Distinct molecular structures and hydrogen bond patterns of α,α-diethyl-substituted cyclic imide, lactam, and acetamide derivatives in the crystalline phase

High pressure Raman study of isobutyramide

Isobutyramide (IBA) has attracted considerable attention due to its expansive prospects for practical applications in the synthesis of drugs, dyes and other organic compounds. Herein we perform the high-pressure studies of IBA crystal by Raman spectral measurements at room temperature from ambient pressure to 30 GPa by using diamond anvil cells (DACs) to gain comprehensive insights into its structure and stability. Raman vibrational modes of IBA crystal at ambient pressure are resolved based on the experimental results and the first-principles theoretical calculations. High-pressure Raman scattering results show the Raman bands splitting, emergence/disappearance of the Raman bands and discontinuous wavenumber shifts at 1, 2 and 10 GPa, which indicate that IBA crystal undergoes three structural phase transitions at corresponding pressure. In addition, softening of the C = O and N-H stretching vibrational modes of IBA with increasing pressure can be interpreted by the reorganization of the hydrogen bond network of IBA molecules due to phase transition.

Remarkable similarity of molecular packing in crystals of racemic and enantiopure 2-phenylpropionamide: Z' = 4 structures, molecular disorder, and the formation of a partial solid solution

Substituted acetamides (many of which are chiral) are known to be pharmacologically active. 2-Phenylpropionamide (2PPA) is one of the simplest chiral α-substituted acetamides and thus is of interest as a model compound in the growth and design of pharmaceutical crystals. In this study, the crystal structures of racemic and enantiopure forms of 2PPA were determined for the first time using single crystal X-ray diffraction at 100 K. The relationship between the signs of optical rotation and the absolute configurations is (+)-(S)-2PPA and (-)-(R)-2PPA. Four symmetrically independent molecules with different conformations are observed in crystals of both racemic and enantiopure forms. Remarkably, all forms adopt very similar supramolecular structures, H-bonded corrugated layers, that can be described using a R 2 2 ( 8 ) R 6 4 ( 16 ) graph set. The outer surfaces of these layers are built of nonpolar phenyl groups, and their inner structures are composed of H-bonded amide groups. The presence of these layers determines the thin plate shape of 2PPA crystals. Spectroscopically, the racemic and enantiopure forms substantially differ only in the low-frequency Raman region. X-ray diffraction data suggest that the racemic form of 2PPA is a partial solid solution made possible by statistical occupancy of molecular positions by (R)- and (S)-enantiomers.

Unusual polymorphs of rac-3-phenylpyrrolidine-2,5-dione with Z′ = 1, 2, and 3

We report two new polymorphs of rac-3-phenylpyrrolidine-2,5-dione and discuss the structural aspects of variable-Z′ polymorphism in this compound.

Molecular arrangements in crystals of racemic and enantiopure forms of N-carbamoyl-2-phenylbutyramide and 2-phenylbutyramide: differences and similarities

As solid drugs may be regarded as “pharmaceutical materials”, molecular pharmaceutics of such drugs is expected to benefit from application of materials science concepts. In this paper, we used a structural chemistry approach to explain the dramatic difference in solubility between two structurally related antiepileptic drugs, N-carbamoyl-2-phenylbutyramide (NC2PBA) and 2-phenylbutyramide (2PBA). Since both of these compounds are chiral, we chromatographically separated the enantiomers and examined them along with the racemic forms. A combination of experimental (single-crystal X-ray diffraction, IR spectroscopy) and computational (crystal lattice energy calculations, Hirshfeld surface analysis) techniques was employed to determine the structural differences between these two compounds in the crystalline state. We found that while NC2PBA and 2PBA have similar molecular packing arrangements, the former compound is distinguished by a more extensive network of hydrogen bonds. Thus, the higher density, higher melting point, and lower solubility of crystalline NC2PBA compared to crystalline 2PBA may be largely explained by the differences in hydrogen bonding. We also found that for each of these compounds there are no major differences in molecular packing (and, correspondingly, in crystal lattice energies) between racemic and enantiopure forms.

Racemic and enantiopure forms of 3-ethyl-3-phenylpyrrolidin-2-one adopt very different crystal structures

3-Ethyl-3-phenylpyrrolidin-2-one (EPP) is an experimental anticonvulsant based on the newly proposed α-substituted amide group pharmacophore. These compounds show robust activity in animal models of drug-resistant epilepsy and are thus promising for clinical development. In order to understand pharmaceutically relevant properties of such compounds, we are conducting an extensive investigation of their structures in the solid state. In this article, we report chiral high-performance liquid chromatography (HPLC) separation, determination of absolute configuration of enantiomers, and crystal structures of EPP. Preparative resolution of EPP enantiomers by chiral HPLC was accomplished on the Chiralcel OJ stationary phase in the polar-organic mode. Using a combination of electronic CD spectroscopy and anomalous dispersion of X-rays we established that the first-eluted enantiomer corresponds to (+)-(R)-EPP, while the second-eluted enantiomer corresponds to (-)-(S)-EPP. We also demonstrated that, in the crystalline state, enantiopure and racemic forms of this anticonvulsant have considerable differences in their supramolecular organization and patterns of hydrogen bonding. These stereospecific structural differences can be related to the differences in melting points and, correspondingly, solubility and bioavailability.