ReviewMallotusrevised

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Journal of Chemistry, Vol 45 (Special issue), P 111 - 121, 2007

REVIEW:

STUDY ON BENZOPYRANS AND OTHER ISOLATED

COMPOUNDS FROM MALLOTUS APELTA

Received 15 October 2007

NGUYEN HOAI NAM1, NGUYEN HAI DANG1, PHAN VAN KIEM1, LUU VAN CHINH1,

PHAN THI BINH2, LA DINH MOI3, AND CHAU VAN MINH1

1 Institute of Natural Products Chemistry, VAST

2 Institute of Chemistry, VAST

3 Institute of Ecology and Biological Reources, VAST

SUMMARY

During the last decades, dozens of compounds have been isolated from Mallotus apelta These compounds which are classified under the categories viz terpenoids, steroids, flavonoids, cumarino-lignoids, cembrane diterpenoids, and benzopyranoids They were known to exhibit interesting biological activities The phytochemical investigations revealed that malloapelta B which was the major component of M apelta, showed strong NF- B and NFAT transcription factor inhibitory and cytotoxic activities Numerous studies on the synthesis of some derivatives of malloapelta B were carried out This paper reviews the progress on the isolation, structure elucidation and biological activities of secondary metabolites from M apelta, especially, the new structures of benzopyrans Chemical modifications of malloapelta B and structure-activity relationship were also discussed

Keywords: Mallotus, mallotus apelta, benzopyran, malloapelta b.

I - INTRODUCTION

Ba bet (Mallotus) genus comprises about

140 species, distributed in regions from South to

South-East Asia, such as in Malaysian region

(about 75 species), in China (about 40 species)

and in Vietnam (about 40 species) [1] Mallotus

species have been used in traditional medicine

to treat various diseases For example Mallotus

apelta has been used to treat chronic hepatitis,

hepatalgia, enteritis, diarrhea, lymphopathy,

Mallotus repandus has been used to treat

influenza and fever, Mallotus barbatus has been

used in both Vietnamese and Chinese folk

medicine as antipyretic, diuretic, relieving pain

and curing cholera, Mallotus macrostachyus has

been used to treat wounds and pimple, Mallotus paniculatus has been used to treat traumatic

injuries and swelling [2, 3] To improve the

efficiency of using Mallotus species in

traditional medicine, it is neccesary to know their chemical components and pharmaceutical activity However, herbal medicine and its extracts contain hundreds of unknown components, which are often only present in a low amount Moreover, variability usually exists within the same herbal materials [4, 5] The chemical components may vary depending on harvest seasons, plant origins, drying processes and other factors [6] Therefore, investigation on the chemical components of the plant is important for pharmaceutical studies Since the

last decades the scientists have been searching

for the chemical components, pharmaceutical

activity of Mallotus species and synthesizing

derivatives from isolated compounds from these

species The purpose of this review is to present

an overview of the studies on M apelta

including the isolation and structure elucidation

of bioactive compounds, chemical modifications

and synthetic processes.

II - PHYTOCHEMISTRY

The chemistry of M apelta has been widely

examined and the biological activity

investigations were carried out from all over of

the world The efforts have led to the isolation

of a number of physiologically active compounds viz terpenoids, steroids, flavonoids, cumarino-lignoids, cembrane diterpenoids, benzopyranoids We are actively working on the synthesis of some new derivatives of

malloapelta B, a major component of M apelta,

with an aim to find new derivatives having stronger bioactivity Various compounds

isolated from M apelta in different areas have

been classified under the categories terpenoids, steroids, flavonoids, cumarino-lignoids, cembrane diterpenoids, benzopyranoids and miscellaneous compounds as listed in figures

1-5

R

R HO

R1

R2

H O

O

R1

H3C R3

R2

1. CH3(CH)CH2OH H

3 CH2(C)CH2OH H

5. 3 - OH, 3 -H

6. 3 - OH, 3 -H

7. OH H CH3

8. COOH CH3 H

R2

HO

R

9. 3 -OH -(CH2)2CH(C2H5)CH(CH3)2

10 3 -OGla -(CH2)2CH(C2H5)CH(CH3)2

11 3 -OH -CH2=CH-CH(C2H5)CH(CH3)2

12 O

Figure 1: Terpenoids and steroids isolated from M apelta

Terpenoids and steroids

The phytochemistry of M apelta has been

extensively studied since the early 1980s One

of the earlier phytochemical reports was

published in 1985, it described the isolation of

four triterpenes 3 , 29-dihydroxylupane (1),

erythrodiol-3-acetate (7), acetylursolic acid (8)

and -sitosterol (9) from the roots of this plant

[7] This plant also contains a variety of other

pentacyclic terpenoids Based on the spectral

and chemical evidence, their structures were

determined to be hennadiol (3), friedelin (4),

friedelanol (5), epifriedelanol (6), taraxerone (12), and epitaraxerol (13) [14, 16] and a new pentacyclic triterpene, named malloapelta A (2)

[15, 16, 19] From the methanol extract of M

apelta, daucosterol (10), stigmasterol (11), and

ergosterol (14) were isolated and purified using column chromatography over silica gel [19] Flavonoids

Two flavonoids quercitrin (15) and astilbin

(16) were identified from M apelta collected in

Vietnam [19]

OH OH

OH

HO

O O

OH HO

OH

O

15

OH OH

OH

HO

O O

OH HO

OH

O

16

Figure 2: Flavonoids isolated from M apelta

Cumarino-lignoids

A method for the isolation and purification

of three coumarino-lignoids aquillochin (17),

cleomiscosin A (18) and 5’-demethylaquillochin

(19) from M apelta has been reported by Cheng

et al [12]

Cembrane diterpenoids

Recently, several cembrane diterpenoids

10-hydroxycembren-5-one (20),

6-hydroxycembrene-5,10-dione (21) [9]

10,14-

Dihydroxy-5-isopropenyl-2,8,12-trimethyl-cyclotetradeca-2,8,12-trienone (22) [10,11] have

been isolated from M apleta

O O

O

CH2OH

H3CO

O

R2

R3

R1

17 OCH 3 OH OCH3

18 OCH 3 OH H

19 OCH 3 OH OH

R1 O

R2

20. OH,H H

22 OH,H OH

Figure 3: Cumarino-lignoids and cembrane diterpenoids isolated from M apelta

Benzopyranoids

In 2001, An et al have isolated seven

benzopyran derivatives

4-hydroxy-2,6-dimethyl-

6-(3,7-dimethyl-2,6-octadienyl)-8-(3-methyl-2-butenyl)-2H-1-benzopyran-5,7(3H,6H)-dione

(23),

4-hydroxy-2,6,8-trimethyl-6-(3,7-

dimethyl-2,6-octadienyl)-2H-1-benzopyran-5,7(3H,6H)-dione (24),

5-hydroxy-2,8-

dimethyl-6-(3-methyl-2-butenyl)-8-(3,7-

dimethyl-2,6-octadiennyl)-2H-1-benzopyran-4,7(3H,8H)-dione (25),

5-hydroxy-2,8,6-

trimethyl-8-(3,7-dimethyl-2,6-octadiennyl)-2H-1-benzopyran-4,7-(3H,8H)-dione (26),

2,3-

dihydro-5,7-dihydroxy-2,6-dimethyl-8-(3-methyl-2-butenyl)-4H-1-benzopyran-4-one (27),

2,3-dihydro-5,7-dihydroxy-2,8-dimethyl-6-(3-methyl-2-butenyl)-4H-1-benzopyran-4-one (28),

and

2,3-dihydro-5,7-dihydroxy-2,6,8-trimethyl-4H-1-benzopyran-4-one (29) from the leaves of

M apelta [13]

O

OH

O

O

R1

CH3

R2

CH3

23 CH2-CH=C(CH3)-(CH2)2-CH=C(CH3)2 CH2-CH=C(CH3)2

24 CH2-CH=C(CH3)-(CH2)2-CH=C(CH3)2 CH3

O O

O

CH3

OH

R2

R1

25 CH2-CH=C(CH3)2 CH2-CH=C(CH3)-(CH2)2-CH=C(CH3)2

O

OH O

R1

O

CH3

CH3

R1

H3CO

OCH3

R2

O

R2

R1

Figure 4: Benzopyranoids isolated from M apelta

In recent years, a number of biologically

active secondary metabolites have been isolated

from M apelta which is widely distributed in

the northern areas of Vietnam From the

methanol extract of the M apelta, a new

chromene derivative with benzopyran skeleton

was isolated and identified as 1-(5,7-dimethoxy-2,2-dimethyl-2H-chromen-8-yl)-but-2-en-1-one

or malloapelta B (30) with high yield Different

chromatographic techniques were applied to

purify compounds

8-(1’-oxo-3’(R)-hydroxy-

butyl)-5,7-dimethoxy-2,2-dimethyl-2H-1-benzopyran (31), 8-(acetic acid

1’-oxo-3’(R)-hydroxy-butyl

ester)-5,7-dimethoxy-2,2-dimethyl-2H-1-benzopyran (32);

6-(1’-oxo-2’-

en-butyl)-5,7-dimethoxy-2,2-dimethyl-2H-1-benzopyran (33),

6-[1'-oxo-3'(R)-hydroxy-

butyl]-5,7-dimethoxy-2,2-dimethyl-2H-1-benzopyran (34),

6-[1'-oxo-3'(R)-methoxy-

butyl]-5,7-dimethoxy-2,2-dimethyl-2H-1-benzopyran (35), and

6-(1’-oxo-2’,3’-epoxy-

butyl)-5,7-dimethoxy-2,2-dimethyl-2H-1-benzopyran (36) from the leaves of M apelta

which were named as malloapelta C, D, E, F, G,

and H, respectively [17, 18] These compounds

were evaluated their NF- B inhibitory, NFAT

transcription factor inhibitory and cytotoxic

activities (The cytotoxic assay was evaluated on

two cancer cell lines (Human hepatocellular

carcinoma, Hep-G2) and rhabdosarcoma, RD)

Interestingly, malloapelta B showed strong

NF-B inhibitory activity, NFAT transcription

factor inhibitory and cytotoxic activities, the

other compounds showed significant cytotoxic

activities against the two mentioned human

cancer cell lines [17, 18]

Miscellaneous compounds

Recently, the Chinese scientists carried out

an extensive screening for effective anti-HIV natural products Notably, the extract of the roots

of M apelta showed significant activity [8]

Based on the bioassay guided fractionation,

malloapeltine (37), 4-methoxy-3-cyano-pyridine 1-oxide (38), along with 4,5,4’-trimethyl-ellagic acid (39) were purified from the roots of this

plant [9] These compounds were evaluated for

their anti-HIV activity, among them 37

demonstrated a significant inhibitory activity [9] The following phytochemical examination of the

M apelta by Cheng et al led to the isolation of

two compounds named as 2 ,4

,15,16-tetrahy-droxydolabradan (40), malloapeltin (37) [10,11]

By repeated chromatography on silicagel column,

isopimpinellin (41), -tocopherol (42), trans-phytol (43), squalene (44) -carotene (45), lutein (46), and betulaprenol 10 (47) were isolated from

the methanol extract of this plant which were identified by comparison with the spectral data reported in the literatures [15, 16, 19]

N

OCH3

CN

O

N +

N O

O

-O

O

OH

H3CO

OCH3

H 3 CO

O

O

OH OH

OH HO

O

OCH 3 OCH3

41

OH

3

CH 2 OH

n n=9

O H

OH 2

2

44

45

46

O HO

42

Figure 5: Other compounds isolated from M apelta

III - CHEMICAL MODIFICATIONS AND SYNTHESES

To investigate the relationship between the structure and their bioactivity as well as to find new derivatives having stronger effect, Binh et al synthesized the derivatives of malloapelta B by using electro organic synthetic method (see scheme 1) As a result, a new compound named as

bimalloapelta (48) and a known compound

8-[1’-oxo-3’(R)-methoxy-butyl]-5,7-dimethoxy-2,2-dimethyl-2H-1-benzopyran (49) were afforded (see scheme 2) [22, 23]

O

CH 3

H 3 C

H 3 C

O

O

CH 3

H 3 C

O

CH 3

H 3 C

O

CH 3

H 3 C

O

O

CH 3

H 3 C

+ClO 3

-+ClO 4- + 2e

e

-2

48

Scheme 1: Modifications of malloapelta B to produce 48 [22]

O

CH 3

H 3 C

H 3 C

O

O

CH 3

H 3 C

H 3 C

O

O

CH 3

H 3 C

H 3 C

O

H 3 CO

49

Scheme 2: Modifications of malloapelta B to produce 49 [23]

O

H3CO

OCH3

O

(C2H5)2NH

O

H3CO

OCH3

O CHOR3

(C2H5)2NH O

C H

H2 C

H3CO

OCH3

49 R=CH3 50 R=CH2CH3 51 R=CH2CH2CH3 52 R=CH(CH3)2

Scheme 3: Modifications of malloapelta B to produce 49 - 52 [25]

O

H3CO

OCH3

O

HNO3/H2SO4 CHCl3

O

H3CO

OCH3

O

NO2

53

Scheme 4: Modification of malloapelta B to produce 53 [25]

As part of our ongoing studies to look for new derivatives having stronger bioactivities, nine

benzopyrans 8-[1’-oxo-3’(R)-methoxy-butyl]-5,7-dimethoxy-2,2-dimethyl-2H-1-benzopyran (49), ethoxy-butyl]-5,7-dimethoxy-2,2-dimethyl-2H-1-benzopyran (50), 8-[1’-oxo-3’(R)-propoxy-butyl]-5,7-dimethoxy-2,2-dimethyl-2H-1-benzopyran (51),

8-[1’-oxo-3’(R)-isopropoxy-butyl]-5,7-dimethoxy-2,2-dimethyl-2H-1-benzopyran (52),

8-[1’-oxo-2’-en-butyl]-5,7-dimethoxy-3-nitro-2,2-dimethyl-2H-1-benzopyran (53), 8-[1’-oxo-3’(R)-methyl-4’-acetyl-5’-oxo-hexyl]-5,7-dimethoxy-2,2-dimethyl-2H-1-benzopyran (54), (methylformiate)-5’-oxo-hexyl)-5,7-dimethoxy-2,2-dimethyl-2H-1-benzopyran (55),

8-(1’-oxo-3’(R)-methyl-4’(S/R)-(ethylformiate)-5’-oxo-hexyl)-5,7-dimethoxy-2,2-dimethyl-2H-1-benzopyran (56), and 1-(5,7-dimethoxy-2,2-dimethyl-2H-chromen-8-yl)butan-1-one (57) were synthesized by addition, nitration

and Michael reactions from malloapleta B [24, 25, 26]

O

H3CO

OCH 3

O

CH3COCH2R Michael reaction

OCH 3

CH 3

O

O

CH3

CH3 O

O

O

H 3 CO

OCH 3

O

R

H 3 C

OCH 2 CH 3

CH3 O

O

55 R=

Scheme 5: Modifications of malloapelta B to produce 54 - 56 [24]

O C H 3

CH 3

O CH 3

H 3 C O O

C O O C H 3

CO O C H 3

O C H 3

C H 3

O C H 3

H 3 C O O

O

C H 3

C H 3

C H 3

O C H3

H 3 C O

O

C H 3

C H 3

C H 3

O C H 3

H 3 C O

H O

O

C H 3

C H 3

C H 3

O C H 3

H 3 C O O O

C H 3

C H 3

C H 3

O C H 3

H 3 C O

O

O

C H 3

C H 3

C H 3

O C H 3

H 3 C O

H 3 C O O

C H 3

C H 3

C H 3

O C H 3

H 3 C O

O

H2/Pd-C

30 psi

H2/Pd-C

15 psi

D im ethyl m alonate

N aO C H3

1) C H3C H2M gB r 2) N H4C l/H2O

1) L iA lH 4

2) H 2 O /M eO H

N aB H4/M eO H 10-15 0 C

57 58 59

62 63

Scheme 6: Modifications of malloapelta B to produce 57 - 63 [26]

The other method for synthesizing of malloapelta B’s derivatives has been reported by Chinh

et al [26] Compounds 1-(5,7-dimethoxy-2,2-dimethyl-2H-chromen-8-yl)butan-1-one (57) was

produced by reducing malloapelta B in sodium borohydride environment (see scheme 6),

1-(5,7-dimethoxy-2,2-dimethylchroman-8-yl)butan-1-one (58), 8-butyl-5,7-dimethoxy-2,2-dimethylchroman (59) were produced by reducing malloapleta B in catalytic hydrogenation (see

Scheme 6) The other compounds

dimethoxy-2,2-dimethyl-2H-chromen-8-yl)-3-methoxy-1-butene (60), dimethoxy-2,2-dimethyl-2H-chromen-8-yl)-3-hydroxy-1-dimethoxy-2,2-dimethyl-2H-chromen-8-yl)-3-methoxy-1-butene (61), 1-(5,7-dimethoxy-2,2-dimethyl-2H-chromen-8-yl)-3-methylpentan-1-one (62), and

2-(4-(5,7-dimethoxy-2,2-dimethyl-2H-chromen-8-yl)-4-oxobutan-2-yl)malonate (63) were synthesized from malloapelta

B by addition reaction (see Scheme 6) By applying this method we also obtained the same

compounds The summary of synthesized compounds from malloapelta B by these methods is

exhibited in Figure 6

CH3

O

OR

CH3

H3CO

49. CH3

50 CH2CH3

51 CH2CH2CH3

52 CH(CH3)2

CH3

O

R

CH3

H3CO

OCH3

CH3

CH3 O O

CH3 O O

OCH2CH3

CH3 O O

O

CH3

H3C

CH3

H3C

CH3

H3C

O O

OCH3H3CO

H

48

O

H3CO

OCH3

O

NO2

O O

H3CO

OCH3

O O

H3CO

OCH3

O

H3CO

OCH3

59

O

H3CO

OCH3

O CH3

O

H3CO

OCH3

OH

O O

H3CO

OCH3

O

O

-O O

OCH 3

H 3 CO

Figure 6: Synthesized compounds from malloapelta B

IV - STRUCTURES AND BIOLOGICAL

ACTIVITIES OF SYNTHESIZED

COMPOUNDS

It is well-known that LPS or TNF induced

NF-.B activation are related to septic shock,

autoimmune disorders, and inflammatory

diseases [27,28] It is also agreed that

compounds containing , -unsaturated carbonyl

moiety usually exhibit good inhibitory activity

against these factors [26, 27] Interestingly,

benzopyrans, with , -unsaturated carbonyl

group, also display this characteristic [20, 21]

Malloapleta B with , -unsaturated

carbonyl motiety in side chain presented a

potent NF-.B inhibitory activity (IC50 = 5.0

µM), and NFAT transcription factor inhibitory

(IC50 = 2.48 µM) and cytotoxic activity against

two human cancer cell lines (Hep-G2 and KB

with IC50 = 0.49 µg/ml and 0.54 µg/ml,

respectively) [20, 21] Therefore, malloapelta B

were selected for studying the relationship

between structure and biological activity In the

structure of malloapelta B (see Figure 7) we

proposed three positions A, B, C which seem to

be active in organic reaction To investigate the

relationship between structure and biological

activity of this compound, we modified its

structure by changing individual A, B, C

position or both A and B, A and C, B and C to

produce some derivatives Then the biological

activity of the derivatives were retested by the

same method applied for malloapelta B [18, 20,

21, 25] The relationship studies between

structure and biological activity of the

derivatives and malloapleta B were carried out

by comparing their biological activity with that

of malloapelta B

Binh et al [22] modified the structure of

malloapelta B by using cyclic voltammetry to

oxidize the C position, in this method, a

mechanism of electro organic oxidation of

malloapelta B was proposed as shown in scheme

1 Firstly, malloapelta B transferred one electron

to convert into corresponding cation radical,

which was further oxidized by LiClO4 then

converted into 48 This compound was tested on

Hep-G2 and RD cells As a result, 48 exhibited

strongly cytotoxic activity on both tested cell lines with the IC50 values of 0.46 µg/ml and 0.33

µg/ml, respectively

O

H 3 CO

OCH 3

O

2

3 4 5

6 7

8 9 10

11

12

1' 2' 3' 4'

Figure 7: Three positions A, B, C in the

structure of malloapelta B Comparing these results with those of

malloapelta B, the IC50 values of both compounds were similar and that the oxidation

of double bond at C-3/C4 (C position) did not affect to their cytotoxic activity The other method [25] to modify the C position (double

bond at C-3/C4) of the structure of malloapelta

B was applied by using nitration reaction (see

scheme 3) to produce 53 In this method the

hydrogen atom (H-3) of the C position on structure of malloapleta B was replaced by the

NO2 group This compound also exhibited strongly cytotoxic activity on both tested cell lines Hep-G2 and RD with the IC50 values of 0.87 µg/ml and 0.62 µg/ml, respectively

Comparing these resutls with those of

malloapelta B, the IC50 values of this compound was slightly higher The observed result suggested that the addition of NO2 group into

the position C in the structure of malloapelta B

was not affect the cytotoxic activities The decreasing cytotoxic activity of this compound due to NO2group obstructed the access into the

C position of structure of malloapelta B [25] The B position on the structure of malloapelta B

was also modified by an electro organic

oxidation method to produce compound 49 (see scheme 2) [23] The B position was made partial changes by an addition reaction and Michael

reaction with different reagents to produce

C

compounds 49-52, 54-57, 62-63 (see scheme

3,5,6) [24,25,26] The reduction of double bond

(B position) and carbonyl group (A position)

was attempted by catalytic hydrogenation of

malloapelta B at 30 psi of H2gas in the presence

of 10% Pd/C resulted in the conversion of ,

-unsaturated carbonyl to butyl yielded compound

59 (see scheme 6) However, by carrying out the

same reaction at 15 psi of H2gas in the presence

of 10% Pd/C, this reaction reduced the double

bond (B position) and saturation of 3,4 double

bond (C position) in the pyran ring to yield

compound 58 (see scheme 6) Compound 60

and 61 were produced by treating malloapelta B

with lithium aluminum hydride The results are

much complicated with the formation of

isomerizied products 60 and 61 as showed in the

scheme 6 [26] Those compounds above were

assayed for their inhibitory activity on

TNF-induced, NF-.B using transfected Hela cell and

the results are described in the literature [26]

The authors concluded that only 59 showed the

slightly decreased inhibitory activity The rest of

compounds showed the NF- B inhibitory

activity at nearly the same concentration of cell

cytotoxicity Thus these compounds are

considered to be inactive The results revealed

that , -unsaturated carbonyl moiety and the

C=C double bond plays an important role to the

activity of malloapelta B According to the

discussion above, mallotapelta B still showed

the most interesting biological activities

V - CONCLUDING REMARKS

Numerous compounds have been isolated

and identified from Mallotus apelta since the

last two decades They were classified under the

categories triterpenoids, steroids, flavonoids,

cumarino-lignoids, cembrane diterpenoids,

benzopyranoids and miscellaneous compounds

Among them, benzopyranoids were known to be

the most identified components from this plant

Interestingly, the results of biological activity

assay showed that malloapelta B, a major

component of M apelta, exhibited considerable

NF- B and NFAT transcription factor inhibitory

and cytotoxic activities Consequently, great

efforts have been made to modify the structure

of malloapelta B for pharmaceutical tests As a result, sixteen derivatives were obtained Structure-activity relationship studies on the synthesized compounds revealed that , -unsaturated carbonyl moiety and the C=C double bond plays an important role to the activity of malloapelta B It is clear that for a number of studies on the synthesis of some new derivatives, malloapelta B presented the most interesting biological activities

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