Chemistry of southeast asian plants 1

Primary and secondary metabolites Natural products, as the term implies, are those biological molecules which originate from living organisms such as plants, animals, and insects.. Howev

CHEMISTRY OF SOUTHEAST ASIAN PLANTS

LE CONG THUAN

(B Sc (Hons.), Vietnam National University)

A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

DEPARTMENT OF CHEMISTRY NATIONAL UNIVERSITY OF SINGAPORE

2005

I would like to thank the following people for their help and support, without whom this project would not have been possible:

I am indebted to my supervisor, Associate Professor Leslie John Harrison, for providing

me with the chance to carry out research in his group and for the valuable trainings that I had received during my journey to discover the richness of nature His patiently guidance and constant encouragement has guided me to the very end of this project

I express my sincere gratitude to Professor Shinro Tachibana of the National University Hospital and Dr Chidambaram of the National Cancer Center for their guidance in biological assays I am also grateful to the Director and staff of the WHO Immunology Center and the Paediatric Laboratory for the permission of using their facilities for cell culture and bioassay screening

I also wish to thank my fellow researchers A S Md Sofian, Wong Chek Ming, Wang Yanmei, Teo Ee Ling, Wu Ji-en, Ge Xiaowei, and Zhang Guodong, for the time that we shared in the laboratory and for their support throughout my project

Special thanks also go to Miss Shanon Sng Poh Tee, Miss Wong Siew Ying, Mdm Han Yanhui, and Miss Peggy Ler of the NMR Laboratory, and last but not least, Mdm Wong Lai Kwai and Mdm Chen Lijun of the NUSChem/Finnigan-MAT Mass Spectrometry Laboratory for their hard work and precious assistance

Table of Contents

Chapter 1 - General introduction……… 1

Chapter 2 - The Celastraceae……… 30

2.1 Introduction……… 30

2.2 Constituents of Salacia chinensis L……… 48

2.3 Experimental……… 68

Chapter 3 - The Hepaticae……… 79

3.1 Introduction……… 79

3.2 Chemistry of Pallavicinia……… 93

3.3 Constituents of Singaporean Pallavicinia cf lyellii……… 95

3.4 Experimental……… 131

3.5 Constituents of Vietnamese Pallavicinia lyellii……… 137

3.6 Experimental……… 165

Chapter 4 - Pain and nociceptin receptor……… 170

4.1 Introduction……… 170

4.2 Results and discussion……… 190

4.3 Experimental……… 193

References……… 195

Appendix……… 214

Summary

From the stem bark of Salacia chinensis L collected in Vietnam, eight triterpenoids have

been isolated and identified, including the new natural product 28-oic acid The known compounds are oleanolic acid, betulin 3-caffeate, krukovine A, morolic acid, 28,30-dihydroxylup-20(29)-en-3-one, 3β-hydroxy-11-oxoolean-12-en-28-oic acid, and 3β,28-dihydroxy-12-oleanen-11-one

3,11-dioxoolean-12-en-Phytochemical investigation of the Singaporean liverwort Pallavicinia cf lyellii led to the

isolation of five novel diterpenoids, together with two known diterpenoids, clerodadien-13-ol and (-)-sacculatal The five novel diterpenoids were named pallavicinins B-F In addition, three bis(bibenzyls) were also obtained, two of which are novel The known compound is perrottetin E, and the two novel bis(bibenzyls) are 8-hydroxymarchantin C and 7-oxoriccardin D

(-)-3,14-The novel compounds isolated from the Vietnamese Pallavicinia lyellii are two highly

modified labdane diterpenoids (pallavicinins G-H), two dimeric diterpenoids (pallavicidine A-B), and a clerodane, 4β-hydroxyclerodane Four known diterpenoids,

pleuroziol, levierol, ent-kaur-16-en-19-ol, and (-)-manool were also isolated and

identified

List of Tables

Table 2-1 The Celastraceae s.l.……… 31

Table 2-2 The Celastraceae in Vietnam ……… 31

Table 2-3 Distribution of triterpenoid quinonemethides in Salacia species …… 40

Table 2-4….……… 53

Table 2-5….……… 64

Table 2-6….……… 66

Table 3-1……… 100

Table 3-2……… 104

Table 3-3……… 109

Table 3-4……… 112

Table 3-5……… 115

Table 3-6……… 126

Table 3-7……… 130

Table 3-8……… 141

Table 3-9……… 147

Table 3-10……… 150

Table 3-11……… 155

Table 3-12……… 158

Table 4-1 Chemical mediators and their effect on C-fibres.……… 173

Table 4-2 Metabolism fate of OFQ/N in different tissues ……… 188

Table 4-3……… 190

Table 4-4……… 190

List of Schemes

Scheme 1-1 Biosynthetic relationship of terpenoids 9

Scheme 1-2 Non-mevalonate pathway to isoprenoids 11

Scheme 1-3 Biosynthetic relationship of cyclic monoterpenoids 13

Scheme 1-4 Biosynthesis of gibberellin A3……… 16

Scheme 1-5 Formation of β-amyrin 17

Scheme 1-6 Biosynthesis of cholesterol from squalene oxide 18

Scheme 1-7 Biosynthetic relationship of flavonoids 21

Scheme 1-8……… 23

Scheme 1-9 Biosynthesis of cocaine in Erythroxylon coca……… 24

Scheme 1-10 Biosynthesis of retronecine……… 25

Scheme 1-11 Biosynthesis of nicotine……… 26

Scheme 1-12 Biosynthesis of mescaline……… 27

Scheme 2-1 Biosynthetic relationship between quinonemethides and 7-oxoquinonemethides……… 39

Scheme 2-2 Proposed biosynthetic pathway of celastrol and pristimerin……… 43

Scheme 2-3.……… 67

Scheme 4-1 Proposed biosynthetic pathway of morphine in Papaver somniferum……… 179

Scheme 4-2 Total synthesis of morphine……… 181

List of spectra in appendix

Chapter 1 General introduction

Humans have extracted natural products from plants and animals for a long time, and used them in crude form to treat diseases, as poisons for warfares and hunting, as

stimulants, etc Papaver somniferum (poppy juice) and Glycyrrhiza glabra (licorice)

are some examples which are still in use today, for the same purpose The natural compounds shown in Fig 1-1 have been used in crude form for centuries before they were isolated in pure form in the 19th century Natural products have attracted and motivated chemists of many generations, due to the fact that mother nature seems to

be an unlimited source of structurally and pharmacologically interesting compounds

H N Me OH

Primary and secondary metabolites

Natural products, as the term implies, are those biological molecules which originate from living organisms such as plants, animals, and insects The study of natural products is the investigation of their structure, formation, use, and purpose in the organism There are two major classes of metabolites, primary and secondary metabolites Compounds that make up the fundamental and essential process of life,

e.g nucleic acids, carbohydrates, etc are primary metabolites Secondary metabolites

are small molecules, e.g terpenoids, alkaloids, phenols, pigments, etc., which are not

vital for the survival or well-being of the organism, usually confined to a particular group of closely related species, or to a single species, or even to a single strain growing under certain conditions The term “natural products” is usually reserved for these secondary metabolites Many of these compounds have certain function in the organisms that produce them: they can act as repellents, attractants, allelopathic reagents, phytoallexins, sex pheromones, etc Tens of thousands of natural products have been characterized, and there are certainly many more thousands of compounds still waiting to be discovered

However, as scientists discover novel natural products, the difference between primary and secondary metabolites becomes less clearly defined: there are amino acids that do not play any vital role at all, while many sterols must be considered as primary metabolites, since they play an essential role in many organisms

The biosynthesis of natural products involves three key building blocks: amino acids, shikimate, and acetate Amino acids are biosynthetic precursors of alkaloids and

peptide antibiotics (e.g penicillins) Shikimate is the key starting material of many

aromatic compounds, such as aromatic amino acids, cinnamic acids, and certain polyphenols Acetate, in its active form (thioester with coenzyme A), is the precursor

of many classes of compounds including polyacetylenes, polyphenols, prostaglandins, and macrocyclic antibiotics A derivative of acetyl thiocoenzyme A, mevalonate, is the branch point leading to the terpenoids, steroids, and carotenoids

Role of enzymes in the biosynthesis of natural products

The metabolites are synthesized (and degraded) in living organisms via series of chemical reactions (metabolic pathways) These reactions are theoretically reversible and are well-known in any laboratory However, these reactions are performed in nature at a much higher rate, efficiency, and selectivity The reason is that each reaction is mediated by a specific biological catalyst called an enzyme Most enzymes catalyse the known types of reaction: oxidation, reduction, elimination, hydroxylation, hydrolysis, etc Enzymes can increase the rates of these reactions by as much as 109 to

1012 fold, due to certain advantages:

- Specificity: Each enzyme only catalyses one particular type of reaction, and only accepts substrates that feature a stereospecific structure Furthermore, the enzyme-substrate complex may be formed in such a way that the substrate is forced into transition-state configuration

- The aprotic environment of the active site necessitate the transfer of H+, thereby enhance reaction rate, since most enzymic reactions are acid-base catalysis

- The enzyme-substrate complex is stabilized by non-covalent interactions,

Elucidation of the metabolic pathway

Once the structure of the metabolite has been established, a possible biosynthetic precursor may be proposed To validate this, a labelled precursor is administered to the organism; the metabolite is then isolated, purified and analyzed to see if there is any incorporation of isotope Another way is using 13C metabolic flux analysis (MFA), which is useful for the metabolism of single cell.1 Validating a proposed metabolic pathway is usually more difficult and time consuming than that of a specific precursor, especially when it does not contain any known pathway and none

of the proposed intermediates are available for labeling Mutant organisms may be helpful in these case: the enzyme that mediates a particular step of the metabolic pathway may be absent or defective, resulting in the accumulation of the intermediate prior to that conversion step However, it is possible that the intermediate is diverted

to a different pathway Beside this, there are two other difficulties:

- The labelled precursor must be incorporated at a percentage high enough to produce meaningful results

- The process of analysis of the labelled metabolite to identify the centres of enrichment

Both intact organisms and cell-free extracts of them can be used for study of biosynthetic pathway However, the latter tend to have a higher percentage of incorporation, probably because in the plants, the precursor remains intact but unable

to reach the active site, or is degraded before reaching its destination Thus, more and more scientists have turned to the cell-free extracts and tissue cultures In case of tissue cultures, one can produce undifferentiated callus cells, which may still be able

to biosynthesize certain secondary metabolites, but unable to produce the others

β-Emitters like tritium (3

H) and carbon-14 (14C) are radioisotopes regularly used to label the precursors To locate the enrichment position in such labelled metabolites, one needs to degrade the compounds, using a sequence of common degradative reactions Such reactions result in a smaller quantity of products, and often terminate the degradative sequence before it finishes, leaving investigators with an incomplete labeling pattern This is where the isotope 13C shows its potential Since 13C can be seen in nuclear magnetic resonance (NMR) spectroscopy, a non-destructive analysis method, the 13C labeling pattern can be obtained by comparing NMR spectrum of the natural metabolite and that of the enriched compound

Secondary metabolites from acetate

From acetate, there are two major, and totally different, biosynthetic pathways: stepwise addition of C2 units leads to the formation of fatty acids and polyketides, while condensation of C5 units produces terpenoids, steroids, and carotenoids

The simplest metabolites of acetate are fatty acids Saturated fatty acids with chain length ranging from C8 to C16 are quite common and are constituents of natural waxes and seed oils They also occur in glycerides and phospholipids, such as phosphatidyl

choline (5) These polar lipids, as well as many common fatty acids, are considered as

primary metabolites, since they play vital roles in organisms Only the unusual and uncommon fatty acids are classified as secondary metabolites The most abundant unsaturated fatty acids are also those with C18 chain length, such as oleic acid (6) and

R'COO CH

CH2OPOCH2CH2NMe3

O O

Another large group of secondary metabolites is the polyketides, which can consist of four to nine C2 units Cyclization of linear polyketides chain gives rise to the polyphenols, which contain aromatic ring(s) and at least one phenolic hydroxyl group The lichens are rich sources of polyphenols, especially those contain several aromatic

rings Usnic acid (8), an anti-tumor compound, is commonly found in lichens.6 The

carcinogenic antimicrobial citrinin (9), which occurred in Penicillium and Aspergillus,

is the cyclization product of a pentaketide precursor (10) The biosynthetic precursor

of citrinin in the filamentous fungus Monascus ruber, however, was proved to be a

tetraketide instead.2

O

OH

COMe OH

Terramycin (11), tetracycline (12), and aureomycin (13) are probably derived from

cyclization of a nonaketide The biosynthetic pathway of these effective broad spectrum antibiotics was proposed based on experiments using mutated strains of

Isoprene (17) was suggested as the building block for terpene biosynthesis: The

terpenes can be formed by condensation of successive isoprene units in a head-to-tail fashion This so-called “isoprene rule” was later replaced by Ruzicka’s “biogenetic isoprene rule”, in which the C5 unit is isoprene-like, and the basic skeletal type formed by condensation of these units undergo further modifications to create the

known skeletal types The discovery of mevalonic acid (MVA) (18) in 1956 3 lead to

the identification of isopentenyl pyrophosphate (IPP) (19) in 1959,4-5b and its isomer,

dimethylallyl pyrophosphate (DMAPP) (20),5c as the activated C5 units These are the biological equivalents of isoprene Formation of the main structural types of terpenoid are shown in Scheme 1-1.6

H PPO

(IPP)

H

(E,E)-Farnesyl PP (FPP)PPO

It is noteworthy that the C30 intermediate squalene (21) was formed by joining two

units of farnesyl pyrophosphate (FPP) tail-to-tail, instead of the regular head-to-tail fashion

(21)

Recently, a mevalonate-independent IPP biosynthetic pathway, i.e from pyruvate

(22) and D-glyceraldehyde-3-phosphate (23), was discovered.7 Condensation of these

two compounds produces 1-deoxy-D-xylulose-5-phosphate (DOXP) (24), the first

precursor of this novel pathway

OH

OPO3O

OH

OPO3

OH HO

In higher plants, the mevalonate route is found to operate in the cytoplasm and mitochondria, where sterols and sesquiterpenes are formed; while the non-mevalonate pathway operates in plastids, where hemiterpenes, monoterpenes, diterpenes and carotenoids are biosynthesized However, this compartmental separation is not absolute Under normal conditions, crosstalk between the two biosynthesis pathways

in intact plants is small (< 1%) In plant cell cultures, the crosstalk contributions are generally higher.17 This novel pathway is also present in bacteria and algae, but not in mammals

Monoterpenoids are widespread in plants, but rarely encountered in animals Studies

on Salvia officianalis, Tanacetum vulgare, Mentha spicata, among others, revealed

that linalyl pyrophosphate (LPP) (25) or its ion-pair (26) is the major precursor on the

pathway to other cyclic monoterpenes LPP could be formed from GPP or NPP under mediation of an isomerase-cyclase enzyme, and subsequent rearrangements and hydride shifts lead to the many different skeleton types of cyclic monoterpenes encountered in nature A few selected rearrangement are shown in Scheme 1-3.18a

OH

Thujone Terpinen-4-ol

(c)

(a) (b)

metabolites are chemotaxonomically important, since certain sesquiterpene structural types are genus-specific For example, most of the sesquiterpene lactones were found

in one plant family, the Compositae Many of these lactones, especially vernolepin

(27), plenolin (28), and elephantin (29), are potent anti-tumor agents.6

O O

(29)

Geranyl geranyl pyrophosphate (GGPP) (30), and in certain cases geranyl linalyl pyrophosphate (GLPP) (31), is the progenitor of diterpenoids Most of the diterpenes

encountered in nature are cyclic metabolites, derived from two hypothetical cationic

species (32, 33) via further modifications

OPP

OPP

H+

OPP H

H

OPP H

Among the diterpenoids, gibberellins, which were first isolated from the fungus

Gibberella fujikuroi, are of particular interest, because they seem to be endogenous

hormones in various plant species, responsible for increased growth, induction of

flowering, etc Their biosynthesis, which is believed to proceed via kaurene (34), is a

complex route Key steps in the currently accepted biosynthetic pathway of gibberellin A3 (35), the most active among these plant hormones, are shown in

Scheme 1-4.19

O

(34)

(35)

Scheme 1-4 Biosynthesis of gibberellin A3

As mentioned earlier, the precursor of triterpenoids (C30) and steroids (C27-C29) is

squalene (21), which is abundant in shark liver oil The first step in the pathway is the selective epoxidation of squalene to form 2,3-oxidosqualene (36) Subsequent

cyclization via two different folding modes of squalene oxide, the boat and chair-chair-chair-boat conformations, and further modifications (rearrangement, oxidation, loss of carbon atoms, etc.) give rise to many skeletal types

chair-boat-chair-of triterpenoids as well as steroids While β-amyrin (37) biosynthesis employed the

chair-chair-chair-boat conformation (Scheme 1-5),6 the well established biosynthetic

pathway of cholesterol (38) proceeds via the chair-boat-chair-boat conformation

(Scheme 1-6),18a with the formations of ring A, and probably ring B, are concerted with the initial epoxide protonation.18b

H H

HO

H

ring expansion

cyclization, rearrangement, elimination

(38)

Scheme 1-6 Biosynthesis of cholesterol from squalene oxide

Secondary metabolites from shikimic acid

The shikimate pathway leads to many classes of natural aromatic compounds, mostly polyphenols These polyphenols, however, are different from those biosynthesized via acetate pathway The characteristic substitution patterns of the shikimate pathway are

p-hydroxy-, o-hydroxy-, or 1,2,3-hydroxy-, in opposition to the m-hydroxy pattern of

the acetate pathway The most important products of the shikimate pathway are not

secondary metabolites; they are the aromatic amino acids phenylalanine (39), tyrosine (40), and tryptophan (41) Further metabolism of either phenylalanine or tyrosine, mediated by phenylalanine ammonia lyase (PAL), yields cinnamic acid (42) or 4- hydroxycinnamic acid (43), respectively

PAL

N H

COOH

NH2H

COOH

R COOH

R

NH2H

(40) R = OH (43) R = OH

It is noteworthy that many classes of secondary metabolites from the shikimate pathway are of mixed biosynthetic origin Two of such examples are the furanocoumarins and flavonoids

The furanocoumarins are derived via incorporation of dimethylallyl pyrophosphate (from the mevalonate pathway), with subsequent loss of three carbon atoms, as shown

in the formation of psoralen (44).18a

OH OH

O O

DMAPP

HO O

HO O

O HO

O

O O

HO

O H

(44)

Flavonoids, which usually occur as glycosides, derive part of their structures from the shikimate pathway and the other part from the polyketide pathway Their biosynthesis starts with the condensation of 4-hydroxycinnamyl-SCoA and a triketide, as shown in Scheme 1-7.18a

SCoA

O

OH HO

Ar

O HO

Ar

OH

O RO

OR

Ar

OH

O HO

3 x

Scheme 1-7 Biosynthetic relationship of flavonoids

interesting pharmacological properties Strychnine (2) is a stimulant of the central

nervous system: it causes increased reflex excitability in the spinal cord that results in

convulsions and hyperreactivity to stimuli Morphine (3) is famous because of its powerful analgetic properties; while codeine (45) is known as an effective cough

suppressant However, both are also well-known due to their narcotic properties

Although relatively harmless when use in moderation, caffeine (4) can cause a feeling

of well-being and alertness, increases heart rate and blood pressure, and stimulates secretion of stomach acids Most alkaloids are biosynthesized from the amino acids

ornithine (46), lysine (47), phenylalanine (40), tyrosine (41), and tryptophan (42)

Alkaloids synthesized from the two aliphatic amino acids can be divided into five major structural types, namely pyrrolidine, pyrrolizidine, piperidine, quinolizidine, and pyridine alkaloids Representative structures of these types are shown in Fig 1-2

Lupinine (QUINOLIZIDINE)

N

N Me

Nicotine (PYRIDINE) N

Rectronecine (PYRROLIZIDINE)

Fig 1-2 Typical alkaloids derived from aliphatic amino acids

Early stages of the metabolism route from lysine is probably via a complex between the amino acid and cofactor pyridoxal phosphate, subsequent decarboxylation and

transamination lead to the actual alkaloid precursor, 5-amino-pentanal (48) or its correspondent iminium salts (49) (Scheme 1-8) The N-methyl group can be

introduced within these early steps, but the point of introduction varies according to the particular pathway

N POH2C

OH

Me H

H2O HN

The pyrrolidine alkaloids contain a C4N unit, which is also derived from ornithine in a

similar fashion A typical biosynthesis of this type is that of cocaine (52), a

pyrrolidine alkaloid from the Coca plant Erythroxylon coca of South America

Comprehensive labelling studies using 15N, 13C, and 14C isotopes carried out for more than ten years by Leete and coworkers, lead to the currently accepted pathway illustrated in Scheme 1-9.20

N

Me H2C

O

SCoA H

B

N Me

CO2Me

O

O Ph

Scheme 1-9 Biosynthesis of cocaine in Erythroxylon coca

Coca leaves have been used for at least 2000 years, as a general stimulant and appetite suppressant, and around 8 million South America Indians still chew the quids made from dried coca leaves everyday Cocaine operates as an inhibitor of the re-uptake

process of dopamine, leaving an excess amount of this excitatory neurotransmitter in the nervous system

The formation of pyrrolizidine alkaloids advances through a route that is analogous to that of quinolizidine alkaloids, which involved the condensation of two ornithine

molecules The biosynthetic pathway of retronecine (53), as shown in Scheme 1-10,

was proposed based on 13C and 15N labelled experiments.21

NH2

NH2

N

CH2OH HO

N

CHO CHO

N CHO

H

+ 2 [H]

(53)

Scheme 1-10 Biosynthesis of retronecine

The biosynthesis of pyridine alkaloids, such as nicotine (54) from tobacco plants, is

straightforward: an electrophilic aromatic substitution at C-3 of the pyridine moiety,

which comes from nicotinic acid (55) or its enzyme-bound thiol ester, by the C4N unit originates from ornithine The overall pathway is shown in Scheme 1-11, together

6

S-adenosyl methionine

Putrescine N-methyl transferase

N-methyl putrescine oxidase CHO

NHMe N

Me X

an ArylC2N unit originated from phenylalanine or tyrosine, and an ArylC2 or ArylC1

unit, arises from partial degradation the above amino acids A large number of compounds, some of which are very complex, can be derived from the basic skeleton

by further modifications such as transamination, hydroxylation, decarboxylation, oxidative phenolic coupling, etc Some representative examples are shown in Fig 1-3

MeO

OMe

NMe MeO

HO

(56)

(57)

Fig 1-3 Alkaloids derived from phenylalanine and tyrosine

Mescaline (57) was isolated as the major psychoactive constituent of the peyote

cactus Lophophora williamsii The biosynthesis of this hallucinogenic alkaloid, as

shown in Scheme 1-12, involve decarboxylation of tyrosine, followed by oxidation to

form dopamine (58), an intermediate commonly found in biosynthesis of many

alkaloids of this group, including the isoquinolines and the benzylisoquinolines

COOH

NH2

The isoquinoline alkaloid lophocereine (59), also isolated from L williamsii, was

probably formed by condensation of dopamine (58) and the keto acid (60) (from the

(59) (60) Labelling studies of the biosynthesis of papaverine (61), also showed the incorporation of dopamine (58) into the final metabolite.6

N MeO

Papaver somniferum, was also formed in a similar fashion The analgetic and narcotic

properties of morphine (3), the representative opium alkaloid, and its derivatives are

well documented Biosynthesis of morphine will be discussed in more details in Chapter 4 Studies using [3H]-morphine in the 1970s also revealed the existence of specific binding sites for morphine in the brain, leading to the identification of opioid

receptors and the endogenous peptides with similar pharmacological properties to morphine These will also be discussed further in Chapter 4

Chapter 2 The Celastraceae

2.1 Introduction

The Celastraceae, generally known as bittersweet, is a family of herbs, lianas, shrubs, and small trees, distributed primarily in tropics and subtropics, lesser in temperate regions, and totally absent in the arctic regions.22 The Celastraceae sensu lato

(including Hippocrateaceae) contains about 55-98 genera and 850-1300 species.23-26These estimated numbers are inconsistent due to the fact that not much taxonomic work has been done on the family and the morphological characters used for classification are controversial Since the original descriptions of Celastraceae and Hippocrateaceae, taxonomists either classify them as two distinct families or one

unified family (with the Hippocrateaceae nested within Celastraceae sensu stricto)

According to Hallé,23,27 the Celastraceae s.l consists of two sub-families and four

tribes (Table 2-1) Lately, phylogenetic analyses, using both morphological and molecular characters, have consistently supported that Hippocrateaceae is nested

within Celastraceae s.str.22 However, these analyses are still insufficiently sampled to propose a new classification of Celastraceae

Celastraceae species are commonly found in woodlands, from dry forests to swamps,

although some genera and species prefer different habitats, e.g Gymnosporia and Salacia are found on grasslands, while Acanthothamnus, Canotia and Mortonia tend

to grow in deserts.22 Among the Celastraceae, Maytenus and Salacia are the two

largest genera with around 200 species each.28

A number of reviews on the chemistry of Celastraceae have been published.29-32

Table 2-1 The Celastraceae s.l

Sub-family Hippocrateoideae

Salacieae Helictonemeae Hippocrateae Campylostemoneae Sub-family Celastroideae

There are 76 species of Celastraceae in Vietnam, of which Euonymus (17 species) and Salacia (13 species) are the most abundant genera (Table 2-2).33

Table 2-2 The Celastraceae in Vietnam

Genus

Glyptopetalum Euonymus Microtropis Maytenus Gymnosporia Celastrus Bhesa Lophopetalum Cassine Pleurostylia Quadripterygium Reissantia Arnicratea

Salacia species have been reported to have medicinal value The roots of S oblonga

Wall have been used in India to cure gonorrhea, rheumatism, asthma and diabetes.34

The root extract of S reticulata is used in Sri Lanka to prevent hyperglycemia,35 and

as a herbal remedy for glycemic control even during pregnancy.36 Due to their uses in

traditional medicine, various Salacia species have been chemically investigated The majority of secondary metabolites isolated from Salacia species are terpenoids,

with fewer flavones, xanthones, catechins, proanthocyanidins

Salacia species only produce certain types of triterpenoids, namely friedelane, lupane,

oleanane, ursane, and quinonemethide (nor-triterpenoid) Among them, friedelane and quinonemethide are the most predominant types Both skeletal types are restricted in their distribution: while friedelanes can be found in the Celastraceae and Buxaceae,37Celastraceae is the single natural source of quinonemethide triterpenoids.38

The root bark of S prinoides DC was found to contain both 3-ketofriedelane and

1,3-diketofriedelane derivatives such as friedelin (62), friedel-1-en-3-one (63),

friedelane-1,3-dione (64), 1,3-dioxofriedelan-24-al (65), and 7α-hydroxyfriedelane-1,3-dione

(66).39-41

Tewari et al 40 also reported the isolation of six closely related 1,3-diketofriedelane

derivatives (64-69) from the root bark of the Indian S prinoides DC; their structures

have been established base on the combination of spectroscopic data and chemical

degradation The structure of compound (69), however, was determined by an X-ray study of its dibromo derivative (70), due to the difficulty of correlating (69) to known

friedelane derivatives by chemical means.40,42

Two new friedelanes, 2lα,30-dihydroxyfriedelin (71) and epi-kokoodiol (72), were

isolated from S reticulata Wight var β-diandra, along with eight known friedelanes

(62, 73-79).43-46

From the stem bark of S beddomei (Gamble), a new friedelane, lβ,15α