiral source D-quinic acid.

The CERT antagonist HPA-12: first practical synthesis and individual binding evaluation of the four stereoisomers

The first unified synthetic route to the four enantiopure HPA-12 stereoisomers in multi-gram scale is reported based on Crystallization-Induced Asymmetric Transformation (CIAT) technology. This preparative stereoselective synthesis allowed the unprecedented comparative evaluation of HPA-12 stereoisomers regarding their interaction with the CERT START domain. In vitro binding assay coupled to in silico docking approach indicate a possible interaction for the four derivatives. The first TR-FRET homogeneous-phase assay was developed to quantify their binding to the START domain, allowing complete determination of HPA-12 EC₅₀. Results indicate that not only the (1R,3S) lead to the strongest binding, but that both 1R and 3S stereocenters similarly contribute to extent of recognition This automated homogenous assay further opens up promising prospect for the identification of novel potential CERT antagonist by means of high throughput screening.

TiCl4-promoted condensation of methyl acetoacetate, isobutyraldehyde, and indole: a theoretical and experimental study

A combined DFT and experimental study of the title reaction points to a three-step mechanism: (1) titanium enolate formation, (2) Knoevenagel condensation of enolate and aldehyde, and (3) Michael addition of indole.

Yonemitsu-type condensations catalysed by proline and Eu(OTf)3

The reaction of aromatic heterocycles, aldehydes, and active methylene compounds affords polyfunctionalised heterocycles. The reaction is green and provides access to compounds that cannot be obtained by classical methodology.

Thermodynamic epimeric equilibration and crystallisation-induced dynamic resolution of lobelanine, norlobelanine and related analogues

The step-economical synthesis of lobelanine involving a ring closing double aza-Michael (RCDAM) reaction is revisited and successfully extended to the synthesis of various configurationally more stable analogues.

Design, preparation, and implementation of an imidazole-based chiral biaryl P,N-ligand for asymmetric catalysis

A new strategy for increasing the barrier to rotation in biaryls has been developed that allows for the incorporation of 5-membered aromatic heterocycles into chiral atropisomers. Using this concept, an imidazole-based biaryl P,N-ligand has been designed and prepared as a single enantiomer. This ligand performs exceptionally well in the enantioselective A(3)-coupling, demonstrating the potential of this new design element.

Stereoselective, nitro-Mannich/lactamisation cascades for the direct synthesis of heavily decorated 5-nitropiperidin-2-ones and related heterocycles

A versatile nitro-Mannich/lactamisation cascade for the direct stereoselective synthesis of heavily decorated 5-nitropiperidin-2-ones and related heterocycles has been developed. A highly enantioenriched substituted 5-nitropiperidin-2-one was synthesised in a four component one-pot reaction combining an enantioselective organocatalytic Michael addition with the diastereoselective nitro-Mannich/lactamisation cascade. Protodenitration and chemoselective reductive manipulation of the heterocycles was used to install contiguous and fully substituted stereocentres in the synthesis of substituted piperidines.