Prospecting New Limonoids From The Mangrove Plant,Xylocarpus Moluccensis,Biomimetic Synthesis,and Antitumor Activity Evaluation

Mangroves are salt-tolerant arbors or shrubs in the intertidal zone of the tropical and subtropical area.Mangrove swamps,mainly composed of mangrove plants,are usually named‘coast guard’.Globally,mangrove plants have about 84 species,which can be divided into two categories,viz.true mangrove and semi-mangrove.The plant genus Xylocarpus is the only one ture mangrove genus belonging to the family Meliaceae,which are known for the production of limonoids with variable frameworks and various bioactivities.Limonoids have attracted considerable interests for natural product researchers and synthetic chemists over the past decade.The seeds of X.moluccensis in this thesis were collected in the mangrove swamps of Trang Province,Thailand,and Krishna estuary,India,respectively.A total of 39 limonoids were obtained from the Thai X.moluccensis by extraction with 95%ethanol,followed by suspension in water and extraction with ethyl acetate,and then purification with silica gel column chromatography,C₁₈ revised-phase column chromatography,and high performance liquid chromatography.The structures of these compounds were determined by ESI-MS,NMR,Single-crystal X-ray diffraction,quantum-chemical electronic circular dichroism（ECD）calculations,and Cotton effects comparison of ECD spectroscopy.22 compounds were identified as new limonoids,whereas 17 ones are known compounds.22 new limonoids were trangmolins A-F（1-6）,thaixylomolin Q（7）,xylomolins A₁,A₃-A₆（8-12）,xylomolins B₁-B₂（13-14）,xylomolins D-E,H-I（15-18）,6R-hydroxymoluccensin A（19）,compounds 20-22,respectively.Among the new limonoids,the C1–C30 linkage in 1–3 and C3–C30 connection in 4 formed two unprecedented types of the ring-A/B-fused carbo-bicyclic cores in limonoids.The oxidative cleavage of the C2–C3 bond in 5 was found for the first time in the construction of limonoid architectures.Compound 21 is a new skeletal limonoid derived from the oxidative cleavage of the C9?C10 bond and then the linkage of C-3 and C-9 via an oxygen bridge;whereas compound 22 is a limonoid with a novel backbone derived from the oxidative breakage of the C2?C1 bond and then linkage of C-2 and C-6 through an oxygen bridge to form an additional ring.In total,five types of novel limonoid skeletons were identified.Krishnadimer A（40）,a C₂-symmetric limonoid dimer representing a new class of axially chiral nonbiaryl natural products,was firstly isolated from the seeds of X.moluccensis,collected in the estuary of Krishna,India.Krishnadimer A was biomimetically semisynthesized by atroposelective 2,3-dichloro-5,6-dicyanobenzoquinone（DDQ）-mediated dienol-oxidative coupling,together with a pair of atropisomers of its derivatives（40a/40b）.Single-crystal X-ray diffraction and ECD spectroscopy analysis of the synthesized krishnadimer A（40）unambiguously established its absolute configuration,including the（P）-configuration of the C15-C15′central axis.Antitumor investigations of the synthesized dimers 40,40a,and 40b revealed the large impact of axial chirality on bioactivities.Scaffold diversity-oriented synthesis under various oxidative coupling conditions lead to the production of ten new limonoid dimers（41-45,41’-43’,46a,and46b）with six different types of architectures.Anti-breast cancer bioassays revealed that the dimer 46b with a（M）-configured C15–C15’central axis exhibited selective and potent cytotoxicities against human triple-negative breast cancer（TNBC）MDA-MB-231 and MDA-MB-453 cells with IC₅₀ values of 5.57±1.48 and 3.93±0.75μM,respectively.It induces cell-cycle arrest at G2/M phase and apoptosis,and enhances the accumulation of reactive oxygen species（ROS）in cells.Importantly,anti-TNBC assay on Balb/c nude mice indicated that after intravenous injection with 46b at the dose of 30 mg/kg every two days for four weeks,the inhibitory rate for TNBC MDA-MB-453 tumor xenografts was63.83%.Calculations of Gibbs free energy changes from（M）-to（P）-configuration concluded that the stable（M）-configuration of the C15–C15’central axis is pivotal for in vivo anti-TNBC activity of 46b.This is the first report of an axially chiral limonoid dimer with in vivo anticancer activity.