Investigation of the Binary System of Proxyphylline Enantiomers: Structural Resolution and Phase Diagram Determination.

The solid-state landscape of proxyphylline (PXL), a chiral derivative of theophylline crystallizing as a racemic compound, was extensively investigated by means of thermal analyses and diffraction techniques. This study revealed the presence of five distinct polymorphic forms that were characterized: two polymorphs of the racemic mixture and three polymorphs of the pure enantiomer. The nature of each solid phase was confirmed by combining the different analytical techniques, revealing the presence of a thermodynamically stable racemic compound, RI (TFus= 134 °C), in equilibrium with the stable enantiopure crystal form, EI (TFus = 148.3 °C). Additionally, other crystal forms could be evidenced: a polymorph of the racemic compound, RII (TFus= 111.5 °C), as well as two metastable conglomerates, cEI and cEII, and two other polymorphs of the pure enantiomer, EII and EIII. The crystal structures of RI and EI are reported and discussed, highlighting the diversity of molecular conformations that can be adopted by the PXL molecule, which accounts for the versatility of the crystallization behaviors observed in this system. These findings enhance our understanding of the crystallization behavior of chiral pharmaceutical compounds and have implications for optimizing their crystallization processes in the pharmaceutical industry.

Intra- and Intermolecular Cation-π Interactions between Onium Salts and Alkynes/Acetylene: Experimental and Theoretical Insights.

Cation-π interactions between various onium salts, alkynes, and acetylene were studied, taking into account the substituents of the triple bond, the nature of the anions, and the polarity of the solvent, through a combination of MP2 calculations and experiments. In an intramolecular setting, these data (including single-crystal X-ray crystallography) concurred with the stability of folded conformers of alkynyl onium salts, even substituted with electron-withdrawing groups. To examine the contribution of these interactions on the alkyne electronic population, a thorough in silico study was carried out using natural bonding orbital analysis of the conformers. Intramolecular interactions from sulfonium salt tethered to phenylalkyne were highlighted, as illustrated above by the computed folded conformation (MP2) along with noncovalent interaction (NCI) analysis. Furthermore, investigations of intermolecular interactions, involving acetylene or phenylacetylene with various onium ions, revealed the high energy interactions of their complexes with phenyldimethylsulfonium chloride, as illustrated above with the complex PhC≡CH/PhMe2SCl (MP2 calculations and NCI analysis).

Critical Influence of Water on the Polymorphism of 1,3-Dimethylurea and Other Heterogeneous Equilibria.

It is shown that the presence of hundreds of ppm of water in 1,3-dimethylurea (DMU) powder led to the large depression of the transition temperature between the two enantiotropically related polymorphic forms of DMU (Form II → Form I) from 58 °C to 25 °C, thus explaining the reported discrepancies on this temperature of transition. Importantly, this case study shows that thermodynamics (through the construction of the DMU-water temperature-composition phase diagram) rather than kinetics is responsible for this significant temperature drop. Furthermore, this work also highlights the existence of a monohydrate of DMU that has never been reported before with a non-congruent fusion at 8 °C. Interestingly, its crystal structure, determined from X-ray powder diffraction data at sub-ambient temperature, consists of a DMU-water hydrogen bonded network totally excluding homo-molecular hydrogen bonds (whereas present in forms I and II of DMU).

Dichotomic Dearomatizations of Benzene vs Pyridine Rings of Sulfonyloxypyridine via (3 + 2) Cycloaddition.

Electron-poor arenesulfonyloxypyridines are selectively dearomatized whether on the pyridine or on the phenyl group through 1,3-dipolar cycloaddition (1,3-DC) involving non-stabilized azomethine ylides (AMY). Electronic effects of substituents on the aromatic rings allow to induce the regioselectivity of the transformation. Novel pyrrolidinic polycyclic heterocycles are thereby produced under mild acidic conditions at room temperature.

Evidence of a New Crystalline Phase of Prednisolone Obtained from the Study of the Hydration-Dehydration Mechanisms of the Sesquihydrate.

The dehydration of prednisolone sesquihydrate is studied and characterized by different physico-chemical analysis methods. The meticulous study of this dehydration led to the highlighting of a new solid form (form 3), metastable, never identified before. In a second step, the rehydration of anhydrous forms 1 and 2 of prednisolone is studied, in particular by Dynamic Vapor Sorption. It is then demonstrated that neither of the two forms is sensitive to humidity. By means of solid-gas equilibria, the sesquihydrate can only be obtainable from the isomorphic anhydrous form. Finally, a classification of the sesquihydrate is made, taking into account, in particular, the activation energy determined during dehydration.

High pressure promoted dearomatization of nitroarenes by [4+2] cycloadditions with silyloxydienes.

Simple nitroarenes such as nitronaphthalenes and nitroquinolines smoothly undergo dearomatizing [4+2] cycloadditions with silyloxydienes under 16 kbar. Highly functionalized 3-dimensional polycyclic adducts bearing a tetrasubstituted carbon centre at the ring junction are obtained in one step from simple raw materials. This unprecedented dearomative Diels-Alder process is performed at room temperature without any chemical promoter, illustrating the exceptional role of high pressure as a physical promoter.

Diastereoselective addition of redox active esters to azomethine imines by electrosynthesis.

Thanks to metal- and catalyst-free electrochemical conditions in an undivided cell, a series of readily available redox-active N-(acyloxy)phthalimide esters led to an efficient and highly stereoselective addition (85 : 15 to 95 : 5 dr) of putative radical species to chiral (racemic and enantioenriched) C5-substituted azomethine imines to provide an array of 31 polyaminated hydrazine derivatives as a single diastereoisomer.

The Catalytic Regio- and Stereoselective Synthesis of 1,6-Diazabicyclo[4.3.0]nonane-2,7-diones.

A straightforward synthesis of original 1,6-diazabicyclo[4.3.0]nonane-2,7-diones was achieved through a DBU-organocatalyzed multicomponent Knoevenagel-aza-Michael-Cyclocondensation reaction which takes advantage of an unprecedented highly regio- and diastereoselective conjugate addition of pyridazinones to alkylidene Meldrum's acid intermediates. The key reactive intermediates of this complex process were analyzed by means of electrospray ionization mass spectrometry coupled to ion mobility spectrometry, allowing us to validate the proposed mechanism.

Solid-State Overview of R-Baclofen: Relative Stability of Forms A, B and C and Characterization of a New Heterosolvate.

A new polymorphic form (Form C) of enantiopure Baclofen was isolated and characterized. Crystal structures of R-Baclofen Form A and Form C were resolved from powder diffraction data, and cell parameters by profile matching for Form B. The relative stability of these three forms is proposed based on structural data, thermal analyses and solvent-mediated conversions. The experiments highlight the stability order A < C < B at 25 °C (A is the most stable form), whereas above 180 °C it would likely be: C < A < B (C being the stable modification). Moreover, a new heterosolvate of the molecule is observed in N,N-DMF/water mixture. This heterosolvate offers a new pathway to isolate pure R-Baclofen Form B provided the lactam impurity does not exceed 3%. Upon mechanical stress Form B tends to evolve to Form C.

Organocatalytic Multicomponent Synthesis of α/ß-Dipeptide Derivatives.

A straightforward multicomponent Knoevenagel-aza-Michael-cyclocondensation reaction involving readily available hydroxamic acid-derived from naturally occurring α-amino acids allows a diversity-oriented synthesis of novel isoxazolidin-5-ones possessing an N-protected α-amino acid pendant with good to high diastereoselectivities thanks to a match effect with a chiral organocatalyst. These diversely substituted heterocycles, easily isolated as a single diastereoisomer, proved to be versatile platforms for the formation of an array of α/ß-dipeptide fragments.

The facile dearomatization of nitroaromatic compounds using lithium enolates of unsaturated ketones in conjugate additions and (4+2) formal cycloadditions.

The dearomatization of conventional nitroarenes by lithiated enolates derived from methyl vinyl ketones easily takes place, following a formal (4+2) cycloaddition process. While nitroindoles react readily with in situ generated conjugated enolates, the deaggregation of these latter species using HMPA extends the reaction scope to the more aromatic nitronaphthalenes and pyridines.

Correction: Metal-free oxidative ring contraction of benzodiazepinones: an entry to quinoxalinones.

Correction for 'Metal-free oxidative ring contraction of benzodiazepinones: an entry to quinoxalinones' by Hasan Mtiraoui, et al., Org. Biomol. Chem., 2017, 15, 3060-3068.

Metal-free oxidative ring contraction of benzodiazepinones: an entry to quinoxalinones.

A novel and practical synthesis of 3-benzoylquinoxalin-2(1H)-ones from benzodiazepin-2-ones in two steps from commercially available starting materials is reported. The reaction was achieved in the presence of N-bromosuccinimide in DMSO which served both as a solvent and an oxidant. Significantly, the yet unknown ketone to alcohol fluorescence turn-on of benzoylquinoxalinones was unveiled through the preparation of a fluorescently labelled cholesterol conjugate.

Original Mitsunobu-Triggered Sequence Involved in a One-Pot Domino Process toward Tetracyclic Systems Bearing a Bis-N,O-acetal Junction.

Herein is reported an efficient, one-pot domino process through a 1,6-aza-Michael addition-triggered sequence and an original Mitsunobu-type concerted sequence for the synthesis of tetracyclic systems containing a bis-N,O-acetal junction. This methodology led to the construction of four new bonds, the cleavage of three C-O bonds, and the generation of an asymmetric center. Mitsunobu activation afforded final ring closure involving the creation of two bonds, which remains unprecedented among reported Mitsunobu-type sequences. The latter occurred in a regioselective fashion at the challenging C6-position of 2-pyridone intermediates. In the case of adequately substituted enantiopure amino alcohols, up to 95:5 of diastereoisomeric excess was achieved. Computational studies allowed the discrimination of a favored pathway for Mitsunobu sequence and supported the regioselectivity as well as the diastereoselectivity observed for this step.

Organocatalyzed Multicomponent Synthesis of Isoxazolidin-5-ones.

An unprecedented multicomponent organocatalyzed Knoevenagel-aza-Michael-cyclocondensation reaction between Meldrum's acid, hydroxylamines, and aldehydes afforded a straightforward entry to a large array of racemic and syn-diastereoenriched isoxazolidinones as synthetically useful scaffolds. This process revealed a markedly facile aza-Michael-cyclocondensation sequence as a key domino reaction between RCO2NHOH and transient alkylidene Meldrum's acid upon Brønsted base catalysis.

Simple access to highly functional bicyclic γ- and δ-lactams: origins of chirality transfer to contiguous tertiary/quaternary stereocenters assessed by DFT.

This paper describes the synthesis of both polysubstituted oxazolo-pyrrolidinones and -piperidinones by a domino process. The methodology is based on the reaction between hydroxyl halogenoamides and Michael acceptors, which leads efficiently to bicyclic lactams. The process is compatible with unsymmetrical electron-withdrawing groups on the Michael acceptor, which allows the formation of two contiguous and fully controlled tertiary and quaternary stereocenters. In the case of tetrasubstituted Michael acceptors, two adjacent quaternary stereocenters are formed in good yield. Starting from (R)-phenylglycinol derived amides results in the formation of enantioenriched bicyclic lactams in low to good yields and with high levels of stereoselectivity, thus greatly increasing the scope and interest of this strategy. The origins of chirality transfer and diastereoselectivity were studied by DFT calculations and have been attributed to a kinetic control in one of the last two steps of the reaction sequence. This selectivity is dependent upon both the substituents on the Michael acceptor and the sodium cation chelation.

Organocatalysed multicomponent synthesis of pyrazolidinones: Meldrum's acid approach.

We discovered a novel organocatalysed multicomponent domino Knoevenagel-aza-Michael-cyclocondensation reaction leading to an unprecedented straightforward synthesis of 1,5-diazabicyclo[3.3.0]octane-2,6-diones. The specific capability of the (DHQ)2PHAL organocatalyst in this process was also highlighted to eventually furnish the corresponding bicyclopyrazolidinones with up to 96 : 4 er.

Developments in Meyers' lactamization methodology: en route to bi(hetero)aryl structures with defined axial chirality.

Highly atroposelective Meyers' lactamization promoted by pivalic acid under microwave irradiation is reported which allows the construction of nonracemic substituted-dibenzo(di)azepine derivatives through a center to axial chirality transfer principle, controlling the otherwise configurationally labile biaryl axis. This approach provides a straightforward entry to enantioenriched analogues of biorelevant architectures.

A hybrid mechanism in chiral discrimination induced by crystallization of supramolecular compounds.

Host-guest complexes formed in aqueous medium between permethylated ß-cyclodextrin (TMß-CD) and racemic 1-(p-fluorophenyl)ethanol (p-F-PE) are studied. The crystalline complexes are characterized and their crystal structures are determined, revealing two sets of solid phases with specific abilities for chiral discrimination: on the one hand, a stable complete solid solution with two independent complexes per asymmetric unit exhibits a limited chiral recognition, and on the other hand, two metastable partial solid solutions with unusual 1:2 host-guest stoichiometries behave as diastereomeric complexes. The structural features of the 1:2 complexes and their study by means of molecular modeling show that these solid phases, described as cocrystals formed between one host-guest inclusion complex and one non-engulfed p-F-PE molecule, present a significant chiral discrimination occurring both inside the cyclodextrin and outside the macrocycle in a crystal lattice cavity. Therefore, the enantioselectivity observed in this system results from an interplay between molecular inclusion in the cyclodextrin and lattice inclusion. To our knowledge, it is the first report of such a hybrid mechanism. An overview of the crystal structures of the literature containing TMß-CD is also achieved and allows their classification in four structural groups in relation to their crystal packing features.

N,N'-Dimethyl-ethylenediammonium dioxalatocuprate(II).

The asymmetric unit of the title salt, (C(4)H(14)N(2))[Cu(C(2)O(4))(2)], consists of one complex anion and two cationic half-mol-ecules, the other halves being generated by inversion symmetry. The Cu(II) atom in the anion is coordinated by two bidentate oxalate ligands in a distorted square-planar geometry. Inter-molecular hydrogen bonds, involving the NH groups as donors and O atoms as acceptors, are observed, which lead to the formation of a three-dimensional network structure.