Diterpenoids from Lycopus europaeus and Nepeta septemcrenata: Revised structures and new isopimarane derivatives

TETRAHEDRON Tetrahedron 55 (1999) 7375-7388

Pergamon

Diterpenoids from Lycopus europaeus and Nepeta septemcrenata: Revised Structures and New Isopimarane Derivatives Ahmed A. Hussein,°Benjamfn Rodriguez,""* Maria de la Paz Martfnez-Alcfizars and F61ix H. Canoc alnstituto de Qui.rnicaOrg,~nica, CSIC, Juan de la Cierva 3, E-28006 Madrid, Spain; bDepartamento de Ciencias Bdsicas, Facultad de Ciencias Experimentales y T~cnicas, Universidad San Pablo-CEU, E-28668 Boadilla del Monte, Madrid; CDepartamentode Cristalografia, Instituto "Rocasolano", CSIC, Serrano 119, E-28006 Madrid

Received 26 February 1999; revised 30 March 1999; accepted 15 April 1999

Abstract: Seven isopimarane diterpenoids have been isolated from Nepeta septemcrenata (3) and Lycopus europaeus (4-9), both species belonging to the Labiatae family. Compound 3 had been reported as the sole diterpene constituent of N. septemcrenata, whereas the physical and spectroscopic data of 4 were identical to those of a compound previously isolated from L europaeus to which structure 1 had been attributed. As a result of exhaustive spectroscopic studies some chemical correlations between 3 and 4, we definitely conclude that structure 1 must be amended to 4. In addition, the structures of five new diterpenoids found in L. europaeus (5-9) were established by chemical and spectroscopic means, and a normal isopimarane absolute configuration for all the isolated compounds, except for 7, was supported by CD data, application of the Horeau's method and chemical correlations. © 1999 Elsevier Science Ltd. All fights reserved. Keywords: Terpones; isopimaranes; stereochemistry; X-ray crystal structure.

Introduction Previous works on the constituents of Lycopus europaeus L. (Labiatae) reported the isolation of two diterpenoids, suggesting a pimarane-type structure (1 and 2) for these substances on the basis of ~H and 13C N M R spectroscopic d a t a ) '2 Recently, Voehler and co-workers 3 isolated from Nepeta septemcrenata Ehrenb. (Labiatae) an isopimarane derivative whose structure (3, except for the absolute configuration) was rigorously established by spectroscopic studies. These authors indicated 3 that "surprisingly, although the proposed configurations at C-13 and C-14 in 3 and 1 are different, their t3C N M R data are very close"; however, no additional results concerning this point have been published hitherto. In view of these facts, it was obvious that some aspects of the reported structures 11 and 33 needed to be reexamined. Dedicated to Prof. Dr. Manuel Lora-Tamayo on the occasion of his 95th birthday. ' Author to whom correspondence should be addressed. Phone 34 91 5622900, Fax 34 91 5644853, e-mail iqorl07 @fresno.csic.es

0040-4020/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved. PII: S0040-4020(99)00361-0

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A. A. Hussein et al. /Tetrahedron 55 (1999) 7375-7388

Results and discussion A reinvestigation of the acetone extract of the aerial parts of Nepeta septemcrenata allowed us to isolate 3 as the sole detectable diterpene constituent of this plant, thus confirming previous results) On the contrary, we have found in the acetone extract of Lycopus europaeus five new isopimarane derivatives (5-9) together with another diterpenoid (4), which showed identical IH and ~3C NMR spectra (Tables 1+ and 2, respectively) that those reported ~ for 1. However, attempts at isolating the other pimarane constituent 2 (2) from our extract of L. europaeus were unsuccessful. The variation observed in the diterpene content of L. europaeus could be attributed

to the fact that the plant material used for the first study ~'2came from Central Europe (near Belgrade, Yugoslavia) whereas our material was collected near Madrid, Spain. In the case of N. septemcrenata, in which no variation of the diterpene composition was observed, the plant materials extracted in both studies (reference 3 and this work) were collected in the same place (South Sinai, Egypt). 16

17

" 17

"OR 4

4

R1

R2

R3

1:14

a-OH,fill

COOMe

Ac

Ac

-o,,c Is ~

oooM..c ~2

6

COOMe

Ac

H

O

CHK)Ac

Ac

Ac

11

(x-OH,~H

CH2OH

H

H

12

(x-OAc,l~-H

CH2OAc

Ac

Ac

COOMe 18

10

1 R=H 2 R=OAc

o:-OAC,13-H

The ~H and ~3C NMR and NOESY spectra of 3 (Tables 1, 2 and 3, respectively) obtained by us for this work were in complete agreement with previous results 3'5 regarding its isopimarane-type hydrocarbon skeleton. Furthermore, a single-crystal X-ray determination of 3 was undertaken in order to elucidate its structure conclusively. Figure 1 shows the result of the X-ray analysis of 3, thus confirming the previous deduction on the structure and relative stereochemistry of this compound (see reference 3 and above). Bond lengths and angles of 3 are in good agreement with those found in analogous compounds. 7,s In the crystalline state, the conformation of ring A is a chair with cndocyclic torsion angles between 53° and 57 °, whereas ring B possesses a 1,2-diplanar form and ring C has a half-chair conformation, as can be seen by the values of the torsion angles 9 of 3 [C6-C7C8-C9-3.2(4)°; C7-C8-C9-C10 -3.3(4)°; C9-C8-C14-C13 28.3(4)°; C14-C8-C9-C11 -5.8(4)°; C8-C9-C11-C12 10.3(4)°]: The previously unknown 3 absolute stereochemistry of 3 was established by application of the Horeau's method, l° which defined as S the configuration of the C-1 stereogenic centre (see Experimental) and consequently, a normal isopimarane absolute configuration for this diterpenoid. This conclusion was also supported" by the negative Cotton effect (As303-1.53) shown by the 1-keto derivative 10, obtained by oxidation of 3. 'Listsof atomic coordinates,thermal parameters,structurefactors,bond lengths,bond angles and torsionangles are deposited as supplementary material at the Cambridge CrystallographicData Centre.

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A. A. Hussein et al. /Tetrahedron 55 (1999) 7375-7388

Acetic anhydride-pyridine ueatment of

( ~ ~ ~ e lC?l

Ct6

diterpenoids

4

and 6,

isolated

from L.

europaeus, yielded the same peracetyi derivative

5, another constituent of the plant. Combustion analysis and low resolution mass spectrometry established the molecular formula C2~H3808 for 5 and its IR spectrum was devoid of any hydroxyl absorption. The ~H and ~3C NMR spectra of this compound (5, Tables 1 and 2) were in agreement ~3 with an 8,15-pimaradiene or

isopimaradiene

ditetpene

structure,

possessing acetoxyl groups at the C-lc~, C-7t~ and C-14¢t or 13 positions

and an

18-

carbomethoxyl group. NOE experiments (Table Figure 1. Molecular structure and conformation of 3. Non-H atoms are relm:se~tedby displacement ellipsoids at the 30% probability level and H atoms as spheres of an

arbitraryradii.6

3) established a cis spatial relationship between the H-11[~, H-12[~, H-14[~, Me-17, Me-19 and Me-20 protons of 5, only compatible with the isopimarane structure depicted in its formula.

Compounds 4 and 6 are the l-deacetyl and 14-deacetyl derivatives of 5, respectively, as was supported by their tH NMR spectra [Table 1: three acetates in 5, only two in 4 and 6; diamagnetic shift of H-II~ (AS -1.4 ppm) in 4 and of H-14I~ (AS -1.58 ppm) in 6 with respect to 5]. This conclusion was also in agreement with the HMBC spectra of 4 and 6, which showed connectivities through three bonds between the carboxyl carbons of the acetates (8 170.5 and 170.3, and 8 170.6 and 170.3 in 4 and 6, respectively) and the H-7I~ and H-14I~ protons in 4 (8 5.42 ddd and 5.47 br s), and the H-113 and H-TI~ protons in 6 (8 4.91 t and 5.18 ddd, respectively). Reduction of 5 with LiA1H~ gave the derivative 11, identical to the compound obtained by alkaline hydrolysis of 3 (see Experimental). Moreover, acetylation of 3 with Ac20-pyridine for 48 hours at room temperature gave the tetraacetyl derivative 12, whereas treatment of the reduction product (11) of 5 with the same reagent during 12 hours at room temperature yielded 12 and minor quatities of another substance identical in all respects (mp, mixed mp, TLC, [o~]o, ~H NMR and mass spectra) with the natural diterpenoid (3) isolated from N. septemcrenata.

From all the above data, it was evident that the diterpenoids 4, 5 and 6 found in L. europaeus possessed a normal isopimarane structure like 3, and we definitely conclude, therefore, that structure 1, previously assigned ~ to the diterpene isolated from this plant, must be amended to 4.12 Another of the diterpenoids isolated from L. europaeus (compound 7, CzsH360~) showed IH and 13C NMR spectra very similar to those of 5 (Tables 1 and 2) and the observed differences were consistent with the absence in 7 of the lct-acetoxyl substituent of 5. In particular, the almost identical chemical shifts for the C-4, C6 - C-8 and C-11 - C-20 carbons in both compounds, as well as the up-field resonance of the C-1, C-2 and C-10 carbons [~- and [I-effects, A8 = 8(7) - 5(5), -37.4, -4.4 and -3.9 ppm, respectively] and the paramagnetic shifts of the C-3, C-5 and C-9 carbons (y-effect, A8 +6.0, +5.4 and +4.1 ppm, respectively) in 7 with respect to 5,

A. A. Hussein et al. / Tetrahedron 55 (1999) 7375-7388

7378

Table 1. ]H NMR Spectral data of compounds 3-9" H lot 113 2or 213 3or 313 5a 6ot 615 71~ ! 1~ 1113 12(x 1213 146 15 16A 16B Me-17 18A 18B Me-19 Me-20 lot-OH' 11ot-OH~ OAt:

C'OOMc J.~ (Hz) lot, l~ lct,2ct 1ct,213 l[~,2ot 113,2[~ 2ot,2~ 20.,3 ot 2ct,313 2~,3ct 2[3,3[3 3ct,313 5Ct,6ct 5ot,6~ 6ct,6~ 6ot,715 613,713 1 l a , ll[3 11ct,12ot

3 3.94 mc 1.68 dddd 1.93 dddd 1.84 ddd 1.18 dt 2.15 dd 1.76 m 1.76 m 5.15 br dd 2.48 ddd 2.15 ddd 1.96 ddd 1.51 ddd 5.09 br st 5.79 dd 5.00 ddt 5.02 dd~ 0.90 s 3.68 d 3.83 d 0.89 s 1.05 s 1.35 d 2.04 s 1.96 s 1.94 s

4 3.54 mc -1.36 b 1.60 dddd 2.35 ddd 1.34 ddd 3.06 dd 1.78 ddd 1.64 ddd 5.42 ddd 2.40 ddd 1.87 dddd 2.05 ddd 1.43 dddd 5.47 br st 6.11 dd 5.18 dds 5.20 dd~ 0.91 s

5

6

4.94 t ~1.87 b -1.87 b -1.82 b 1.48 ddd 2.78 dd 1.43 ddd 1.77 ddd 5.06 ddd ~ 1.92 ~ 2.04 dddd 1.78 ddd ~1.47 b 5.16 br st 5.80 dd 5.00 dd~ 5.04 dd~ 0.89 s

4.91 t ~1.84 b ~1.84 b ~1.80 ~ 1.51 ddd 2.72 dd !.41 ddd ~1.78 b 5.18 ddd -1.95 ~ -1.85 ~ -1.75 ~ 1.39 ddd 3.58 br st 5.89 dd 5.05 ddh 5.07 dde 0.87 s

- 1.67 ~' 1.88 ddd

9 ~ 1.676 1.90 ddd

~ 1.63 b

- 1.65 b

~ I. 6 5 b

~1.63 b

.i.65 b - 1.74 b - 1.58 ~ 2.39 dd 1.41 ddd 1.80 ddd 5.11 dd

~1.65 b

~ 1.74b 1.58 ddd 2.34 dd 1.41 ddd 1.81 ddd 5.08 ddd 2.29 ddd 2.06 dddd 1.90 ddd 1.47 dddd 5.06 br st 5.78 dd 4.98 dda 4.99 dd~ 0.87 s

4.26 ddd" 2.26 dd 1.56 dd 3.63 s 6.07 dd 5.16 ddJ 5.18 dd~ 0.88 s

1.18 s 1.03 s

1.17 s 1.01 s

1.17 s 0.96 s

1.17 s 0.93 s

2.00 s 1.87 s

2.08 2.00 1.95 3.62

2.05 s 2.03 s

1.96 s 1.95 s

2.54 d 2.00 s 1.99 s

2.62 d 2.07 s

3.61 s

3.59 s

3.61 s

3.61 s

3.44 s

s s s s

12.2 6.0 8.3 b

12.6

12.4

b

b

b

b

b

b

b

b

b

1 ll~,12ct

2.8 2.8 ~ b 3.0 b 2.8 12.8 1.9 13.2 14.7 2.2 4.4 18.2 6.1 ~ 9.2

2.8 2.8 b ~ 3.0 b 2.9 12.9 2.0 13.2 14.6 2.3 4.4 b ~ 3.4 ~

1113,1213

2. I

2.1

2.3

2.1

13.0 11.1 17.4 1.5

13.3 11.0 17.7 1.4

14.8 17.5 11.2 1.3

3.4 b 2.4 12.9 2.0 13.2 14.5 2.2 4.6 18.8 6.8 3.2 9.3 2.0 13.2 10.7 17.8 1.2

2.0 1.0