The structures and absolute configurations of macrophyllic acids A-E, five new rearranged eudesmane sesquiterpene acids from the bark of Inula macrophylla

Article abstract of DOI:10.1016/S0040-4039(00)00172-6

Macrophyllic acids A-E, five novel sesquiterpene acids with a new rearranged carbon skeleton, have been isolated from the bark of Inula macrophylla. Their structures were determined on the basis of spectral evidence (especially by HREIMS and 2D NMR) as well as chemical transformations. The structure of macrophyllic acid A was finally confirmed by an X-ray analysis. The absolute configuration of macrophyllic acid A was determined by appropriate chemical conversion and the means of PGME. The possible biosynthetic pathway for macrophyllic acids A-E is also discussed. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00172-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 2395–2400
The structures and absolute configurations of macrophyllic acids
A–E, five new rearranged eudesmane sesquiterpene acids from
the bark of Inula macrophylla
Bao-Ning Su,a Yoshihisa Takaishi,a,∗ Tetsuya Yabuuchi,a Takenori Kusumi,a Motoo Tori b and
Shigeru Takaoka b
aFaculty of Pharmaceutical Sciences, University of Tokushima, Shomachi 1-78, Tokushima 770-8505, Japan
bFaculty of Pharmaceutical Sciences, Tokushima Bunri University, Yamashiro-cho, Tokushima 770-8514, Japan
Received 16 December 1999; revised 14 January 2000; accepted 21 January 2000
Abstract
Macrophyllic acids A–E, five novel sesquiterpene acids with a new rearranged carbon skeleton, have been
isolated from the bark of Inula macrophylla. Their structures were determined on the basis of spectral evidence
(especially by HREIMS and 2D NMR) as well as chemical transformations. The structure of macrophyllic acid
A was finally confirmed by an X-ray analysis. The absolute configuration of macrophyllic acid A was determined
by appropriate chemical conversion and the means of PGME. The possible biosynthetic pathway for macrophyllic
acids A–E is also discussed. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Inula macrophylla; compositae; macrophyllic acids A–E; sesquiterpene acid; new carbon skeleton; NMR data;
chemical transformation; X-ray analysis; absolute configuration; biosynthetic pathway.
Aristophyllides A–D, the derivatives of the rearranged ent-elemane sesquiterpenes with a new carbon
skeleton, have been reported recently.1 We report herein five novel rearranged eudesmane sesquiterpene
acids, macrophyllic acids A–E, which also possess a new kind of carbon skeleton and were isolated
from the bark of Inula macrophylla. The structures of macrophyllic acids A–E were determined on
the basis of spectral evidence, especially by HREIMS and 2D NMR. The structure of macrophyllic
acid A (1) was finally confirmed by an X-ray analysis. The absolute configuration of macrophyllic acid
A was determined by appropriate chemical conversion and the means of PGME. Two pairs of stereo-
isomers, macrophyllic acids B–E (2–5), are the derivatives of macrophyllic acid A (1), and their absolute
configurations were established by comparison of the spectral data with those of macrophyllic acid A (1).
The MeOH extract of the powdered air-dried bark (1.6 kg) of I. macrophylla was partitioned between
H2O and CHCl3. The CHCl3-soluble fraction was chromatographed over a silica gel column and further
∗
Corresponding author. Tel: 0081-88-6337275; fax: 0081-88-6339501; e-mail: takaishi@ph.tokushima-u.ac.jp (Y. Takaishi)
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00172-6
tetl 16452

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purified by HPLC and GPC (general permeation chromatography), to give macrophyllic acids A–E (1–5)
(950 mg, 25 mg, 14 mg, 10 mg and 11 mg, respectively), (Fig. 1).
Fig. 1. The structures of compounds 1–5
24
Macrophyllic acid A (1)2 was obtained as colorless needles, [α] −8.0 (CHCl3, c=0.85). Its HREIMS
D
(m/z 234.1608) indicated a molecular formula of C15H22O2 (calcd 234.1620). The 1H NMR spectrum of
1 showed signals of two methyls at δH 1.60 (3H, s, H-14) and 1.04 (3H, s, H-15), a trans double bond
which should be conjugated with a carbonyl group based on their chemical shifts and coupling constants
(δH 7.08, 1H, dd, J=15.7, 7.0 Hz, H-11; δH 5.82, 1H, dd, J=15.7, 0.7 Hz, H-12), as well as the signals
of other methylenes and methine. Its 13C NMR and DEPT spectra (Table 1) displayed signals of four
quaternary carbons, three methines, six methylenes and two methyls. Among these signals, the chemical
shifts of C-11 (δC 156.8, d) and C-12 (δC 118.6, d) further supported that this double bond should be
conjugated with the carbonyl group (δC 172.6, s). There are no other oxygenated carbons except for
the carbonyl group in 1, based on its NMR data, suggesting this carbonyl group was a carboxy group.
Compound 1 is an acid, which also can be inferred from experience since it was a long spot on TLC.
In the 1H–1H COSY spectrum of 1, the correlations of H-12 to H-11, H-11 to H-7, H-7 to H-6 and H-8,
H-8 to H-9, H-1 to H-2, and H-2 to H-3, suggested the structure as shown. This structure was confirmed
by the observed correlations of H-11 to C-12, C-13, C-7, C-6 and C-8, H-12 to C-13, C-11 and C-7, H-15
to C-5, C-10, C-1 and C-9, H-14 to C-3, C-4 and C-5, and H-6 to C-7, C-8, C-11, C-4, C-5 and C-10
in the HMBC spectrum. All of the spectral data were assigned on the basis of the correlations of 1H–1H

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Table 1
The 13C NMR and DEPT spectral data of macrophyllic acids A–E (1–5)
COSY, NOESY, HSQC and HMBC spectra. The structure of macrophyllic acid A was finally verified by
an X-ray analysis (Fig. 2).3
Fig. 2. ORTEP drawing of compound 1
In order to determine the absolute configuration of macrophyllic acid A, 1 was converted to 1a (Fig. 3,
the structure was confirmed by 1H, 13C NMR, 1H–1H COSY and HSQC spectral data). The (S)- and (R)-
PGME (phenylglycine methyl ester) amides were obtained after 1a was treated with (R)- and (S)-PGME,

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respectively (Fig. 3). Thus, the absolute configuration of 7R and 10R for 1 can be elucidated according
to the ∆δ values (∆δ=δS−δR) (Fig. 3).4
Fig. 3. The structures of 1a and (S)- and (R)-PGME amides and their ∆δ values
The NMR data of macrophyllic acid B (2)5 were similar to those of compound 1. Its 1H NMR spectrum
exhibited signals of exomethylene hydrogens at δH 5.05 (1H, s, H-14) and 4.74 (1H, s, H-14), and
one methyl signal at δH 0.91 (3H, s, H-15). The 13C NMR and DEPT spectra (Table 1) of 2 indicated
an oxygenated quaternary carbon at δC 86.5 (C-5). These NMR data suggested the exocyclic double
bond was located between C-4 and C-14, and a hydroxyl group attached to C-5 in 2. The HIEIMS (m/z
250.1587) gave a molecular formula of C15H22O3, which was identical with the proposed structure. The
observed HMBC correlations of H-14 to C-3, C-4 and C-5, H-6 to C-4, C-5, C-7, C-8, C-10 and C-11,
and H-15 to C-1, C-9, C-5 and C-10 confirmed its structure.
The HREIMS (m/z 250.1561) of macrophyllic acid C (3)6 indicated a molecular formula of C15H22O3,
the same as that of 2. The NMR spectral data of 3 were very similar to those of 2, and the evident
difference between them were the chemical shifts of C-5 (Table 1). The hydroxyl groups should adopt
an equatorial orientation and an axial orientation in 2 and 3 based on the chemical shifts of C-5, because
the axial orientation hydroxyl group has a stronger shielding effect. For confirming the structures of 2
and 3, they have been treated with MeOH and DCC in a CH2Cl2 solution at rt for 2 h, which gave their
methyl esters 2a and 3a. The methyl esters 2a and 3a were further acetylated using acetic anhydride and
pyridine in the presence of a catalytic amount of 4-dimethylamoniopyridine at rt overnight; 2b and 3b
were obtained.
The HREIMS of 4 (m/z 250.1556) and 5 (m/z 250.1580) gave the same molecular formula of C15H22O3
and their NMR data were also very similar to each other. The NMR data of compounds 47 and 58
indicated that two methyls were attached to the quaternary carbons. The chemical shifts of C-4 (4: δC
64.2, s; 5: δC 63.6, s) and C-5 (4: δC 67.9, s; 5: δC 68.9, s) suggested the existence of epoxy groups
in 4 and 5, which were also identical with their molecular formulas. The HMBC correlations of H-
14 to C-3, C-4 and C-5, and H-15 to C-1, C-5, C-9 and C-10 were observed for both 4 and 5; these
correlations verified that their epoxy groups were between C-4 and C-5. In the NOESY spectrum of 5,
H-14 correlated to H-15, suggested the two methyls were in cis relationship in 5. The signal of H-14 of
4 (δH 1.33) showed a downfield shift relative to that of 5 (δH 1.26), and the NOESY correlation between
H-14 and H-15 was not observed for 4. Thus, macrophyllic acid D (4) is an isomer of macrophyllic acid
E (5).
All of the H-7 of 1–5 should adopt the axial orientations by analysis of the splitting patterns and
coupling constants of H-6 (H-6eq: 1, dd, J=13.7, 1.4 Hz; 2, dd, J=12.6, 2.4 Hz; 3, dd, J=13.2, 2.8 Hz; 4,
dd, J=13.8, 3.2 Hz; 5, dd, J=14.0, 3.7 Hz; H-6ax: 1, dd, J=13.7, 12.6 Hz; 2, dd, J=13.2, 12.6 Hz; 3, dd,
J=13.2, 12.7 Hz; 4, dd, J=13.8, 12.5 Hz; 5, dd, J=14.0, 12.8 Hz). The absolute configurations of C-7 of
2–5 should be the same as that of 1, since all of these compounds were isolated from the same plant and
all of the H-7 of 1–5 adopt the axial orientations. Thus, the absolute configurations of 2–5 are as shown
(Fig. 1).

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11(13)-Eudesmen-12-oic acid is a common natural product, and a number of these types of sesqui-
terpenes have been isolated from various plant materials.9–14 However, macrophyllic acids A–E (1–5)
possess a new rearranged carbon skeleton, and this is the first report of this kind of sesquiterpene acids.
We speculate the possible biosynthetic pathway of macrophyllic acid A (1) from the related 11(13)-
eudesmen-12-oic acid to be as shown in Fig. 4.
Fig. 4. The possible biosynthetic pathway of macrophyllic acid A (1)
It is worth to point out that macrophyllic acid A (1) is one of the main components (950 mg 1 has
been isolated from 1.6 kg material) of the bark of Inula macrophylla. This suggests the biological
transformation from general 11(13)-eudesmen-12-oic acids to the presently reported new rearranged
carbon skeleton sesquiterpene acids (1–5) is a main and easy biological reaction in the studied plant.
Macrophyllic acids B–E (2–5) were more possible derived from macrophyllic acid A (1) than from the
related 11(13)-eudesmen-12-oic acids, due to 1 is one of the main components of this plant.
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