Genepolide, a sesterpene γ-lactone with a novel carbon skeleton from mountain wormwood (Artemisia umbelliformis)

Article abstract of DOI:10.1021/np800468m

The sesterpene γ-lactone genepolide (5) has been isolated from a Swiss horticultural variety of mountain wormwood (Artemisia umbelliformis) developed as a thujones-free alternative to native Western Alps wormwoods for the production of liqueurs. Genepolid

Full text of DOI:10.1021/np800468m

340
J. Nat. Prod. 2009, 72, 340–344
Genepolide, a Sesterpene γ-Lactone with a Novel Carbon Skeleton from Mountain Wormwood
(Artemisia umbelliformis)
Giovanni Appendino,*,† Orazio Taglialatela-Scafati,*,‡ Adriana Romano,‡ Federica Pollastro,† Cristina Avonto,† and
Patrizia Rubiolo§
Dipartimento di Scienze Chimiche, Alimentari, Farmaceutiche e Farmacologiche, UniVersita` del Piemonte Orientale, Via BoVio 6,
28100 NoVara, Italy, Dipartimento di Chimica delle Sostanze Organiche Naturali, UniVersita` di Napoli “Federico II”, Via Montesano 49,
80131 Napoli, Italy, and Dipartimento di Scienza e Tecnologia del Farmaco, UniVersita` di Torino, Via Giuria 9, 10125 Torino, Italy
ReceiVed July 27, 2008
The sesterpene γ-lactone genepolide (5) has been isolated from a Swiss horticultural variety of mountain wormwood
(Artemisia umbelliformis) developed as a thujones-free alternative to native Western Alps wormwoods for the production
of liqueurs. Genepolide is the formal Diels-Alder adduct of the exomethylene-γ-lactone costunolide (2) and the diene
myrcene (6), two poorly reactive partners in cycloaddition reactions, and its structure was elucidated through a combination
of spectroscopic methods. An investigation on the thermal stability of mixtures of 2 and 6, as well as considerations on
the sensitivity of 2 to Brønsted and Lewis acids, suggests that 5 is a genuine natural product and that the Swiss chemotype
of A. umbelliformis contains Diels-Alderase enzymatic activity that is lacking in native mountain wormwoods from
Western Alps. Remarkable differences in thermal and acid-catalyzed reactions of the cyclodecadiene moiety of 2 and
5 suggest that quaternarization at C-11 has far-reaching effects on the reactivity of their homoconjugated medium-sized
diene system. The wide occurrence of this structural motif in sesquiterpenoids makes this issue worth a systematic
investigation.
The past two decades have witnessed an explosion of interest
for secondary metabolites from food plants.1 These studies were
triggered by epidemiological correlations between semivegetarian
diets and a reduced incidence of chronic and degenerative diseases,
and have fostered a growing interest for healthy food by consumers
and food companies.2 Within this context, the Mediterranean diet
has emerged as the archetypal healthy food style,3 and enormous
biomedical work has been done on olive oil and wine, the hallmarks
of this dietary regime.4 Surprisingly, little attention has been given
to another important feature of the Mediterranean diet, namely, the
consumption of wild, often bitter, herbs.5 This trait has long been
perceived of exclusive relevance for taste and flavor, but the recent
discovery that taste receptors are expressed in the gastrointestinal
tract6 and are involved in the secretion of GLP-1 and in glucose
homeostasis7 suggests that taste-active compounds can have a
biological profile that goes beyond chemoreception and involves
glucose regulation, appetite control, and weight management.7 Thus,
bitter herbs have been traditionally used to stimulate appetite and
treat cachexia,8 while the beneficial effects of bitter wormwood
extracts for the management of Crohn’s disease suggests an even
broader involvement in gastrointestinal health.9
Alps (A. umbelliformis and A. genipy Weber).13 These plants are
used for the production of genepy, the celebrated alpine bitter
liqueur recently granted a Geographical Indication status by the
European Community.14 The wild chemotype of A. umbelliformis
is characterized by the accumulation of bitter11 sesquiterpene
γ-lactones of the cis 8-olide type and of the polymethoxylated
flavonoid eupatilin (1),10c,d a registered antiulcer drug in some Far
East countries.15 While retaining the presence of 1, the Swiss
chemotype of A. umbelliformis contained instead sesquiterpene
γ-lactones of the trans 6-olide type. Thus, large amounts of the
germacranolide costunolide (2) and minor amounts of its related
oxygenated derivatives verlotorin (3a), artemorin (3b), reynosin
(4a), and santamarin (4b) were isolated, a trait otherwise typical
of A. genipy.10a Along with these compounds, relatively large
amounts of a new lactone were present. This compound was absent
in A. genipy and was characterized as the sesterpene γ-lactone 5,
named genepolide in consideration of the use of mountain worm-
woods to produce the liqueur genepy.
Bitter vegetables and liqueurs prepared from bitter aromatic herbs
are the major dietary sources of bitter compounds in the Western
diet. Our studies in this area have focused on bitter terpenoids,
characterizing a host of new sesquiterpenoids and diterpenoids from
dietary plants,10 and eventually identifying a human taste receptor
(hTAS2R46) capable of binding this class of compounds.11 As part
of these studies, we have investigated a horticultural Swiss variety
of mountain wormwood [Artemisia umbelliformis Lam. (Aster-
aceae)] characterized by the lack of thujones,12 the major constitu-
ents of the essential oil of mountain wormwoods from Western
Dedicated to Dr. David G. I. Kingston of Virginia Polytechnic Institute
and State University for his pioneering work on bioactive natural products.
* To whom correspondence should be on addressed. Tel: +39 0321
373744 (G.A.); +39 081 678509 (O.T.S). Fax: +39 0321 375621 (G.A.);
+39 081 678552 (O.T.S.). E-mail: appendino@pharm.unipmn.it (G.A.);
scatagli@unina.it (O.T.S.).
Results and Discussion
† Universita` del Piemonte Orientale.
‡ Universita` di Napoli.
Genepolide (5) was obtained as a colorless foam from fractions
less polar than those containing costunolide (2). Its molecular
§ Universita` di Torino.
10.1021/np800468m CCC: $40.75
2009 American Chemical Society and American Society of Pharmacognosy
Published on Web 11/20/2008

Sesterpene γ-Lactone from Mountain Wormwood
Journal of Natural Products, 2009, Vol. 72, No. 3 341
formula, C25H36O2 (HREIMS), implied eight unsaturation degrees,
while the IR spectrum retained the γ-lactone absorption band at
1766 cm-1, typical of other bitter compounds from this plant.10c,d
The 1H NMR spectrum of 5 (Table 1) showed the presence of four
relatively downfield shifted and broad methyl singlets (δ 1.38-1.67),
a series of partially overlapped multiplets between δ 1.45 and 2.40,
and five methine signals in the mid-field region of the spectrum
(from δ 4.60 to 5.35). The 13C NMR spectrum of 5 (Table 1)
disclosed the presence of four carbon-carbon double bonds (eight
signals between δ 117.3 and 139.5) and of one ester (lactone)
carbonyl (δ 180.8), accounting for both oxygen atoms required by
the molecular formula and for five of its eight degrees of
unsaturation. Both the IR and 13C NMR features of the carbonyl
suggested a γ-lactone and not an ester moiety,16 and therefore a
C25 carbon connectivity was present. Taken together, these
observations suggested that genepolide was a tricyclic sesterpene
lactone, a very rare class of natural products.
Table 1. 1H (500 MHz) and 13C (125 MHz) NMR Data of
Genepolide (5) in CDCl3
pos. δH, mult., J in Hz δC, mult. pos. δH, mult., J in Hz δC, mult.
1
4.80, bd, 10.7
126.8, d 13a 1.95a
30.5, t
2a 2.24a
2b 2.13a
3a 2.25a
3b 1.99a
4
26.0, t 13b 1.65a
14
1.38, s
1.64, s
16.2, q
17.1, q
27.1, t
39.4, t 15
16a 2.20a
139.5, s 16b 2.14a
5
6
7
4.61a
128.3, d 17
79.3, d 18
57.4, d 19
25.1, t 20
21
5.35, bs
117.3, d
138.1, s
37.6, t
4.60a
1.78, bt, 7.8
1.99a
8a 1.85, bdd, 15.1, 7.8
8b 1.45, bdd, 15.1, 7.5
2.06a
26.2, t
5.06, bt, 6.4
124.2, t
131.5, s
17.7, q
26.0, q
24.8, t
9a 2.32, dd, 13.1, 7.5
41.6, t 22
23
137.0, s 24
9b 1.97a
1.61, s
1.67, s
10
11
12
44.5, s 25a 2.40, dd, 15.1, 7.0
180.8, s 25b 1.94a
a Overlapped with other signals.
A 2D NMR gradient-HSQC experiment made it possible to
associate all the proton signals with those of the relevant carbon
atoms. As a result, six carbon atoms resulted to be unprotonated
(five sp2 and a single sp3 carbon, resonating at δ 44.5), while the
remaining 19 carbons were all protonated. Among them, four mid-
field methines were vinylic, while the fifth signal in this region
was an oxymethine (δH 4.60, δC 79.3). The proton multiplets of
the 1H NMR spectrum of 5 could be arranged in five distinct spin
2,3
Figure 1. 2D NMR COSY and key
genepolide.
J
H,C
HMBC cross-peaks of
of costunolide and myrcene. Myrcene (6) has been reported to be a
very poor diene,19 while dienophilic behavior for sesquiterpene
exomethylene-γ-lactones has been postulated to explain the occur-
rence of some bis-sesquiterpenoids.20 No chromatographic or
spectroscopic evidence of a reaction occurred when a toluene
solution of myrcene and costunolide was kept in a sealed tube for
1 month, while prolonged (36 h) heating at 150 °C in o-
dichlorobenzene resulted only in the formation of traces of the
elemanolide derivative 7 (1H NMR evidence), the result of a Cope
rearrangement of the diallylic system of costunolide.21 Diels-Alder
reactions can be catalyzed by the addition of Lewis acids,22 but, in
our case, the addition of zinc chloride, a Lewis acid reported to
increase the dienophilic behavior of myrcene,19 to a cooled (0 °C)
mixture of costunolide and myrcene led only to the formation of
the cyclocostunolides 8a-c,23 without any trace of a Diels-Alder
adduct, while heating in benzene gave only degradation products.
systems (A-E, shown in bold in Figure 1) through the use of a 2D
2,3
COSY spectrum. The
J
C,H
gradient-HMBC experiment was
instrumental in connecting these moieties and assembling the
structure of 5. In particular, the three gHMBC cross-peaks between
both H3-14 and H3-15 methyls (Figure 1) established two links
between fragments A and B, defining a dimethylcyclodecadiene
subunit, while a γ-lactone ring was merged to this system on the
basis of gHMBC cross-peaks between H-6 and H-7. The R-carbon
of this lactone ring, the quaternary sp3 carbon resonating at δ 44.5,
should be the spiro carbon atom connecting the lactone ring with
a cyclohexene ring, as suggested by gHMBC cross-peaks between
H2-13 and H2-16. Finally, the whole series of cross-peaks between
H-17, H2-25, H3-23, and H3-24 located the prenyl moiety at C-18,
thus completing the structure of genepolide (5).
The E configuration at the endocyclic ∆1(10)- and ∆4-double bonds
was deduced on the basis of the 2D ROESY cross-peaks H-1/H-9
and H-5/H-3, respectively, while R-orientation of H-7 was assumed,
as typical for sesquiterpene lactones from asteraceous plants.10a The
ROESY experiment was also helpful in defining the relative
configuration of the three adjacent stereogenic carbons C-6, C-7,
and C-11: Finally, the cross-peak H-6/H-8a suggested the trans
junction of the lactone ring, while the cross-peaks H-7/H2-13 and
H-6/H2-16 defined the relative configuration at C-11.
Genepolide (5) can be viewed as the result of a Diels-Alder
cycloaddition between costunolide (2), the most abundant constitu-
ent of the plant, and the monoterpene myrcene (6). Since the
existence of Diels-Alderase enzymes is still debated,17 it was not
clear if 5 could be considered a genuine natural product or rather
an artifact from the isolation and/or fractionation steps. Myrcene
was detected only in trace amounts in the essential oil from the
Swiss variety of A. umbelliformis,18 but this observation was not
perceived as relevant since the thermal treatment required for
obtaining the essential oil could have trapped myrcene as a
Diels-Alder adduct with costunolide, the major constituent of the
plant. We therefore investigated the thermal stability of a mixture
Faced with these observations and the lack of reactivity of
mixtures of costunolide and myrcene, we eventually investigated
if costunolide, when confronted with reactive dienes, could indeed
behave as a dienophile.20 Unsurprisingly, costunolide did not react
with isoprene, but evidence of a reaction was observed with
cyclopentadiene, and gentle heating of a mixture of costunolide
and artabsin (9), a reactive cyclopentadienyl derivative,24 cleanly
afforded the Diels-Alder adduct 10 (Scheme 1) in a rewarding
31% conversion yield, showing that the exomethylene γ-lactone
group can engage in cycloaddition reactions only with highly
reactive partners. The structure of compound 10 was determined
by inspection of MS and NMR data. In particular, the 2D NMR
COSY, gHSQC, and gHMBC experiments were used to assign all
1H and 13C resonances (see Experimental Section) and to define
the regiochemistry of the addition. In particular, COSY cross-peaks

342 Journal of Natural Products, 2009, Vol. 72, No. 3
Appendino et al.
Scheme 1. Formation of the Dimeric Lactone 10 by Thermal
Cycloaddition of Costunolide (2) and Artabsin (9)a
In conclusion, the isolation of a formal Diels-Alder adduct
between an unreactive diene such as myrcene (6) and an acid and
thermally unstable poor dienophile such as costunolide (2) suggests
that genepolide (5) is not an isolation artifact but a genuine natural
product and that the biogenetic machinery of A. umbelliformis
contains Diels-Alderase enzymatic activity. This observation is
important for the hot debate on the existence of enzymes promoting
cycloaddition reactions, since all previously reported Diels-Alder
adducts involving exomethylene sesquiterpene-γ-lactones involve
reactive dienophiles and not a poor partner such as myrcene.17 In
addition, remarkable differences were observed in the thermal and
acid-catalyzed reactivity of the cyclodecadiene moiety of costuno-
lide and genepolide, suggesting that quaternarization at C-11 has a
dramatic effect on the reactivity of the medium-sized diene system
of these compounds. Finally, the absence of genepolide in native
wormwoods from Western Alps qualifies this compound and its
degradation products 11 and 12 as critical markers for the detection
of the alien Swiss chemotype in genepy from Western Alps.
a Regioisomer 10a was not detected.
Scheme 2. Acid-Catalyzed Cyclization and Thermal
Rearrangement of Genepolide (5)
Experimental Section
General Experimental Procedures. Low- and high-resolution EIMS
spectra (70 eV) were performed on a VG Prospec (Fisons) mass
spectrometer. ESIMS spectra were performed on a LCQ Finnigan MAT
mass spectrometer. Optical rotations (CHCl3) were measured at 589
nm on a Perkin-Elmer 192 polarimeter equipped with a sodium lamp
1
(λ ) 589 nm) and a 10 cm microcell. H (500 MHz) and 13C (125
MHz) NMR spectra were measured on a Varian INOVA spectrometer.
Chemical shifts were referenced to the residual solvent signal (CDCl3:
δH 7.26, δC 77.0). Homonuclear 1H connectivities were determined by
the COSY experiment. One-bond heteronuclear 1H-13C connectivities
were determined with the HSQC experiment. Two- and three-bond
1H-13C connectivities were determined by HMBC experiments opti-
mized for a 2,3J of 7 Hz. Through-space 1H connectivities were
evidenced using a ROESY experiment with a mixing time of 500 ms.
Two- and three-bond 1H-13C connectivities were determined by
gradient 2D HMBC experiments optimized for a 2,3J of 9 Hz. Silica
gel 60 (70-230 mesh) and Lichroprep RP-18 (25-40 mesh) were used
for gravity CC. Reactions were monitored by TLC on Merck 60 F254
(0.25 mm) plates, which were visualized by UV inspection and/or
staining with 5% H2SO4 in ethanol and heating. Organic phases were
dried with Na2SO4 before evaporation.
3
of H-2′ with H2-13 and the JH,C gHMBC cross-peak of H3-15′
with C-11 excluded the alternative structure 10a (Scheme 1). The
2D NMR ROESY spectrum of 10 was used to assess the relative
configuration of the newly created stereogenic carbons, whose
configuration shows that interactions occur in an endo fashion and
according to the ortho rule. The triterpene dilactone 10 is structurally
similar to arteminolides, a class of farnesyl-protein transferase
inhibitors of relevance for cancer research.25 Since artabsin is easily
available in synthetically useful amounts from wormwood (A.
absinthium L.)26 and a vast array of exomethylene-γ-lactones can
be isolated from commercially available medicinal and dietary
plants,27 the successful preparation of 10 points to the possibility
of expanding the limited structure-activity relationships of artemi-
nolides.25
Plant Material. The Swiss chemotype of A. umbelliformis was
purchased from the Alpine farm La Freido (Stroppo, CN, Italy) of the
Associazione Genepi Occitan. A voucher specimen (GA20082) is kept
at the Novara Laboratories.
Extraction and Isolation of Genepolide (5). The powdered plant
material (nonwoody aerial parts, leaves, and flowers, 631 g) was
extracted with acetone (3 × 7.5 L) at room temperature, affording 57 g
(9.0%) of a black syrup, part of which (6.0 g) was purified by filtration
over RP-18 silica gel (30 g) with MeOH to remove fats and pigments.
The methanol filtrate was evaporated, affording 3.52 g of a brownish
paste. The latter was fractionated by gravity column chromatography
(CC) on silica gel (90 g, petroleum ether-EtOAc gradient). Fractions
eluted with petroleum ether-EtOAc (9:1) afforded 127 mg of crude
5, further purifed by gravity CC on neutral alumina to obtain 80 mg of
5 (0.12%). Fractions eluted with petroleum ether-EtOAc (8:2) afforded,
after crystallization from ether, 1.20 g of costunolide (2) (1.8%).
Fractions eluted with petroleum ether-EtOAc (7:3) were crystallized
from ether to afford 41 mg (0.06%) of eupatilin (1). The mother liquors
and the fractions eluted with petroleum ether-EtOAc (6:4) were further
purified by gravity CC on silica gel to afford 10 mg of crude verlotorin
(3a), a ca. 1:1 mixture of santamarin (4a) and reynosin (4b) (60 mg,
0.09%), and artemorin (3b, 29 mg, 0.04%).
During the study on the reactivity of costunolide in cycloaddition
reactions, we became aware of a marked reactivity difference
between the germacradienolide systems of costunolide (2) and
genepolide (5). For example, while treatment of 2 with protic acids
gave the mixture of eudesmane derivatives 8a-c,23,28 under similar
conditions 5 cleanly afforded the single cyclization product 11
(Scheme 2). Interestingly, even more marked differences were
observed in the thermal treatment of 2 and 5. Thus, while heating
of 2 at 200 °C in a sealed tube gave mainly a mixture of degradation
products, under similar conditions 5 quantitatively afforded the
elemanolide-type compound 12 (Scheme 2). In the realm of
germacradienolides, this behavior has previously been reported for
compounds having the C-15 methyl replaced by a formyl group29
or with a cis-6-olide configuration.30 It is not clear how quaterna-
rization at C-11 could promote the Cope rearrangement of the
germacradiene system, since the configuration of the elemanolide-
type compound 12 (ꢀ-methyl at C-10 and R-hydrogen at C-5, as
suggested by ROESY cross-peaks H3-14/H-6 and H-5/H-7) shows
the involvement of the same [1D14,15D6] reactive conformation31
of the cyclodecadiene ring as also observed in the transannular
cyclization of both compounds.
Genepolide (5): colorless foam; [R]25D -7 (c 0.2, CHCl3); IR (KBr)
1
νmax 1766, 1438, 1278, 1208, 1139, 979 cm-1; H NMR (500 MHz,
CDCl3) and 13C NMR (125 MHz, CDCl3), see Table 1; ESIMS
(positive-ion) m/z 391 [M + Na]+; HREIMS m/z 368.2723 [M]+ (calcd
for C25H36O2, 368.2715).
Cycloaddition of Costunolide (2) and Artabsin (9). An equimo-
lecular mixture of artabsin (9, 107 mg, 0.43 mmol) and costunolide
(2, 100 mg, 0.43 mmol) in toluene (1 mL) was heated at 60 °C. The
course of the reaction was followed by TLC (petroleum ether-EtOAc,
8:2), monitoring the disappearance of 2. After 48 h, the reaction was

Sesterpene γ-Lactone from Mountain Wormwood
Journal of Natural Products, 2009, Vol. 72, No. 3 343
worked up by evaporation, and the residue was purified by gravity CC
on silica gel (petroleum ether-EtOAc gradient, from 8:2 to 6:4) to
afford 29 mg of recovered 2 and 34 mg (31% based on the conversion
of costunolide) of 10 as a white powder: mp 120-122 °C; IR (KBr)
Under similar conditions, costunolide (2) yielded a resinous mass,
partly insoluble in organic solvents, and probably resulting from a
polymerization process. The soluble fraction, when analyzed by 1H
NMR, showed a complex mixture.
1
νmax 1767, 1455, 1229, 1172, 1000, 943 cm-1; H NMR (CDCl3, 500
Acknowledgment. We are grateful to M. Amerio (Associazione “Zio
John”, Pietraporzio, CN, Italy) for the picture of A. umbelliformis shown
in the table of contents graphic.
MHz) δ 5.50 (1H, bd, J ) 8.0 Hz, H-6′); 4.86 (1H, bd, J ) 10.5 Hz,
H-1); 4.63 (1H, d, J ) 7.8 Hz, H-5); 4.47 (1H, t, J ) 7.8 Hz, H-6);
3.03 (1H, bs, H-2′); 2.42 (1H, m, H-9a); 2.35 (1H, m, H-7); 2.32 (1H,
m, H-3a); 2.26 (1H, m, H-11′); 2.25 (1H, m, H-2a); 2.23 (1H, m, H-3′a);
2.10 (1H, m, H-2b); 2.00 (1H, m, H-3b); 2.00 (1H, m, H-9b); 1.93
(1H, m, H-8a); 1.86 (1H, m, H-7′); 1.71 (1H, m, H-8′a); 1.67 (3H, s,
H-15); 1.65 (1H, m, H-13a); 1.57 (1H, m, H-8′b); 1.51 (1H, m, H-8b);
1.46 (3H, s, H-15′); 1.45 (1H, m, H-3′b); 1.40 (3H, s, H-14); 1.36 (3H,
s, H-14′); 1.32 (1H, m, H-13b); 1.20 (1H, d, J ) 7.0 Hz, H-13′); 13C
NMR (CDCl3, 125 MHz) δ 182.0 (s, C-12); 179.8 (s, C-12′); 150.5 (s,
C-1′); 146.1 (s, C-5′); 140.1 (s, C-4); 137.5 (s, C-10); 128.3 (d, C-5);
128.1 (d, C-1); 88.9 (d, C-6′); 79.4 (d, C-6); 73.1 (s, C-10′); 62.3 (s,
C-11); 57.0 (s, C-4′); 53.7 (t, C-3′); 51.4 (d, C-7); 51.4 (d, C-7′); 42.1
(d, C-11′); 41.3 (t, C-9); 40.6 (t, C-9′); 39.9 (t, C-3); 35.9 (t, C-13);
32.5 (2′); 27.2 (14′); 26.8 (t, C-8); 25.8 (t, C-2); 25.6 (t, C-8′); 20.4
(15′); 17.8 (q, C-15); 16.9 (q, C-14); 13.2 (q, C-13′); ESIMS (positive-
ion) m/z 503 [C30H40O5 + Na]+.
Transannular Cyclization of Genepolide (5). A solution of 5 (50
mg, 0.13 mmol) in CHCl3 (5 mL) was irradiated in a quartz
photochemical reactor using a low-pressure mercury lamp. The course
of the reaction was monitored by TLC (petroleum ether-EtOAc, 95:
5; Rf(5) ) 0.50; Rf(11) ) 0.60). After 20 min, the solvent was evaporated
and the residue, analyzed by 1H NMR, indicated complete conversion
to 11. An analytical sample was obtained by gravity CC on silica gel
(petroleum ether-EtOAc, 98.5:1.5): IR (KBr) νmax 1770, 1505, 1288,
1270, 1198, 980 cm-1; 1H NMR (CDCl3, 500 MHz) δ 5.38 (1H, bt, J
) 7.0, Hz, H-17); 5.05 (1H, bt, J ) 7.0 Hz, H-21); 4.91 (1H, bs, H-15a);
4.75 (1H, bs, H-15b); 4.10 (1H, t, J ) 8.2 Hz, H-6); 2.30 (1H, m,
H-25a); 2.20 (1H, m, H-16a); 2.12 (1H, m, H-16b); 2.12 (1H, m, H-5);
2.08 (2H, m, H-20a,b); 2.00 (2H, m, H-19a,b); 1.99 (1H, m, H-25b);
1.98 (1H, m, H-3a); 1.95 (1H, m, H-13a); 1.82 (1H, m, H-7); 1.67
(3H, bs, H-23); 1.65 (1H, m, H-13b); 1.64 (1H, m, H-3b); 1.60 (3H,
bs, H-24); 1.51 (1H, m, H-9a); 1.45 (1H, m, H-8a); 1.40 (1H, m, H-2a);
1.35 (1H, m, H-1a); 1.30 (1H, m, H-8b); 1.22 (1H, m, H-9b); 1.21
(1H, m, H-2b); 1.06 (1H, m, H-1b); 0.90 (3H, s, H-14); 13C NMR
(CDCl3, 125 MHz): δ 179.9 (s, C-12); 137.8 (s, C-18); 136.8 (s, C-4);
132.0 (s, C-22); 124.5 (d, C-21); 118.0 (d, C-17); 109.3 (t, C-15); 77.9
(d, C-6); 57.0 (d, C-7); 56.0 (d, C-5); 44.5 (s, C-11); 37.9 (t, C-19);
37.8 (t, C-1); 37.7 (t, C-9); 36.2 (t, C-3); 30.8 (t, C-13); 30.8 (t, C-25);
27.1 (t, C-16); 26.4 (q, C-23); 26.2 (s, C-10); 26.2 (t, C-20); 24.5 (t,
C-8); 24.0 (t, C-2); 22.0 (q, C-14); 18.2 (q, C-24); ESIMS (positive
ion) m/z 391 [C25H36O2 + Na]+.
References and Notes
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Under similar conditions, the reaction with costunolide (2) required
a longer time (90 min) and afforded a ca. 3:6:1 mixture (1H NMR
analysis) of R-, ꢀ-, and γ-cyclocostunolides (8a-c), which was
fractionated by gravity CC on silica gel (petroleum ether-EtOAc, 98:
2, to separate the R- and ꢀ-isomers, and petroleum ether-EtOAc, 9:1,
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(16) Bud⊃esˇ´ınsky´, M.; Saman, D. Annu. Rep. NMR Spectrosc. 1995, 231–
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Cope Rearrangement of Genepolide (5). A sample of 5 (30 mg,
0.08 mmol) was heated under vacuum at 220 °C (oil bath) for 3 min.
1H NMR analysis indicated quantitative conversion to 12. Evaporation
of the solvent afforded an off-white, amorphous foam: IR (KBr) νmax
1770, 1440, 1376, 1213, 1135, 1001 cm-1; 1H NMR (CDCl3, 500 MHz)
δ 5.76 (1H, dd, J ) 17.3, 10.8 Hz, H-1); 5.36 (1H, bt, J ) 7.0, Hz,
H-17); 5.05 (1H, bt, J ) 7.0 Hz, H-21); 4.99 (1H, bs, H-3a); 4.95 (1H,
bd, J ) 10.8 Hz, H-2a); 4.91 (1H, bd, J ) 17.3 Hz, H-2b); 4.66 (1H,
bs, H-3b); 4.21 (1H, t, J ) 11.2 Hz, H-6); 2.40 (1H, m, H-25a); 2.24
(1H, d, J ) 11.2 Hz, H-5); 2.20 (1H, m, H-16a); 2.14 (1H, m, H-16b);
2.06 (2H, m, H-20a,b); 1.99 (2H, m, H-19a,b); 1.95 (1H, m, H-13a);
1.95 (1H, m, H-25b); 1.79 (1H, m, H-7); 1.76 (3H, s, H-15); 1.67 (1H,
m, H-13b); 1.66 (3H, bs, H-23); 1.63 (1H, m, H-8a); 1.59 (3H, bs,
H-24); 1.57 (1H, m, H-9a); 1.45 (1H, m, H-8b); 1.39 (1H, m, H-9b);
1.04 (3H, s, H-14); 13C NMR (CDCl3, 125 MHz): δ 181.7 (s, C-12);
148.6 (d, C-1); 141.2 (s, C-4); 137.7 (s, C-18); 131.7 (s, C-22); 124.2
(d, C-21); 116.8 (t, C-2); 115.1 (t, C-3); 111.5 (d, C-17); 78.8 (d, C-6);
56.6 (d, C-5); 54.5 (d, C-7); 42.6 (s, C-10); 44.7 (s, C-11); 40.1 (t,
C-9); 37.6 (t, C-19); 30.5 (t, C-13); 26.5 (t, C-16); 26.2 (q, C-23); 25.9
(t, C-20); 24.8 (t, C-25); 24.0 (q, C-15); 23.0 (t, C-8); 18.6 (q, C-14);
18.2 (q, C-24); ESIMS (positive-ion) m/z 391 [C25H36O2 + Na]+.
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