This paper reviews a substantial amount of research related to the Eschweiler–Clarke methylation reaction, summarizing its mechanism, development, and applications, to present the chemical essence and history of this reaction and its importance and application in modern synthetic chemistry. In particular, this review focuses on advances regarding the Eschweiler–Clarke methylation reaction since 2000. This work will provide researchers with a comprehensive and in-depth perspective of the Eschweiler–Clarke reaction, as well as guidance and inspiration for future research and application.

We developed a solvent-suppressed ¹H nuclear magnetic resonance (NMR) method for the quantitative analysis of the components of rotigotine prolonged-release microspheres prepared for injection. Dimethyl terephthalate was used as an internal standard and dimethylsulfoxide -d₆ as the solvent. The analysis was performed using a Bruker Avance III HD 600 MHz NMR spectrometer, employing the noesygppr1d pulse sequence at a controlled temperature of 25 °C. Nuclear magnetic resonance spectra were acquired with a relaxation delay time (D1) of 40 s to simultaneously determine the content of rotigotine and the excipients mannitol and stearic acid in the rotigotine prolonged-release microspheres. Using the proposed approach, we successfully quantified the active pharmaceutical ingredient rotigotine and excipients in the prolonged-release microspheres. This method demonstrated excellent linearity, high precision, and strong repeatability. The solvent-suppressed ¹H NMR method developed in this study allows for the simultaneous quantification of rotigotine and the key excipients mannitol and stearic acid in the prolonged-release microspheres. This approach is accurate, simple, efficient, and environmentally friendly.

Background/Objectives: Rotigotine is a conformationally flexible chiral dopamine agonist, but its solution-phase chiroptical behavior and conformational heterogeneity have not been systematically assessed. This study characterized its electronic circular dichroism (ECD), optical rotatory dispersion (ORD), and vibrational circular dichroism (VCD) responses and evaluated their consistency with its absolute configuration. Methods: ECD and ORD data were acquired for rotigotine ((S)-1) and its simplified analog (S)-2 in acetonitrile and methanol, and VCD spectra were recorded in appropriate deuterated solvents. Conformational searches, Boltzmann weighting, and density functional theory/time-dependent density functional theory calculations were performed. A principal-component-analysis/torsion (PCA-Tor) workflow selected a compact but structurally representative conformer ensemble for (S)-1. Results: In acetonitrile, (S)-1 exhibited a positive Cotton effect (CE) at 208.5 nm and a negative CE at 194.0 nm. The Boltzmann-weighted ECD spectrum reproduced the intense negative band below 200 nm and the positive band near 210 nm. The calculated VCD spectrum agreed with the signs and positions of the main experimental bands in the 1500–1100 cm⁻¹ region. Experimental ORD curves of both compounds were negative in acetonitrile and methanol. The calculations reproduced the sign and overall trend, although their magnitudes were protocol- and population-sensitive. Ring puckering strongly affected the calculated ECD and ORD data, whereas side-chain flexibility expanded the accessible conformational space. Conclusions: The combined results support the known S configuration of rotigotine and demonstrate the value of representative conformational sampling for flexible chiral pharmaceuticals. This study also defines the practical utility and limitations of simplified molecular models in chiroptical analysis.

Tramadol acts via μ-opioid receptor agonism and monoamine reuptake inhibition but is clinically limited by metabolic dependence, interindividual variability, and addiction risks. Structural modification aims to resolve these limitations. This review systematically summarizes tramadol’s structure–activity relationships and mechanisms, focusing on key strategies for structural optimization. Major advances include: (i) synergistic strategies, such as tramadol–celecoxib cocrystals (tramadol and celecoxib coexist in the supramolecular crystal network at a 1:1 molar ratio), achieving multimodal analgesia at lower doses; (ii) mechanism-balancing strategies such as tapentadol (derivatives of tramadol with a dual mechanism of action), which enhance μ-opioid agonism and norepinephrine reuptake inhibition while attenuating serotonergic effects to improve efficacy; (iii) metabolic optimization utilizing M1 analogues to circumvent CYP2D6 polymorphisms (tramadol is metabolized by this enzyme into the active metabolite M1 to exert analgesic effects); and (iv) pharmacophore optimization leveraging tramadol–morphine homology and “message–address” concepts to design selective ligands. Novel derivatives demonstrate improved potency and metabolic stability but continue to face challenges regarding opioid risks and clinical translation. Future research should integrate rational drug design, delivery systems, and personalized medicine to facilitate the development of safer next-generation analgesics.