Recent progress in chemistry and bioactivity of monoterpenoid indole alkaloids from the genus gelsemium: a comprehensive review

Abstract Monoterpenoid indole alkaloids (MIAs) represent a major class of active ingredients from the plants of the genus Gelsemium. Gelsemium MIAs with diverse chemical structures can be divided into six categories: gelsedine-, gelsemine-, humantenine-, koumine-, sarpagine- and yohimbane-type. Additionally, gelsemium MIAs exert a wide range of bioactivities, including anti-tumour, immunosuppression, anti-anxiety, analgesia, and so on. Owing to their fascinating structures and potent pharmaceutical properties, these gelsemium MIAs arouse significant organic chemists’ interest to design state-of-the-art synthetic strategies for their total synthesis. In this review, we comprehensively summarised recently reported novel gelsemium MIAs, potential pharmacological activities of some active molecules, and total synthetic strategies covering the period from 2013 to 2022. It is expected that this study may open the window to timely illuminate and guide further study and development of gelsemium MIAs and their derivatives in clinical practice.


Introduction
Derivatives of monoterpenoid indole alkaloids (MIAs) represent a class of active secondary metabolites mostly isolated from the plants of the Gelsemium genus (Loganiaceae), mainly in three MIAs are particularly concentrated in the roots of Gelsemium plants, accounting for the content of approximately 0.5%, whereas the stems, fruits, branches, and leaves also contain smaller amounts 3 . Since the discovery of the first MIA in 1959, more than 100 kinds of representative MIAs with complex structures have been widely extracted from these Gelsemium plants 4 . The majority of gelsemium MIAs have been found to exhibit a plethora of notable pharmacological properties, especially anti-tumour, immunosuppressive, anxiolytic, and analgesic characteristics, reflecting their great potential as lead compounds in new drug development [5][6][7][8] .
In terms of structure type, gelsemium MIAs possess characteristic chemical structures containing polycyclic monoterpene portions and indole, oxindole, or bisindole nuclei 9 . Gelsemium MIAs can be classified into six categories: gelsedine-, gelsemine-, humantenine-, koumine-, sarpagine-and yohimbane-type, on the basis of their structural features 10 (Figure 1). Among them, gelsemine-type, humantenine-type and gelsedine-type alkaloids bear peculiar spiro-indolinone nuclei, while koumine-, sarpagine-and yohimbane-type alkaloids have normal indole groups. The structural skeletons of their monoterpene parts incorporate sterically compact and dense polycyclic architectures and multiple stereocenters, forming privileged chemical diversity and structural complexity of gelsemium MIAs. These exceptional structural properties of gelsemium MIAs render them sophisticated challenges for total synthesis and structure modification and attracted considerable attention from synthetic scientists. Historically, a vast number of total synthetic works on gelsemium MIAs have been reported [11][12][13] .
Jin's group (2014) previously reviewed the phytochemistry, pharmacology, and toxicology together with their traditional use of the genus Gelsemium, whereas Carter's group (2019) described the synthetic strategies towards the gelsemine-and gelsedinetype MIAs between 2005 and 2016 14,15 . However, these reviews did not provide a comprehensive review of all types of gelsemium MIAs, especially in aspects of their frontier pharmacological effects as well as chemical structures and syntheses. A large number of breakthroughs in their novel compounds, biological activities, and more elegant total syntheses have been reported over the past decade. As such, this review is intended to comprehensively summarise the representative examples covering from 2013 to 2022 with the following novel objectives: (1) a comprehensive presentation of novel gelsemium MIAs and more elegant total synthetic methodologies; (2) a focus on recent their bioactivities of gelsemium MIAs, mainly involving specific biotarget and mechanism of actions; (3) an overview of advice how gelsemium MIAs can be utilised as promising candidates in further studies. Finally, we hope that this review will provide an insight into rational study and development of gelsemium MIAs and their derivatives in further clinical practice.

Novel chemical structures of gelsemium MIAs
Since 2013, a total of 70 novel MIAs have been isolated from the Gelsemium genus, mainly G. elegans. The structural types of gelsemium MIAs are mainly focussed on gelsedine-type, humanteninetype, and koumine-type. These gelsemium MIAs groups will be discussed in the following paragraphs (Table 1).

G. elegans
HR-FAB-MS, 13  respectively, which structurally featured a particular trans-butenyl group at C20 19,20 . Gelsekoumidines A (15) and B (16) were two pairs of atropoisomeric bisindole alkaloids from the roots of G. elegans. Gelsekoumidines A and B represented an unprecedented class of seco-koumine-gelsedine-type alkaloids containing a unique 20,21-seco-koumine skeleton fused with a gelsedine scaffold via a double bond bridge. The only difference between these two compounds was that gelsekoumidine A had a hydroxyl group at C1421. 14b-Hydroxygelsedethenine (17) and 14-hydroxygelseziridine (18) were obtained from the aerial parts of G. elegans. Structural comparison with gelsedine (1) displayed that the two compounds had an a-configuration of the 1,2-oxaziridine group located between C20 and N419, 22 . Gelseleganin E (19) and 11-methoxy-14-hydroxygelsedilam (20) were isolated from the leaves and branches of G. elegans, which possessed a particular lactam ring 17,23 . Gelseleganin C (21) and gelsepyrrodines A (22), B (23), and C (24) were isolated from the leaves, branches, and roots of G. elegans, in which a pyrrole ring was incorporated into their gelsedine-type skeleton. The main difference was the replacement of an additional aldehyde in the pyrrole ring in the former two by an acetyl group in the latter two 17,24 . Gelsecorydines A (25), B (26), C (27), D (28), and E (29), five bisindole alkaloids with novel chemical skeleton, were obtained from the fruits of G. elegans. Their heterodimeric framework incorporated a gelsedine-type alkaloid and a modified corynanthe-type monomer. Especially, gelsecorydine B possessed an unprecedented caged structure with a 6/5/7/6/5/6 heterohexacyclic ring system via a direct pyridine ring linkage 25 . Gelserancines A (30), B (31), C (32), D (33), and E (34), five unusual gelsedine-type derivatives, were separated from the roots of G. elegans. The structure of gelserancine A incorporated a rare trimethyl-dihydrofuranone building block at C20. In gelserancines B and C, their gelsedine-type frameworks are bound with an additional pyridine ring with a 5-hydroxy-2-(hydroxymethyl)-3methylcyclopentyl moiety at N4 and C19. Gelserancines D and E were a pair of E/Z tautomer, in which the 14-hydroxygelsenicine unit was connected to a 2-hydroxymethyl furan ring via a C19-C1' conjugated bridge 26 . Geleganimines A (35) and B (36), two trace nonsymmetric bisindole alkaloids, were isolated from the aerial parts of G. elegans. Geleganimines A and B belonged to two epimers consisting of gelsenicine and gelseziridine moieties via a 3carbon alkanoic chain 27 . 11-Methoxy-14,15-dihydroxygelsedine (37) and 11-methoxy-14,15-dihydroxy-19-oxogelsenicine (38) were got from the ethanol extracts of the leaves and branches of G. elegans. The difference between them was that the ethyl moiety in compound 37 was replaced by an acetyl group in compound 3823. Gelselegandines A (39), B (40), and C (41), isolated from the roots of G. elegans, possess an unprecedented gelsedine-type core structure incorporating an additional C 9 aromatic unit as a side chain. Gelselegandines A and C belonged to a pair of cisand trans-isomers, whereas gelselegandine B existed in the replacement of the ethyl group by a vinyl moiety that was not in accordance with the two formers 28 (Figure 3).

Humantenine-type alkaloids
The chemical structures of humantenine-type alkaloids are quite similar to those of gelsemine-type alkaloids, having an oxindole group, but adding a C21 carbon and a C19-C20 double bond. 19,20-Epoxyhumantenine (42), gelselegandines D (43), and E (44), with a rare epoxypropyl ring at C19 and 20, were isolated from the roots and stems of G. elegans. The difference was that the configuration of C19 near the epoxypropyl ring was S and R in gelselegandines D and E, respectively 20,29 . Geleganidines A (45) and C (46), two unusual humantenine-type alkaloids, were isolated from the roots of G. elegans. Particularly, geleganidine A carried a formamide moiety at N4 in its molecular structure. Geleganidine C was a novel dimer of geleganidine A connected by a carbonyl group to form a rare ureacontaining substructure 30 . 11-Hydroxyhumantenine N 4 -oxide (47) and N-desmethoxyhumantenine N 4 -oxide (48) were isolated from the stems of G. elegans. Both of them owned a N-O coordinate linkage at N4, however, the distinct difference was the methoxy group and hydroxy substitution at C11 and N1 in compound 4718. N 4 -methyl-19,20-dihydrorankinidine (49) and gelstriamine A (50) were isolated from the roots and stems of G. elegans, respectively. In compound 49, the C20 olefin moiety was changed to an ethyl group, compared with humantenine. As mimics of compound 49, gelstriamine A featured a unique hexahydrooxazolo [4,5-b]pyridin-2(3H)-one moiety at C20 and C21, forming an abnormal 6/5/7/6/6/5 heterohexacyclic core 16,18 . 14b,20a-Dihydroxydihydrorankinidine (51), 11-methoxy-19,20a-dihydroxydihydrorankinidin (52), and norhumantenine A (53) were purified from the leaves and vine stems of G. elegans. The structure of compounds 51 and 52 was similar to that of rankinidine, except for the reduction of the C19-C20 double bond with a location of a hydroxy group at C20. A comparison of structural differences showed the replacement of the C18/C21 subunits by those from an a,b-unsaturated formyl functionality in norhumantenine A 31 ( Figure 4).

Sarpagine-type alkaloids
Sarpagine-type alkaloids feature an exocyclic (E)-ethylidene side chain and a cage-shaped scaffold that is made up of two bridged substructures, namely indole-fused azabicyclo [  high not only in Gelsemium genus, but also in Gardneria, Rauwolfia, and Alstonia genera (Apocynaceae) 35,36 . epi-Koumidine N 4 -oxide (71), isolated from the stems of G. elegans, had the presence of one more oxygen atom than that of epi-koumidine, indicating that the formation of a coordination bond was formed among oxygen and nitrogen atoms. It represented the first example of N 4 -oxide sarpagine-type alkaloid from the Gelsemium genus 18 (Figure 6(B)).    46 Anxiolytic activity koumine (2) i.g.

Pharmacological activity
In recent years, a large number of studies have proven that gelsemium MIAs exhibit extensive beneficial pharmacological activities, which primarily focus on analgesic, anti-tumour, anxiolytic, immunosuppressive, and anti-inflammatory aspects. Among general gelsemium MIAs, the activity evaluations of gelsemine and koumine are the most intensively studied. These pharmacological activities could be briefly summarised as follows ( Table 2).

Analgesic activity
Several research has indicated that gelsemium MIAs exert analgesic properties in vivo and in vitro. Koumine (2) is the most abundant MIA of G. elegans. Its treatment displayed efficient analgesic activity against inflammatory and neuropathic pain in a variety of rodent models 37 . Mechanistic studies revealed that koumine could function as a high-affinity ligand that interacted with translocator protein 18kda positive (TSPO) protein in microglia, thereby inducing TSPO allostery 37,38 . TSPO allostery triggered the biosynthesis of neurosteroids, such as allopregnanolone in the spinal cords, which mediated the reduction of neuropathic pain 39 . Moreover, in vivo and in vitro studies also found that koumine enabled to inhibit the production of proinflammatory cytokines and glial activation 40,41 . Because TSPO is the typical marker of activated microglia, we conjectured that this anti-inflammatory action of koumine might also be related to TSPO allostery. Besides, koumine also increased astrocyte autophagy occurrence and decreased astrocyte-related inflammation, which was the mechanistic basis for its analgesic activity 42 . In terms of improvement in diabetic neuropathic pain (DNP), the neuropathic pain behaviour and the injury of axon and myelin sheath of the sciatic nerve were greatly ameliorated in streptozocin (STZ)-induced diabetes rats after subcutaneous treatment with koumine (0.28, 1.4, and 7.0 mg/kg, for 7 days) 43 . Its analgesic effects could be attributed to the modulation of spinal microglial M1 polarisation and proinflammatory mediators via inhibiting the Notch-RBP-Jj signalling pathway 44 . Moreover, the study on pharmacokinetics indicated that koumine elimination was decreased in STZ-induced rats, suggesting koumine was retained for the treatment of DNP in vivo 45 . Gelsemine (3), is the principal active alkaloid from G. sempervirens. Like koumine, gelsemine also was able to inhibit nociceptive pain and tonic pain in different pain models. Its mechanism for analgesic activity was that gelsemine might serve as a potential a3 glycine receptor (a3-GlyR) agonist to modulate the function of spinal a3-GlyRs and stimulate the biosynthesis of allopregnanolone through upregulation of the mRNA expression of 3a-hydroxysteroid oxidoreductase [46][47][48] . Where after, gelsemine's analgesic effect was reported in partial sciatic nerve-ligated mice as its administrations (2.0 and 4.0 mg/kg, i.p.) alleviated both neuropathic pain and sleep disturbance, and upregulated c-Fos expression in the neurons of the anterior cingulate cortex 49 . Additionally, intraperitoneal N-desmethoxyhumantenine N 4 -oxide (48) treatment at lower doses of 0.04 and 0.2 mg/kg alleviated acetic acid intraperitoneal injectioninduced writhing of mice with inhibition rates of 67.6 and 76.1%, respectively, which were even stronger than those of the positive control, morphine. Additionally, gelstriamine A (50) treatment (1.0 mg/kg, i.p.) showed potent analgesic activity with a reduced rate of 64.7% in the same model 18 .

Antitumor activity
Sempervirine (6)    the prefrontal cortex and hippocampus and a decrease in ACTH and CORT levels in plasma, suggesting that its anxiolytic mechanism was related to mediating neurosteroids-HPA axis 55 . In 2013, an in vitro study performed in rats reported that gelsemine (3) treatment at low doses (0.0001 and 1 lM, i.p., for 7 days) significantly improved anxiety-specific parameters, some of which even approached the activity of the positive control, benzodiazepine diazepam 56 . Also, gelsemine when administered to chronic unpredictable mild stress-induced ICR mice by gavage (0.4, 2.0, and 10.0 mg/kg, for 9 days) substantially altered anxiety-like behavioural performance via inhibiting NLRP3-inflammasome and upregulating CREB and BDNF expression in the hypothalamus 57 .

Anti-rheumatoid arthritis and immunosuppressive activities
Gelsevirine (77) has been well described to have an excellent antiosteoarthritis effect. In IL-1b-stimulated mouse primary chondrocytes, its treatment (6.25 and 50.0 lM) dose-dependently enhanced cell viability and mitigated cell apoptosis. Moreover, it could downregulate the mRNA expression of MMPs and inflammatory factors and upregulate the mRNA expression of Col2A and IL-10 via suppression of STING activation. In an in vivo experiment, chronic exposure to gelsevirine (5.0 mg/kg, i.p., every 3 days for 10 weeks) could markedly reduce OARSI scores and MMP13 expression levels and increased cartilage area and Col2A expression levels in STING-deficient mice and the destabilisation of the medial meniscus-operated mice. Its latent mechanism was in conformity with the promotion of the K48-ubiquitination of STING 59 . Recent studies on collagen-induced rats of arthritis disclosed that treatment with koumine (2) alone (0.6, 3.0, or 15.0 mg/kg, i.g., for 10 days) exerted an inhibitory effect on joint pain, that concomitantly occurred with an improvement in the arthritis index scores, mechanical allodynia and volume of injected hind paw as well as the destruction of bone and cartilage. Moreover, koumine effectively ameliorated the production of proinflammatory cytokines in joint tissues and astrocyte activation in the spinal cords. Studies on its antirheumatic mechanism revealed that koumine suppressed the secretion of anti-CII antibody, which was produced by B lymphocytes and could damage joints via the occurrence of the inflammatory response 60,61 . In 2022, koumine treatment decreased T cell-dependent and T cell-independent B cell immune response in vivo and in vitro, which might be an alternative mechanism for its anti-rheumatoid arthritis bioactivity 62 . Such evidence was also

Total synthetic chemistry
Due to profuse and diverse effects along with their distinctive chemical structures, tremendous efforts have been devoted to synthetic approaches towards the total synthesis of gelsemium MIAs. Gelsemine and koumine-type alkaloids, the flagship members of gelsemium, have been widely studied by several synthetic chemists. Conversely, yohimbane-type alkaloids with relatively simple structures have received relatively less attention. According to the different molecular skeletons of gelsemium MIAs, these synthetic approaches were classified as follows. (Figure 8 and 9)

Total syntheses of gelsedine-type alkaloids
Carreira's total synthesis of (±)-gelsemoxonine (2013, 2015) Carreira and co-workers achieved the total synthesis of (±)-gelsemoxonine using a ring contraction approach of a spirocyclopropane isoxazolidine to introduce the b-lactam intermediate, providing access to the unusual azetidine 64,65 . They began with aldehyde 78 (5 steps from cyclopropanone hemiacetal), which was converted into nitro-alcohol 79 in high yield through Henry reaction. Then, it was prepared to isoxazoline 82 via elimination followed by intramolecular Huisgen dipolar cycloaddition induced Scheme 4 Ferreira's total synthesis of (-)-gelsenicine.  In parallel, the instalment of the ethyl group onto 99 using EtMgBr followed by IBX-induced hydroxyl oxidation afforded another key intermediate carbonyl 103. Then, Pd(OAc) 2 -catalysed conjugate reduction of the unsaturated ketone with in situ trapping as its silyl enol ether at base condition yielded the resulting 104. It was exposed to TBAF, and then the liberated amine and ketone were facilely cyclized to achieve the total synthesis of (-)-gelsenicine (105). It was further subjected to hydrogenation reaction by Adams' catalyst to afford (-)-gelsedine (1). In addition, the exposed double bond of 103 was oxidised by catalytic OsO 4 / NMO in acetone/H 2 O to yield diol 106, which underwent similar two-step Pd(OAc) 2 /Et 3 SiH treatment and concomitant dehydrative cyclisation yielded (-)-14,15-dihydroxygelsenicine (107) in 43% yield over 2 steps. Meanwhile, the epoxy moiety was diastereoselectively introduced in treating TBHP/Triton B on C14 and C15 in 103 to afford epoxy 108. After the loss of the Cbz group by TMSI in DCM, the treatment of this substrate with reductive SmI 2 in THF at À78 C allowed the reduction of a,b-epoxy ketone to b-hydroxy ketone, further forming the samarium enolate 109.
The following aldol reaction between 113 and aldehyde 114 occurred to construct compound 115 (dr ¼ 1:1) using a mild base of K 2 CO 3 . Oxidation using Dess-Martin periodinane (DMP) and rereduction subsequent were performed to switch the b-hydroxyl group to the a-hydroxyl group, and the diastereoselective ratio was noticeably improved from 1:1 to 5:1. Then, an intramolecular condensation employing TFA occurred to smoothly deliver lactone 116, which was further selectively reduced by DIBAL-H at À78 C and removed the ketal group by p-TSA in acetone to yield pyrone 117 in 60% yield over 3 steps. Next, aldehyde 118 was synthesised over 3 steps. Cs 2 CO 3 -promoted Michael addition of thiol and conjugate addition-aldol reaction as key steps produced thiolated 119, which was removed from the thiolate group in the presence of AIBN/n-Bu 3 SnH in benzene yielded the corresponding 120 as a single diastereoisomer in moderate yield. Upon acetylation protection, the carbonyl group in 120 was transformed into triflyl enol in 121 by treatment with KHMDS. Subsequent Pd(OAc) 2 -catalysed carbonylation reaction in the presence of CO obtained unsaturated ester 122. Deprotection of the acetate group along with DMP oxidation furnished the ketone product, which was treated by hydroxylamine hydrochloride to obtain oxime 123. It was further elaborated to complete the reduction with NaBH 4 with the use of NiCl 2 and in situ lactam cyclisation in one pot process to yield the final gelsedilam (102). (Scheme 3) Ferreira's total synthesis of (-)-gelsenicine (2016, 2022) Ferreira and co-workers reported the shortest approach towards the total synthesis of gelsenicine in 13 steps 69,70 . Their synthesis commenced with the alkylation of (Z)-but-2-ene-1,4-diol 124 with 3-bromoprop-1-yne 125, along with Cu-catalysed oxidation and olefin isomerisation to forge aldehyde 127 (E/Z > 20:1) in satisfactory yield over 3 steps. Then, it underwent a Horner-Wadsworth-Emmons olefination with phosphonate 128 and phosphine-mediated alkene E/Z isomerisation to synthesise (E,E)-dienyne 129 in a Scheme 6 Continued.
high (E,E)/(E,Z) ratio of 8.2:1. Subsequent Cadiot-Chodkiewicz coupling of 129 with 1-bromo-1-propyne 130 allowed access to (E,E)diyne 131. Under the optimised condition, Au-catalysed cycloisomerization provided the resulting product 132 with an outstanding yield in a 3.2:1 dr ratio, whereupon a strain-release Cope rearrangement was performed to obtain bicycle 135 in MeOH at 60 C in 75% yield. Regioselective Kucherov alkyne hydration using HgSO 4 /H 2 SO 4 catalysis then transformed 135 into enone 136. It was directly subjected to conjugate reduction to afford ketone 137 (dr ¼ 4:1) using Stryker's reagent, followed by a series of hydrolysis, acyl chloride, and amidation, thereby generating amide 138 with an overall yield of 73%. Oxime formation with hydroxylamine and benzoylation proceeded to install the oxindole unit, giving benzoyl-oxime 139. The oxindole moiety was then assembled through a ring closure of amide using PhI(OTFA) 2 in TCM at 0 C, thus giving rise to oxindole 140. Finally, the radical ring closure between benzoyl-oxime and olefin using Bu 3 SnH/AIBN at 120 C successfully favoured ( to the first total synthesis of (-)-gelselegandine C (179), albeit in 23% yield. (Scheme 6)

Total syntheses of gelsemine-type alkaloids
Qiu's total synthesis of (þ)-gelsemine (2015) In 2015, Qiu and co-workers completed the asymmetric total synthesis of (þ)-gelsemine using an organocatalytic Diels-Alder starting strategy 73 . The synthesis was initiated by the linkage of methyl (Z)-4-oxobut-2-enoate 180 with dihydropyridine 181 through an organocatalytic Diels-Alder reaction. This process provided the intermediate 183 and by-product 182. Fortunately, 182 could be converted to lactone 183 through an intramolecular cyclisation in the presence of DBU in 97% yield. 183 was then transformed into hemiacetal 184 through selective reduction using DIBAL-H at À78 C. It was then subjected to a Wittig reaction to yield a racemic mixture that occurred an electrophilic addition reaction by catalytic p-TSA in DCM to afford (S)-acetal 185 (dr ¼ 13:1) in 93% overall yield. Subsequent ozonolysis of 185 employing ozone in DCM, accompanied by trans-annular aldol condensation of resulting dicarbonyl groups using sodium methanol afforded ketone 186. On subjecting reduction of 186 using NaBH 4 led to hydroxyl 187, whose hydroxyl group was further methanesulfonylated with MsCl to afford disulfonate 188. Upon treatment of 188 with DBU in heating toluene, reduction of the Cbz protecting group to methyl group with LiAlH 4 in THF resulted in the formation of olefin 189. Hemiacetal 190 was prepared via acid hydrolysis with HCl in THF in a 2:1 dr ratio. Then, 1-MOM-oxindole 191 was installed onto 190 via condensation reaction by catalytic piperidine to generate the resulting product 192. Using their optimised conditions, its treatment with LDA and subsequent S N 2 substitution reaction using Et 2 AlCl constructed the configuration of the C7 quaternary carbon stereochemical centre and afforded the desired 193 as a single diastereoisomer in only 32% yield. Finally, acid hydrolysis of the methyl group from the MOM group and removal of the resulting hydroxymethyl group using Et 3 N furnished (þ)-gelsemine (3) in 70% yield over 2 steps. (Scheme 7) Vanderwal's synthetic route to the polycyclic core of gelsemine (2015) In the same year, Vanderwal's and co-workers relied on a Zinckealdehyde-based approach to prepare the polycyclic core Kerr's total synthesis of isodihydrokoumine and (4 R)-isodihydroukoumine N 4 -oxide (2018) In 2018, Kerr's total syntheses of isodihydrokoumine and (4 R)-isodihydroukoumine N 4 -oxide utilised an intramolecular [3 þ 2] nitrone olefin cycloaddition and a Lewis acid-mediated cyclisation as the key steps to prepare their core structure 76 . They commenced this synthetic study with the preparation of dihydropyranone 227. Hydrostannylation of alkyne 225 with n-Bu 3 SnH followed by Stille coupling with methyl (Z)-3-iodoacrylate 226 provided dihydropyranone 227 in 60% yield. Then, the copper-catalysed conjugate addition of 228 with vinyl magnesium bromide 228 led to the formation of lactone 229. After the smooth reduction of lactone, the allylic alcohol of the resulting product was substituted through Mitsunobu reaction to yield hydroxylamine 232 in 83% yield with high regioselectivity. Upon removal of the Boc groups, the relevant product was condensed with N-tosyl indole-3-acetaldehyde 233 in situ to provide nitrone 234, which underwent an intramolecular N-alkenyl nitrone dipolar cycloaddition upon heating in toluene to produce isoxazolidine 235 in a 23% yield of a 2:1 mixture of cis-diastereomer. Removal of the Ntosyl protecting group with Mg, followed by Swern oxidation, acetal protection as well as SmI 2 -mediated isoxazolidine reduction, and ring-opening smoothly delivered acetal 236 in 48% yield over 4 steps. Treatment of 236 with TMSCI in MeCN induced the cascade Friedel-Crafts and Conia-Ene cyclizations that forged the polycyclic cage skeleton in 240. Eschweiler-Clarke reaction installed a methyl group on an N4 atom and then furnished natural isodihydrokoumine (241) in a 57% yield. Oxidisation of the N4 atom in 241 with mCPBA generated the separable diastereomeric products in a dr ratio of 1.8:1, which was separated by chiral chromatography to afford the natural product (4 R)-isodihydrokoumine-N 4 -oxide (66)  hand, there was an opportunity to employ this key core as the progenitor needed for the synthesis of koumine (2). It was envisioned that koumine eventually could be completed by subsequent closure of the piperidine and tetrahydropyrane ring as well as the assembly of the indole portion. (Scheme 11) Tanja's total synthesis of koumidine (2019) In Tanja's synthesis of koumidine, the late-stage enol-oxonium cyclisation sequence was used to construct the hexacyclic cage framework on a gram scale 78 . In their work, a highly diastereoselective 1,3-dipolar cycloaddition reaction of trans-2-methylene-1,3-dithiolane 1,3-dioxide 249 (from available 1,1,2-trimethoxyethane in 4 steps) with 3-oxidopyridinium 250 generated an inseparable mixture of tropane 251 and 252 in 2.5:1 dr ratio. Next, the bissulfoxides in the mixture of 251 and 252 were simultaneously reduced using TFAA/NaI to afford two separable regioisomers dithiolanes 253 and 254, with the latter capable of being transformed to ketone 255 via 1,4-reduction using L-selectride. Then, the Palladium-catalysed intramolecular coupling of vinyliodide and ketone fused the piperidine ring using potassium phenoxide in THF, thus affording tetracycle 256. Wittig reaction of 256 with triphenylmethylmethoxy-chloride in the presence of KHMDS happened to smoothly deliver the corresponding enol ether 257 with high efficiency. After removal of the ethylenethioacetal group using Meerwein's reagent, the resulting bissulfonium intermediate was hydrolysed with CuSO 4 , followed by basification with NH 4 OH to yield the crude mixture, which was readily converted to keto-aldehyde 258 via acid hydrolysis in 82% yield over 3 steps. Subsequently, the chemoselective reduction of the aldehyde proceeded to yield alcohol 259 with excellent chemoselectivity that participated in a TMSCH 2 N 2 -involved expansion reaction followed by subsequent resulting TMS enol ether hydrolysis to furnish 6-membered ketone 260. With sufficient 260 in hand, the total synthesis of koumidine (5) was eventually completed by Fischer indole synthesis reaction with phenylhydrazine 261. (Scheme 12) Zhang's asymmetric total syntheses of (-)-koumimine, (-)-N-demethylkoumine, and (-)-koumine (2021) In 2021, Zhang and co-workers disclosed a tandem sequential oxidative cyclopropanol ring-opening cyclisation and a cooperative organo/metal-assisted ketone a-allenylation for constructing the core skeleton of koumine 79  The synthesis was further manipulated by a Gold-catalysed intramolecular coupling reaction between C7 and C20 positions, DBUinduced isomerisation of the aldehyde group, and sequential aldol condensation reaction with trapping of the hydroxyl group newly generated in situ to obtain the required hemiacetal 275 as a single diastereomer. Next, the hemiacetal and imine moieties were reduced by TFA/Et 3 SiH to produce the resulting 276. Removal of the Troc group and successive PhIO-induced oxidation gave (-)-koumimine (277) in 45% yield over 2 steps. Meanwhile, PhIO-exposed oxidation and subsequent deprotection of the Troc group took place to complete the synthesis of (-)-N-demethylkoumine (278) in 77% yield. Finally, a HCHO/NaBH 3 CN-assisted reductive methylation happened to afford (-)-koumine (2) in good yield. (Scheme 13) Zhang's total syntheses of akuammidine, 19-(Z)-akuammidine, komidine, dihydrokoumine and koumine (2022) In 2022, Zhang and co-workers exploited a unified approach towards the asymmetric synthesis of sarpagine-and koumine-type alkaloids. Among them, akuammidine, 19-Z-akuammidine, and dihydrokoumine are synthesised for the first time 80 6 with the use of DIPEA and (TMS) 2 NH under blue LED at À60 C in 47% yield. Koumidine (5) could be obtained from the removal of the PMB group of 298 with TFA along with reduction with LiAlH 4 in 80% yield over 2 steps. Treatment of 5 with methyl chloroformate triggered the formation of amide 299 in a 69% isolated yield. NIS-induced cyclisation simultaneously set up two vicinal all-carbon quaternary stereocenters, thereby providing iodide 300 in 88% yield. Next, olefin 301 was obtained by eliminating the iodide in 300 with AgOAc in acetic acid. Upon reduction with LiAlH 4 , the first synthesis of dihydrokoumine (302) was eventually completed in 79% yield. Oxidation of 302 with PhIO in DCM gave rise to koumine (2) in nearly quantitative yield. (Scheme 14) Total syntheses of sempervirine-type alkaloids Malhotra's total synthesis of sempervirine (2013) In 2013, Malhotra and co-workers accomplished the concise total synthesis of sempervirine under microwave irradiation in one pot process 81 . Quaternization of 1-methyl-9H-pyrido [3,4-b]indole 303 with ethyl bromoacetate 304 occurred to give the corresponding quaternary 305 in excellent yield. In a one-pot reaction, a Westphal condensation reaction with 1,2-cyclohexanedione 306 under microwave heating in MeOH in the presence of sodium methoxide, followed by ester hydrolysis and decarboxylation yielded a separable mixture of sempervirine precursor 309 in 53% yield. After acidification with dilute hydrochloric acid, the mixture was separated through silica gel chromatography to afford the expected sempervirine (6) in a 78% isolated yield. (Scheme 15) Bannister's total synthesis of sempervirine triflate (2016) In 2016, Bannister and co-workers used Pd-catalysed Sonagashira coupling and Larock indole annulation reaction to efficiently synthesise sempervirine and its analogs 82 . Regioselective semi-reduction of 3-isoquinolone 310 using PtO 2 catalysis in TfOH/TFA generated the corresponding hydrogenated product, whose amide group was further alkylated to provide triflate 311 in 94% yield over 2 steps. Under optimised Sonagashira conditions, the addition of 311 to butyne-1-ol 312 smoothly gave alkyne 311 using Pd(PPh 3 ) 2 Cl 2 as a catalytic agent in 92% yield. Upon Larock indole synthesis of 313 with o-bromoaniline 314 catalysed by Pd(OAc) 2 , the 2-heteroaryl indole product 315 was prepared with a good yield. Subsequently, a triflate-mediated cyclisation cleanly furnished the intermediate pyridinium salt 316. Ultimately, it was converted to sempervirine triflate (317) through DDQ-promoted oxidation in 96% yield. (Scheme 16)

Conclusions and perspectives
At present, this review has summarised a total of 70 novel gelsemium MIAs, which greatly complemented the library of compounds from the Gelsemium genus. Although remarkable accomplishments to synthesise several gelsemium MIAs are successful, the separation from plants are a more economical route for access to these gelsemium MIAs and their analog, due to their abundance in plant sources. An increasing number of gelsemium MIAs are essential for mapping out their possible biosynthesis mechanisms and molecule transformations. Once a reasonable biosynthesis is proposed, it is conducive to satisfying the need of synthetic chemists for addressing the synthetic challenges of complex gelsemium MIAs depending on biomimetic synthesis inspired by nature. Therefore, it is necessary to deeply dig and identify unknown trace gelsemium MIAs from natural sources on a large scale. It is noteworthy that the structure-activity relationship of many gelsemium MIAs remains unclear, partly because of limited material availability. The structural modification of gelsemium MIAs using semi-synthesis directly starting from several key intermediates could effectively solve this issue as possible. Besides, these synthetic tactics also enable to provide novel gelsemium MIAs derivatives with better bioactivities. Further innovative total synthetic strategies with more concise steps and higher yields should be designed to construct the complicated skeletons of gelsemium MIAs.
Despite gelsemium MIAs' promising bioactive potentials both in vitro and in vivo, their exact molecular mechanisms and specific targets in many types of diseases have long been limited; thus, such further studies are required. Bioinformatics and the integration analyses of transcriptome, genome, intestinal flora, and proteome are ripe for wide prediction and exploration of singling pathways and precise target proteins [83][84][85] . Molecular docking tools, which create ligand-target interaction, aid the prediction of binding sites of compounds and the delineation of SARs 86 . These approaches greatly support the in-depth understanding of gelsemium MIAs-regulated molecular mechanisms. In addition, the extensive clinical applications of gelsemium MIAs and plants are still largely challenging due to their toxicity. Our study has reported that the combination treatment of koumine and Glycyrrhiza uralensis showed a significant low-toxic effect by upregulating cytochrome enzymes and mediating pharmacokinetics 87 . Thus, the synergistic application with another detoxifying agent would be crucial to partly enhancing the curative effect and reducing the toxicity of individual gelsemium MIA or gelsemium extract. Regarding the tissue distribution, the koumine and gelsemine peak concentrations in the intestines and livers are higher than that in other tissues, thus indicating that gelsemium MIAs may bring greater advantages into full play in the treatment of digestive system diseases 88,89 .
In conclusion, a historical account of relevant studies research advances on the structural diversity, potential bioactivity, and total syntheses of gelsemium MIAs covering the period from 2013 to 2022 has been described and discussed. We hope this review will help drive the future drug development of gelsemium MIAs as promising lead compounds with better safety and potency forward.

Disclosure statement
No potential conflict of interest was reported by the author(s).