Characterization of serrulatane diterpenoids in Eremophila phyllopoda subsp. phyllopoda by triple high-resolution α-glucosidase/PTP1B/radical scavenging profiling, NMR spectroscopy, DFT-GIAO NMR, and electronic circular dichroism calculations

Extracts of Eremophila phyllopoda subsp. phyllopoda showed α-glucosidase and PTP1B inhibitory activity with IC₅₀ values of 19.6 and 13.6 μg/mL, respectively. High-resolution α-glucosidase/PTP1B/radical scavenging profiling was performed to establish a triple high-resolution inhibition profile that allowed direct pinpointing of the constituents responsible for one or more of the observed bioactivities. Subsequent targeted isolation and purification by analytical-scale HPLC led to the identification of 21 previously undescribed serrulatane diterpenoids, eremophyllanes A-U, as well as two known serrulatane diterpenoids, 1β-trihydroxyserrulatane (8) and 1α-trihydroxyserrulatane (10d), and five known furofuran lignans, (+)-piperitol (6), horsfieldin (7e), (―)-sesamin (9), (+)-sesamin (10h), and asarinin (10i). Their structures were elucidated by extensive analysis of HRMS and 1D and 2D NMR spectroscopic data. The relative configurations of the previously undescribed compounds were established by analysis of ROESY spectra as well as by DFT-GIAO NMR calculations followed by DP4+ probability analysis. The absolute configurations were determined by comparison of experimental and calculated ECD spectra. Serrulatane diterpenoids 7b and 14 exhibited α-glucosidase inhibitory activity with IC₅₀ values of 28.4 and 64.2 μM respectively, while 11, 12, 14, and 15 exhibited PTP1B inhibitory activity with IC₅₀ values ranging from 16.6 to 104.6 μM. Hypothetical routes for formation of all identified serrulatane diterpenoids are proposed.