Endiandric Acid Derivatives and Other Constituents of Plants from the Genera Beilschmiedia and Endiandra (Lauraceae)

Plants of the Lauraceae family are widely used in traditional medicine and are sources of various classes of secondary metabolites. Two genera of this family, Beilschmiedia and Endiandra, have been the subject of numerous investigations over the past decades because of their application in traditional medicine. They are the only source of bioactive endiandric acid derivatives. Noteworthy is that their biosynthesis contains two consecutive non-enzymatic electrocyclic reactions. Several interesting biological activities for this specific class of secondary metabolites and other constituents of the two genera have been reported, including antimicrobial, enzymes inhibitory and cytotoxic properties. This review compiles information on the structures of the compounds described between January 1960 and March 2015, their biological activities and information on endiandric acid biosynthesis, with 104 references being cited.

of double bonds and terminal carboxylic acid groups. They can be grouped according to the carbon skeleton layout into three groups. The first group is that of compounds characterized by a 13 carbon atom fused tetracyclic ring system containing ǻ 8,9 and ǻ 4,5 or ǻ 5,6 (1); the second group is that of compounds with tetracylic ring system formed with 11 carbon atoms with ǻ 10,11 (2) and the last group contains compounds that possess bi-, tri-or tetracyclic ring systems other than skeleton 1 and 2. The side chain attached to C-11 in 1 or C-4 in 2 contains in some case double bonds, phenyl residues, or a methylenedioxyphenyl moiety. The substituents at C-6 in 1 or C-8 in 2 are usually a carboxylic acid, a phenyl ring or a methylenedioxyphenyl fragment. (Table 1) The first members of this class of compound, endiandric acid A and B (3)(4) were isolated from the leaves of Endiandra introrsa [35][36][37][38][39]. Endiandric acid A (3) was also obtained from the leaves of other species such as B. obtusifolia and B. oligandra [40]. Endiandric acid B (4) was also isolated from E. jonesii, E. baillonii and B. tooram [40]. In addition to endiandric acid A (1), a new derivative, 3'',4''-methylenedioxy endiandric acid A (5) was obtained from B. oligandra after methylation of the extract and isolation of the non-natural methylated derivative 6 [40]. Endiandric acid H (7), a derivative with a hydroxyl group at C-4, was isolated from the stem of Beilschmiedia fulva [53,54]. Other endiandric acid analogues of this group with C8 alkyl side chain attached to the carbon C-11, named beilschmiedic acid A-E (8)(9)(10)(11)(12), in addition with beilschmiedic acid F (13) were isolated from the stem bark of B. anacardioides [17,19,20].

Endiandric Acid Derivatives with an 13 Carbon Atoms Fused Tetracyclic Ring System
From the leaves of an unidentified Beilschmiedia species from Gabon, eight new beilschmiedic acid derivatives, named beilschmiedic acid H-O (14)(15)(16)(17)(18)(19)(20)(21) were isolated using high-throughput natural products chemistry methods [48]. These compounds possess a phenylalkyl side chain at C-11, containing generally two trans-configured double bonds. A cis double bond, not reported previously in the side chain of endiandric acid derivatives was observed in beilschmiedic acid M (18) [48]. Beilschmiedic acid N (20) contains an unusual endoperoxide phenyl moiety that might have been formed during the process of isolation [48].
Four beilschmiedic acid derivatives with different oxidation states at C-4, cryptobeilic acids A-D (33)(34)(35)(36), together with the known tsangibeilin B (29) were isolated from the bark of B. cryptocaryoides collected in Madagascar [52]. Other endiandric acid analogues, named ferrugineic acids A-J (37-46) were isolated from the leaves and flowers extracts of B. ferruginea with the help of 1 H and 13 C HSQC NMR screening of ethyl acetate extracts and fractions [24].
A total of 69 endiandric acid derivatives were isolated from 11 Beilschmiedia and four Endiandra plant species with the majority of these secondary metabolites having skeleton 1.

Biosynthesis of Endiandric Derivatives
Endiandric acids are polycyclic fatty acid derivatives with particular scaffolds isolated until date only in Beilschmiedia and Endiandra species of the Lauraceae family. They are products of electrocyclic ring closures of naturally occurring polyketides, resulting from both the shikimate and acetate pathways (Scheme 1).

Scheme 1. General biosynthesis scheme of polyketides.
Their biosyntheses from the polyketide contain two consecutive non-enzymatic electrocyclic reactions, followed by an intramolecular Diels-Alder reaction [35,36,55]. As a result of the whole reaction sequence, an open-chain compound is converted into a tetracyclic compound. The starting product contains a conjugated tetraene system, as well as a conjugated diene system. Thus, it already displays the ʌ electron systems required for the three pericyclic reactions; they are the two electrocyclizations and the Diels-Alder reaction [35,36,55] (Scheme 2).
Biogenesis of compounds of the kingianin family of natural products isolated from E. kingiana also involve a key Diels-Alder cycloaddition via a tandem 8ʌ/6ʌ electrocyclisation. In fact, an arylpolyene undergoes a conrotatory 8ʌ e electrocyclization followed by a disrotatory 6ʌ e electrocyclization of the formed cyclooctatriene. Radical cation formal Diels-Alder reaction between two bicyclo[4.2.0]octa-2,4-diene monomers led to unique and complex pentacyclic derivatives as shown in the Scheme 3 [42].

Spectroscopic Characterization
The structures of endiandric acid derivatives have mainly been proposed on the basis of modern spectroscopic methods and sometimes X-ray diffraction analysis. The majority of these compounds possess a C11 or C13 tetracyclic ring system along with a number of double bonds and substituents that display characteristic spectroscopic properties.

Mass Spectra
The mass spectrum of endiandric acid derivatives with C13 fused tetracyclic ring system having C 4 =C 5 double bond generally exhibits an unusual fragmentation pattern where a strong M-78 ion is often observed, due to the loss of a benzene moiety (Scheme 4) [35,36]. Most of the EI spectra exhibited base peaks at m/z 172 or m/z 129 like those of beilschmiedic acid A and C (Scheme 5). Other fragments corresponding to cleavage of the side chain and the dehydration have also been observed (Scheme 5) [17,19,20].
The chemical investigation of the bark of E. kingiana led to the isolation of a series of polyketides as a racemic mixtures, having each an amide function and named kingianins A-N (112-125). These amides possess an unusual pentacyclic carbon skeleton and was described for the first time in nature by Leverrier and coll. [42,43]. Seemingly, no alkaloids have been reported to date for the genus Endiandra.

Lignans and Neolignans
Plants of the Lauraceae family are sources of bioactive lignans [70][71][72]. The majority of lignans and neolignans isolated from the genus Beilschmiedia were obtained from the species B. tsangii. The investigation of the roots, leaves and stem of this species afforded the lignans ambo- The investigation of the dichloromethane extract of the leaves of B. kunstleri yielded an antioxidant neolignan, (+)-kunstlerone (140) [30].

Terpenoids
Sesquiterpenes and triterpenoids are two classes of terpenoids mostly isolated from plants of Beilschmiedia and Endiandra genera.

Cyanoglycosides
Chemotaxonomic studies indicated that terpenoids and alkaloids are common among plants of the Lauraceae family. In contrast, organic cyanides are very rare in Lauraceae and have only been reported from few species such as Cinnamomum camphora, Litsea glutinosa and Nectandra megaptamica. The investigation of 39 Australian Lauraceae species indicated that only B. collina was cyanogenic.
In fact, from the methanol extract of B. collina, the first cyanogenic compound taxiphillin (208) was isolated from this genus [82].

Biological Activities
Several interesting biological activities for the Beilschmiedia and Endiandra constituents have been reported, including antibacterial, anticancer, antifungal, antiinflammatory, antileishmanial, antiplasmodial and cytotoxic properties as well as Į-glucosidase inhibiting activity.

Anticancer and Cytotoxic Activities
Cancer cells that avoid apoptosis continue to proliferate uncontrollably. Apoptosis is an ordered and orchestrated cellular process that occurs in physiological and pathological conditions. An understanding of the underlying mechanism of apoptosis is important as it plays a pivotal role in the pathogenesis of many diseases. Degenerative diseases are characterized by too much apoptosis, while in the case of cancer, too little apoptosis occurs. Thus, resisting apoptosis is a key process in cancer development and progression [83]. Targeting the antiapoptotic proteins such as those of the Bcl-2 family members (Bcl-2, Bcl-xL, Bcl-w, Mcl-1, and A1) is essential for cancer treatment or preventive drug discovery. In addition, it has been shown that most cancers depend on more than one antiapoptotic Bcl-2 member for survival. The discovery of new selective inhibitors of antiapoptotic proteins is thus important for the search for anticancer drugs [84][85][86][87].
Endiandric acids derivatives isolated from Beilschmiedia and Endiandra species were screened for Bcl-xL and Mcl-1 binding affinities (Table 5). Amongst the tested compounds, ferrugineic acid B (38) exhibited the best binding affinity for Mcl-1 (85% inhibition at 100 ȝM) while ferrugineic acid C (39) showed the highest binding affinity to Bcl-xL (93% inhibition at 100 ȝM). Two compounds, ferrugineic acids B (38) and C (39), exhibited significant binding affinities for both antiapoptotic proteins. Apart from tsangibeilin B (29) and ferrugineic acid J (46), the compounds that exhibited good binding affinity to Mcl-1 possess a C13 fused tetracyclic ring system with ǻ 4,5 and ǻ 8,9 double bonds. In the group of compounds with an C11 fused tetracylic ring system, only kingianic acid C (61) showed significant binding affinity to Mcl-1. No binding was detected from the compounds of this last fused tetracylic ring system for Bcl-xL. After the correlation between the structures and activities of compounds with C13 fused tetracyclic ring system, Appel and collaborators postulated that the length of the saturated carbon side chain, the ȕ-oriented C-4 hydroxy group and the terminal 4-hydroxyphenyl ring, play a crucial role for Bcl-xL and Mcl-1 binding affinities [24,34]. The binding affinity of the racemic mixtures of kingianin A-N (112-125) isolated from E. kingiana was evaluated on Bcl-xL by competition against the fluorescently tagged BH3 domain of the protein Bak. Racemic mixtures of kingianins G-L (118-123) exhibited good binding affinity with kingianin G (118) exhibiting the best potency with a Ki value of 2 ± 0 ȝM ( Table 6). The pure enantiomers of these active racemates obtained using chiral preparative HPLC were evaluated for their binding affinity. Taking into account the stereochemistry of the compounds, the binding affinity was significantly higher for the (í)-enantiomers compared to the (+)-enantiomers, as illustrated by the comparison of the Ki for (í) and (+)-kingianin derivatives (Table 6) [43]. Endiandric acid analogues isolated from unidentified Gabonese Beilschmiedia species were screened for their cytotoxicity against NCI-H460 (human lung cancer cell lines); PC-3 (prostate adenocarcinoma cell lines), and M14 (amelanotic melanoma cell lines) using an MTT assay. All the isolated compounds were inactive against PC-3, and M14 cell lines. Beilschmiedic acids K (17), L (19), M (18), N (20) and A (8) exhibited moderate cytotoxicity against NCI-H460 human lung cancer cells with IC50 values of 5.5; 5.9; 4.4; 8.7; 19; 6.1 ȝM, respectively. This was the first report of the cytotoxicity of this class of secondary metabolites [48]. Subsequently, other endiandric acid derivatives kingianic acids A (59), C (61), E (63), F (47), G (48), endiandric acid M (56), and tsangibeilin B (29) isolated from E. kingiana were evaluated for their cytotoxicity activity against A549 (lung adenocarcinoma epithelial cell lines), HT29 (colorectal adenocarcinoma cell lines) and PC3 cell lines. As reported by Williams et al. [48], all compounds were inactive against prostate adenocarcinoma cancer cell lines. Kingianic acid A (59) showed weak activity against HT-29 and A549 cell lines with IC50 values of 35 ± 0.2 ȝM and 85.4 ± 0.2 ȝM, respectively. Kingianic acid E (63) showed moderate cytotoxic activity against A549 and HT-29 cell lines with IC50 values of 15.36 ± 0.19 ȝM and 17.10 ± 0.11 ȝM, respectively [34]. The other tested compounds showed very weak or were devoid of cytotoxic activity against the cell lines tested.
Ferrugineic acids A-J (37-46) and K (58), isolated from B. ferruginea, were screened for cytotoxicity against HCT-116 (Human colorectal carcinoma) and K562 (human leukemia) cancer cell lines. All these compounds were devoid of cytotoxicity on the two cancer cell lines tested at concentrations up to 50 μM [24].
Endiandrin A (143), endiandrin B (144), (í)-dihydroguaiaretic acid (145) isolated from E. anthropophagorum and the synthesized derivative cinbalansan (223) were also evaluated for their cytotoxicity against A549 cell line. In high-content screening (HCS) assays, (í)-dihydroguaiaretic acid (145) was found to be the most potent compound, displaying cytotoxicity against the A549 cell line with an IC50 of 7.49 ȝM after 24 h incubation in both propidium iodide and Yo-PRO-1 assays. It effect was less pronounced in the mitotracker assay with IC50 of 31.2 ȝM. Endiandrin A (143), and B (144) were found to have moderate effects with an inhibition of 76% and 75% at 100 ȝM, respectively. Cinbalansan (223) was found to have much less effect with a maximum inhibition of 34% [32].
Alkaloids isolated from the leaves of B. brevipes exhibited cytotoxicity activity against P-388 murine leukemia cell lines.  [45].

Antimalaria Activity
Malaria remains one of the most notorious infectious diseases in the world. It constitutes a public health problem in more than 90 countries, inhabited by about 40% of the world's population. The World Health Organisation estimates that there are 300-500 million malaria cases annually, causing 2-3 million deaths, mostly among children under five years old. In the last decades, resistance of Plasmodium falciparum, the causative agent of the most severe form of the disease, to several antimalarials, especially chloroquine and antifolates, became widely disseminated, while the cost of effective treatment is prohibitive for the large majority of the population in developing countries. For these reasons, new effective and affordable antimalarials are urgently needed [89][90][91]. In this perspective, extracts and some compounds isolated from Beilschmiedia species were screened for their antiplasmodial potency.
The antiplasmodial bioassay guided separation of the chemical constituents of the wood of the Indonesian medicinal plant B. madang led to the isolation of the bisbenzylisoquinoline dehatrine (100), that exhibited potent antiplasmodial activity against the chloroquine-resistant strain P. falciparum k1 with IC50 value of 0.17 ȝM, and which is comparable to that of the reference drug quinine against the same strain in vitro [12].
Lupeol (171), which showed in vitro inhibitory activity against the P. falciparum 3D7 strain with an IC50 value of 27.7 ± 0.5 ȝM, was shown to cause a transformation of the human erythrocyte shape toward that of stomatocytes [92].

Anti-Asthmatic and Other Anti-Inflammatory Activities
Asthma is a disease of the immune system, which is expressed for example as bronchial asthma in the form of acutely occurring, paroxysmal dyspnea with expiratory ventilation disability. Studies reported that persistent inflammation is central to the pathogenesis of asthma. So far, asthma therapy uses drugs which alleviate the symptoms but do not inhibit the mechanism responsible for the expression of inflammatory mediators such as the cytokines interleukin-4 (IL-4), interleukin-13 (IL-13) and interleukin-5 (IL-5) [93,94]. Endiandric acid H (7), obtained from the plant Beilschmiedia fulva, and its synthetic derivatives, known as c-maf, and NFAT inhibitors are used for producing a medicament, in particular for the treatment of allergic disorders, asthmatic disorders, inflammatory concomitant symptoms of asthma and/or of diseases which can be treated by inhibiting c-maf and NFAT [53,54].
Synthetic glucocorticoids are widely used as drugs to treat inflammatory conditions such as rheumatoid arthritis or dermatitis and as adjunctive therapy for conditions such as autoimmune diseases. However, current glucocorticoid drugs act non-selectively, with the potential of long-term impairment of many physiological anabolic processes. Therefore, research aiming at the discovery of selective novel glucocorticoid receptor (GR) binders may provide new and improved drug therapies [95][96][97][98]. The bioguided fractionation of the dichloromethane extract of Endiandra anthropophagorum based on GR binding assay resulted in the isolation of the active lignans endiandin A (143), nectantin B (146) and (í)-dihydroguaiaretic acid (145) which displayed IC50 values of 0.9, 27 and 35 ȝM, respectively. The di-acetylated (224) and di-methylated (225) derivatives of endiandrin A also exhibited good activities with IC50 of 34 and 13 μM, respectively. From the structure-activity correlation, David and collaborators suggested that the constrained four-membered which has implications in the spatial arrangements of the substituents is important for the potent GR activity. In addition, increasing the steric bulk of the C-4/C-4' substituents in the cyclobutane series was shown to significantly reduce the activity [33].

Antimicrobial Activity
Antibacterial activity of extracts and a number of endiandric acid derivatives and other constituents isolated from Beilschmiedia and Endiandra species have been studied.
Beilschmiedic acid A (8), B (9), and C (10), isolated from the stem bark of B. anacardioides, exhibited antibacterial activities against a wide range of microorganisms (Bacillus subtillis, Micrococcus luteus, Streptococcus faecalis) with minimum inhibitory concentrations (MICs) of 0.7-364 ȝM. Compound 10 was found to be the most active derivative against the three tested strains with MICs of 5.6, 0.7 and 22.7 ȝM, respectively. Compounds 9 (MIC value of 11.3 ȝM) and 10 (MIC value of 5.6 ȝM) were found to be more active than the reference drug ampicillin (MIC value of 89.5 μM) against B. subtillis. Compound 10 was also more active than the reference drug ampicillin (MIC value of 5.58 μM) against M. luteus [17].
Cryptobeilic acids A (33) and B (34) isolated from B. cryptocaryoides displayed antibacterial activity against Escherichia coli 6r3 with MIC values of 10 and 20 ȝg/mL, respectively. Their activity were moderate compared to that of the reference drug ampicillin (MIC value of 5 ȝg/mL) [52].
Endiandric acid erythrophloin C (24) with phenyl in the side chain isolated from B. erythrophloia exhibited antitubercular activity against Mycobacterium tuberculosis with MIC of 50 ȝg/mL [41].
In addition to endiandric acid derivatives, other constituents of Beilschmiedia and Endiandra genera have also exhibited antibacterial activity in vitro against some strains of bacteria. The amide pipyahyine (107) and beilschmieflavonoid B (156) isolated from the stem of B. zenkeri exhibited antibacterial activity in vitro against Bacillus subtilis, P. agarici and S. minor with MICs between 81.5-197.5 ȝM [18].
The methanol extract of the wood of B. tovarensis showed significant antibacterial activity results against Staphylococcus aureus and Enterococcus faecalis [13]. The methanol extract of the fruits of B. obscura showed antibacterial activity against multi-resistant drugs strains of Escherichia coli, Enterobacter aerogenes, Klebsiella pneumoniae, Enterobacter cloacae, Pseudomonas aeruginosa, and Providencia stuartii with MICs between 16-128 ȝg/mL [100].
Essential oils from the leaves and bark of B. madang showed moderate antibacterial activity towards B. subtilis and S. aureus with identical minimum inhibitory concentrations (MIC), 125 ȝg/mL. They also exhibited activity towards E. faecalis with MIC value of 250 ȝg / mL. Both oils were also found to be active against Gram-negative bacteria, K. pneumoniae with MIC value of 250 ȝg/mL [77].
The antifungal activities of extracts and the isolates from Beilschmiedia and Endiandra were also reported. The essential oils from the bark of B. madang showed strong antifungal activity towards Aspergillus niger and A. fumigatus with identical MIC values, 62.5 ȝg/mL [78].
The essential oils from the leaves of B. tilaranensis and B. brenesii exhibited enzyme inhibitory activities against cruzain, a potential therapeutic target for Chagas' disease, a parasitic disease caused by Trypanosoma cruzi and that occurs mostly in South and Central American countries, with IC50 values of 23.6 ȝg/mL and 61.9 ȝg/mL, respectively [102][103][104].

Conclusions
The genera Beilschmiedia and Endiandra include ca. 250 and ca. 125 species, respectively. Only 31 species of Beilschmiedia and 11 species of Endiandra have been investigated phytochemically, indicating that there are still many species that have received little or no attention. Secondary metabolites isolated from the two genera, can be classified as endiandric acid derivatives (30.9%), alkaloids and amides (23.7%), lignans and neolignans (9.5%), flavonoids and chalcones (5.8%) and others (30.21%: terpenoids, benzene derivatives, steroids, cyanoglycoside, fatty acids). Although being the majority of the isolates, the endiandric acid derivatives were not isolated from all species of Beilschmiedia or Endiandra investigated. This class of compounds and alkaloids is more widespread and the investigation of the remaining species could led to new derivatives. Bioassay on extracts and secondary metabolites of these species revealed pronounced biological properties, such as Bcl-xL and Mcl-1 binding affinity, Į-glucosidase inhibiting activity, antimicrobial, anti-inflammatory, antiplasmodial, and cytotoxic properties. In addition to these bioactive secondary metabolites, essential oils from these species displayed good biological activities against a wide range of microorganisms and also enzyme inhibitory properties. The structural diversity of Beilschmiedia and Endiandra constituents and their interesting biological activities indicate that they are two potential sources of other new drugs such as that used in the treatment of asthma.