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.

Ginsenoside Re was the major bioactive component found rich in Panax ginseng C. A. Meyer, which exerted excellent cardiovascular protection, anti-inflammatory, and anti-oxidation effects. The generation of unexpected degradation products (DPs) may influence the therapeutic effect of Re, or even bring toxic effects to patients. However, to date, only a few reports were available about the stability of Re. The present study aims to systematically investigate the degradation behaviors of Re under different stress conditions, including hydrolysis (acidic, basic, and neutral), oxidation, humidity, thermal, and photolytic (ultraviolet and visible light) conditions. A total of thirteen DPs were putatively identified, and among them, nine were discovered for the first time in our study. The results showed that Re was sensitive to exposure to acidic, basic, and oxidation conditions. It underwent a series of chemical degradation reactions, including deglycosylation, dehydration, addition, oxidation at the double bond, and isomerization under various stress conditions. Structural characterization of these DPs was carried out by UHPLC-DAD-CAD and LC-LTQ/Orbitrap. A plausible mechanism of their formation was proposed to support the structures of all DPs of Re. In silico toxicity prediction and metabolism behavior assessment were done by Derek Nexus and Meteor Nexus software. Re and DP-1 to DP-6 were predicted to possess potential skin irritation/corrosion toxicity. DP-11 and DP-12 bear the potential for carcinogenicity, mutagenicity, irritation, hepatotoxicity, and skin sensitization. The observation of these DPs updates our knowledge regarding the stability of Re, which provides valuable information for quality control and to choose suitable storage conditions.