Prenylstilbenes and prenylflavanones from Schoenus nigricans

Pergamon

0031~9422(94)EOOO2-A

PRENYLSTILBENES

Phymhemmry. Vol. 36. No 3, pp. 803406. 1994 Copynfit Q 1994 Ekwr B&ma Ltd Pnnkd m Great Britam All n&u reiervcd 0031 9422/w 1700+0.00

AND PRENYLFLAVANONES NIGRICANS

FROM SCHOENUS

A. M. DAWIDAR, J. JAKUPOVIC,* M. ABDEL-MOGIB and I. A. MASHALY~ Department Technical

of Chemistry, University

Faculty

of Science,

of Berlin, D-1000

Berlin

Mansoura 12, Germany;

University,

Mansoura-35516,

TDepartment

Mansoura-35516,

of Botany,

Egypt; Faculty

*Institute of Organic of Science, Mansoura

Chemistry, University,

Egypt

(Receioed in revised form 7 December 1993) Key

Word

Index-Schoenus

nigricans;

Cyperacea;

stilbenes; prenylstilbenes;

prenylflavanones.

Abstract-The extract of Schoenus nigricans afforded, in addition to a known stilbene and a known prenylflavanone, four new stilbenes and two new prenylflavanones. The structures were elucidated on the basis of spectral data.

INTRODUCTION

The genus Schoenus is represented in Egypt only by Schoenus nigricans. The chemistry of the genus Schoenus is still not well known. Only the leaf flavonoids have been studied [ 11. Tricin has been reported as well as flavone Cglycosides. The present paper describes the isolation and structural elucidation of new stilbene and flavanone derivatives from Schoenus nigricans. RF.SULTS AND Dl!SCUSION

The extract of Schoenus nigricans L. afforded four new stilbenes, 2-5, in addition to the known one 1 [Z] and three flavanones, 6-8, of which two are new, 7 and 8. The structure of 1 was established by comparing its spectral data with those of 3,5,4’-trimethoxystilbene [2], which has recently been reported from Virola elongata (Myristicaceae). For comparison, we added the NMR data in Tables 1 and 2. The mass spectrum of 2 showed a molecular ion peak at m/z 300 corresponding to C,,H,,O,, with one CH,O more than 1. The ‘HNMR spectrum of 2 (Table 1) showed the 3’,4’-dioxygenation of ring B (67.06 d, H-2’; 67.04 dd, H-6’ and 66.86 d, H-5’) instead of 4’-oxygenation in 1. Additionally, one methoxyl group signal more than in the spectrum of 1 indicated the 3’-methoxyl group. The structure was finally confirmed by comparing the spectral data with those of 3,5,3’,4’-tetramethoxy stilbene which ..1OC I..~~;~ CL ot US. “1 r71 W(lJ r.rntknr;Tn~ JJLllllrJlUU h,, “J .V”Ur,= L-‘J tn .” k_ rnnvprtprl W”.LS”..V.. intr\ &.I.” the corresponding dihydrocompound. Stilbene 2 was also synthesized by Cushman et al. [4] along with other stilbene and dihydrostilbene derivatives as anticancer agents. Compound 3 gave a M + at m/z 338 due to CzzHz,O,. The molecular formula indicated C,H, more than 1. The ‘H NMR spectrum of 3 (Table 3) revealed the presence of a prenyl group (63.43 br d, H-9; 65.12 ~44, H-10; 6 1.81 br

s, H-13 and 6 1.68 d, H-12). The signals of ring B were similar to those of I, indicating the location of the prenyl group in ring A. The nonequivalence of the three methoxyl groups (63.85,3.83 and 3.81) and also of the aromatic A-ring signals required the prenyl group to be in the ortho position. The proposed structure was confirmed by the 13C NMR spectrum (Table 2). Compound 4 showed a M’ at m/z 324 due to CZ1Hz403, with one CH, unit less than 3. The ‘H NMR ,G.. .* ..\ spectrum ( 1 aote 5) was very simiiar to ihai of 3 iviih one methoxyl group singlet no longer present. This was in agreement with an absorption band at 3620 cm- ’ in the IR spectrum due to a free hydroxyl group. Examination of the data in Table 3 indicated that the chemical shifts of the signals of ring B were similar in both 3 and 4. Thus, the hydroxyl group was located on ring A. The upfield shift (0.05 ppm) of H-4 rather than H-6, as well as the downfield shift (0.07 ppm) of H-10 suggested the location of the hydroxyl group at C, rather than Cg. Additionally, these are the only notable shifts (Table 3). This was confirmed by the “CNMR data (Table 2). Compound 5 exhibited M+ at m/z 310 due to CzoH,,03 with a CH, unit less than 4. The ‘HNMR spectrum (Table 3) showed only one methoxyl group signal at 6 3.80. Acetylation of 5 with acetic anhydride yielded the corresponding diacetate 5a, which showed M + at m/z 426 due to C,,H,,O,. The ‘H NMR spectrum of !%I(Table 3) showed a singlet of two acetate groups at d2.31. position _5 nf _. the .___mcthnxvl ________ ._,- cn111d _____ he _- easilv _-___, decided -__.-_from the examination of the data in Table 3. H-6 has taken the same value in the compounds 3-5, whereas H-4 in compound 3 is more downfield than in 4 or 5. Thus, the oxygen function at position 3 in 4 and 5 is as hydroxyl groups whereas at position 5 it is as methoxyl groups. The signals of ring B in compound 5 were shifted upfield with respect to the other compounds indicating hydroxyl group at position 4’. 803

A. M. DAWIDAR et al.

804

R’

R2

1

H

OMC

3

OMe

OMe

““‘rf-y-“o/”

R’

R2

3

OMe

OMe

4

OH

OMe

5

CH

!ia OAc

a! OAc

HO

i)H

0

R 4

H

7

OH

The ‘H NMR spectra of 6-8 (Table 4) showed a flavanone structure having two prenyl groups in each. The structure of 6 was elucidated by direct comparison with literature data of euchrestaflavanone A [S]. For reason of comparison we added the ‘H NMR data in Table 4. Both 7 and 8 showed M’ at m/z 424 corresponding to C,,H,,O,. The ‘HNMR spectra of both compounds showed the same pattern of signals with small differences in the chemical shifts of some signals. Thus, these two compounds are isomers having one oxygen (as a hydroxyl group) more than 6. The ring B, in both compounds, was represented by two doublets with mefa coupling of 2 Hz, indicating a 3’,4’,5’-substitution. The appearance of the aromatic proton of ring A in compound 8 more upfield

8

(65.93) than in 6 and 7 (65.99 and 5.98, respectively) indicated that the prenyl group of ring A was present in compound 7 at position 6, while in compound 8 at position 8. The MS fragmentation (Experimental) confirmed the proposed structures. Reviewing the structures of stilbenes 3-5, we could conclude the biosynthetic ease of methylation of the hydroxyl groups of 3,5,4’-trihydroxystilbenes with prenyl group at C-2 to be 5, then 4’ and then 3. Natural stilbenes are more frequent in hardwood genera [6-lo], so that King er al. suggested that stilbenes may prove to be characteristic of certain hardwood genera [6]. This paper represents the first report of flavanonesfrom Cyperaceae. However, some related flavanones have been reported from Leguminosae [S].

Prenylstilbenes and prenylflavanones from Schoenw niqricanr Table 1. ‘H NMR data of stilknes 1 and 2 (400 MHs CDCl,, d values)

805

Table 3. ‘H NMR data of stilbenes 3-S (400 MHz, CDCl,, 6 values)

H

1

2

H

3

4

5

sa

Multiplicity

2.6

6.65 d 6.38 r 7.04 d 6.91 d 7.45 AA’BB 7.45 6.90 AA’BB 6.98 3.83 s (9H)

6.66 d 6.38 r 7.03 d 6.90 d 7.06 d

4 6 7 8 2’,6

6.41 6.73 7.23 6.92 7.44

6.36 6.73 7.18 6.89 7.43

6.37 6.72 7.17 6.88 7.38

6.55 7.02 7.23 6.93 7.48

d d d d

7.04 dd

3.5 9 10 12 13 OMe

6.90 3.43 5.12 1.68 1.81 3.85 3.83 3.81

6.90 3.43 5.19 1.73 1.83 3.83 3.80

6.83 3.44 5.19 1.73 1.83

7.08 3.29 5.05 1.68 1.77

3.80

-

S

3.81 2.31 (6H)

s s

4 7 8 2 6 3’ 5 OMe

AA’BB -

6.86 d 3.83 s (6H) 3.90 s (3H) 3.95 s (3H)

J [Hz]: compound 1: 2,6= 2,4= 4.6 = [ZS]; 2’,3’= 5’,6’= [9]; 7.8 = [16.5-J; compound 2: 2,4= 4,6= 2,6= [2.5]; 2’,6 = 123; 5’,6’= 8; 7.8 = [16.5].

Table 2. r3CNMR data of stilbenes 1,3 and 4 (CDCl,) C

1

3

1 2 3 4 5 6 7 8 1’ 2 3 4 5 6 9 10 11 12 13 OMe

129.9 s 104.3 d 160.9 s 99.6 d 160.9 s 104.3 d 128.7 d 126.5 d 139.6 d 127.7 d 114.1 d 159.3 s 114.1 d 127.7 d

OAc

J [Hz]: SS and !k 4.6 = [2.5]; 7.8 = [ 16.51; 2.3’ = 5’6 = [9]; 9,10=[7-J; 10,12=10,13=[1.5].

Table 4. ‘H NMR data of flavanones 6-8 (400 MHz, CDCI,, 6 values)

4

3

H

-

55.3 q 55.3 q (2 x )

130.8 s 120.9 s .158.5 s 97.9 d 158.3 s 101.4 d 129.8 d 124.8 d 137.9 s 127.7 d 114.1 d 159.2 s 114.1 d 127.7 d 24.4 t 123.5 d 130.4 s 25.7 q 17.9 q 55.6 q 55.3 q 55.3 q

br d br I d br s s

130.3 s 117.6 s 158.6 s 101.2 d 155.3 s 103.8 d 130.6 d 124.6 d 138.7 s 127.7 d 114.1 d 159.3 s 114.1 d 127.7 d 25.0 t 122.4 d 133.7 s 25.7 q 17.9 q -

55.3 q (2 x) -

2 3-Ha 3-HP 5-OH 6 8 2 5 6 Prenyl at C-6 (or C-8) CH, CH= 2xMe Prenyl at C-3’ CH, CH= 2xMe

6 5.30 3.09 2.77 Il.36

I 5.23 3.04 2.75 12.32 -

8

Multiplicity

5.22

dd dd dd

298 2.72

1201 5.93

s s si

5.99 7.17 6.84 7.19dd

5.98 6.72 -

6.74

6.86d

6.70 d

3.35 5.25 1.78 1.76

3.34 5.24 1.78 1.75

3.20 5.16 1.62 1.61

br br br br

d t s s

3.38 5.32 1.82 1.78

3.37 5.32 1.81 1.78

3.32 5.29 1.71 1.69

br br br br

d t s s

J [Hz]: 6-8: 2a, 3a=[13]; 2a. k=[3]; = [2]; 5’,6’= [S]; prenyl: I.2 = [7J.

d

3a, k=[17.5];

2.6

EXPERIMENTAL The underground tubers of Schoenus nigricans L. g), collected from the Mediterranean coastal strip at Baltim, Egypt, in January 1992 and identified by Dr I. A.

(390

Mashaly, Botany soura University, room temp. with with cold MeOH. tionated by silica

Department, Faculty of Science, ManMansoura, Egypt, were extracted at EtOH. The extract (15 g) was defatted The defatted extract (9.24 g) was fracgel CC into 3 frs. Fr. I [80 mg, eluted

with petrol-methyl tert. butyl ether (MTBE) 9: l] was sepd by TLC (silica gel, petrol-MTBE 9: 1) to give 3 (23 mg, R, 0.29) and 1 (30 mg, R, 0.19). Fr. II (300 mg, eluted with petrol-MTBE 2: 1) was sepd by TLC (silica gel, petrol-MTBE 4: 1) to give 3 (18 mg, R, 0.56), I (26 mg, R, 0.47) and 4 (20 mg, R, 0.24). Fr. III (800 mg, eluted with petrol-MTBE 1: 1) was sepd (200 mg) using

806

A. M. DAWIDAR et al.

HPLC (RP8, MeOH-H,O 17:3) to give 5 (35mg, R, 3.7 min), 8 (28 mg, R, 6.4 min) and the remainder unresolved material was collected and resepd by HPLC (RPS, MeOH-H,O 4: 1) to give 7 (25 mg, R, 6.6 min), 4 (18 mg, R, 8.7 min) and 6 (30 mg, R, 9.3 min). 3,5,3’,4’-Tetramethoxystilbene (2). IR YL!!‘~ cm- ‘: 3020 (GCH), 2860 (OMe), 1610 and 1525 (aromatic ring), 1480, 1280, 1175. I 130,980 (trans CH=CH); MS m/z (rel. int.): 300.157 [M]’ (100) (talc. for C,,H,,O,: 300.157). 285 [M-Me]’ (7), 270(3), 254(3), 226(5), 150(6.5), 121 (2.8). 99 (6), 85 (6). 3,5,4’-Trimethoxy-2-prenylstilhene (3). IR ~~~~‘3 cm I: 3020 (C=CH), 2860 (OMe), 1610 and 1525 (aromatic ring), 1595 (C=C), 1475,1260,1190,1165,1100,1050,980 (trans CH=CH), 850 (2 adjacent ArH,); MS m/z (rel. int.): 338.118 CM]’ (IO)(calc. for C,,Hz60,: 338.205), 323 [M -Me]’ (ll), 308 [M-CH,O]’ (60), 295 (59). 283 [M -C,H,]+ (8), 280 (52), 269 [M-C,H,](8), 217 (58), 176 (68), 162 (lOO), 149 (70), 121 (86.5). 69 [C,H,] + (50), 55 [C,H,]’ (57). 3-Hydroxy-5,4’-dimethoxy-2-prenylsrilbene (4). IR vzF’3 cm - .‘ . 3620 (free OH), 3020 (C=CH), 2860 (OMe), 1620 and 1525 (aromatic ring), 1590 (C=C), 1480, 1270, 1190, 1160, 1050, 980 (rrans CH=CH), 850 (2 adjacent ArH,); MS m/z (rel. int.): 324.189 [M]’ (5.5) (talc. for C,,H,,O,: 324.189), 309 [M - Me] + (7), 281 [309 -CO]+ (28.2), 205 (22), 192 (23), 162 (lOO), 121 (61). 69 CC&l+ (31), 55 [C,H,]’ (31). 3,4’-Dihydroxy-5-methoxy-2-prenylstilbene (5). MS m/z (rel. int.): 310.173 [M]’ (7.5) (talc. for C,,H,,O,: 310.173). 295 [M-Me]’ (II), 267 [295-CO]’ (37.5), 203 (38). 178 (46), 148 (loo), 107 (59), 69 [C,H,] + (9.5), 55 (14). 3,4’-Diacetoxy-5-methoxy-2-prenylsrilbene (Sa). IR vz:‘3 cm - *: 3020 (C=CH), 2860 (OMe), 1770 and 1385 (OAc), 1620 and 1520 (aromatic ring), 1585 (C=C), 1480, 1200, 1150, 1050, 980 (rrans CH=CH), 850 (2 adjacent ArH,); MS m/z (rel. int.): 394.204 CM]- (4.7) (talc. for C24H2605: 394.204). 379 [M-Me] + (0.7). 351 [379 -CO] + (7), 309 (36), 267 (42), 192 (48), 178 (78), 162 (64). 148 (lOO), 107 (39). 5,7,4’,5’-Telrahydroxy-6,3’-diprenplflacanone (7). IR \‘:;;I, cm 1: 3550 (free OH), 3380 (bonded OH), 1640 (H-

bonded C=O), 1610 and 15 15 (aromatic ring), 1590 (C=C), 1455, 1345, 1315, 1160, MS m/z (rel. int.): 424.188 [M]’ (22)(calc. for C,,H,,O,: 424.219),407 [M-OH]+ (4), 369 [M-C,H,]+ (5.5). 356 [M-C,H,]+ (98), 323 (13). 295 (43), 283 (IOO), 252 (lo), 221 (6.5), 220 (5.5), 217 (21), 205 (13.5). 204 (5.5). 165 (23). 149 (32). 121 (43); [z];~’ - 22.14 (CHCI,; c 0.28) 5,7,4’,5’-Tetrahydroxy-8,3’-diprenyljfauanone (8). IR $:;‘I m- ‘1 3680,359O and 3540 (free OH), 3300 (bonded OH), 1640 (H-bonded C=O), 1610 and 1510 (aromatic ring), 1440, 1345, 1310, 1160, 1085; MS m/z (rel. int.): 424.188 [M]’ (89) (talc. for C,,H,,O,: 424.219), 407 [M -OH]’ (13), 369 [M-C,H,]+ (20), 340 (20), 330 (17), 222 (88), 221 (22), 220 (21), 205 (72). 204 (36.5), 162 (lOO), 121 (6Y), 57 (92); [x];~’ - 16.83 (CHCI,; c 0.82). Acknowledgement-The authors thank Prof. Dr M. A. Zahran, the P.1. of the project number 90009 (FRCU), for financial support.

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