Mechanistic studies on the autoxidation of α-guaiene: Structural diversity of the sesquiterpenoid downstream products

Article abstract of DOI:10.1021/np500819f

Two unstable hydroperoxides, 6b and 10a, and 13 downstream sesquiterpenoids have been isolated from the autoxidation mixture of the bicyclic sesquiterpene α-guaiene (1) on cellulose filter paper. One of the significant natural products isolated was rotundone (2), which is the only known impact odorant displaying a peppery aroma. Other products included corymbolone (4a) and its C-6 epimer 4b, the (2R)- and (2S)-rotundols (7a/b), and several hitherto unknown epimers of natural chabrolidione A, namely, 7-epi-chabrolidione A (3a) and 1,7-epi-chabrolidione A (3b). Two 4-hydroxyrotundones (8a/b) and a range of epoxides (9a/b and 5a/b) were also formed in significant amounts after autoxidation. Their structures were elucidated on the basis of spectroscopic data and X-ray crystallography, and a number of them were confirmed through total synthesis. The mechanisms of formation of the majority of the products may be accounted for by initial formation of the 2- and 4-hydroperoxyguaienes (6a/b and 10a/b) followed by various fragmentation or degradation pathways. Given that α-guaiene (1) is well known to exist in the essential oils of numerous plants, coupled with the fact that aerial oxidation to form this myriad of downstream oxidation products occurs readily at ambient temperature, suggests that many of them have been overlooked during previous isolation studies from natural sources.

Full text of DOI:10.1021/np500819f

Article
pubs.acs.org/jnp
Mechanistic Studies on the Autoxidation of α‑Guaiene: Structural
Diversity of the Sesquiterpenoid Downstream Products
An-Cheng Huang,† Mark A. Sefton,† Christopher J. Sumby,§ Edward R. T. Tiekink,‡
and Dennis K. Taylor*,†
†School of Agriculture, Food and Wine, The University of Adelaide, Waite Campus, 5064, Adelaide, South Australia, Australia
§School of Chemistry and Physics, The University of Adelaide, North Terrace, 5000, Adelaide, South Australia, Australia
‡Department of Chemistry, University of Malaya, Kuala Lumpur 50603, Malaysia
S
* Supporting Information
ABSTRACT: Two unstable hydroperoxides, 6b and 10a, and
13 downstream sesquiterpenoids have been isolated from the
autoxidation mixture of the bicyclic sesquiterpene α-guaiene
(1) on cellulose filter paper. One of the significant natural
products isolated was rotundone (2), which is the only known
impact odorant displaying a peppery aroma. Other products
included corymbolone (4a) and its C-6 epimer 4b, the (2R)-
and (2S)-rotundols (7a/b), and several hitherto unknown
epimers of natural chabrolidione A, namely, 7-epi-chabroli-
dione A (3a) and 1,7-epi-chabrolidione A (3b). Two 4-
hydroxyrotundones (8a/b) and a range of epoxides (9a/b and
5a/b) were also formed in significant amounts after
autoxidation. Their structures were elucidated on the basis of
spectroscopic data and X-ray crystallography, and a number of them were confirmed through total synthesis. The mechanisms of
formation of the majority of the products may be accounted for by initial formation of the 2- and 4-hydroperoxyguaienes (6a/b
and 10a/b) followed by various fragmentation or degradation pathways. Given that α-guaiene (1) is well known to exist in the
essential oils of numerous plants, coupled with the fact that aerial oxidation to form this myriad of downstream oxidation
products occurs readily at ambient temperature, suggests that many of them have been overlooked during previous isolation
studies from natural sources.
While these cationic intermediate-based cyclizations and
rearrangements have been well studied and provide a
comprehensive understanding of the diverse scaffolds of
sesquiterpene formation, free radical-associated reactions
including autoxidation should also be able to modify
sesquiterpenes at various stages throughout their formation.
Such processes would boost the natural diversity of these
compounds. A growing body of work has demonstrated the
potential of harnessing free radical approaches for the synthesis
of terpenes and also provide further insight into their
biosynthesis.17 However, in comparison to the understanding
of the biogenesis of sesquiterpene hydrocarbons, knowledge of
postcyclization chemical transformations of sesquiterpenoids is
far from complete. One possible free radical associated
mechanism exploited by Nature in the biosynthesis of
postcyclization sesquiterpeniods is oxidative transformations
catalyzed by oxidases, such as cytochrome P450 monoox-
ygenases,18 the mechanisms of which resemble those of free
radical autoxidation. Indeed, a recent study has demonstrated
he vast structural diversity of sesquiterpenoids and their
Taccompanying intriguing biological properties have driven
longstanding research interests in understanding their biosyn-
thesis. Sesquiterpenoids are known to be derived from the
achiral precursor farnesyl pyrophosphate (FPP) biosynthesized
via the mevalonate pathway in plastids.1,2 The enzymatically
generated intermediate carbocations of FPP can subsequently
cyclize and rearrange in a number of ways to furnish a myriad of
structurally diverse sesquiterpenoids, which often display highly
complex stereochemical arrays.3 The position of the carboca-
tion formation on FPP is governed by the conformation and
geometric constraints of the various substrates and the active
sites of the natural cellular cyclase enzymes.4 Furthermore, the
way the carbocations cyclize via their various transition states to
often yield their thermodynamically favored products has been
studied in great detail theoretically through the use of quantum
chemical calculations.5−12 These calculations have also
permitted the prediction of the formation of downstream
products based on the transition states of the intermediate
carbocations,13 and the findings have been supported from
studies on the acid-catalyzed cyclization of sesquiterpene
precursors.14−16
Received: October 18, 2014
Published: January 12, 2015
© 2015 American Chemical Society and
American Society of Pharmacognosy
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Figure 1. α-Guaiene (1) and several major autoxidation products (2−5a/b) of 1 previously identified.
Figure 2. GC chromatogram of the aerial oxidation of α-guaiene (1, neat, coated on filter paper) at 50 °C after 15 h.
that oxidases are able to modify sesquiterpene substrates and
produce miscellaneous oxygenated sesquiterpenoids.19
extent, will lead to downstream oxygenated sesquiterpene
products which (a) may have been or are being formed in
Nature and thus can be considered as natural products and (b)
may display yet to be discovered important aroma or
biochemical properties. Four other major products formed
during the natural autoxidation of 1 were identified as 7-epi-
chabrolidione A (3a), corymbolone (4a), and the epoxides 5a/
b, with the latter three derivatives being previously identified
natural products, Figure 1.20,21 Moreover, we hypothesized that
the key initiation step in the formation of these downstream
oxidation products was the initial formation of the secondary
radical at C-2 of 1 followed by formation of the hydroperoxides
6a/b and subsequent decomposition/fragmentation. The
formation of such sesquiterpene hydroperoxides in Nature is
not unusual, with a number previously being isolated from
natural sources including Magnolia grandiflora, Pogostemone
cablin, and Aster spathulifolius.23−25 Intrigued by the fact that
the GC chromatogram of the autoxidation mixture of 1
indicated the presence of a large number of additional oxidation
We have recently reported that the sesquiterpene α-guaiene
(1), which is known to exist in the essential oils of numerous
plants, undergoes aerial oxidation to form a host of downstream
oxidation products.20,21 One of the major oxygenated products
formed was identified as the natural product (−)-rotundone
(2), which is the only known impact odorant displaying a
peppery aroma and which has recently been identified in some
wine varieties and in a large range of common herbs and spices
(Figure 1).22 Although the concentration of 2 found in
products of natural origin is known to be small (from ng/L
amounts in red wines up to 2 mg/kg in peppercorns), the odor
detection threshold (ODT) of rotundone (8 ng/L in water, 16
ng/L in red wine) is among the lowest of any known natural
product.22 Thus, a trace of rotundone imparts strong pepper
notes to foods and beverages.
This discovery serves as an excellent example and highlights
that the postoxidation of natural sesquiterpenes, even to a small
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Figure 3. Additional downstream oxygenated sesquiterpenoids isolated from the autoxidation of α-guaiene (1).
products, many of which were potentially known natural
products, coupled with the fact the hydroperoxides 6 were yet
to be identified and characterized as the key intermediates, we
have now explored the product mixture in great detail and
report our findings herein along with mechanistic rationales for
product formation.
to obtain in a pure state for characterization. Consequently, 3b
was confirmed unequivocally to be an authentic autoxidation
product via total synthesis (vide infra). The 13C NMR spectrum
of β-ketol 4b displayed 15 carbon signals including one
carbonyl (δC 212.7), two olefinic (δC 149.8 and 111.1), and one
1
oxygenated carbon (δC 79.0). The H NMR spectrum showed
two vinyl protons, two methyl singlets, and one methyl doublet.
1
Both the H and 13C NMR spectra of 4b closely resembled
RESULTS AND DISCUSSION
■
those of corymbolone (4a), indicating that 4b was highly likely
to be an isomer of 4a. Analysis of the 2D NMR spectra
confirmed that 4b shared a carbon skeleton with 4a except for
the configuration of the bridged carbon C-6. On the basis of the
known stereochemistry of 1 and the possible mechanism of
oxidative cleavage of the bridged double bond to form a
diketone, which then cyclizes to form 4b, C-5 and C-8 of 4b
should remain intact during its formation and thus maintain the
same R configurations. Moreover, key ROESY correlations
between both H3-14 and H3-15 and the hydroxyl proton of 4b
were observed, suggesting the hydroxy moiety at C-6 and Me-
15 should both adopt the same β-orientation as Me-14.
Therefore, the structure of 4b was assigned as 6-epi-
corymbolone (4b) as depicted in Figure 3. (2R)-Rotundol
(7a) and (2S)-rotundol (7b) are aroma compounds with
peppery and woody notes and have been recently synthesized
by reduction of rotundone 2 and identified as components from
the laccase-catalyzed oxidation products of essential oils
containing 1.26 Consequently their identification was confirmed
by direct comparison of experimental and reported spectro-
scopic data.26
Autoxidation of α-Guaiene (1). The autoxidation of 1 had
previously been carried out by bubbling O2 through a solution
containing 1 at ambient temperature, and it was found that the
complete consumption of 1 was rather slow and took several
weeks.20 However, when the same autoxidation was conducted
on cellulose filter paper, a noticeable acceleration was seen,
particularly if the temperature was raised to 50 °C.
Consequently filter paper was once again exploited as the
supporting medium to perform a large-scale autoxidation of 1
in air at 50 °C for 20 h, after which time the substrate-coated
filter paper was extracted (Soxhlet) with CH2Cl2 under a N2
atmosphere. Analysis by GC-MS (after 15 h) once again
revealed the formation of rotundone (2), 7-epi-chabrolidione A
(3a), corymbolone (4a), and the epoxides 5a/b along with
numerous yet to be identified oxidation products (Figure 2).
These latter products were also observed previously during GC
analysis of the autoxidation of 1 in solution after an extended
period of time.23 The crude extract of the products was first
purified by silica column chromatography (SCC, pretreated
with 5% Et3N/hexanes due to the acid sensitivity of a number
of the products) to obtain seven fractions, which were further
separated by repeated chromatography including SCC, multi-
layer coil counter-current chromatography (MLCCC), and
preparative TLC. A total of five oxygenated sesquiterpenes
previously described (2, 3a, 4a, and 5a/b) and eight new
analogues (3b, 4b, 7a/b, 8a/b, and 9a/b) were successfully
isolated from the reaction mixture (Figures 1 and 3,
respectively). The fact that the majority of the major
compounds seen within the GC chromatogram (Figure 2)
can be attributed to the isolated compounds themselves
indicates that these are true products of the autoxidation
process and are not simply artifacts of decomposition of more
reactive intermediates in the GC injector block (vide infra).
Diketone 3a was previously isolated as one of the major
products resulting from the autoxidation of 1 in solution
(CH2Cl2) and again found here to form during oxidation on
filter paper.20 Related epimer 3b was also found as an
autoxidation product in these studies; however, it was difficult
The two hitherto unknown β-ketols 8a and 8b displayed
1
similar H and 13C NMR spectra that closely resembled those
of rotundone (2) except for the presence of one additional
oxygenated carbon and the replacement of one methyl doublet
by a methyl singlet at approximately δH 1.00, suggesting that
they could be isomeric hydroxy derivatives of 2. Analysis of
their 2D NMR spectra (COSY, HSQC, and HMBC) confirmed
the presence of an additional epimeric hydroxy moiety at C-4 of
rotundone. The configurations of C-4 of 8a and 8b were
determined by analysis of their ROESY spectra. A key ROESY
correlation between H3-14 and H-10 for 8b indicated that Me-
14 is α-oriented. No such correlation between H3-14 and H-10
was observed for 8a. Therefore, the C-4 configurations of 8a
and 8b were established as R and S, respectively. Finally, the
two new epoxides 9a and 9b displayed highly similar 13C and
1H NMR spectra. Both of them displayed a pair of 13C signals
between δC 70−80 and 60−70 and two other pairs at ca. δC 150
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and 110, suggesting the presence of two C−O bonds along with
HRMS analysis of 6b and 10a showed [M + H]+ m/z values
of 237.1840 and 237.1837, respectively, indicative of a common
molecular formula, C15H24O2 (calcd for C15H25O2 237.1855).
Both hydroperoxides 6b and 10a displayed characteristic
1
the presence of two CC bonds for each analogue. Their H
NMR spectra indicated the presence of two methyl groupings,
one as a deshielded singlet around δH 1.6 and one more
shielded doublet near δH 1.0. Two additional deshielded vinyl
proton signals at around δH 5.0 for each isomer also indicated
the presence of two CH2 groupings for each isomer. HMBC
spectra of both analogues showed correlations from the vinylic
H-14 to quaternary carbon C-4 and the oxygenated C-5 and
from the shielded Me-15 to the other oxygenated tertiary
carbon C-1, supporting the presence of an epoxy ring at the
bridged carbons. On the basis of further analysis of their 2D
NMR spectra, the structures of 9a and 9b were elucidated as
shown in Figure 3. The orientations of the epoxy moiety in
these two sesquiterpenoids were not resolved due to the lack of
any NOE associations. However, as noted in our previous study
showing that the β-epoxides of guaiene and guaiol displayed
shorter GC retention times than their α-isomers and the fact
that 9a displayed a shorter retention time than 9b, the epoxy
moiety of 9a was tentatively characterized as being β-oriented
and that of 9b α-oriented.27
Identification of the C-2 and C-4 Hydroperoxides as
Intermediates during the Autoxidation of α-Guaiene (1).
The diverse structures of the oxygenated sesquiterpenoids
formed during the autoxidation of 1 as described above display
one or more of three types of oxygen moieties, namely,
carbonyl, hydroxy, and epoxy moieties, the majority of which
could be foreseen as being generated by the decomposition/
consumption of the C-2 or C-4 hydroperoxides (6a/b and 10a/
b) of α-guaiene (1) (Figure 4). Taking into account the
1
−OOH moieties, as indicated by their H signals at δH 7.00
and 6.71 and 13C signals at δC 92.6 and 97.6 in the 1H and 13
C
NMR spectra, respectively. Compound 6b differed from 10a in
having an extra signal of an allylic proton on the carbon bearing
the −OOH at δH 4.96 and a more shielded methyl doublet at
δH 0.84 as opposed to the methyl singlet of 10a appearing at δH
1
1.21 in the H NMR spectra. The connectivity of all carbons
1
and full assignment of H and 13C signals were established on
the basis of 2D NMR analysis including COSY, HSQC, and
HMBC correlations. The C-2 configuration of 6b was
determined as S with the −OOH α-oriented based on the
analysis of the ROESY spectrum, which showed correlations
between H-2 and H3-15, H-2, and H-3b, and H-3b and H3-14
(Supporting Information). Moreover, reduction of 6b with
LiAlH4 afforded (2S)-rotundol (7b), confirming the stereo-
chemical assignments made for 6b. The ROESY spectrum of
10a showed interactions between H3-15 and H-9b, H-9a and
H-6b, and H-6a and H3-14, indicative of opposite orientations
of Me-14 and Me-15. Since the orientation of Me-15 would
remain β from the starting material 1, Me-14 of 10a was
therefore assigned as α-oriented. Furthermore, the observation
that 10a, with a β-oriented hydroperoxy moiety, decomposed
upon GC-MS analysis to afford significant amounts of the β-
epoxide 9a with a retention time at 16.86 min without the
formation of 9b was consistent with the aforementioned
tentative assignment of β-epoxy 9a and supported the previous
stereochemical assignments of 9a/b.
The two hydroperoxides 6b and 10a decomposed readily
upon GC-MS analysis (Figure 5A and B) but were relatively
stable in the pure state, with only slow decomposition seen by
TLC analysis. Decomposition of 2-hydroperoxyguaiene (6b)
during GC analysis afforded rotundone (2) and the (2S)-
rotundol (7b) as the major products, while decomposition of 4-
hydroperoxyguaiene (10a) under the same conditions afforded
the diketone chabrolidiones A 3a/b and the monoepoxide 9a as
significant products. It is clear that the secondary hydro-
peroxide 6b prefers to undergo the common decomposition
routes to afford an alcohol and/or ketone, while the tertiary
hydroperoxide 10a prefers fragmentation or elimination
pathways, as discussed in greater detail in the mechanistic
section below.
Apart from the pure isolated hydroperoxides 6b and 10a, we
also obtained three fractions of hydroperoxide mixtures (MF2,
-4, and -5) that were inseparable due to their extremely similar
polarities. Among the three mixtures, the (2R)-hydroperoxide
6a and the (4S)-hydroperoxide 10b could be tentatively
identified as two major components in fractions MF2 and
MF4, respectively. Reduction of MF2 with LiAlH4 furnished
(2R)-rotundol (7a) as a major product, whereas GC-MS
analysis of MF4 gave the α-bridged monoepoxide 9b as the
main product with a small proportion of β-bridged epoxide 9a.
MF5 contained mainly peroxides that were relatively nonpolar
and displayed one spot on TLC (Et2O/n-hexane, 20%) with an
Rf of 0.93 and one major peak (ca. 80%) at tR = 22.02 min upon
Figure 4. C-2 and C-4 hydroperoxides of α-guaiene (1).
likelihood of decomposition of any unstable hydroperoxy
intermediates on silica-based chromatography during purifica-
tion, we performed another large-scale isolation of the
autoxidation products of 1 employing MLCCC, which is a
neutral method of isolation, as it does not require a solid
support phase, in order to search for the presence of the C-2
and C-4 guaiene hydroperoxides. The solvent system MeCN/t-
BuOMe/n-hexane (10:1:10, descending mode) was found to be
effective for separation. Two pure hydroperoxides, 6b and 10a,
substrates 2, 4a, and 7a/b, and the acid-sensitive guaiene-
bridged monoepoxides 5a and 5b were successfully isolated
along with several inseparable mixtures of hydroperoxides
containing 6a and 10b. The pair of epoxides 5a and 5b
accounted for 2% and 9% of the products by GC and were
identical to authentic standards.20
1
GC-MS analysis. However, H and 13C NMR spectra of this
fraction indicated it to be a complex mixture of peroxide
isomers displaying typical double-bond and hydroperoxy
moieties. The identities of these peroxide species were not
established.
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Figure 5. (A) GC chromatogram of hydroperoxide 6b; (B) GC chromatogram of hydroperoxide 10a.
Overoxidation of Rotundone (2) and the Rotundols
(7a/b). In our previous preliminary study on the autoxidation
of α-guaiene (1) and the formation of rotundone (2) we noted
that overoxidation of 2 may be occurring during prolonged
periods of autoxidation.20 Furthermore, the formation of β-
ketols 8a and 8b in the current autoxidation studies also
suggests that the initially formed oxidation product 2 could be
undergoing further oxidation via the formation and decom-
position of the 4-hydroperoxyrotundone. With a view to
identifying any additional secondary oxidation products of 2
and the rotundols 7a/b such as the epoxides 11a/b, 12a/b, and
13a/b, we carried out the formal syntheses of these compounds
as depicted in Scheme 1.
Epoxidation of 2 with excess m-CPBA in CH2Cl2 yielded
only the terminal epoxide epimers 11a and 11b in average
yields without the formation of bridged epoxides 13a/b.
Attempts to synthesize 13a/b by epoxidation of the electron-
poor bridged enone CC bond of 2 with other epoxidizing
reagents (e.g., DMDO, H2O2, or TBHP with bases) gave
predominantly terminal epoxides in low yields. In order to
prepare 13a/b, we therefore resorted to utilizing the hydroxyl
directing effect of the hydroxy moiety within the rotundols 7a/
b to realize the epoxidation of the centrally bridged double
bond. Both 7a/b were prepared in one pot via the reduction of
rotundone (2), with the structure of 7a confirmed by X-ray
analysis (Supporting Information). Epoxidation of 7a and 7b
employing stoichiometric m-CPBA in CH2Cl2 permitted
installation of the bridged epoxy moiety in both a regio- and
stereoselective manner, yielding β-epoxy-(2R)-rotundol (12a)
and α-epoxy-(2S)-rotundol (12b) in 76% and 95% yields,
respectively. Further oxidation of 12a/b with pyridinium
chlorochromate (PCC) in CH2Cl2 furnished the bridged β-
epoxyrotundone (13a) and α-epoxyrotundone (13b) in
excellent yields of 91% and 94%, respectively. The stereo-
chemistries of the bridged and terminal epoxides within these
substrates were established by X-ray diffraction analyses of
(11R)-epoxyrotundone (11a) and α-1,5-epoxyrotundone
(13b) (Supporting Information). Finally, attempts to synthe-
size 8a via simple allylic oxidation with SeO2 and tert-
butylhydroperoxide (TBHP) gave terminal aldehyde 14 as
the dominant product in 54% yield.
Scrutiny of the GC-MS chromatograms of the crude
autoxidation products by comparing the retention times and
MS of these synthetic standards failed to identify the epoxides
11a/b and 13a/b as autoxidation products of α-guaiene (1),
possibly due to low levels of formation of these compounds
after a relatively short period (20 h) of oxidation. However,
since pure rotundone (2) was readily available, we left neat 2
under a pure O2 atmosphere at ambient temperature and
monitored its autoxidation by GC-MS over a 46-day period,
after which time only 26% of 2 had been consumed. At this
time point, the major products of autoxidation were the β-
ketols 8a/b (10%) along with the terminal epoxides 11a/b
(8%) and minor amounts of the bridged epoxides 13a/b
(0.4%). Formation of 8a/b and 11a/b was observed to be
much faster than the formation of 13a/b. After one year,
approximately 75% of 2 was consumed in the neat state with
many other unidentified highly oxidized products observed by
GC-MS, as indicated by their longer retention times and
increased molecular weights. The autoxidation of rotundols 7a
and 7b was also monitored; both readily afforded 2 as the
dominant product at varying reaction rates. For example,
oxidation of neat 7a under a pure O2 atmosphere at room
temperature was complete within 72 h and afforded 2 (32%),
8a (6%), and 8b (8%) as the major products. Oxidation of 7b
was much slower and took up to 17 days to initiate; however it
was completed quickly within 2 days after initiation had
commenced. This demonstrates the dynamics of these
autoxidation reactions.
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Scheme 1. Synthesis of Epoxides 11a/b, 12a/b, and 13a/b and Aldehyde 14
Synthesis of 7-epi-Chabrolidione A (3a), 1,7-Di-epi-
chabrolidione A (3b), and Ketoaldehyde (3c) as
Potential Hock Cleavage Products. The formation of the
chabrolidiones 3a/b appeared to result from the oxidative
cleavage of the five-membered ring of α-guaiene (1), and they
feature the presence of two carbonyl moieties. The formation of
dicarbonyls from allylic hydroperoxides has long been known to
proceed via the acid-catalyzed Hock cleavage mechanism.28
Given that the presence of the hydroperoxides 6a/b and 10a/b
had been established as autoxidation products, coupled with the
fact that thermal decomposition of pure 10a during GC-MS
analysis afforded 3a/b, we suspected that exposure to trace
amounts of acid during workup or during the autoxidation
reactions themselves was inducing Hock cleavage of the C-4
guianene hydroperoxides of guaiene 10a/b and resulting in the
formation of the epimers of chabrolidione A 3a/b as depicted
in Scheme 2. Furthermore, Hock cleavage of the C-2 guianene
hydroperoxides of 6a/b has a similar propensity to form the
downstream ketoaldehydes 3c/d under acid conditions,
although the latter derivatives were not isolated from the
original autoxidation mixture (Scheme 2).
In order to establish whether the ketoaldehydes 3c/d were
present as products of autoxidation of 1 and to further confirm
the structures of diketones 3a/b, we carried out short syntheses
of three of these four compounds (3a−c). The key protocol
involved the oxidative cleavage of various olefins with ozone
(Scheme 3). The appropriate precursors 15, 19, and 21 were
prepared from α-guaiene (1) and guaiol (15) by ring-opening
of their bridged epoxides followed by selective dehydration as
described recently.27 Ozonolysis of benzylate 16 gave the
diketone 17 in 54% yield, which upon hydrogenolysis furnished
the diketoalcohol 18 quantitatively. Dehydration of 18 with
SOCl2 in benzene afforded the 7-epi-chabrolidione A (3a) in
1
51% yield. The H and 13C NMR spectra of synthetic 3a were
identical to reported data.20 Attempted epimerization of 3a into
3b under either basic or acidic conditions [e.g., Na2CO3/
MeOH, NaOH/EtOH, or Amberlite resin (H+)] failed,
presumably due to competing facile aldol condensation
reactions. Alternatively, and with synthetic (1R)-4-isoguaiol
(19) in hand,27 3b was readily prepared via ozonolysis of 19
followed by dehydration utilizing SOCl2 in benzene in 42%
yield over two steps (Scheme 3). The identity of 3b as one of
the Hock cleavage products of tertiary hydroperoxide 10a or
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employing several different GC columns precluded the
presence of 3c as a product of the autoxidation of 1.
Scheme 2. Formation of 3a/b and 3c/d by Hock Cleavage of
10a/b and 6a/b, Respectively
Mechanistic Rationale of the Autoxidation of α-
Guaiene (1). The autoxidation of α-guaiene (1) resembles
that of lipids whose mechanisms have recently been reviewed.29
On the basis of the autoxidation products isolated and
identified, several mechanistic pathways may be proposed for
the formation of the autoxidation products reported herein and
are outlined in Scheme 4.
The autoxidation may begin with H atom abstraction by
oxygen of an allylic hydrogen from four potential sites within
the bicyclic [5,7]-fused structure of α-guaiene (1). There was
no clear evidence for the formation of oxidized products
resulting from allylic hydrogen abstraction within the seven-
membered ring; however the isolation of the C-2 and C-4
hydroperoxides 6a/b and 10a/b clearly suggests that
abstraction is preferred from within the smaller five-membered
ring of 1 as observed previously.30,31 Once formed, the
secondary hydroperoxides 6a/b may decompose via O−O
bond cleavage to furnish alcohols and ketones as major
products as typically expected.28,29 Consequently, (2R)-hydro-
peroxyguaiene (6a) would afford (2R)-rotundol (7a), and (2S)-
hydroperoxyguaiene (6b) would lead to the formation of (2S)-
rotundol (7b). Both epimeric C-2 hydroperoxides 6a/b would
also lead to the formation of rotundone (2). In addition, once
the rotundols 7a/b are formed, they may themselves undergo
further oxidation to form 2, as was confirmed herein through
overoxidation studies on pure samples of the rotundols 7a/b.
Moreover, the formation of the observed 4-hydroxyrotundones
8a/b presumably results from the overoxidation of 2 itself.
Indeed, we found that allowing pure 2 to oxidize under an
oxygen atmosphere led to the formation of 8a/b and
presumably occurred via the requisite 4-hydroperoxy inter-
mediates.
Unlike the secondary 2-hydroperoxides 6a/b, the tertiary C-4
hydroperoxides 10a/b lack an allylic hydrogen on the carbon
bearing the OOH moiety and consequently decompose in a
different fashion (Scheme 4). The preferential decomposition
route for 10a/b appears to be via the known Hock cleavage
pathway, as outlined above, and would be catalyzed by trace
amounts of acid, resulting in the formation of 3a/b. Direct
thermal decomposition of 10a during GC analysis also afforded
3a/b, suggesting that a thermally induced fragmentation
pathway may also be operating in the background. The
formation of 9a/b is also quite intriguing and requires both an
epoxidation and elimination step. Given that 10a thermally
decomposed during GC analysis to also afford a significant
amount of 9a, it appears that the 4-hydroperoxides 10a/b either
are the direct precursors to epoxides 9a/b or at least suggest
that activation of the C-4 position as a hydroperoxide can lead
to a pathway of elimination upon decomposition and the
installation of the requisite double bond within 9a/b.
Presumably, the formation of the bridged epoxide moiety
within these derivatives is simply a result of the fact that one or
more of the hydroperoxides being formed in situ is acting as a
peroxidizing agent and provides for the formation of the
epoxides in a similar fashion as has been seen for the
epoxidization of α-guaiene (1) itself with various peracids.27
Furthermore, the observed monoepoxides 5a/b presumably are
being formed in a similar fashion and once formed may also be
able to undergo additional oxidation at C-4 followed by
elimination to provide for an alternative route to 9a/b.
10b was established by co-injection of pure synthetic
derivatives with the autoxidation crude products and by
comparison of their retention time and MS data by GC-MS
analysis utilizing several different columns.
Since the presence of 3a and 3b as potential Hock cleavage
products of 10a and 10b was now confirmed, we shifted our
focus to the synthesis of 3c. Synthesis of 3c using ozonolysis
would require the installation of a C-1−C-2 double bond via
dehydration of C-1 alcohol 21.27 However, the additional
terminal double bond would require protection prior to
ozonolysis. Consequently the terminal double bond was
converted into epoxide 22, the C-1 alcohol dehydrated, and
epoxide 23 ring-opened with AlH3 to afford 24 in 41% yield
over three steps (Scheme 3). The stereochemistry of guaia-
1(2)-en-11-ol (24) was confirmed by X-ray crystallography
(Supporting Information). Ozonolysis of 24 proceeded
smoothly to yield the ketohydroxyaldehyde (25) in 58%
yield. The terminal hydroxyl group was eliminated with SOCl2
in toluene to form a mixture of the diketo-alkenes 3c and 26 in
a ratio of 1:1 in 94% yield. Purification of this mixture by SCC
resulted in significant mass loss due to tight separation and
acid-induced decomposition. Repeated separation of the
reaction products employing MLCCC (MeCN/t-BuOMe/PE,
10:1:10, descending mode) afforded 3c in pure form. The
configuration at C-5 of 3c was established as R on the basis of
ROESY correlations displayed between H-5 and H3-13, H3-14,
and H3-15 and the confirmed stereochemistry of its precursor
(24). The identity of 26 was tentatively elucidated as shown
based on EIMS showing m/z 236 as the [M]+ peak and the 1H
NMR spectrum showing the presence of an additional methyl
singlet at δH 1.45 and the absence of two terminal olefinic
proton signals at δH 4.76 and 4.67 as compared to that of 3c.
Examination by GC-MS of the retention time, MS of 3c, and
the GC chromatograms after injection of a mixture of 6a/b and
also with the crude autoxidation reaction products of 1
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Scheme 3. Synthesis of 7-epi-Chabridione A (3a), 1,7-Di-epi-chabridione A (3b), and Ketoaldehyde 3c
Finally, the formation of corymbolone (4a) and its C-6
epimer 4b required the oxidative cleavage of the central CC
bond of α-guaiene 1. While the oxidizing species that performs
this cleavage is currently unknown, it is highly likely that this
oxidative process proceeds via the 1,2-dioxetane of 1 and results
in the formation of the cyclic diketone, which simply undergoes
an aldol-type cyclization to form the observed corymbolones
4a/b. Indeed such aldol-type cyclizations have been reported
previously for these systems.28
Autoxidation has been an important topic in lipid and steroid
chemistry due to the wide exposure of lipids to molecular
oxygen in Nature.29 However, its role in terpene chemistry has
not been extensively explored. We have demonstrated herein a
case study of the autoxidation of α-guaiene (1), showing that
this could lead to the generation of a range of oxygenated
sesquiterpenoids. Two unstable hydroperoxides, 6b and 10a,
and 13 downstream sesquiterpenoids have been isolated from
the autoxidation mixture of the bicyclic sesquiterpene α-guaiene
(1) on cellulose filter paper. One of the natural products
isolated of significance was rotundone (2), which is the only
known impact odorant displaying a peppery aroma. Other
products included corymbolone (4a) and its C-6 epimer 4b,
the (2R)- and (2S)-rotundols (7a/b), and several hitherto
unknown epimers of natural chabrolidione A, namely, 7-epi-
chabrolidione A (3a) and 1,7-di-epi-chabrolidione A (3b). Two
4-hydroxyrotundones (8a/b) and a range of epoxides (9a/b
and 5a/b) were also formed in significant amounts during
autoxidation. The mechanisms of formation of the majority of
the products may be accounted for by initial formation of the
C-2- and C-4 hydroperoxyguaienes (6a/b and 10a/b) followed
by various fragmentation or degradation pathways. Given that
α-guaiene (1) is well known to exist in the essential oils of
numerous plants, coupled with the fact that aerial oxidation to
form this myriad of downstream oxidation products occurs
readily at ambient temperature, suggests that many of them
have been overlooked during previous isolation studies from
natural sources.
EXPERIMENTAL SECTION
■
General Experimental Procedures. Melting points are un-
corrected and were obtained on a Buchi B-540 melting point
apparatus. All reagents were purchased from commercial sources and
used directly unless otherwise stated. Solvents for synthesis were
purified according to known procedures where necessary.32 Solvents
for general chromatography were AR grade, and those used for GC-
MS and HRMS analysis were HPLC grade. All reactions were
138
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Scheme 4. Mechanistic Rationale of the Autoxidation of α-Guaiene (1)
conducted under a N2 atmosphere except for ozonolysis. Silica column
chromatography was performed using either LC60A 40−63 μm silica
(Grace Davison) or silica gel 60 (0.015−0.040 mm) from Merck. TLC
and preparative TLC were conducted with TLC silica gel 60 F254
plates (Merck KGaA) using standard vanillin stain for visualization.
Silver nitrate impregnated silica (SNIS) was prepared according to the
literature.33 GC-MS/FID analysis was performed on a 6890 GC
coupled with a 5973N MSD or a 7890A GC-FID (Agilent
Technologies). VF-Wax (60 m × 0.25 mm, i.d. × 0.25 μm film
thickness), DB-5 (60 m × 0.32 mm, i.d. × 0.25 μm film thickness), and
HP-5 (30 m × 0.32 mm, i.d. × 0.25 μm film thickness) capillary
columns (Agilent Technologies) were used for GC-MS/FID analyses
in this study. HRMS (ESI-TOF) analysis was performed on a Triple
TOF 5600 mass spectrometer from AB Sciex Instruments. MLCCC
separation was carried out on an MK5 LabPrep 1000 machine (AECS
QuikPrep) coupled with two LC1110 HPLC pumps (GBC) and a
FRAC-100 fraction collector (Phamacia Fine Chemicals). NMR data
were recorded on Varian-Inova 500/600 MHz spectrometers. All
compounds for NMR analysis were dissolved in either CDCl3 or
benzene-d6. All 1H and 13C NMR spectra were calibrated with residual
deuterated solvent signals set at 7.26 and 77.0 ppm for CDCl3 and
7.16 and 128.06 ppm for benzene-d6, respectively. COSY, ROESY,
HSQC, and HMBC were common 2D NMR techniques used for full
Isolation of Autoxidation Products by SCC. α-Guaiene (1, 2 g,
9.8 mmol) was loaded onto a single sheet of filter paper (Whatman
grade 1, 180 mm diameter) that was prerolled and placed into a 10 mL
test tube. The test tube was initially filled with CH2Cl2, and the solvent
evaporated under a N2 stream to enable an even coating of 1 onto the
filter paper. The test tube was placed in an oven (50 °C) for ca. 20 h,
and the autoxidation monitored by TLC and GC-MS regularly. The
reaction products were extracted with CH2Cl2 using continuous
Soxhlet extraction under N2 for 12 h. The extract was concentrated in
vacuo, and the residue loaded onto base-treated silica (pretreated with
5% Et3N). Gradient elution with Et2O/n-hexane (2:98, 5:95, 6:94,
8:92, 12:88, 15:85, 16:84, 17:83, 19:81, 20:80, 23:77, 26:74, and
50:50) gave mixtures of compounds in different fractions, which were
combined accordingly (based on TLC and GC-MS) to give seven
main fractions: F1 (from 2:98 to 6:94), F2 (from 8:92 to 12:88), F3
(from 15:85 to 17:83), F4 (from 19:81 to 20:80), F5 (from 20:80 to
23:77), F6 (from 23:77 to 26:74), F7 (50:50). F1, containing mainly
compounds of retention times 16.86 and 17.06 min, was subjected to
repeated silica chromatography (15−40 μm, Et2O/n-hexane, 0.5:99.5)
to furnish pure 9a (2 mg, tR = 16.86 min) as a colorless liquid and 9b
(2.5 mg, tR = 17.07 min) as a colorless oil. F2 contained ca. 10% 4b (tR
= 25.6 min) based on the GC chromatogram and several other known
compounds including rotundone (2) and the epimeric rotundols (7a
and 7b). This mixture was subjected to MLCCC utilizing the solvent
system MeCN/t-BuOMe/n-hexane (10:1:10, ascending mode) to
1
assignment of H and 13C NMR signals.
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afford 4b of ca. 60% purity, pure 2 (108 mg) as a pale yellow oil, 7a
(13 mg) as white crystals, and 7b (5 mg) as a colorless liquid. 4b of
60% purity was further purified by SCC (15−40 μm, Et2O/n-hexane,
8:92) to give pure 4b (5 mg) as a colorless liquid. F3, containing
diketone 3a as the relatively more abundant compound in the mixture,
was purified by repeated SCC (15−40 μm, Et2O/n-hexane, 15:85) to
afford 3a (2 mg) as a colorless oil. F4 contained a complex mixture of
minor products, and so no separation was attempted. F5, containing
mainly corymbolone (4a), was recrystallized from Et2O/n-hexane
(8:92) to give pure 4a (52 mg) as colorless crystals. F6, containing 8a
and 8b, was purified by repeated SCC (Et2O/n-hexane, 25:75) to give
a relatively pure mixture of the keto alcohols 8a and 8b, which were
further purified by repeated preparative TLC (MeOH/EtOAc/n-
hexane, 4:20:76) to furnish pure 8a (5.3 mg) and 8b (4.1 mg) as
colorless liquids. F7 simply contained small amounts of column
residues.
93 (66), 69 (77), 55 (78); HRMS (ESI-TOF) m/z 237.1827 [M +
H]+ (calcd for C15H25O2 237.1855).
(1R,3S,5R,8S)-3,8-Dimethyl-5-(prop-1-en-2-yl)-
1,2,3,4,5,6,7,8-octahydroazulen-1-ol (7a) and (1S,3S,5R,8S)-3,8-
Dimethyl-5-(prop-1-en-2-yl)-1,2,3,4,5,6,7,8-octahydroazulen-
1-ol (7b). To a solution of rotundone (2, 114.2 mg, 0.5 mmol) in
MeOH (10 mL) was added CeCl3·7H2O (200.1 mg, 0.537 mmol).
The resulting mixture was stirred under N2 for 0.5 h and cooled to
−78 °C, followed by the addition of NaBH4 (19.9 mg, 0.53 mmol) in
one portion. After 2 min, the reaction was quenched with saturated
NH4Cl solution (10 mL), and the reaction temperature allowed to rise
to ambient temperature. The reaction mixture was extracted with ether
(3 × 25 mL), and the combined ether layers were further washed with
brine (20 mL), dried over anhydrous MgSO4, and filtered. The filtrate
was concentrated in vacuo, and the residue separated by SCC (15−40
μm, Et2O/hexanes, 6:94) to recover rotundone (2, 25.0 mg, 21%), 7a
(63.8 mg, 71% based on 21% recovery) as colorless crystals, and 7b
(14.8 mg, 16% based on 21% recovery) as a colorless oil.
Spectroscopic data of 7a and 7b were in agreement with those
reported previously.26 7a: mp 78−80 °C; 1H NMR (CDCl3, 600
MHz) δ 4.62 (1H, br s, H-12a), 4.57 (1H, br s, H-12b), 4.47 (1H, dq,
J = 1.8, 1.2 Hz, H-2), 2.48 (1H, qdd, J = 7.2, 4.2, 3.6 Hz, H-10), 2.46
(2H, m, H-3a, H-4), 2.30 (1H, dd, J = 15.0, 12.6 Hz, H-6a), 2.02 (1H,
dd, J = 15.0, 2.4 Hz, H-6b), 1.90 (1H, ddd, J = 12.6, 10.8, 2.4 Hz, H-7),
1.85 (1H, dddd, J = 12.0, 11.4, 10.8, 1.5 Hz, H-8a), 1.66 (3H, br s, H-
13), 1.65 (1H, m, H-8b), 1.61 (1H, m, H-9a), 1.58 (1H, dddd, J =
14.4, 11.4, 3.6, 2.1 Hz, H-9b), 1.18 (1H, m, H-3b), 1.17 (3H, d, J = 7.2
Hz, H-15), 1.04 (3H, d, J = 7.0 Hz, H-14); 13C NMR (CDCl3, 150
MHz) δ 152.2 (C-11), 143.9 (C-5), 143.6 (C-1), 108.14 (C-12), 81.4
(C-2), 46.4 (C-7), 42.6 (C-4), 41.9 (C-3), 33.7 (C-9), 3.6 (C-6), 31.6
(C-10), 31.1 (C-8), 21.1 (C-13), 20.4 (C-15), 19.2 (C-14); EIMS m/z
(rel intensity) 220 ([M]+, 20), 205 (10), 187 (16), 176 (12), 163
(100), 145 (30), 138 (22), 119 (61), 107 (40), 95 (49), 79 (30), 67
(28), 55 (33). 7b: 1H NMR (benzene-d6, 600 MHz) δ 4.80 (1H, s, H-
12a), 4.74 (1H, s, H-12b), 4.57 (1H, dd, J = 7.2, 4.2 Hz, H-2), 2.65
(1H, m, H-10), 2.52 (1H, m, H-4), 2.20−2.13 (2H, m, H-6a, and H-
7), 2.01 (1H, d, J = 13.8 Hz, H-6b), 1.80 (1H, ddd, J = 13.8, 8.1, 4.2
Hz, H-3a), 1.74−1.71 (2H, m, H-8a, H-8b), 1.69 (1H, m, H-9a), 1.65
(3H, s, H-13), 1.65 (1H, ddd, J = 13.8, 7.2, 3.6 Hz, H-3b), 1.62- 1.57
(1H, m, H-9b), 1.01 (3H, d, J = 7.2 Hz, H-15), 0.84 (3H, d, J = 7.2 Hz,
H-14); 13C NMR (benzene-d6, 150 MHz) δ 152.2 (C-11), 144.5 (C-
5), 143.3 (C-1), 109.1 (C-12), 79.0 (C-2), 47.3 (C-7), 43.7 (C-4),
43.1 (C-3), 34.8 (C-9), 34.4 (C-6), 31.8 (C-8), 30.9 (C-10), 20.8 (C-
13), 20.7 (C-14), 18.4 (C-15); EIMS m/z (rel intensity) 220 ([M]+,
20), 205 (10), 187 (10), 177 (14), 163 (100), 145 (24), 137 (19), 119
(41), 105 (29), 95 (33), 79 (21), 67 (19), 55 (23).
(3R,5R,8S)-3-Hydroxy-3,8-dimethyl-5-(prop-1-en-2-yl)-
3,4,5,6,7,8-hexahydroazulen-1(2H)-one (8a): 1H NMR (benzene-
d6, 600 MHz) δ 4.73 (1H, dq, J = 1.8, 0.9 Hz, H-12a), 4.70 (1H, dq, J
= 1.8, 1.5 Hz, H-12b), 3.22 (1H, qdd, J = 7.2, 4.2, 3.6 Hz, H-10), 2.45
(1H, dt, J = 15.6, 2.4 Hz, H-6a), 2.31 (1H, d, J = 18.0 Hz, H-3a), 2.28
(1H, dd, J = 15.6, 12.0 Hz, H-6b), 2.23 (1H, d, J = 18.0 Hz, H-3b),
1.77 (1H, tt, J = 12.0, 2.4 Hz, H-7), 1.70 (1H, dddd, J = 14.4, 12.0,
12.0, 2.4 Hz, H-8a), 1.63−1.58 (1H, m, H-8b), 1.58 (3H, s, H-13),
1.54 (1H, dddd, J = 14.4, 6.0, 4.2, 2.4 Hz, H-9a), 1.34 (1H, dddd, J =
14.4, 12.0, 3.6, 2.4 Hz, H-9b), 1.01 (3H, d, J = 7.2 Hz, H-15), 0.99
(3H, s, H-14); 13C NMR (benzene-d6, 150 MHz) δ 202.3 (C-2), 172.2
(C-5), 150.9 (C-11), 145.6 (C-1), 109.3 (C-12), 75.7 (C-4), 51.2 (C-
3), 46.8 (C-7), 32.7 (C-9), 32.4 (C-6), 31.1 (C-8), 26.9 (C-10), 26.2
(C-14), 20.3 (C-13), 17.0 (C-15); EIMS m/z (rel intensity) 234 (4),
216 (78), 201 (20), 188 (22), 173 (32), 159 (43), 145 (34), 132 (52),
119 (33), 111 (57), 95 (100), 91 (72), 67 (41); HRMS (ESI-TOF)
m/z 235.1684 [M + H]+ (calcd for C15H23O2, 235.1698).
(3S,5R,8S)-3-Hydroxy-3,8-dimethyl-5-(prop-1-en-2-yl)-
3,4,5,6,7,8-hexahydroazulen-1(2H)-one (8b): 1H NMR (benzene-
d6, 600 MHz) δ 4.76 (1H, dq, J = 1.8, 0.9 Hz, H-12a), 4.74 (1H, dq, J
= 1.8, 1.5 Hz, H-12b), 3.24 (1H, qdd, J = 7.2, 4.2, 3.6 Hz, H-10), 2.56
(1H, d, J = 15.6 Hz, H-6a), 2.27, 2.22 (2H, ABq, J = 18.0 Hz, H-3a and
H-3b), 2.11 (1H, dd, J = 15.6, 12.0 Hz, H-6b), 1.89 (1H, m, H-7),
1.67−1.63 (2H, m, H-8a and H-8b), 1.62 (3H, s, H-13), 1.58−1.52
Isolation of Autoxidation Products Using MLCCC. α-Guaiene
(1, 1.0 g, 5 mmol) was coated onto a single sheet of filter paper in a
test tube according to the procedure described above. The α-guaiene-
coated filter paper was kept in a 50 °C oven for 16 h. The oxidation
products were extracted by Soxhlet extraction from refluxing CH2Cl2
under N2 for 12 h. The extracts were concentrated in vacuo, and the
residue was separated by MLCCC (MeCN/t-BuOMe/n-hexane,
10:1:10, descending mode) to give five main fractions of hydro-
peroxides (MF1−MF5), monoepoxides 5a (3 mg) and 5b (11 mg),
unreacted 1 (280 mg), and several of the previously isolated
compounds 2, 4a, 7a, and 7b. MF1 (80 mg) and MF3 (86 mg)
contained mainly hydroperoxides 6b and 10a, respectively. MF2 (93
mg) contained mixtures of hydroperoxides 6b, 10a, and 10b, judging
from the presence of their corresponding decomposition products 7b,
9a, and 9b in the GC chromatogram. MF4 (220 mg) contained
mixtures of hydroperoxides including 6a, 10a, and 10b, with 6a being
the main product identified by the formation of 7a as one main
product after the reduction of the mixture with LiAlH4. MF5 (39 mg)
showed one main spot upon TLC analysis and one main peak upon
GC-MS analysis. All fractions (MF1−MF5) were further purified by
MLCCC separately using the same solvent system. Pure hydro-
peroxide 6b (45 mg) and 10a (50 mg) were obtained as colorless oils
from MF1 and MF3, respectively. Repeated separation of MF2, MF4,
and MF5 with MLCCC failed to furnish pure compounds. MF2 and
MF4 gave mixtures of hydroperoxides in different ratios. Purified MF5
(17 mg) appeared to be pure, as it displayed only one single spot on
TLC (n-hexane) and one major peak accounting for 83% in GC-MS
1
analysis. However, H and 13C NMR analysis indicated the purified
fraction MF5 was a mixture of peroxides that were not able to be
characterized herein due to the large extent of overlapping signals in
the NMR spectra.
(4S,4aR,6R,8aR)-4a-Hydroxy-4,8a-dimethyl-6-(prop-1-en-2-
yl)octahydronaphthalen-1(2H)-one (corymbolone) (4a): EIMS
m/z (rel intensity) 236 (10), 218 (18), 203 (33), 194 (9), 175 (28),
161 (16), 147 (13), 135 (43), 124 (38), 109 (100), 93 (48), 69 (59),
55 (52). All physical and chemical properties were identical to those
previously reported.21
(4S,4aS,6R,8aR)-4a-Hydroxy-4,8a-dimethyl-6-(prop-1-en-2-
yl)octahydronaphthalen-1(2H)-one-methane (4b): 1H NMR
(benzene-d6, 600 MHz) δ 4.88 (1H, s, H-12a), 4.66 (1H, s, H-12b),
2.57 (1H, d, J = 1.8 Hz, -OH), 2.37 (1H, ddd, J = 15.0, 13.2, 7.8 Hz,
H-3a), 2.32 (1H, ddd, J = 15.0, 14.4, 4.2 Hz, H-10a), 2.25 (1H, ddd, J
= 15.0, 6.0, 1.8 Hz, H-3b), 1.98 (1H, qd, J = 13.2, 6.0 Hz, H-4a), 1.89
(1H, br t, J = 6.6 Hz, H-8), 1.81 (2H, m, H-7a, H-9a), 1.60 (1H, qdd, J
= 6.6, 4.8, 1.8 Hz, H-5), 1.58 (3H, s, H-13), 1.52 (1H, ddd, J = 15.0,
4.5, 3.0, H-10b), 1.42 (1H, dddd, J = 15.0, 14.4, 6.6, 4.5 Hz, H-9b),
1.33 (1H, dddd, J = 13.2, 7.8, 4.8, 1.8 Hz, H-4b), 1.15 (1H, dd, J =
14.4, 6.6 Hz, H-7b), 0.87 (3H, s, H-15), 0.86 (3H, d, J = 6.6 Hz, H-
14); 13C NMR (benzene-d6, 150 MHz) δ 212.7 (C-2), 149.8 (C-11),
111.1 (C-12), 79.0 (C-6), 52.1 (C-1), 37.2 (C-8), 36.5 (C-3), 34.0 (C-
5), 30.7 (C-7), 29.4 (C-4), 24.9 (C-10), 22.8 (C-13), 22.1 (C-9), 20.8
(C-15), 14.6 (C-14); EI-MS m/z (rel intensity) 236 (25), 218 (10),
203 (28), 185 (5), 175 (38), 161 (28), 147 (16), 137 (85), 109 (100),
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(1H, m, H-9a), 1.41 (1H, dt, J = 14.4, 3.6 Hz, H-9b), 0.96 (3H, d, J =
7.2 Hz, H-15), 0.93 (3H, s, H-14); 13C NMR (benzene-d6, 150 MHz)
δ 202.7 (C-2), 171.9 (C-5), 151.1 (C-11), 145.6 (C-1), 109.3 (C-12),
75.7 (C-4), 51.0 (C-3), 46.8 (C-7), 32.7 (C-9), 32.1 (C-6), 31.0 (C-8),
27.1 (C-10), 26.1 (C-14), 20.5 (C-13), 17.3 (C-15); EIMS m/z (rel
intensity) 234 (57), 219 (62), 201 (25), 191 (27), 176 (71), 59 (54),
148 (54), 133 (65), 119 (51), 111 (97), 10 (70), 91 (100), 79 (72);
HRMS (ESI-TOF) m/z 235.1683 [M + H]+ (calcd for C15H23O2,
235.1698).
anhydrous MgSO4, and filtered. The filtrate was concentrated in vacuo,
redissolved in CH2Cl2, and analyzed by TLC and GC-MS. TLC
analysis showed the full consumption of 6b, whereas GC-MS analysis
showed the formation of (2S)-rotundol (7b) as the major product,
accounting for ca. 40% along with two other unidentified products.
(1S,4S,7R)-1,4-Dimethyl-7-(prop-1-en-2-yl)-1,2,3,4,5,6,7,8-
1
octahydroazulen-1-yl hydroperoxide (10a): H NMR (benzene-
d6, 600 MHz) δ 6.71 (1H, s, −OOH), 4.83 (1H, s, H-12a), 4.75 (1H,
dq, J = 1.8, 1.5 Hz, H-12b), 2.38 (1H, ddt, J = 16.2, 9.0, 4.2 Hz, H-2a),
2.31 (1H, ddd, J = 13.2, 9.0, 4.2 Hz, H-3a), 2.27 (1H, d, J = 15.6, 1.8
Hz, H-6a), 2.23−2.15 (2H, m, H-6b and H-10), 2.04−1.95 (2H, m, H-
2b and H-7), 1.81−1.69 (2H, m, H-8a and H-8b), 1.69 (3H, s, H-13),
1.65 (1H, ddd, J = 13.2, 9.0, 4.2 Hz, H-3b), 1.59 (1H, ddd, J = 13.8,
7.2, 3.0 Hz, H-9a), 1.51 (1H, dddd, J = 13.8, 10.2, 7.2, 3.0 Hz, H-9b),
1.22 (3H, s, H-14), 1.00 (3H, d, J = 7.2 Hz, H-15); 13C NMR
(benzene-d6, 150 MHz) δ 151.7 (C-11), 147.7 (C-1), 135.8 (C-5),
108.8 (C-12), 97.6 (C-4), 47.1 (C-7), 34.9 (C-10), 34.3 (C-2), 33.6
(C-3), 33.2 (C-9), 31.5 (C-8), 30.5 (C-6), 22.1 (C-14), 20.7 (C-13),
17.6 (C-15); HRMS (ESI-TOF) m/z 237.1837 [M + H]+ (calcd for
C15H25O2, 237.1855), decomposed upon heating.
(3aS,4S,7R,8aS)-4-Methyl-1-methylidene-7-(prop-1-en-2-yl)-
1
hexahydro-1H,4H-3a,8a-epoxyazulene (9a): H NMR (benzene-
d6, 600 MHz) δ 5.09 (1H, dd, J = 2.7, 1.2 Hz, H-14a), 4.99 (1H, dd, J
= 2.7, 1.2 Hz, H-14b), 4.74 (1H, dq, J = 2.4, 0.9 Hz, H-12a), 4.69 (1H,
dq, J = 1.5, 1.2 Hz, H-12b), 2.34 (1H, d, J = 14.4 Hz, H-6a), 2.27 (1H,
dddt, J = 15.0, 9.6, 9.0, 2.7 Hz, H-3a), 2.12 (1H, m, H-7), 1.97 (1H,
ddt, J = 15.0, 9.0, 1.2 Hz, H-3b), 1.89 (1H, dqd, J = 9.0, 7.2, 2.4 Hz, H-
10), 1.85 (1H, d, J = 14.4 Hz, H-6b), 1.83 (1H, dd, J = 13.2, 9.0 Hz, H-
2a), 1.61−1.56 (1H, m, H-9a), 1.57 (3H, s, H-13), 1.52−1.48 (2H, m,
H-8a and H-8b), 1.35 (1H, ddd, J = 13.2, 9.6, 9.0 Hz, H-2b), 1.22 (1H,
dtd, J = 15.0, 4.8, 2.4 Hz, H-9b), 1.05 (3H, d, J = 7.2 Hz, H-15); 13C
NMR (benzene-d6, 150 MHz) δ 151.8 (C-4), 150.4 (C-11), 109.1 (C-
12), 108.4 (C-14), 74.6 (C-1), 69.1 (C-5), 44.0 (C-7), 34.8 (C-10),
31.6 (C-8), 30.4 (C-2), 30.0 (C-6), 29.3 (C-9), 28.2 (C-3), 20.1 (C-
13), 17.5 (C-15); EIMS m/z (rel intensity) 220 (1), 205 (13), 187
(29), 177 (18), 162 (22), 147 (33), 135 (11), 123 (46), 107 (100), 95
(58), 81 (70), 67 (51), 55 (47); HRMS (ESI-TOF) m/z 219.1732 [M
+ H]+ (calcd for C15H23O1, 219.1749).
(3S,5R,8S)-3,8-Dimethyl-5-[(2R)-2-methyloxiran-2-yl]-
3,4,5,6,7,8-hexahydroazulen-1(2H)-one (11a) and (3S,5R,8S)-
3,8-Dimethyl-5-[(2S)-2-methyloxiran-2-yl]-3,4,5,6,7,8-hexahy-
droazulen-1(2H)-one (11b). To a solution of rotundone (2, 11 mg,
50 μmol) in CH2Cl2 (2 mL) was added m-CPBA (46 mg, 210 μmol).
After stirring for 1 h, the reaction was quenched with solid KI (10 mg),
aqueous Na2S2O3 (2 mL), and saturated aqueous NaHCO3 (5 mL).
The products were extracted with Et2O (3 × 5 mL), and the combined
ether extracts washed with brine (5 mL), dried over anhydrous
MgSO4, and filtered. The filtrate was concentrated in vacuo, and the
residue purified by SCC (15−40 μm, Et2O/petroleum ether, 8:92) to
give pure 11a (4.5 mg, 38%) as a colorless solid and 11b (3.5 mg,
33%) as an oil. 11a: mp 94.5−95.0 °C; 1H NMR (CDCl3, 600 MHz) δ
3.00 (1H, qt, J = 7.2, 3.6 Hz, H-10), 2.75 (1H, quint, J = 6.6 Hz, H-4),
2.65 (1H, d, J = 6.6 Hz, H-12a), 2.62 (1H, d, J = 6.6 Hz, H-12b), 2.58
(1H, dd, J = 18.6, 6.6 Hz, H-3a), 2.58 (1H, dd, J = 15.6, 3.6 Hz, H-6a),
2.53 (1H, dd, J = 15.6, 12.0 Hz, H-6b), 1.97 (1H, d, J = 18.6 Hz, H-
3b), 1.83−1.76 (3H, m, H-8a, H-8b, and H-9a), 1.49−1.45 (1H, m, H-
9b), 1.30 (3H, s, H-13), 1.50 (3H, d, J = 6.6 Hz, H-14), 1.08 (1H, tt, J
= 12.0, 3.0 Hz, H-7), 1.01 (3H, d, J = 7.2 Hz, H-15); 13C NMR
(CDCl3, 150 MHz) δ 207.9 (C-2), 176.4 (C-5), 145.4 (C-1), 60.6 (C-
11), 55.4 (C-12), 45.9 (C-7), 42.9 (C-3), 37.7 (C-4), 33.0 (C-6), 32.3
(C-9), 28.3 (C-8), 26.8 (C-10), 19.1 (C-14), 17.5 (C-15), 16.5 (C-
13); EI-MS m/z (rel intensity) 234 (1), 219 (3), 205 (24), 187 (23),
177 (100), 161 (47), 147 (70), 133 (37), 119 (41), 105 (52), 91 (44),
77 (25), 55 (20); HRMS (ESI-TOF) m/z 235.1672 [M + H]+ (calcd
for C15H23O2, 235.1698).
3aR,4S,7R,8aR)-4-Methyl-1-methylidene-7-(prop-1-en-2-yl)-
1
hexahydro-1H,4H-3a,8a-epoxyazulene (9b): H NMR (benzene-
d6, 600 MHz) δ 4.99 (1H, dd, J = 2.7, 1.2 Hz, H-14a), 4.94 (1H, dd, J
= 2.7, 1.2 Hz, H-14b), 4.76 (1H, dq, J = 1.8, 0.9 Hz, H-12a), 4.72 (1H,
dq, J = 1.8, 1.5 Hz, H-12b), 2.51 (1H, br d, J = 14.4 Hz, H-6a), 2.46
(1H, tt, J = 11.4, 2.0 Hz, H-7), 2.36 (1H, dtt, J = 15.0, 9.0, 2.7 Hz, H-
3a), 2.22 (1H, qd, J = 7.2, 6.0 Hz, H-10), 1.99 (1H, ddt, J = 15.0, 9.0,
1.2 Hz, H-3b), 1.96 (1H, m, H-9a), 1.82 (1H, dd, J = 14.4, 11.4 Hz, H-
6b), 1.68 (1H, dt, J = 13.2, 9.0 Hz, H-2a), 1.61 (3H, s, H-13), 1.59
(1H, dd, J = 13.2, 9.0 Hz, H-2b), 1.50 (1H, m, H-8a), 1.32 (1H, dddd,
J = 13.8, 6.0, 4.2, 2.0 Hz, H-9b), 1.28 (1H, ddd, J = 14.4, 11.4, 2.0 Hz,
H-8b), 0.84 (3H, d, J = 7.2 Hz, H-15); 13C NMR (benzene-d6, 150
MHz) δ 153.7 (C-4), 151.5 (C-11), 109.1 (C-12), 108.0 (C-14), 76.4
(C-1), 67.8 (C-5), 43.7 (C-7), 33.6 (C-10), 31.6 (C-6), 31.2 (C-9),
29.8 (C-8), 29.4 (C-2), 28.1 (C-3), 20.9 (C-13), 16.0 (C-15); EIMS
m/z (rel intensity) 218 (18), 203 (24), 189 (9), 175 (48), 161 (35),
147 (35), 133 (100), 119 (46), 107 (43), 95 (39), 81 (34), 67 (41), 55
(38); HRMS (ESI-TOF) m/z 219.1732 [M + H]+ (calcd for
C15H23O1, 219.1749).
Epoxides 5a/b. Spectroscopic data were identical to those
1
reported by us recently.20
11b: H NMR (CDCl3, 600 MHz) δ 2.99 (1H, qt, J = 7.2, 3.6 Hz,
H-10), 2.70 (1H, qd, J = 7.2, 6.6 Hz, H-4), 2.64 (1H, d, J = 4.8 Hz, H-
12a), 2.63 (1H, d, J = 4.8 Hz, H-12b), 2.58 (1H, dd, J = 18.6, 6.6 Hz,
H-3a), 2.44−2.37 (2H, m, H-6a and H-6b), 2.02 (1H, dddd, J = 13.8,
6.0, 2.4, 1.8 Hz, H-8a), 1.97 (1H, d, J = 18.6 Hz, H-3b), 1.84−1.79
(2H, m, H-9a and H-8b), 1.48 (1H, dddd, J = 15.6, 13.8, 3.6, 1.8 Hz,
H-9b), 1.31 (3H, s, H-13), 1.18 (1H, dddd, J = 12.6, 11.4, 4.2, 2.4 Hz,
H-7), 1.14 (3H, d, J = 7.2 Hz, H-14), 1.00 (3H, d, J = 7.2 Hz, H-15);
13C NMR (CDCl3, 150 MHz) δ 207.2 (C-2), 175.5 (C-5), 145.6 (C-
1), 60.5 (C-11), 54.4 (C-12), 45.2 (C-7), 42.9 (C-3), 37.8 (C-4), 34.0
(C-6), 32.2 (C-9), 27.2 (C-8), 26.9 (C-10), 19.2 (C-14), 17.6 (C-15),
17.4 (C-13); EIMS m/z (rel intensity) 234 (9), 219 (6), 205 (35), 187
(42), 177 (100), 161 (58), 147 (94), 133 (29), 119 (54), 105 (67), 91
(73), 77 (35), 55 (25); HRMS (ESI-TOF) m/z 235.1670 [M + H]+
(calcd for C15H23O2, 235.1698).
(1R,3S,3aS,5R,8S,8aR)-3,8-Dimethyl-5-(prop-1-en-2-yl)-
hexahydro-1H,4H-3a,8a-epoxyazulen-1-ol (12a). To a solution
of m-CPBA (77%, 16 mg, 72 μmol) in CH2Cl2 (1 mL) at 0 °C was
added a solution of 7a (10 mg, 45 μmol) in CH2Cl2 (1 mL). The
resulting mixture was stirred at 0 °C until TLC showed the complete
consumption of the starting alcohol (1 h). The reaction was quenched
with solid KI (5 mg), saturated aqueous Na2S2O3 (2 mL), and
saturated aqueous NaHCO3 (5 mL), and the mixture was stirred for 1
(1S,3S,5R,8S)-3,8-Dimethyl-5-(prop-1-en-2-yl)-
1,2,3,4,5,6,7,8-octahydroazulen-1-yl hydroperoxide (6b): 1H
NMR (benzene-d6, 600 MHz) δ 7.00 (1H, d, J = 1.8 Hz, −OOH),
4.95 (1H, ddd, J = 7.2, 2.4, 1.8 Hz, H-2), 4.75 (1H, dq, J = 2.4, 1.2 Hz,
H-12a), 4.72 (1H, dq, J = 1.8, 1.5 Hz, H-12b), 2.70−2.64 (2H, m, H-4
and H-10), 2.32 (1H, ddd, J = 13.8, 7.8, 2.4 Hz, H-3a), 2.18 (1H, tq, J
= 12.0, 1.8 Hz, H-7), 2.15 (1H, dd, J = 12.0, 3.6 Hz, H-6a), 2.04 (1H,
d, J = 12.0 Hz, H-6b), 1.75−1.60 (4H, m, H-8a, H-8b, H-9a and H-
9b), 1.62 (3H, s, H-13), 1.45 (1H, ddd, J = 13.8, 7.2, 5.4 Hz, H-3b),
1.02 (3H, d, J = 6.6 Hz, H-15), 0.84 (3H, d, J = 7.2 Hz, H-14); 13C
NMR (benzene-d6, 150 MHz) δ 151.6 (C-11), 148.1 (C-5), 139.3 (C-
1), 108.9 (C-12), 92.6 (C-2), 46.5 (C-7), 43.7 (C-4), 37.6 (C-3), 34.0
(C-9), 33.8 (C-6), 31.5 (C-8), 31.2 (C-10), 20.4 (C-13), 20.2 (C-15),
18.4 (C-14); HRMS (ESI-TOF) m/z 237.1840 [M + H]+ (calcd for
C15H25O2, 237.1855), decomposed upon heating.
Reduction of (2R)-Hydroperoxyguaiene 6b with LiAlH4.
LiAlH4 (6 mg, 158 μmol) was suspended in dry THF (0.8 mL) at
room temperature under N2. To the stirred suspension was added
slowly a solution of 6b (2 mg, 8 μmol) in THF (0.5 mL) followed by
quenching of the reaction with H2O after a further 10 min. The
resulting aqueous mixture was extracted with Et2O (3 × 5 mL), and
the combined ether layers were washed with brine, dried over
141
DOI: 10.1021/np500819f
J. Nat. Prod. 2015, 78, 131−145

Journal of Natural Products
Article
h. The mixture was extracted with Et2O (3 × 10 mL), and the
combined ether layers were washed with brine (10 mL), dried over
anhydrous MgSO4, and filtered. The filtrate was concentrated in vacuo
and purified by SCC (Et2O/hexanes, 18:82) to yield 12a (8.1 mg,
resulting mixture was stirred for 2 h under N2, and the reaction
mixture was loaded on a silica pipet and purified by SCC (CH2Cl2) to
afford pure 13b (4.1 mg, 94%) as a white solid: mp 90.9−91.3 °C; 1H
NMR (benzene-d6, 600 MHz) δ 4.70 (2H, s, H-12a, H-12b), 2.84
(1H, qt, J = 7.2, 3.6 Hz, H-10), 2.47 (1H, dd, J = 11.4, 8.4 Hz, H-3a),
2.31 (1H, tt, J = 11.4, 1.8 Hz, H-7), 1.95−1.91 (2H, m, H-4 and H-6a),
1.86 (1H, dd, J = 15.0, 11.4 Hz, H-6b), 1.69 (1H, dddd, J = 14.4, 13.8,
3.6, 1.8 Hz, H-9a), 1.57 (3H, s, H-13), 1.42−1.36 (2H, m, H-3b and
H-8a), 1.30 (1H, dddd, J = 14.4, 6.0, 3.6, 1.8 Hz, H-9b), 1.15 (1H,
dddd, J = 14.4, 13.8, 11.4, 1.8 Hz, H-8b), 0.91 (3H, d, J = 7.2 Hz, H-
15), 0.50 (3H, d, J = 7.2 Hz, H-14); 13C NMR (benzene-d6, 150 MHz)
δ 210.8 (C-2), 151.6 (C-11), 109.9 (C-12), 74.7 (C-5), 70.6 (C-1),
44.0 (C-7), 42.0 (C-3), 35.0 (C-6), 34.6 (C-4), 30.7 (C-9), 30.5 (C-8),
26.5 (C-10), 21.4 (C-13), 18.7 (C-14), 15.9 (C-15); EIMS m/z (rel
intensity) 218 (9), 203 (8), 191 (9), 177 (25), 163 (100), 149 (28),
135 (28), 121 (40), 109 (70), 69 (81); HRMS (ESI-TOF) m/z
235.1672 [M + H]+ (calcd for C15H23O2, 235.1698).
2-[(3S,5R,8S)-3,8-Dimethyl-1-oxo-1,2,3,4,5,6,7,8-octahydroa-
zulen-5-yl]prop-2-enal (14). To a stirred suspension of SeO2 (4.6
mg, 41 μmol) in CH2Cl2 (1 mL) at 0 °C was added slowly a solution
of TBHP (5−6 M in decane, 20 μL) in CH2Cl2 (1 mL) and a solution
of 2 (9 mg) in CH2Cl2 after 2 min. The reaction was allowed to warm
to RT over 1 h and stirred at RT for 30 h. The reaction mixture was
filtered through a pad of silica and eluted with CH2Cl2 (2 × 6 mL).
The filtrate was concentrated in vacuo, and the residue purified by SCC
(Et2O/n-hexane, 15:85) to give 14 (5.4 mg, 54%) as a colorless liquid:
1H NMR (CDCl3, 600 MHz) δ 9.53 (1H, s, −CHO), 6.35 (1H, s, H-
12a), 6.04 (1H, s, H-12b), 3.05−3.00 (1H, m, H-10), 2.75 (1H, dq, J =
7.2, 7.2 Hz, H-4), 2.62 (1H, m, H-7), 2.61 (1H, dd, J = 18.6, 6.6 Hz,
H-3a), 2.56 (1H, dd, J = 15.6, 12.0 Hz, H-8a), 2.34 (1H, br d, J = 15.6
Hz, H-8b), 1.97 (1H, d, J = 18.6 Hz, H-3b), 1.95 (1H, ddd, J = 14.4,
10.8, 1.8 Hz, H-6a), 1.81−1.75 (2H, m, H-6b and H-9a), 1.59 (1H,
dddd, J = 15.0, 12.0, 3.6, 1.8 Hz, H-9b), 1.09 (3H, d, J = 7.2 Hz, H-14),
1.03 (3H, d, J = 7.2 Hz, H-15); 13C NMR (CDCl3, 150 MHz) δ 208.0
(C-2), 194.0 (C-13), 175.7 (C-5), 155.4 (C-11), 145.8 (C-1), 133.2
(C-12), 43.0 (C-3), 37.8 (C-4), 36.4 (C-8), 36.2 (C-7), 32.5 (C-9),
30.4 (C-6), 26.7 (C-10), 19.1 (C-14), 17.5 (C-15); EIMS m/z (rel
intensity) 232 (81), 217 (25), 203 (52), 189 (63), 175 (62), 161 (63),
147 (73), 133 (38), 119 (48), 105 (65), 91 (100), 77 (66); HRMS
(ESI-TOF) m/z 233.1528 [M + H]+ (calcd for C15H21O2, 233.1542).
Autoxidation of Rotundone (2). Neat rotundone (2, 112 mg, 0.5
mmol) was allowed to oxidize under a pure O2 atmosphere with
stirring. A small portion of the crude reaction mixture was dissolved in
CH2Cl2 and analyzed by GC-MS at different time points. After 6
months, the pure O2 atmosphere was removed, and the crude mixture
was left in the open air for the remaining time of the oxidation and
monitored by GC-MS.
Autoxidation of Rotundols 7a/b. (2R)-Rotundol (7a, 2 mg, 9
μmol) and (2S)-rotundol (7b, 2 mg, 9 μmol) were dissolved with
CH2Cl2 (1 mL), respectively. The solvent was allowed to evaporate
under an O2 atmosphere, and the neat compounds allowed to oxidize
under pure O2 and monitored periodically by GC-MS. A small volume
of CH2Cl2 (ca. 100 μL) was added to the reaction mixture, and a small
aliquot (ca. 1 μL) was further diluted with CH2Cl2 (ca. 200 μL) for
GC-MS analysis. The solvent in the reaction mixture was again
removed under the O2 stream at each analysis time point.
1
76%) as a pale yellow oil: H NMR (CDCl3, 600 MHz) δ 4.66 (1H,
dq, J = 1.2, 0.9 Hz, H-12a), 4.64 (1H, dq, J = 1.8, 1.5 Hz, H-12b), 4.07
(1H, dd, J = 8.0, 7.8 Hz, H-2), 2.53 (1H, qdd, J = 7.2, 6.6, 3.0 Hz, H-
10), 2.09 (1H, ddd, J = 12.6, 7.8, 7.8 Hz, H-3a), 2.04 (1H, m, H-7),
2.04 (1H, d, J = 14.4 Hz, H-6a), 1.97 (1H, ddq, J = 9.6, 7.8, 6.6 Hz, H-
4), 1.69 (3H, s, H-13), 1.68−1.56 (5H, m, H-6b, H-8a, H-8b, H-9a
and H-9b), 1.05 (3H, d, J = 7.2 Hz, H-15), 1.01 (3H, d, J = 6.6 Hz, H-
14), 0.88 (1H, ddd, J = 12.6, 9.6, 7.8 Hz, H-3b); 13C NMR (CDCl3,
150 MHz) δ 150.6 (C-11), 108.7 (C-12), 75.4 (C-2), 73.4 (C-1), 71.4
(C-5), 44.6 (C-7), 37.7 (C-3), 33.6 (C-6), 33.4 (C-4), 30.77 (C-9),
30.73 (C-8), 30.2 (C-10), 20.2 (C-13), 15.9 (C-15), 13.3 (C-14);
EIMS m/z (rel intensity) 221 (13), 203 (9), 193 (23), 175 (23), 165
(57), 152 (75), 135 (57), 123 (70), 107 (78), 97 (100), 81 (67), 69
(98); HRMS (ESI-TOF) m/z 237.1829 [M + H]+ (calcd for
C15H25O2, 237.1855).
(1S,3S,3aR,5R,8S,8aS)-3,8-Dimethyl-5-(prop-1-en-2-yl)-
hexahydro-1H,4H-3a,8a-epoxyazulen-1-ol (12b). To a solution
of m-CPBA (77%, 17 mg, 78 μmol) in CH2Cl2 (1.5 mL) at 0 °C was
added dropwise a solution of 7b (13 mg, 59 μmol) in CH2Cl2 (0.5
mL). The resulting mixture was stirred under N2 for 1 h and quenched
with solid KI (5 mg), saturated aqueous Na2S2O3 (2 mL), and
saturated aqueous NaHCO3 (5 mL). The mixture was extracted with
Et2O (3 × 10 mL), and the combined ether layers were washed with
brine (10 mL), dried over anhydrous MgSO4, and filtered. The filtrate
was concentrated in vacuo and purified by SCC (Et2O/hexanes, 24:76)
1
to yield 12b (13.3 mg, 95%) as a colorless liquid: H NMR (CDCl3,
600 MHz) δ 4.69 (1H, br s, H-12a), 4.65 (1H, dq, J = 1.5, 1.2 Hz, H-
12b), 4.31 (1H, t, J = 7.8 Hz, H-2), 2.67 (1H, qt, J = 7.2, 3.6 Hz, H-
10), 2.18 (1H, tdd, J = 11.4, 3.6, 1.8 Hz, H-7), 2.15 (1H, dq, J = 7.8,
7.2 Hz, H-4), 1.98−1.91 (2H, m, H-9a and H-9b), 1.76−1.71 (1H, m,
H-6a), 1.72 (3H, s, H-13), 1.65 (1H, dd, J = 13.2, 7.8 Hz, H-3a),
1.57−1.43 (3H, m, H-3b, H-6b and H-8a), 1.33 (1H, dd, J = 13.2, 11.4
Hz, H-8b), 1.11 (3H, d, J = 7.2 Hz, H-15), 0.91 (3H, d, J = 7.2 Hz, H-
14); 13C NMR (CDCl3, 150 MHz) δ 151.4 (C-11), 108.6 (C-12), 74.8
(C-1), 72.9 (C-5), 72.4 (C-2), 43.3 (C-7), 37.48 (C-4), 37.42 (C-3),
34.1 (C-9), 30.8 (C-6), 29.4 (C-8), 27.7 (C-10), 20.8 (C-13), 17.8 (C-
14), 15.2 (C-15); EIMS m/z (rel intensity) 236 (3), 218 (53), 203
(34), 189 (8), 175 (47), 161 (37), 147 (49), 134 (94), 107 (100), 95
(91), 69 (90); HRMS (ESI-TOF) m/z 237.1828 [M + H]+ (calcd for
C15H25O2, 237.1855).
(3S,3aS,5R,8S,8aS)-3,8-Dimethyl-5-(prop-1-en-2-yl)-
hexahydro-1H,4H-3a,8a-epoxyazulen-1-one (13a). To a stirred
suspension of PCC (26 mg, 121 μmol) in CH2Cl2 (0.8 mL) at 0 °C
was added a solution of 13a (8.0 mg, 34 μmol) in CH2Cl2 (0.4 mL) in
one portion. The resulting suspension was stirred under N2, and the
temperature allowed to rise to ambient temperature overnight. The
reaction mixture was loaded on a silica pipet and purified by SCC
1
(CH2Cl2) to afford pure 13a (7.0 mg, 91%) as a colorless glass: H
NMR (benzene-d6, 600 MHz) δ 4.65 (2H, s, H-12a and H-12b), 2.72
(1H, qdd, J = 7.2, 7.2, 4.2 Hz, H-4), 1.94−1.90 (2H, m, H-6a and H-
7), 1.88 (1H, dd, J = 17.4, 8.4 Hz, H-9a), 1.76 (1H, dd, J = 17.4, 8.4
Hz, H-9b), 1.70 (1H, dd, J = 14.4, 12.0 Hz, H-6b), 1.66 (1H, q, J = 7.2
Hz, H-10), 1.49 (3H, s, H-13), 1.44−1.38 (2H, m, H-3a and H-8a),
1.38−1.31 (1H, m, H-8b), 1.16 (1H, dd, J = 13.2, 4.2 Hz, H-3b), 1.12
(3H, d, J = 7.2 Hz, H-14), 0.83 (3H, d, J = 7.2 Hz, H-15); 13C NMR
(benzene-d6, 150 MHz) δ 210.3 (C-2), 150.1 (C-11), 109.3 (C-12),
74.6 (C-1), 70.4 (C-5), 43.9 (C-7), 40.2 (C-9), 32.6 (C-6), 31.9 (C-8),
31.3 (C-10), 29.5 (C-3), 26.7 (C-4), 20.0 (C-13), 16.4 (C-14), 13.5
(C-15); EIMS m/z (rel intensity) 234 (1), 219 (6), 191 (11), 177
(100), 163 (100), 149 (30), 135 (44), 121 (41), 109 (63), 69 (75);
HRMS (ESI-TOF) m/z 235.1669 [M + H]+ (calcd for C15H23O2,
235.1698).
(2S,3S,6R)-6-[2-(Benzyloxy)propan-2-yl]-3-methyl-2-(3-
oxobutyl)cycloheptanone (17). Ozone was bubbled through a
solution of benzyl 4,5-ene-guaiol (16, 31.5 mg, 101 μmol), CH2Cl2
(2.5 mL), and dry pyridine (10 μL, 124 μmol) at −78 °C via a pipet
until the solution turned blue. After 3 min, O3 purging was ceased and
the solution was sparged with N2 until the blue color of the solution
disappeared. Zn powder (87 mg, 1.3 mmol) and HOAc (100 μL, 1.8
mmol) were added, and the resulting mixture was stirred until TLC
indicated the consumption of ozonide. Saturated aqueous NaHCO3
solution (5 mL) was added, the mixture was extracted with Et2O (3 ×
15 mL), and the combined organic layers were washed with brine (15
mL), dried over anhydrous MgSO4, and filtered. The filtrate was
concentrated in vacuo, and the residue purified by SCC (Et2O/
petroleum ether, 8:92) to yield 17 (18.7 mg, 54%) as a pale yellow
(3S,3aR,5R,8S,8aR)-3,8-Dimethyl-5-(prop-1-en-2-yl)-
hexahydro-1H,4H-3a,8a-epoxyazulen-1-one (13b). To a stirred
suspension of PCC (12 mg, 55 μmol) in CH2Cl2 (0.8 mL) at 0 °C was
added a solution of 13b (4.4 mg, 19 μmol) in CH2Cl2 (0.5 mL). The
142
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Journal of Natural Products
Article
1
liquid: H NMR (benzene-d6, 600 MHz) δ 7.32 (2H, d, J = 7.2 Hz,
17.4, 8.4, 6.0 Hz, H-3a), 2.37 (1H, dd, J = 12.0, 11.4 Hz, H-6b), 2.32
(1H, ddd, J = 17.4, 8.4, 6.6 Hz, H-3b), 2.29 (1H, ddd, J = 10.2, 7.8, 3.6
Hz, H-1), 2.12 (3H, s, H-14), 1.94 (1H, dddd, J = 13.2, 10.2, 8.4, 6.0
Hz, H-2a), 1.78 (1H, dddd, J = 13.2, 8.4, 6.6, 3.6 Hz, H-2b) 1.76−1.57
(4H, m, H-7, H-8a, H-9a and H-10), 1.53 (1H, m, H-8b), 1.21 (3H, s,
H-12), 1.20 (3H, s, H-13), 1.08 (3H, d, J = 7.2 Hz, H-15); 13C NMR
(CDCl3, 150 MHz) δ 214.9 (C-5), 208.6 (C-4), 72.9 (C-11), 58.1 (C-
1), 47.1 (C-7), 45.3 (C-6), 42.2 (C-3), 37.3 (C-10), 32.6 (C-8), 30.0
(C-14), 27.2 (C-12, C-13), 24.3 (C-2), 23.8 (C-9), 21.1 (C-15); EI-
MS m/z (rel intensity): 236 (54), 221 (76), 203 (13), 178 (43), 161
(16), 138 (100), 123 (58), 109 (59), 95 (70), 69 (49); HRMS (ESI-
TOF) m/z 255.1947 [M + H]+ (calcd for C15H27O3, 255.1960).
(2R,3S,6R)-3-Methyl-2-(3-oxobutyl)-6-(prop-1-en-2-yl)-
cycloheptanone (3b). To a stirred solution of 20 (9.0 mg, 35 μmol)
in CH2Cl2 (1.0 mL) was added Et3N (20 μL, 144 μmol). The mixture
was cooled to −78 °C, and SOCl2 (20 μL, 282 μmol) introduced
slowly. After 0.5 h, the reaction was quenched with saturated aqueous
NaHCO3 (3 mL) and extracted with Et2O (3 × 10 mL), and the
combined ether layers were washed with brine (10 mL), dried over
anhydrous MgSO4, and filtered. The filtrate was concentrated in vacuo,
and the residue purified by SNIS column chromatography (Et2O/
petroleum ether, 12:88) to give 3b (5.8 mg, 66%) as a pale yellow
ArH), 7.20 (2H, dd, J = 7.2, 6.6 Hz, ArH), 7.11 (1H, d, J = 6.6 Hz,
ArH), 4.22 (2H, s, PhCH2), 2.73 (1H, ddd, J = 9.6, 4.2, 2.4 Hz, H-1),
2.58 (1H, ddd, J = 17.4, 4.8, 2.4 Hz, H-6a), 2.29 (1H, dd, J = 17.4, 12.0
Hz, H-6b), 2.25 (1H, dddd, J = 13.8, 9.6, 8.4, 6.0 Hz, H-2a), 2.09 (1H,
ddd, J = 16.8, 8.4, 6.0 Hz, H-3a), 1.97 (1H, dddd, J = 12.0, 10.2, 4.8,
1.2 Hz, H-7), 1.87 (1H, ddd, J = 16.8, 8.4, 6.6 Hz, H-3b), 1.72−1.66
(1H, m, H-10), 1.63 (3H, s, H-14), 1.51 (2H, m, H-9a and H-9b), 1.43
(1H, dddd, J = 13.8, 8.4, 6.6, 4.2 Hz, H-2b), 0.98−1.03 (1H, m, H-8b),
0.96 (3H, s, H-12), 0.90 (3H, s, H-13), 0.73 (3H, d, J = 6.6 Hz, H-15);
13C NMR (benzene-d6, 150 MHz) δ 211.4 (C-5), 206.6 (C-4), 140.3
(Ar C), 128.6 (Ar C), 127.5 (Ar C),127.4 (Ar C), 77.4 (C-11), 63.6
(PhCH2), 53.8 (C-1), 47.2 (C-6), 44.5 (C-7), 41.6 (C-3), 37.3 (C-9),
35.3 (C-10), 29.3 (C-14), 25.2 (C-8), 24.9 (C-2), 22.3 (C-12), 21.9
(C-13), 14.3 (C-15); EIMS m/z (rel intensity) 236 (0.6), 195 (8), 180
(4), 149 (18), 124 (4), 109 (11), 91 (100), 79 (5), 67 (6); HRMS
(ESI-TOF) m/z 345.2434 [M + H]+ (calcd for C22H33O3, 345.2429).
(2S,3S,6R)-6-(2-Hydroxypropan-2-yl)-3-methyl-2-(3-
oxobutyl)cycloheptanone (18). To a solution of 17 (76.7 mg, 223
μmol) in MeOH (3 mL) was added 5% Pd/C (73.3 mg, 34 μmol).
The resulting mixture was evacuated under vacuum, refilled with H2
three times, and stirred under a H2 atmosphere (in a balloon) for 16 h.
The reaction mixture was quenched by filtration through a pad of silica
and eluted with CH2Cl2 (10 mL). The filtrate was concentrated in
vacuo, and the residue purified by SCC (Et2O/petroleum ether,
gradient elution from 20:80 to 50:50) to give 18 (56 mg, 99%) as a
1
liquid: H NMR (benzene-d6, 600 MHz) δ 4.71 (1H, dq, J = 1.8, 1.5
Hz, H-12a), 4.69 (1H, dq, J = 1.8, 0.9 Hz, H-12b) 2.49 (1H, ddd, J =
11.4, 5.1, 1.2 Hz, H-6a), 2.31 (1H, t, J = 11.4 Hz, H-6b), 2.21 (1H, m,
H-7), 2.14−2.08 (2H, m, H-1 and H-3a), 2.01−1.93 (2H, m, H-2a and
H-3b), 1.73 (1H, ddd, J = 13.8, 6.6, 3.6 Hz, H-2b), 1.64 (3H, s, H-14),
1.56 (3H, s, H-13), 1.56−1.50 (1H, m, H-8a), 1.42−1.29 (4H, m, H-
8b, H-9a, H-9b, and H-10), 0.86 (3H, d, J = 6.6 Hz, H-15); 13C NMR
(benzene-d6, 150 MHz) δ 211.8 (C-5), 206.3 (C-4), 148.9 (C-11),
109.9 (C-12), 58.2 (C-1), 48.7 (C-6), 44.0 (C-7), 41.5 (C-3), 35.2 (C-
10), 32.9 (C-9), 29.4 (C-14), 28.6 (C-8), 25.0 (C-2), 21.2 (C-15),
20.8 (C-13); EIMS m/z (rel intensity) 236 (24), 221 (16), 203 (9),
179 (27), 161 (23), 150 (16), 135 (20), 123 (62), 109 (75), 95 (100),
67 (81); HRMS (ESI-TOF) m/z 237.1845 [M + H]+ (calcd for
C15H25O2, 237.1855).
1
colorless oil: H NMR (benzene-d6, 600 MHz) δ 2.71 (1H, ddd, J =
9.6, 4.2, 2.4 Hz, H-1), 2.54 (1H, ddd, J = 17.4, 4.2, 2.4 Hz, H-6a), 2.24
(1H, dddd, J = 18.0, 9.6, 7.8, 6.0 Hz, H-2a), 2.18 (1H, dd, J = 17.4,
12.0 Hz, H-6b), 2.11 (1H, ddd, J = 16.8, 7.8, 6.0 Hz, H-3a), 1.88 (1H,
ddd, J = 16.8, 7.8, 6.6 Hz, H-3b), 1.69 (1H, m, H-10), 1.66 (3H, s, H-
14), 1.66−1.65 (1H, m, H-7), 1.61−1.59 (1H, m, H-8a), 1.51 (2H, m,
H-9a and H-9b), 1.43 (1H, dddd, J = 18.0, 7.8, 6.6, 4.2 Hz, H-2b),
0.98−0.92 (1H, m, H-8b), 0.90 (3H, s, H-12), 0.85 (3H, s, H-13), 0.72
(3H, d, J = 7.2 Hz, H-15); 13C NMR (benzene-d6, 150 MHz) δ 211.6
(C-5), 206.7 (C-4), 72.2 (C-11), 53.8 (C-1), 47.3 (C-6), 46.8 (C-7),
41.6 (C-3), 37.3 (C-9), 35.2 (C-10), 29.3 (C-14), 26.6 (C-12), 26.5
(C-13), 25.4 (C-8), 24.8 (C-2), 14.3 (C-15); EIMS m/z (rel intensity)
236 (22), 221 (14), 196 (5), 177 (54), 163 (10), 150 (18), 138 (100),
123 (37), 109 (37), 95 (38), 59 (73); HRMS (ESI-TOF) m/z
255.1949 [M + H]+ (calcd for C15H27O3, 255.1960).
(2S,3S,6R)-3-Methyl-2-(3-oxobutyl)-6-(prop-1-en-2-yl)-
cycloheptanone (3a). To a solution of 18 (19 mg, 75 μmol) in
benzene (1 mL) was added pyridine (25 μL, 0.3 mmol). The resulting
mixture was cooled to 0 °C with stirring followed by the slow addition
of SOCl2 (20 μL, 282 μmol). After stirring for 5 min, the reaction was
quenched with saturated aqueous NaHCO3 (5 mL) and extracted with
Et2O (3 × 10 mL), and the combined ether layers were washed with
brine (10 mL), dried over anhydrous MgSO4, and filtered. The filtrate
was concentrated in vacuo, and the residue purified by SNIS column
chromatography (Et2O/petroleum ether, 12:88) to give 3a (9 mg,
51%) as a pale yellow oil. Spectroscopic data were identical to those
reported by us recently.20
(2R,3S,6R)-6-(2-Hydroxypropan-2-yl)-3-methyl-2-(3-
oxobutyl)cycloheptanone (20). 1-epi-4,5-Ene-guaiol (19, 29 mg,
131 μmol) was dissolved in CH2Cl2 (8 mL), and the resulting solution
cooled to −78 °C under N2. Through this stirred solution was bubbled
a stream of O3 via a pipet (N2 atmosphere was removed) until the blue
color of the solution persisted. The introduction of O3 was ceased, and
the reaction allowed to warm to room temperature (RT) while the
solution was sparged with N2 until the blue color disappeared. Zn
powder (89 mg, 1.4 mmol) and HOAc (30 μL, 0.5 mmol) were added,
and the mixture was stirred at RT until TLC showed the full
consumption of ozonide. Saturated aqueous NaHCO3 solution (5 mL)
was added, the mixture was extracted with Et2O (3 × 15 mL), and the
combined organic layers were washed with brine (10 mL), dried over
anhydrous MgSO4, and filtered. The filtrate was concentrated in vacuo,
and the residue purified by SCC (EtOAc/petroleum ether, 24:76) to
2-[(3S,3aR,5R,8S)-3,8-Dimethyl-2,3,3a,4,5,6,7,8-octahydroa-
zulen-5-yl]propan-2-ol (24). To a stirred solution of 1-α-
hydroxyguaiene 21 (180 mg, 0.81 mmol) in CH2Cl2 (5 mL) was
added m-CPBA (77%, 330 mg, 1.5 mmol) in one portion. The
resulting mixture was stirred under N2 for 30 min and quenched with
solid KI, aqueous Na2S2O3, and aqueous NaHCO3. The products were
extracted with Et2O (4 × 20 mL), and the combined ether layers
washed with brine (20 mL), dried over anhydrous MgSO4, and
filtered. The filtrate was concentrated in vacuo to give the crude
epoxide mixture 22 (192 mg), which was used directly for the next
step without purification. To a stirred solution of 22 (192 mg) in Et2O
(20 mL) was added Et3N (0.5 mL). The resulting mixture was cooled
to 0 °C, and SOCl2 (100 μL, 1.4 mmol) introduced slowly. After 15
min, the reaction was quenched with saturated aqueous NaHCO3 (10
mL) and extracted with Et2O (4 × 20 mL), and the combined ether
layers were washed with brine (20 mL), dried over anhydrous MgSO4,
and filtered. The filtrate was concentrated in vacuo to give the crude
epoxy olefinic mixture 23 (160 mg), which was used for the next step
without purification. To a stirred solution of LiAlH4 (190 mg, 5 mmol)
in dry THF (3 mL) was added dropwise a solution of anhydrous AlCl3
(115 mg, 0.86 mmol) in dry THF (2 mL). To the resulting mixture
was added dropwise a solution of 23 (160 mg) in THF (3 mL). After
stirring for 2 h, the reaction was cooled to 0 °C and quenched with
H2O (5 mL). The products were extracted with Et2O (3 × 20 mL),
and the combined ether layers washed with brine (20 mL), dried over
anhydrous MgSO4, and filtered. The filtrate was concentrated in vacuo,
and the residue purified by repeated SNIS column chromatography
(Et2O/PE, 8:92) to give pure 24 (5.4 mg) and a mixture of 24 and an
uncharacterized isomer (142 mg, with a ratio of 48:52, 41%).
Considering the difficulty in separating these two isomers and the fact
that ozonolysis in the next step would generate more polar compounds
that would be easier to separate, the mixture was used in the next step
directly without further purification. 1H NMR (benzene-d6, 600 MHz)
δ 5.38 (1H, s, H-2), 2.77−2.72 (1H, m, H-10), 2.39−2.34 (2H, m, H-
1
yield 20 (20.8 mg, 63%) as a pale yellow oil: H NMR (CDCl3, 600
MHz) δ 2.62 (1H, ddd, J = 12.0, 5.4, 1.5 Hz, H-6a), 2.45 (1H, ddd, J =
143
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Article
3a and H-4), 1.94−1.89 (2H, m, H-3b and H-5), 1.84 (1H, dt, J =
12.0, 4.8 Hz, H-7), 1.71 (1H, d, J = 13.8 Hz, H-9a), 1.66 (1H, m, H-
6a), 1.49−1.42 (2H, m, H-6b and H-9b), 1.27−1.21 (2H, m, H-8a and
H-8b), 1.09 (3H, d, J = 6.0 Hz, H-14), 1.03 (3H, d, J = 7.2 Hz, H-15),
1.01 (6H, s, H-12 and H-13); 13C NMR (benzene-d6, 150 MHz) δ
152.6 (C-1), 123.8 (C-2), 72.7 (C-11), 50.3 (C-4), 46.9 (C-8), 41.3
(C-5), 39.3 (C-3), 37.0 (C-6), 36.0 (C-10), 32.7 (C-9), 28.1 (C-7),
27.0 (C-12), 26.6 (C-13), 18.8 (C-14), 17.9 (C-15); EIMS m/z (rel
intensity) 207 (1), 204 (27), 189 (13), 175 (1), 161 (100), 147 (30),
133 (11), 122 (44), 107 (54), 93 (33), 81 (24), 59 (42); HRMS (ESI-
TOF) m/z 223.2041 [M + H]+ (calcd for C15H27O1, 223.2062).
(3S)-3-[(1R,3S,6R)-6-(2-Hydroxypropan-2-yl)-3-methyl-2-
oxocycloheptyl]butanal (25). A mixture containing 48% 24 (110
mg, 0.5 mmol) was dissolved in CH2Cl2 (6 mL), and the resulting
mixture cooled to −78 °C under N2. To the stirred solution was
introduced a stream of O3 via a pipet until the blue color persisted.
Bubbling of O3 was ceased, the reaction mixture was sparged with N2,
and the reaction temperature allowed to warm to RT. Zn powder (240
mg, 3.7 mmol) and HOAc (100 μL, 1.7 mmol) were added, and the
resulting mixture was stirred at RT until TLC indicated the complete
consumption of ozonides. Saturated aqueous NaHCO3 solution (10
mL) was added, the resulting mixture was extracted with Et2O (3 × 20
mL), and the combined ether layers were washed with brine (20 mL),
dried over anhydrous MgSO4, and filtered. The filtrate was
concentrated in vacuo and purified by SCC (Et2O/petroleum ether,
50:50) to give 25 (34.8 mg, 58%): 1H NMR (benzene-d6, 600 MHz) δ
9.38 (1H, dd, J = 2.4, 1.2 Hz, CHO), 2.44 (1H, q, J = 7.8 Hz, H-5),
2.42−2.35 (1H, m, H-4), 2.31 (1H, appr sext, J = 7.2 Hz, H-10), 2.12
(1H, ddd, J = 16.8, 4.2, 1.2 Hz, H-3a), 1.91 (1H, ddd, J = 16.8, 7.8, 2.4
Hz, H-3b), 1.54 (1H, ddd, J = 13.8, 7.8, 4.2 Hz, H-6a), 1.48 (1H, m,
H-9a), 1.35−1.24 (3H, m, H-6b and H-8a and H-8b), 1.27 (1H, tt, J =
11.4, 4.2 Hz, H-7), 1.07−1.00 (1H, m, H-9b), 0.90 (3H, d, J = 7.2 Hz,
H-15), 0.88 (3H, s, H-12), 0.86 (3H, s, H-13), 0.80 (3H, d, J = 6.6 Hz,
H-14); 13C NMR (benzene-d6, 150 MHz) δ 215.8 (C-1), 200.7 (C-2),
72.6 (C-11), 50.5 (C-5), 49.5 (C-3), 46.2 (C-7), 45.5 (C-10), 30.7 (C-
8), 29.9 (C-4), 27.83 (C-12), 27.82 (C-6), 26.3 (C-13), 25.9 (C-9),
17.7 (C-15), 17.3 (C-14). EI-MS m/z (rel. intensity): 254 (1), 236
(6), 221 (11), 203 (6), 193 (11), 166 (20), 138 (12), 123 (33), 109
(39), 83 (70), 59 (100); HRMS (ESI-TOF) m/z 255.1946 [M + H]+
(calcd for C15H27O3, 255.1960).
MHz) δ 9.27 (1H, s), 2.52−2.45 (3H, m), 2.18−2.08 (3H, m), 1.78
(1H, ddd, J = 18.0, 9.0, 1.8 Hz), 1.60−1.50 (3H, m), 1.46 (3H, s), 1.44
(3H, s), 1.17 (1H, dd, J = 12.0, 4.5 Hz), 1.02 (3H, d, J = 6.5 Hz), 0.71
(3H, d, J = 6.5 Hz); EI-MS m/z (rel intensity) 236 (12), 218 (13), 207
(25), 193 (14), 165 (65), 138 (50), 123 (100), 109 (39), 95 (48), 67
(50).
ASSOCIATED CONTENT
■
S
* Supporting Information
1
Copies of H and 13C NMR spectra of all new compounds;
ROESY NMR spectra of 8a, 8b, 6b, 10a, 4b, 3a, and 3c; X-ray
crystallographic data for 7a, 11a, 13b, and 24. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: dennis.taylor@adelaide.edu.au. Tel: (+61) 08
83137239. Fax: (+61) 08 83137116.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the School of Agriculture, Food
and Wine, The University of Adelaide. A.C.H. is indebted to
the University of Adelaide and the China Scholarship Council
for a joint Ph.D. scholarship. C.J.S. acknowledges the ARC for
funding the Bragg Crystallography Facility (LE0989336 and
LE120100012). This research is also supported by High Impact
Research MoE Grant UM.C/625/1/HIR/MoE/SC/12 from
the Ministry of Higher Education Malaysia and the University
of Malaya.
REFERENCES
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6570−6579.
(3S)-3-[(1R,3S,6R)-3-Methyl-2-oxo-6-(prop-1-en-2-yl)-
cycloheptyl]butanal (3c). To a stirred solution of alcohol 24 (25
mg, 10 μmol) in dry toluene (1.5 mL) was added pyridine (100 μL)
and SOCl2 (20 μL) successively under N2. After stirring for 10 min,
the reaction was quenched with saturated aqueous NaHCO3 (2 mL),
the products were extracted with Et2O (3 × 10 mL), and the
combined ether layers were further washed with brine (2 × 10 mL),
dried over anhydrous MgSO4, and filtered. The filtrate was
concentrated in vacuo, and the residue filtered through Celite to give
a mixture of two closely related isomers, 3c and 26, in a ratio of 1:1
(22 mg, colorless oil, 94% overall and 47% calculated for 3c) based on
GC-MS analysis. Owing to the decomposition of 3c when using SCC
for purification in previous experiments, the mixture was further
purified by MLCCC (MeCN/t-BuOMe/PE, 10:1:10, descending
mode) to furnish pure 3c (2 mg) as a colorless oil: 1H NMR
(benzene-d6, 600 MHz) δ 9.33 (1H, dd, J = 2.4, 1.2 Hz, CHO), 4.76
(1H, s, H-12a), 4.67 (1H, s, H-12b), 2.42 (1H, ddd, J = 9.9, 7.8, 5.4
Hz, H-5), 2.34 (1H, m, H-4), 2.29 (1H, sext, J = 7.2 Hz, H-10), 2.07
(1H, m, H-7), 2.06 (1H, ddd, J = 16.8, 6.0, 1.2 Hz, H-3a), 1.88 (1H,
ddd, J = 16.8, 8.4, 2.4 Hz, H-3b), 1.56 (1H, ddd, J = 13.8, 7.8, 5.4 Hz,
H-6a), 1.50 (3H, s, H-13), 1.49 (1H, m, H-8a), 1.30 (1H, ddd, J =
13.8, 9.9, 4.5 Hz, H-6b), 1.30−1.26 (3H, m, H-8b, H-9a, and H-9b),
0.88 (3H, d, J = 7.2 Hz, H-15), 0.73 (3H, d, J = 6.6 Hz, H-14); 13C
NMR (benzene-d6, 150 MHz) δ 215.6 (C-1), 200.4 (C-2), 148.3 (C-
11), 110.5 (C-12), 50.3 (C-5), 49.3 (C-3), 46.1 (C-10), 42.3 (C-7),
31.6 (C-6), 29.9 (C-4), 29.8 (C-8), 29.3 (C-9), 21.4 (C-13), 17.7 (C-
15), 17.3 (C-14); EI-MS m/z (rel intensity) 236 (2), 218 (6), 203 (9),
193 (23), 176 (12), 165 (40), 151 (21), 135 (22), 123 (54), 110 (91),
95 (100), 81 (63), 67 (81), 55 (83); HRMS (ESI-TOF) m/z 237.1843
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