Exploring the Conformational Effects of <i>N</i> - and <i>C</i> -Methylation of <i>N</i> -Acylhydrazones

<i>N</i>-Acylhydrazones (NAH) are privileged structures in chemistry and medicinal chemistry. In this study, we describe the conformational effects of <i>N</i>- and <i>C</i>-methylated <i>N</i>-acylhydrazone derivatives, combining theoretical and experimental data analysis. Four <i>N</i>-acylhydrazone (NAH) derivatives (<b>4</b>-<b>7</b>) 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 (<i>E</i>)-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 <i>N</i>-methylation (<b>6</b>) significantly alters the preferred dihedral angle (O=C-N-X), inducing a shift from an antiperiplanar to a synperiplanar conformation. Notably, compound <b>7</b> showed two possible conformers in solution, <i>anti</i> and <i>syn</i> 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.