Eremane, viscidane and isozizaene diterpenoids from the leaves of Eremophila rigida and their absolute configurations

Previously undescribed eremane, viscidane, and isozizaene diterpenoids, eremorigidanes A-F, along with six known O-methylated flavonoids and three known triterpenoids were isolated and identified from the leaves of Eremophila rigida Chinnock by combined use of high-resolution PTP1B inhibition profiling, semipreparative- and analytical-scale HPLC separations, HPLC-PDA-HRMS analysis, and NMR spectroscopy. The absolute configuration of the unreported diterpenoids were determined by comparison of their experimental and calculated ECD spectra as well as by biosynthetic arguments. All isolates were evaluated for their PTP1B inhibitory activities, which revealed the flavonoid penduletin (3) to show inhibition with an IC50 value of 18.3 μM, and the triterpenoids 3,4-seco-olean-12-ene-3,28-dioic acid (15), oleanolic acid (16), and 3-oxo-oleanolic acid (17) to show inhibition with IC50 values of 55.7, 9.9, and 6.3 μM, respectively. The preliminary structure-activity relationship (SAR) of isolated flavonoids and triterpenoids is discussed. Plausible biosynthetic steps involved in eremane and isozizaene metabolism are presented and discussed.

Identification of new PTP1B-inhibiting decipiene diterpenoid esters from Eremophila clarkei by high-resolution PTP1B inhibition profiling, enzyme kinetics analysis, and molecular docking

In this study, an extract of the leaves of Eremophila clarkei Oldfield & F.Muell. showed protein tyrosine phosphatase 1B (PTP1B) inhibitory activity with an IC50 value of 33.0 μg/mL. The extract was therefore investigated by high-resolution PTP1B inhibition profiling to pinpoint the constituents responsible for the activity. Subsequent isolation and purification using analytical-scale HPLC led to identification of eight previously undescribed decipiene diterpenoids, eremoclarkanes A-H, as well as eremoclarkic acid, a biogenetically related new phenolic acid. In addition, one known decipiene diterpenoid and ten known O-methylated flavonoids were isolated. The structures of the isolated compounds were elucidated by extensive analysis of their HRMS and 1D and 2D NMR spectra. The absolute configuration of decipiene diterpenoids was determined by comparison of experimental and calculated ECD spectra. The flavonoid hispidulin (2b) and the four decipiene diterpenoids 13a, 13b, 13f, and 14b exhibited PTP1B inhibitory activity with IC50 values ranging from 22.8 to 33.6 μM. This is the first report of PTP1B inhibitory activity of decipienes, and enzyme kinetics revealed that 13a and 13b are competitive inhibitors of PTP1B, whereas 13f and 14b displayed mixed-type-mode inhibition of PTP1B. Finally, molecular docking indicated that 13a, 13b, 13f, and 14b showed comparable binding affinity towards the active and/or allosteric site of PTP1B enzyme. Structure-activity relationship (SAR) of the identified O-methylated flavonoids and decipiene diterpenoids towards PTP1B is discussed. Plausible enzymatic and photochemically driven routes for the formation of the decipienes and conversion products thereof are presented and discussed.

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.

Serrulatane diterpenoids with unusual side chain modifications from root bark of Eremophila longifolia

The plant genus Eremophila is endemic to Australia and widespread in arid regions. Root bark extract of Eremophila longifolia (R.Br.) F.Muell. (Scrophulariaceae) was investigated by LC-PDA-HRMS, and dereplication suggested the presence of a series of diterpenoids. Using a combination of preparative- and analytical-scale HPLC separation as well as extensive 1D and 2D NMR analysis, the structures of 12 hitherto unreported serrulatane diterpenoids, eremolongine A-L, were established. These structures included serrulatanes with unusual side chain modifications to form hitherto unseen skeletons with, e.g., cyclopentane, oxepane, and bicyclic hexahydro-1H-cyclopenta[c]furan moieties. Serrulatane diterpenoids in Eremophila have recently been shown to originate from a common biosynthetic precursor with conserved stereochemical configuration, and this was used for tentative assignment of the relative and absolute configuration of the isolated compounds. Triple high-resolution α-glucosidase/α-amylase/PTP1B inhibition profiling demonstrated that several of the eremolongines had weak inhibitory activity towards targets important for management of type 2 diabetes.

Serrulatane diterpenoids from the leaves of Eremophila glabra and their potential as antihyperglycemic drug leads

Eremophila (Scrophulariaceae) is a genus of Australian desert plants, which have been used by Australian Aboriginal people for various medicinal purposes. Crude extracts of the leaf resin of Eremophila glabra (R.Br.) Ostenf. showed α-glucosidase and protein tyrosine phosphatase 1B (PTP1B) inhibitory activity with IC₅₀ values of 19.3 ± 1.2 μg/mL and 11.8 ± 2.1 μg/mL, respectively. Dual α-glucosidase/PTP1B high-resolution inhibition profiling combined with HPLC-PDA-HRMS and NMR were used to isolate and identify the compounds providing these activities. This resulted in isolation of seven undescribed serrulatane diterpenoids, eremoglabrane A-G, together with nine previously identified serrulatane diterpenoids and flavonoids. Three of the serrulatane diterpenoids showed PTP1B inhibitory activities with IC₅₀ values from 63.8 ± 5.8 μM to 104.5 ± 25.9 μM.

PTP1B-Inhibiting Branched-Chain Fatty Acid Dimers from Eremophila oppositifolia subsp. angustifolia Identified by High-Resolution PTP1B Inhibition Profiling and HPLC-PDA-HRMS-SPE-NMR Analysis

Ten new branched-chain fatty acid (BCFA) dimers with a substituted cyclohexene structure, five new monomers, and two known monomers, (2E,4Z,6E)-5-(acetoxymethyl)tetradeca-2,4,6-trienoic acid and its 5-hydroxymethyl analogue, were identified in the leaf extract of Eremophila oppositifolia subsp. angustifolia using a combination of HPLC-PDA-HRMS-SPE-NMR analysis and semipreparative-scale HPLC. The dimers could be classified as three types of Diels-Alder reaction products formed between monomers at two different sites of unsaturation of the dienophile. Two of the monomers represent potential biosynthetic intermediates of branched-chain fatty acids. Several compounds were found by high-resolution bioactivity profiling to inhibit PTP1B and were purified subsequently by semipreparative-scale HPLC. The dimers were generally more potent than the monomers with IC₅₀ values ranging from 2 to 66 μM, compared to 38-484 μM for the monomers. The ten fatty acid dimers represent both a novel class of compounds and a novel class of PTP1B inhibitors.