Eudesmanolides from Stevia aff. tomentosum

Phytochemistry, Vol. 29, No. 5, pp. 1689-1690, 1990. Printed in Great Britain.

EUDESMANOLIDES MARIANO

MARTfNEZ-VAZQUEZ,

0

FROM STEVIA

003 l-9422/90 $3.00 + 0.00 1990 Pergamon Press plc

AFF. TOMENTOSUM

ROSA ELENA GALLEGOS and PEDRO JOSEPH-NATHAN*

Instituto de Quimica, Universidad National Autbnoma de Mxico, Circuit0 Exterior, Ciudad Universitaria, Coyoacln 04510, Mtxico, D. F.; *Departamento de Quimica, Centro de Investigacibn y de Estudios Avanzados, Instituto Polittcnico National, Apartado 14-740, MBxico, D.F., 07000 Mbxico (Received 18 July 1989) Key Word Index-Stevia a# tomentosum;Compositae; sesquiterpene lactones; eudesmanolides.

Abstract-Three new eudesmanolides have been isolated from Steuia afJ: tomentosum and identified by chemical and spectral means as 1~,6a,8~-trihydroxy-11,13-dihydroeudesm-4-en-6-olide 8-acetate, l/I,6cc,SP-trihydroxy-11,13dihydroeudesm-4(15)-en-6-olide &acetate and 1/?,6a,8fl-trihydroxy-11,13-dihydroeudesm-3-en-6-olide 8-acetate.

INTRODUCTION

RO

Sfeuia is one of the largest (15&200 species) genera in the tribe Eupatorieae [ 11. However, this morphologically well delimited genus is surprisingly heterogeneous in its chemical composition [2]. Accordingly, we were able to isolate sesquiterpenoids of the longipinane type from S. subpubescens [3] and from S. serrata [4], while from S. pilosa [2] we isolated a guainolide type sesquiterpene lactone. Now we wish to report the isolation of three new isomeric eudesmanolides (l-3) from the aerial parts of S. afl tomentosum H.B.K. RESULTS AND

1 A4

;

2

; K= H

A4(“)

3 A3 4A4

DISCUSSION

1~,6a,8j?-Trihydroxy-ll,l3-dihydroeudesm-4-en-6olide 8-acetate (1) was isolated as crystals. Its structure is assigned on the following spectral evidence: the IR spectrum shows bands for y-lactone (1772 cm- ‘), hydroxyl (3611, 1030 cm-‘) and ester (1737 cm-‘) functions. The mass spectrum gave the M, as 308, which together with the IR data, confirms the molecular formula C, 7H,,05. The presence of hydroxyl and acetyl groups is evidenced by the peaks at [M-IS]‘, [M-42]+, [M-60]+ and the base peak at m/z 43. The ‘H NMR spectrum confirms the eudesmanolide skeleton since it shows two singlets (3H each) at 61.20 and 1.87 assigned to the two Me groups at C-10 and C-4, respectively, a three proton doublet at 6 1.25 assigned to the Me group at C-11 and the signal of the acetoxyl Me group appears as a sharp singlet at 62.07. At low fields (Table l), signals due to three protons assigned to hydrogens geminal to oxygen functions are as a double doublet at 6 3.51 (J = 4.1, 11 Hz, H-l), a broad doublet at 65.35 (J = 12 Hz, H-6P [S]) and an apparent quartet at 65.51 (J=2.6 Hz, H-8/3 [6, 73). Comparison of the chemical shifts of H-6 and H-8 of 1 with those reported for eudesmanolides with the lactone ring closure at C-6 or C-8 show that 1 has the lactone ring closed to C-6 [6-91. This is supported by the fact that H68 in 1 is deshielded due to the /?-OAc group at C-8, the effect being clearly evident upon comparison with the data of the eudesmanolides torrentin [S], 11,13-dihydro/?-cyclotulipinolide [6] and rothin A acetate [lo], all having an !YZ equatorial OAc group at C-8.

; ,

R=H K=H R = pBrC,H,CO

The 13C NMR spectrum of 1 is in full agreement with the proposed structure, the assignments being done by 13C/lH heteronuclear chemical shift APT and (HETCOR) experiments (Experimental). Although 1 was obtained in crystalline form, it was not suitable for a crystallographic study. Therefore, we prepared its la-pBr-benzoate ester (4) which unfortunately was also unsuitable for X-ray analysis. However, ester 4 can be used to elucidate the stereochemistry at C-11, using the method of Narayanan et al. [1 l] for lactones with /36Hct7H configuration. This method is based in the change of the chemical shift of the methyl group at C-11 on going from chloroform-d, to benzene-d,. The methyl group at C-l 1 in 4 shows an upfield shift of 0.38 ppm in benzene-d, relative to chloroform-d,, which is indicative of the porientation. l/I,6a,8/?-Trihydroxy11,13-dihydroeudesm-4( 15)-en6-olide 8-acetate (2) was isolated in very low yields as a gum. Inspection of its IR, MS and ‘HNMR (Table 1) spectra reveals that 2 is an isomer of 1. The one proton triplet at 64.75 (J = 11 Hz) assigned to 6-/?H and the two broad singlets (one proton each) assigned to H-15 in the ‘H NMR spectrum clearly show that the double bond in 2 is located between C-4 and C-15. Comparison of the ‘H NMR data of 2 with those reported for 11,13-dihydroj-cyclopitulipinolide [6], which is the I-deoxy analogue of 2, further support the structure of 2.

1689

1690

Short Reports Table

I. ‘H NMR spectral

H

l(300

MHz)

1 2 3 6 7 8 9

3.51 dd (4.1, 11) 1.75-1.65 m 2H 2.20-2.00 m 2H 5.35 hr d (12) 2.38 ddd (11, 8, 2.5) 5.51 app q (2.6) 2.33 dd (2.6, 15)

9’ 11 13 14 15

1.48 2.73 1.25 1.20 1.87

OAc

2.07 s 3H

data of compounds

l-4 (80 MHz, CDCl,,

TMS as int. standard)

2

3

4

3.45 dd (5, 10)

3.69 dd (3, 8)

4.75 t (I 1)

5.37 br m 4.68 r (12)

5.47 VP 4 (3)

5.51 app q (2.6)

5.01 dd (4.4, 11.5) 1.9551.85 ,n 2H 2.25-2.00 m 2H 5.36 br d (1 1.2) 2.35 ddd (I 1, 8. 2.5) 5.48 upp q (2.6) 2.41 dd (2.6, 15) 1.58 dd (2.6. 15) 2.75 quirl (7) 1.24 d (7) 3H 1.25 s 3H 1.92 hr.\ 3H

_____

dd (2.6, 15) quin (7) d (7) 3H s 3H br s 3H

2.73 1.25 0.95 4.90 4.97 2.05

quin (7) d (7) 3H s 3H hi-s hr s s 3H

p-BrC,H, Coupling

constants

lj?,6a,Sfl-Trihydroxy-l l,i

2.74 1.30 1.05 1.83

quin (7) d (7) 3H s 3H brs 3H

2.04 s 3H

2.08 s 3H 7.59 d (8.6) 2H 7.85 d (8.6) 2H

in Hz are in parentheses.

3-dihydroeudeesm-3-en-6-

elide &acetate (3), was isolated in very low yields as a gum. The IR, MS and ‘H NMR spectra clearly show that 3 is an isomer of 1 and 2. The double bond is located between C-3 and C-4 as evidenced by the ‘HNMR spectrum, which shows a broad one-proton signal at 65.37 assigned to H-3 and a three proton singlet at 6 1.83 assigned to the vinylic methyl group at C-4.

1450, 1430; MS m/z (70 eV) (rel. int.): 308 [M] i (S), 248 (lo), 220 (9), 159 (S), 124 (23), 91 (20), 43 (100). Preparation of 4. To a soln of 8 mg of 1 in 0.6 ml pyridine, 40 mg p-bromobenzoyl chloride was added. The reaction was keep at room temp. for 72 hr, then treated as reported [12]. Purification by crystallization (MeOH. hexane) furnished 6 mg 4, mp 160-163”, IR vLt,C’3 cm- I: 1770, 1730, 1710. 1580: MS hi./-_ (70 eV) (rel. int.): 446 [M -441 ’ (Z),202 (100). 200 (99). 1X5 (82). 183 (86), 167 (37) 165 (33). 75 (25). 43 (10).

EXPERIMENTAL Aerial parts (197 g) of Steuia of/: tomentosum H.B.K. (collected near Valsequillo, Puebla, Mexico, August 1987, voucher Martinez 40 deposited at the Instituto de Biologia Herbarium UNAM) were extracted successively with hexane, EtOAc and MeOH. The EtOAc residue (3.7g), when chromatographed using hexane with increasing proportions of EtOAc, successively gave(4:1)10mg1(R,0.34),8mg2(R,0.29)and(l:1)7mg3(R, 0.22). 1/&6a,8/I-Trihydroxy-l l, 13-dihydroeudesm-4-en-6-elide 8aceture(1). C,,H,,O,,mp 162-164”, IR v~~~‘~ cm-‘: 3611, 1772, 1737, 1458. 1027; MS m/z (70 eV) (rel. int.): 308 [M] + (2), 290 (2), 266 (5), 248 (18). 215 (27), 159 (15), 107 (26), 91 (35), 43 (100); iJCNMR (75.4 MHz, CDCI,, TMS): 6178.7 (C-12). 169.8 (COMe), 128.8 (C-5 or C-4), 127.0 (C-4 or C-5), 77.8 (C-l), 77.4 (C-6), 68.6 (C-8), 4X.9 (C-7), 43.3 (C-9), 41.4 (C-lo), 37.3 (C-l l), 33.4 (C-3), 26.6 (C-2), 21.5 (COMe), 20.9 (C-14), 20.0 (C-15) and 11.5 (C-13); i3C”H heteronuclear chemical shift correlation 75.4 (300 MHz) CDCl,: 77.8 (3.51), 77.4 (5.35), 68.6 (5.51), 48.9 (2.38), 43.3 (2.33 and 1.48), 37.3 (2.73), 33.4 (2.2G2.00), 26.6 (1.75-1.65), 21.5 (2.07), 20.9 (1.20), 20.0 (1.87) and 11.5 (1.25). 18,6a,8/3- Trihydroxy- 11,13-dihydroeudesm-4 (15)-en-6-okde 8ucefate (2). C,,H,,O,, gum IR v’,:‘J cm -’ 3600, 3500, 1770, 1730, 1650, 1450; MS m/s (70 eV) (rel. int.): 308 [M] + (1). 248 (X), 230 (20). 159 (14), I19 (18), 107 (25), 91 (27), 5.5 (28), 43 (100). 1~,6a,8/r’-Trihydroxy-11,13-dihydroeudesm-3-en-6-o1ide B-wetate (3). C,,H,,O,, gum IR v~~~3cm-‘: 3460, 1770, 1730,

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