Exploring the Conformational Effects of N- and C-Methylation of N-Acylhydrazones

Abstract

N-Acylhydrazones (NAH) are privileged structures in chemistry and medicinal chemistry. In this study, we describe the conformational effects of N- and C-methylated N-acylhydrazone derivatives, combining theoretical and experimental data analysis. Four N-acylhydrazone (NAH) derivatives (4-7) were synthesized and structurally characterized to investigate the impact of methylation on their conformational preferences and electronic properties. The structural characterization by NMR spectroscopy, including 2D techniques (HSQC, HMBC, and NOESY), confirmed the exclusive formation of (E)-diastereomers. Theoretical conformational analysis using density functional theory (DFT) calculations (CAM-B3LYP/6-31+G(d,p) with the C-PCM solvation model) revealed that N-methylation (6) significantly alters the preferred dihedral angle (O=C-N-X), inducing a shift from an antiperiplanar to a synperiplanar conformation. Notably, compound 7 showed two possible conformers in solution, anti and syn at the amide bond, and exhibited a greater deviation from planarity due to steric effects imposed by the two methyl groups, which disrupt conjugation within the NAH moiety. This was further supported by natural bond orbital (NBO) analysis, which demonstrated changes in electron density distribution, particularly at the carbonyl and imine carbons, correlating well with the calculated and experimental ¹³C NMR chemical shifts. Noncovalent interaction (NCI) analysis and powder X-ray diffraction provided additional evidence for these conformational trends, reinforcing the influence of methylation on NAH planarity. The findings highlight the steric and electronic consequences of methylation on NAH derivatives, which may have implications for their biological activity and molecular recognition properties.

Figure 1

Bioactivities associated with the privileged NAH structure.

Scheme 1. Synthesis of the NAH Derivatives 4–7 Studied in This Work

Figure 2

Potential energy curves of the compounds evaluated through CAM-B3LYP/6-31+G(d,p) using the C-PCM solvent model for polar organic solvents available in SPARTAN′24.

Figure 3

Spatial correlation between the hydrogens of the NAH moiety of the compounds (4–6).

Figure 4

Noncovalent interaction (NCI) analysis of compounds 4–7. (A–E) Molecular structures with their corresponding reduced density gradient (RDG) isosurfaces and RDG scatter plots of 4, 5, and 6 and 7-syn and 7-anti, respectively. The isosurfaces highlight regions of noncovalent interactions, color-coded according to the interaction type: blue for strong attractive interactions (e.g., hydrogen bonding), green for weak van der Waals interactions, and red for strong steric repulsion. The RDG scatter plots display the sign(λ₂)ρ values, where negative values correspond to attractive interactions, near-zero values indicate van der Waals forces, and positive values represent repulsion. (F) Color scale used in the analysis.

Figure 5

Crystal structures of compounds (4), (5), (6), and (7). (A) and (B) represent the crystal structures of compounds (4) and (6), respectively, as determined in this study. (C), (D), and (E) correspond to crystal structures of (4) (CCDC ID: 1287640), (5) (CCDC ID: 706086), and (7) (CCDC ID: 1006339), respectively, which are already deposited in the CCDC database.

Figure 6

Thin-layer chromatography (TLC) for compounds 4–7. TLC was performed on 2.0 × 6.0 cm aluminum sheets precoated with silica gel 60 (HF-254, Merck) to a thickness of 0.25 mm, using n-hexane/ethyl acetate (70:30) as the solvent system. The spots were visualized under ultraviolet light at 254 nm.