Phytochemical Investigation of Crataegus almaatensis Leaves: Isolation, Structural Characterization, and Antioxidant Activity of Flavonoids, Flavolignans, and Phenolic Glycosides

A systematic phytochemical investigation of the leaves of Crataegus almaatensis Pojark., an endemic hawthorn species native to the mountainous regions of Kazakhstan, was carried out. Multi-step chromatographic fractionation, comprising ultrasound-assisted methanol extraction, sequential liquid–liquid partitioning, silica gel column chromatography, and preparative HPLC, afforded five phenolic compounds as individual, spectroscopically homogeneous constituents from the ethyl acetate fraction. The structures of all isolated compounds were elucidated by comprehensive analysis of high-resolution mass spectrometry (HR-ESI-QTOF-MS) data and one- and two-dimensional NMR spectroscopy (1H, 13C, HSQC, HMBC) in DMSO-d6. The following compounds were identified: (−)-epicatechin, cinchonain Ia, a methoxy- and hydroxy-substituted diaryl ether β-D-glucopyranoside, kakispyrol, and a dimethoxy-substituted diaryl ether β-D-glucopyranoside. The isolated compounds belong to three chemical classes: flavanols, flavolignans, and diaryl ether-type phenolic glycosides. Evaluation of ABTS and DPPH radical-scavenging activity revealed compound-specific antioxidant profiles, with compound 5 (cinchonain Ia) showing activity comparable to the ascorbic acid standard. This study provides, for the first time, a detailed structural characterization of individual phenolic metabolites from C. almaatensis leaves and makes a significant contribution to understanding the chemical diversity of this endemic species.