Crystallization-induced diastereomer transformation of thiohydantoin derivatives

Crystallization-induced diastereomer transformation assists the diastereoselective synthesis of 4-nitroisoxazolidine scaffolds

This communication describes a solution to the vexing problem of synthesis of 4-nitroisoxazolidine rings from cyclic nitrones and β-nitrostyrenes. The trans-endo adduct 2 is quantitatively synthesised from E-β-nitrostyrene under mild conditions avoiding purification, while the trans-exo adduct 4 is obtained at higher temperatures. Furthermore, a Crystallization-Induced Diastereomer Transformation (CIDT) process was used to epimerise the NO2 bond from 2 to the cis-exo adduct 3 with total conversion assisted by the para-halogen substitution of the aromatic ring without any additives. By combining these approaches, we can establish a simple synthetic methodology that allows the diastereoselective synthesis of three diastereoisomers, overcoming the previous problems described in the literature.

Overcoming a solid solution system on chiral resolution: combining crystallization and enantioselective dissolution

Chiral resolution of rac-4-cyano-1-aminoindane, a key intermediate of ozanimod, was successfully achieved through a combination of crystallization and enantioselective dissolution with up to 96% ee. The disastereomeric salt with di-p-toluoyl-L-tartaric acid was characterized by the construction of a binary phase diagram and ternary isotherm. Enantioselective dissolution was then employed to further enrich the enantiomer.

Solvent dependent hindered rotation versus epimerization in axially chiral thiohydantoin derivatives: an experimental and a computational study

5-Benzyl-3-(o-aryl)-2-thiohydantoin and 5-isobutyl-3-(o-aryl)-2-thiohydantoin derivatives (o-aryl = o-tolyl and o-bromophenyl) have been synthesized by reacting o-aryl isothiocyanates with S-phenylalanine methyl ester hydrochloride or with S-leucine methyl ester hydrochloride in the presence of triethylamine (TEA). The synthesized compounds have a chirality center at C5 of the heterocyclic ring and a chirality axis, the N3-C_(aryl) bond. The axially chiral compounds were shown to exist in unequal amounts of SM, SP, RM and RP stereoisomeric forms with a high prevalence of the P isomers over the M isomers. The isomeric assignments were done by comparing the ¹H NMR spectra with the HPLC chromatograms. The stereoisomers were resolved micropreparatively by HPLC on chiral stationary phases and the interconversion of the single isomers has been investigated. The conversion type has been determined as epimerization or rotation by the HPLC analyses. It has been found that although the stereoisomers converted to each other only by rotation in toluene, in ethanol epimerization (racemization at C5 of the heteroring) was accompanied with rotation depending on the duration, temperature of the thermal interconversion experiment and the nature of the ortho substituent. The occurrence of epimerization was also proved through H/D exchange reactions via¹H NMR experiments done in CD₃OD. The rotation and epimerization mechanisms of synthesized compounds were further elucidated by Density Functional Theory (DFT) calculations at M062X/6-311 + G** level of theory and the results were shown to be in harmony with experimental findings.

Recognition in the Domain of Molecular Chirality: From Noncovalent Interactions to Separation of Enantiomers

It is not a coincidence that both chirality and noncovalent interactions are ubiquitous in nature and synthetic molecular systems. Noncovalent interactivity between chiral molecules underlies enantioselective recognition as a fundamental phenomenon regulating life and human activities. Thus, noncovalent interactions represent the narrative thread of a fascinating story which goes across several disciplines of medical, chemical, physical, biological, and other natural sciences. This review has been conceived with the awareness that a modern attitude toward molecular chirality and its consequences needs to be founded on multidisciplinary approaches to disclose the molecular basis of essential enantioselective phenomena in the domain of chemical, physical, and life sciences. With the primary aim of discussing this topic in an integrated way, a comprehensive pool of rational and systematic multidisciplinary information is provided, which concerns the fundamentals of chirality, a description of noncovalent interactions, and their implications in enantioselective processes occurring in different contexts. A specific focus is devoted to enantioselection in chromatography and electromigration techniques because of their unique feature as "multistep" processes. A second motivation for writing this review is to make a clear statement about the state of the art, the tools we have at our disposal, and what is still missing to fully understand the mechanisms underlying enantioselective recognition.

Recent advances in the field of chiral crystallization

Crystallization is one of the largest and most economical bulk purification techniques used in industry today. There has been an increase in demand for enantiomerically pure compound production for research, organic synthesis, pharmaceutical drug production, and other applications. Even after asymmetric synthesis, chiral purification will always be necessary. The focus of this review is on recent advances in chiral crystallization for the purification of enantiomers. A comprehensive discussion of three techniques and their mechanisms is provided, namely: attrition-enhanced deracemization, cocrystallization, and inorganic ionic cocrystallization. Several examples of attrition-enhanced deracemization are discussed. The key advantage of this technique is that it eliminates enantiomeric waste and can be used to produce enantiomeric excesses of greater than 99% from racemic mixtures. Chiral cocrystallization is examined, with over 60 cocrystallizing compounds, as an excellent means for enantiomeric enrichment. Selective chiral inclusion complexation was shown to be a novel approach for the formation of cocrystals. Chiral inorganic ionic cocrystallization is a new technique involving the formation of cocrystals between chiral ligands and certain metal salts in order to produce conglomerate crystal behavior in otherwise racemic compounds.

Stereochemical switch driven by crystallization: Interplay between stoichiometry and configuration of the products

An intriguing example of a crystallization-induced stereochemical switch in the configuration of aza-Michael reaction products is described. Depending on both the stereochemical purity and stoichiometric ratio of the chiral amine used, the reaction delivers crystalline diastereomers of a different stereochemistry. The optically pure diastereomer smoothly converts to its racemic epimer salt upon the addition of a complementary chiral amine.

Catalytic Asymmetric Synthesis of Chiral Thiohydantoins via Domino Cyclization Reaction of β,γ-Unsaturated α-Ketoester and N,N'-Dialkylthiourea

The first catalytic asymmetric route to synthesize chiral thiohydantoins containing a quaternary stereogenic center has been established utilizing a chiral phosphoric acid-catalyzed domino cyclization reaction of N,N'-dialkyl thioureas with β,γ-unsaturated...

Attrition-Enhanced Asymmetric Transformation of Axially Chiral Nicotinamides by Dynamic Chiral Salt Formation

Atroposelective resolution for axially chiral nicotinamides was achieved by dynamic chiral salt formation with L-DBTA using six types of nicotinamides that could not be optically resolved by the preferential crystallization method. Kinetic studies of their racemization indicated that the chiral conformation was retained for a significant period of time. Two methods of crystallization-induced asymmetric transformation were examined by dynamic diastereomeric salt formation: solvent evaporation from a supersaturated solution, and attrition-enhanced asymmetric transformation. The attrition method was more effective for asymmetric amplification of diastereomeric salts of axially chiral materials. Attrition of equimolar amount of the nicotinamide salts with L-DBTA converged to one diastereomer salts, and the corresponding enantiomers in 87-99 % ee were obtained after the chiral acid was removed. Changing the ratio of two of the nicotinamides with L-DBTA to 1 : 2 inverted the axial chirality.