Five undescribed triterpenoids, two unusual omphalane-type sesquiterpenoids together with twenty-five known compounds were isolated from the leaves and stems of Rhododendron minutiflorum Hu. The absolute configurations of 1-3 and 6 were established by single-crystal X-ray diffraction analysis and electronic circular dichroism (ECD). Compounds 6-7 feature the rare omphalane-type sesquiterpene skeleton and are verified by single-crystal X-ray diffraction analysis for the first time. In the biological activity assay, most of the triterpenoids have different degrees of inhibitory effects on α-glucosidase, with IC50 values ranging from 6.97 to 229.3 μM (the positive control drug acarbose has an IC50 value of 3.07 × 10-3 μM). Structure and activity relationship (SAR) study reveals that the oxidation degrees of C-3, C-8, or C-11 to C-13 of the ursane-type triterpenoid influence the inhibitory activity dramatically.
Keywords: Dammarane-type triterpenoid; Ericaceae; Omphalane-type sesquiterpenoid; Rhododendron minutiflorum Hu; Ursane-type triterpenoid; α-glucosidase inhibitory activity.
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