Total synthesis of eudesmane terpenes: Cyclase phase

Article abstract of DOI:10.1016/j.tet.2010.02.088

A full account of synthetic efforts toward dihydrojunenol, one of the lowest oxidized members of the eudesmane family of natural products, is presented. The final synthetic sequence illustrates a nine-step, gram-scale, enantioselective route to this bicyclic terpene with excellent stereocontrol and in 21% overall yield.

Full text of DOI:10.1016/j.tet.2010.02.088

Tetrahedron 66 (2010) 4738–4744
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Total synthesis of eudesmane terpenes: cyclase phase
*
´
´
´
Ke Chen, Yoshihiro Ishihara, Marıa Moron Galan, Phil S. Baran
Department of Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, CA 92037, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 January 2010
Received in revised form 20 February 2010
Accepted 23 February 2010
Available online 26 February 2010
A full account of synthetic efforts toward dihydrojunenol, one of the lowest oxidized members of the
eudesmane family of natural products, is presented. The final synthetic sequence illustrates a nine-step,
gram-scale, enantioselective route to this bicyclic terpene with excellent stereocontrol and in 21% overall
yield.
Ó 2010 Elsevier Ltd. All rights reserved.
Dedicated to Professor Brian Stoltz on the
occasion of his receipt of the Tetrahedron
Young Investigator Award
Me
Me
Me
1. Introduction
Me
Me
Me
Me
Me
Me
Me
Me
Me
The eudesmanes represent a broad family of sesquiterpenoid
natural products containing approximately 1000 members, many
of which exhibit antifungal, antibacterial, and anticancer activ-
ity.1 These bicyclic terpenes are characterized by a 2-isopropyl-
4a,8-dimethyldecahydronaphthalene framework, and most of its
members are adorned with a variety of oxidation patterns. In
order to target a variety of members within a class of terpenoid
natural products using a unified synthetic approach, the logic of
a ‘two-phase approach’ in terpene synthesis was conceived and
demonstrated in the total synthesis of eudesmanes.2 This design
was inspired by terpene biosynthesis3 and was executed in two
separate phasesda ‘cyclase phase’ and an ‘oxidase phase’.4 In the
context of eudesmane total synthesis, four targets were selected
and retrosynthetically traced back to a common precursor bear-
ing a lower oxidation state through the use of a retrosynthetic
pyramid diagram. Thus, dihydrojunenol (1) became the target of
a cyclase phase endpoint, which would then serve as the oxidase
phase starting point toward polyhydroxylated eudesmanes 2–5
(Fig. 1).2 In this full account, the evolution of our synthetic
strategy toward 1 is described, culminating in a short, scalable
and enantioselective route to this bicyclic terpene.
HO
1: dihydrojunenol
HO
HO
HO
HO
2: 4-epi-ajanol
3: dihydroxyeudesmane
(structure reassigned)
"oxidase
phase"
Me
Me
Me
Me
OH
Me
Me
OH
HO
HO
4: pygmol
HO
HO
HO
Me
5: eudesmantetraol
Figure 1. Dihydrojunenol (1), its X-ray structure and the eudesmane family of
terpenes.
prior to the communication of this work,2 it had only been synthe-
sized once in a racemic, stereorandom fashion from farnesol.7 Semi-
synthesis was the preferred route in the 1960s for the synthesis of 1,
astwo suchroutes weredevised from othereudesmane precursors.6,8
Moreover,1 could be accessed in one step from junenol,9 and as such,
the three semi-syntheses6c,10,11 and one racemic total synthesis12 of
junenol could be considered as being formal syntheses of 1.13
Early in this program, in order to obtain authentic samples of 1 for
comparison, we repeated the procedure by Pedro and co-workers,11
which we had judged to be the most efficient synthesis of either
dihydrojunenol (1) or junenol. Although this nine-step synthesis to-
ward 1 would rely on a natural product as the starting material,
santonin (6) was a relativelycheap and commerciallyavailable option
(Fig. 2). In our hands, the sequence9,11 proceeded in 8% overall yield;
along with the desired product 1, two other isomers 7 and 8 were also
obtained and their structures were verified by X-ray crystallography
(crystals obtained astheir p-nitrobenzoyl esterderivatives). Thus, this
route not only provided us with a sample of 1, but also furnished two
valuable stereoisomers that would serve as markers for spectroscopic
comparison in our total synthesis endeavors toward 1.
Although dihydrojunenol (1) wasonly recently isolatedduringthe
study of aroma components of the woody material Bursera graveolens
by Yukawa and co-workers,5 its structure was reported over 50 years
ago as a result of hydrogenation of another natural product, junenol.6
Despite its low molecular weight, 1 contains five contiguous stereo-
centers that pose a challenge for its total synthesis. Consequently,
* Corresponding author. Tel.: þ1 858 784 7373; fax: þ1 858 784 7375; e-mail
address: pbaran@scripps.edu (P.S. Baran).
0040-4020/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.02.088

K. Chen et al. / Tetrahedron 66 (2010) 4738–4744
4739
O
O
6
O
9 steps, 8%
overall yield
(refs 9,11)
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
HO
HO
HO
Me
1
7
8
Figure 2. Previously reported semi-synthesis of dihydrojunenol (1) from santonin (6).9,11
2. Results and discussion
forge the stereotriad C4–C5–C6 (eudesmane numbering,1 Fig. 3a).
Although such a synthesis would rely entirely on a chiral pool
source, this attractive three-step route would render the pro-
duction of 1 highly efficient and scalable. To this end, alkene- and
alkyne-bearing conjugate addition products were prepared, as
well as some of their derivatives (Fig. 3b). To our dismay, the
desired C4–C5 bond-forming reactions did not proceed in our
hands to generate the desired decalin framework, most likely due
to the steric congestion imposed by the adjacent quaternary
center (C10). At this juncture, we devised an alternative strategy
that delayed the introduction of the C14 methyl group until after
the C4–C5 bond formation.
2.1. Initial synthetic strategy
Early forays toward the synthesis of dihydrojunenol (1) were
conducted according to a retrosynthetic blueprint that simplified
the target back to commercially available piperitone (dehy-
dromenthone, 9; Fig. 3a). This monocyclic enone can be procured
in both enantiomeric forms and could, in principle, lead to the
desired eudesmane skeleton in as few as three steps:
(1) Substrate-controlled diastereoselective conjugate addition
onto C10; (2) C4–C5 bond formation; and (3) net reduction to
a
14
Me
Me
Me
Me
1
4
9
6
9
M
2
3
10
5
8
7
10
5
8
7
12
11 Me
5
5
+
6
Me
Me
Me
Me
Me
Me
4
6
4
6
13
H
15 Me OH Me
O
O
O
M = metal
1
9
b
Me
Me
Me
Me
Conditions
Conditions
Me
OR Me
Me
Me
Me
OR Me
Me
Me
R = TMS;
R = TMS;
R
TBS; TIPS
TBS; TIPS
O
O
Conditions: MW, 200 °C; HgCl2, then NaI; AuCl; AuCl3;
Au[N(SO2CF3)2]PPh3; AuClPPh3;
Au(OTf)PPh3; AgOTf
Conditions: MW, 200 °C; Cu(OAc)2; Mn(OAc)3; Pd(OAc)2;
Pd(CH3CN)2Cl2, benzoquinone;
Pd(OAc)2, O2; Pd(OAc)2, NH4HCO2
Me
Me
Me
Me
a) LHMDS
b) [O]
MLn
Me
Me
Me
Me
Me
Me
Me
Me
N2
O
O
O
Me
O
MLn = Rh2(OAc)4; Rh2(OCt)4; Rh2(cap)4; Cu(OTf)2
[O] = I2; Cu(2-ethylhexanoate)2; K3Fe(CN)6; Cp2FePF6
Me
Me
Me
Me
Conditions
Conditions
Me
Me
Me
Me
Me
Me
Conditions: (nBu3Sn)2; AIBN, nBu3SnH
Me
Me
R = CO2Et
R
R
Br
O
O
O
O
Conditions: Pd(CH3CN)2Cl2, TMSCl;
Pd(CH3CN)2Cl2,
Yb(OTf)3; MW, 200 οC
Figure 3. (a) Initial strategy to generate the decalin skeleton; (b) selected approaches.

4740
K. Chen et al. / Tetrahedron 66 (2010) 4738–4744
2.2. Revised strategy and racemic synthesis
and are less efficient, or generate olefin-transposed side products
that are difficult to remove.19,20 This enone procured in multigram
quantities was then iodinated at the -position in near-quantitative
Equipped with the knowledge garnered from failed approaches,
a revised synthetic strategy was conceived (Fig. 4). Retrosyntheti-
cally, net oxidation and C14 methyl group excision from 1 would
lead to dienone 10. With the all-carbon quaternary center at C10 no
longer present, 10 could be disconnected at the C4–C5 linkage via
retro-Heck coupling to evoke the monocyclic enone 11. This enone
could then be thought to arise from 2-iodocryptone (13) by
a Grignard addition followed by a Dauben carbonyl transposition.14
Finally, 13 could be generated from isovaleraldehyde and methyl
vinyl ketone through the well-known Robinson annulation,
followed by an electrophilic iodination.
a
yield to install a functional group handle that would later become
useful for C–C bond formation (Fig. 5b). Iodocryptone 13 thus
generated was further subjected to a two-step alkylative carbonyl
transposition,14 consisting of pentenylmagnesium bromide addi-
tion followed by PCC oxidation. It is of note here that an unexpected
solvent effect was observed during the Grignard addition, in which
the use of THF or ether led to low (<20%) yields whereas toluene
allowed efficient formation (>75%) of 12 as an inconsequential
mixture of diastereomers. PCC oxidation of this mixture sub-
sequently allowed smooth transformation into the desired mono-
cyclic enone 11 (74% over two steps).
Me
The installation of the necessary functional group handles pro-
vided the opportune moment to forge the previously problematic
C4–C5 linkage. Without an adjacent quaternary center hindering
the site of reaction, 11 smoothly underwent intramolecular ring
closure under standard Heck conditions (using catalytic Pd(OAc)2
and PPh3) to generate bicyclic dienone 10 in 74% yield (Fig. 5b). It is
of note that these preliminary reaction conditions gave way to ca.
20% of a 7-endo side product (16). Substituting Pd(OAc)2 by
Pd2(dba)3 eliminated this side product but did not significantly
increase the reaction yield; however, the use of a stoichiometric
Ag2CO3 additive21 improved the formation of 10 to 95% yield. Ac-
cess to bicyclic dienone 10 was thus achieved in short order (six
steps) from cheap (<1 $/g) starting materials in multigram
quantities.
At this point in time, all that separated dienone 10 from the
desired natural product 1 were one methyl group addition and two
strategic reductions, which would set four stereocenters overall.
The remaining C14 methyl group was introduced via copper-
mediated conjugate addition; however, this initially failed to give
product when MeMgBr and CuI were employed (Fig. 6, entry 1).
Substituting the active metal species to Me2CuLi in ether allowed
a diastereoselective 1,4-addition to proceed, albeit in abysmal yield.
Employing Me3SiCl as an additive22 improved the reaction yield to
62%, but resulted in a 1:3 mixture of C10 epimers 17 and 18, fa-
voring the undesired stereoisomer 18 (entry 3). Modifying the re-
action medium to toluene surprisingly shut down the reaction
completely, whereas dichloromethane proved to be the solvent of
choice, improving the combined yield to 78% (33% of 17 and 45% of
18). When Me3SiCl was eliminated in a control experiment (entry
6), the product ratio drastically changed, providing the 1,4-addition
products as a 3:1 ratio of 17 to 18 in 73% combined yield (with 17%
recovered starting material). These reaction conditions were
eventually adopted as the optimal conditions. Performing this same
reaction at a higher temperature for a shorter amount of time yet
again altered reaction yields, but in unfavorable fashion (entry 7:
44% of 17 and 38% of 18).
Me
Me
Me
Me
Me
I
H
Me OH Me
O
O
1
10
11
O
MgBr
OH
O
Me
Me
Me
Me
Me
I
O
Me
Me
I
13
12
Figure 4. Revised strategy to construct the decalin skeleton.
In the forward direction, methyl vinyl ketone and iso-
valeraldehyde were mixed together in the presence of 20 mol %
(diethylamino)trimethylsilane15 to generate racemic 2-isopropyl-
5-oxohexanal (14) in 62% yield (Fig. 5a). This known Michael
addition product16,17 was then treated with base under phase-
transfer conditions18 to afford racemic cryptone (15) in 87% yield.
An efficient synthesis of 15 has been a long-standing problem, since
it is a useful building block for the construction of various natural
products; although more than a dozen other methods for cryptone
synthesis are known, most of them require more than two steps
O
O
a
Et2NTMS
MeCN
nBu4NOH, KOH
O
Me
THF/Et2O/H2O
Me
O
Me
62%
87%
O
14
15
b
OH
O
O
MgBr
I2, pyr
(99%)
3
Me
Me
Me
Me
Me
Conditions A
I
I
Me
15
13
12
PCC, DCM
14
Me
14
Me
Conditions B
Me
Me
Me
Me
Me
10
10
Conditions
Me
Me
I
Me
Me
Me
Me
+
O
then
Me
O
aqueous
work-up
16
10
11
O
Me
O
Me
O
10
18
17
Conditions A a
Yield (%)
Conditions B d
Yield (%)
Entry
Entry
Entry
Conditions
Yield of 17 (%)
e
trace
74
1
2
THF (0.1 M), -78 °C, 4 h
Et2O (0.1 M), -78 °C, 2 h
PhMe (0.1 M), -78 °C, 4 h
1
2
3
Pd(OAc)2, PPh3, Et3N, DMF
Pd2(dba)3, Et3N, PhMe
a
20
b
≤
trace
1
2
3
4
5
6
7
MeMgBr (2.5 equiv.), CuI (2.5 equiv.), THF (0.1 M), 0 °C, 1 h
Me2CuLi (0.5 M in Et2O, 1.5 equiv.), Et2O (0.1 M), 0 °C, 2 h
Me2CuLi (as entry 2), TMSCl (5.0 equiv), Et2O (0.1 M), 0 °C, 2 h
Me2CuLi (as entry 2), TMSCl (5.0 equiv), toluene (0.1 M), 0 °C, 2 h
Me2CuLi (as entry 2), TMSCl (5.0 equiv), DCM (0.1 M), 0 °C, 2 h
Me2CuLi (as entry 2), DCM (0.1 M), 0 °C, 4 h
80
a
10
3
4
≤
≤ 5
b
Pd(OAc)2, Ag2CO3 (1.0 equiv.), 95
PPh3, Et3N, MeCN
c
17 (+ 45 )
PhMe (0.1 M), -78 °C for
74
a
30 min; 0 °C for 30 min
trace
b
33 (+ 45 )
56 (+17 )
44 (+ 38 )
a. The Grignard reagent was prepared
as a 0.7 M ether solution; b.
Conversion was low; c. The yield was
based on two steps (after PCC
oxidation).
d. 0.1 equiv. of Pd(OAc)2 and 0.3 equiv.
of PPh3 (or 0.1 equiv. Pd2(dba)3) were
used, along with 1.2 equiv. of Et3N; e. ca.
20% of 16 was obtained.
b c
b
Me2CuLi (as entry 2), DCM (0.1 M), RT, 1 h
a. Low conversion; b. Yield of the C10-epimer 18; c. 17% recovered 10 was also obtained.
Figure 5. (a) Synthesis of racemic cryptone (15); (b) preparation of dienone 10.
Figure 6. Installation of the C14 methyl group.

K. Chen et al. / Tetrahedron 66 (2010) 4738–4744
4741
With ample quantities of bicyclic enone 17 in hand, all that was
required to generate dihydrojunenol (1) was to forge three con-
tiguous stereocenters via two net reductions. Although one-step
simultaneous reductions of C]C and C]O bonds in enones are
known, many require harsh conditions such as high-pressure hy-
drogenation23 or strong electroreduction,24 and is otherwise non-
selective.25 Furthermore, the necessity for dihydrogen to add to 17
for ensuing mesylation and elimination steps, due to the increased
basicity of the active lithium alkoxide species. Consequently, an
asymmetric synthesis of cryptone (15) was achieved in two steps, in
63% overall and in 89% ee, which proved to be superior to previous
syntheses19 of this simple terpene. Furthermore, this asymmetric
synthesis of 15 fared better than our own racemic counterpart (54%
over two steps; see Fig. 5a). With enantioenriched 15 in hand, the
same synthetic sequence as the racemic synthesis was carried out
(vide supra), resulting in a gram-scale enantioselective total syn-
thesis of dihydrojunenol (1) in nine steps and in 21% overall yield.
from the a-face of the C]C bond but from the b-face of the C]O
bond precluded the use of hydrogenation as a means for one-step
double reduction. Ultimately, we resorted to a known two-step
reduction strategy8 that hydrogenated the olefin first, followed by
a dissolving metal reduction (Fig. 7). Hydrogenation over Pd on
carbon set the stereocenters at C4 and C5, and without isolation,
the resulting ketone was treated with sodium in ethanol, yielding 1
stereoselectively (only a single isomer was observed) in 87% yield
over two steps. The eudesmane thus obtained in nine steps and in
19% overall yield was in excellent spectroscopic agreement with
that reported in the literature.5
Ph
Ph
OMe
(S)-19
O
N
O
O
O
H
5
Me
6
Me
Conditions
5
6
(5 mol%)
7
7
Me
Me
HO
O
4 °C, 36 h, 89%
92-95% ee
O
Me
Me
Me
Me
Me
Me
(S)-14
(S)-15
Yield (%); ee (%)
87; 56
Entry
Conditions
Me
Me
nBu4NOH, KOH, THF/Et2O/H2O, 50 °C, 30 min
pyridine, rt, 12 h
1
2
3
4
5
1) Pd/C, H2
a
N/A
b
Me
Me
Me
LDA (1.0 equiv.), -78 °C, 30 min
6
N/A
N/A
5
2) Na, EtOH
(87% overall)
b
4
silica gel (5% v/v), 50 °C, 12 h
H
Me OH Me
DBU (1 equiv.), DCM, rt, 1 h; MsCl (5 equiv.), Et3N (10 equiv.), rt, 1 h
TBAF (0.5 equiv.), CH3CN, rt, 2 h; then MsCl, Et3N (as Entry 5)
75; 34
34; 0
Me
O
6
7
8
9
1
LiOH (0.1 equiv.), THF/H2O (3:1), 0 °C, 30 min; then MsCl, Et3N (as Entry 5)
LiOH (0.1 equiv.), MeOH, 0 °C, 30 min; then MsCl, Et3N (as Entry 5)
LiOH (0.1 equiv.), iPrOH, rt, 24 h
17
78; 74
71; 80
71; 89
Figure 7. Stereoselective reductions and completion of dihydrojunenol.8
a. No reaction; b. Decomposition of 14.
Figure 8. Enantioselective Michael addition and generation of enantioenriched cryp-
tone (15).
2.3. Enantioselective synthesis
With a short, robust synthesis of racemic 1 in hand, an enan-
tioselective route was sought. Since the synthesis was designed in
such a way that chiral information from the isopropyl-bearing C7
stereocenter is propagated onto the remaining four stereocenters of
the bicyclic molecule, all that was required was the enantioselective
formation of cryptone (15), and in turn, that of the keto-aldehyde
14 (vide supra). Examples of enantioselective Michael additions26
onto methyl vinyl ketone17 are known, which even encompassed
our specific substrate.17c,d Thus, diphenylprolinol methyl ether 1927
was used in catalytic quantities (5 mol %) to generate the Michael
adduct 14 in 89% yield and in 92–95% ee (Fig. 8). However, this
straightforward enantioselective addition was followed by a chal-
lenging transformation, in which aldol condensation was required
to occur without epimerizing the newly formed stereocenter.
Conditions previously employed upon racemic 14 (see Fig. 5a)
resulted in partial epimerization, thus reducing the enantiopurity
to 56% ee. The use of pyridine at room temperature did not effect
any conversion, whereas the use of LDA or silica gel resulted in
decomposition of 14 (Fig. 8, entries 3 and 4). Assuming that the
thermodynamic conditions required for dehydration were the
cause of loss in enantiopurity, milder two-step procedures for aldol
addition and dehydration were attempted. However, this triggered
losses in both yield and in enantiopurity (entries 5 and 6). Intrigued
by Chakraborti’s report on the use of catalytic LiOH as an efficient
base for Claisen–Schmidt condensations,28 we became interested
in similar reaction conditions. Thus, addition of 10 mol % LiOH in
THF–H2O at 0 ꢀC, followed by in situ mesylation and elimination,
resulted in a gratifying 78% yield and a retention of 74% ee (entry 7).
Further investigation revealed that a solvent change to methanol
increases the enantiopurity of 15, and this solvent effect could
perhaps be explained by the formation of a lithium alkoxide species
that serves to deprotonate at the terminal methyl (C5) position
rather than at the more hindered C7 position. Following this logic,
a more bulky alcohol such as 2-propanol was employed, and this
significantly mitigated the loss of enantiopurity due to epimeriza-
tion (entry 9). In addition, these new conditions obviated the need
3. Conclusion and strategic perspective
In summary, a full account of synthetic efforts toward dihy-
drojunenol (1) has been presented. Failed attempts at forging a C–C
bond adjacent to an all-carbon quaternary center have led to a re-
vised route that coupled together simple starting materials, methyl
vinyl ketone and isovaleraldehyde, in enantioselective fashion. Key
steps toward the synthesis of 1 involved an efficient, intramolecular
Heck coupling followed by a copper-mediated conjugate addition
that proceeded with excellent diastereoselectivity. This chemistry
has thus enabled the first enantioselective total synthesis of 1 in
21% overall yield (Fig. 9). Moreover, this protecting group-free,
nine-step route resulted in a gram-scale preparation of 1, which
allowed for the generation of ample quantities of this bicyclic ter-
pene required for its subsequent use as a starting material in
a systematic exploration of C–H oxidation chemistry.2
O
Me
• nine steps
Me
• 21% overall yield
• gram-scale
Me
• enantioselective
O
Me
Me
H
Me OH Me
• protecting group-free
1
Figure 9. Summary of the approach.
The efficiency with which the total synthesis of a representative
eudesmane, dihydrojunenol (1), was completed is partly due to the
low oxidation state of the target, which was an intended feature of
this study. Since there is only one heteroatom present in the target
1, protecting group chemistry is destined to be relinquished or, at
the very least, minimized.29 A great variety of chemistry can be
employed without fear for side reactions, including the use of harsh
organometallic reagents. As demonstrated here, the mere choosing
of a judicious retrosynthetic intermediate (such as 1 in the prepa-
ration of targets 2–5) can free one from chemoselectivity issues30 in

4742
K. Chen et al. / Tetrahedron 66 (2010) 4738–4744
terpene synthesis. Finally, the synthesis outlined herein facilitates
an adherence to the principles of redox economy,31 since the in-
stallation of functional groups, which is commonly plagued by
superfluous redox manipulations, is practically unnecessary in 1.
optimization. The enantiomeric excess (92–95%) was determined
by 1H NMR analysis of the corresponding
imine32].
L-valine methyl ester
4.1.3. (rac)-4-Isopropylcyclohex-2-enone (15). A solution of (rac)-14
(0.50 g, 3.3 mmol, 1.0 equiv) in Et2O (30 mL) and THF (10 mL) was
stirred at room temperature. To this solution was added aqueous
KOH solution (30 mL, 0.1 N) and nBu4NOH (0.52 mL, 40% aq) in one
portion. The mixture was heated to reflux for 3 h before cooling to
room temperature. Et2O (50 mL) was then added and the layers
were separated. The aqueous layer was extracted with Et2O
(2ꢁ50 mL) and the combined organic layers were dried over
MgSO4, filtered, and concentrated in vacuo. Flash chromatography
(silica gel, gradient from 10:1 to 5:1 hexanes/Et2O) yielded cryp-
tone, (rac)-15, as a colorless oil (0.39 g, 87%), and its spectroscopic
data was identical to previous reports.19
4. Experimental section
4.1. General
All reactions were carried out under a nitrogen atmosphere with
dry solvents using anhydrous conditions unless otherwise stated.
Dry tetrahydrofuran (THF), diethyl ether (Et2O), dichloromethane
(DCM), toluene, methanol (MeOH), acetonitrile (MeCN), N,N-dime-
thylformamide (DMF), and triethylamine (Et3N) were obtained by
passing these previously degassed solvents through activated alu-
mina columns. Reagents were purchased at the highest commercial
quality and used without further purification, unless otherwise
stated. Yields refer to chromatographically and spectroscopically (1H
NMR) homogeneous materials, unless otherwise stated. Reactions
were monitored by thin layer chromatography (TLC) carried out on
0.25 mm E. Merck silica gel plates (60F-254) using UV light as the
visualizing agent and an acidic mixture of anisaldehyde, phospho-
molybdic acid, ceric ammonium molybdate, or basic aqueous
potassium permangante (KMnO4), and heat as developing agents. E.
Merck silica gel (60, particle size 0.043–0.063 mm) was used for flash
column chromatography. Preparative thin layer chromatography
(PTLC) separations were carried out on 0.25 or 0.5 mm E. Merck silica
gel plates (60F-254). NMR spectra were recorded on Bruker DRX-
600, DRX-500, and AMX-400 or Varian Inova-400 instruments and
calibrated using residual undeuterated solvent as an internal refer-
ence (CHCl3 at 7.26 ppm in 1H NMR; 77.0 ppm in 13C NMR). The
following abbreviations (or combinations thereof) were used to ex-
plain the multiplicities: s¼singlet, d¼doublet, t¼triplet, q¼quartet,
m¼multiplet, br¼broad. High-resolution mass spectra (HRMS) were
recorded on Agilent LC/MSD TOF time-of-flight mass spectrometer
by electrospray ionization time-of-flight reflectron experiments. IR
spectra were recorded on a Perkin-Elmer Spectrum BX FTIR spec-
trometer. Melting points were recorded on a Fisher-Johns 12-144
melting point apparatus. Optical rotations were obtained on
a Perkin-Elmer 431 Polarimeter.
4.1.4. (S)-4-Isopropylcyclohex-2-enone (15). A solution of (S)-14
(2.21 g, 14.1 mmol, 1.0 equiv) in 2-propanol (47 mL, 0.3 M) was
stirred at room temperature, upon which LiOH (33.7 mg, 1.4 mmol,
0.10 equiv) was added in one portion. After stirring for 24 h, the
reaction was quenched with saturated aqueous NH4Cl (100 mL).
EtOAc (100 mL) was added and the layers were separated. The
aqueous layer was extracted with EtOAc (2ꢁ100 mL) and the
combined organic layers were dried over MgSO4, filtered, and
concentrated in vacuo. Flash chromatography (silica gel, gradient
from 10:1 to 5:1 hexanes/Et2O) yielded cryptone, (S)-15, as a col-
orless oil (1.38 g, 71%), whose spectroscopic data was identical to
previous reports.19 The enantiomeric excess (89%) was determined
by chiral HPLC [AD-H, 2% isopropanol in hexanes, 0.5 mL/min, tr
12.373 min (major), 13.687 min (minor)].
4.1.5. (S)-2-Iodo-4-isopropylcyclohex-2-enone (13). To a solution of
compound (S)-15 (9.05 g, 65.4 mmol, 1.0 equiv) in DCM (80 mL)
and pyridine (80 mL) at 0 ꢀC, iodine (19.9 g, 78.5 mmol, 1.2 equiv)
was added in five portions over 10 min. After stirring at room
temperature for 12 h, the brown solution was poured into satu-
rated aqueous Na2S2O3 (150 mL) and Et2O (500 mL). The layers
were separated and the organic phase was washed with aqueous
HCl solution (1 N, 2ꢁ300 mL), dried over MgSO4, filtered, and
concentrated in vacuo. The crude material was filtered through
4.1.1. (rac)-2-Isopropyl-5-oxohexanal (14). In a flame-dried round-
a short silica plug eluting with DCM to provide 13 as a yellow oil
23
bottom flask,
a
mixture of freshly distilled isovaleraldehyde
(17.3 g, 99%). Rf¼0.5 (silica gel, 9:1 hexanes/EtOAc); [
a
]
ꢂ26.4
D
(20.0 mL, 0.18 mol, 1.0 equiv) and (diethylamino)trimethylsilane
(6.9 mL, 0.037 mmol, 0.2 equiv) in MeCN (600 mL) was cooled to
0 ꢀC. Freshly distilled methyl vinyl ketone (22.5 mL, 0.28 mol,
1.5 equiv) was added, and the resulting yellow, homogeneous so-
lution was heated at reflux under nitrogen for 36 h. The crude
mixture was cooled to room temperature and concentrated in
vacuo, upon which the resulting thick oil was purified by column
chromatography (silica gel, gradient from 10:1 to 1:1 hexanes/
EtOAc) to provide (rac)-14 (17.8 g, 62%), whose spectroscopic data
was identical to previous reports.15a,16
(c 2.19, DCM); IR (film) nmax 2954, 2358, 1687, 1582, 1322,
1154 cmꢂ1 1H NMR (500 MHz, CDCl3)
;
d
7.67 (s, 1H), 2.78 (dt,
J¼16.5, 3.9 Hz, 1H), 2.41–2.51 (m, 2H), 2.03–2.06 (m, 1H),
1.78–1.85 (m, 2H), 0.97 (d, J¼7.0 Hz, 3H), 0.95 (d, J¼7.0 Hz, 3H);
13C NMR (125 MHz, CDCl3)
d 192.3, 163.2, 103.9, 47.0, 36.2, 31.4,
25.2, 19.6, 19.4; HRMS (ESI) calcd for C9H13IO [MþNa]þ 286.9909,
found 286.9906.
4.1.6. (S)-2-Iodo-6-isopropyl-3-(pent-4-enyl)cyclohex-2-enone
(11). In a flame-dried round-bottom flask charged with argon,
iodocryptone 13 (6.04 g, 22.9 mmol, 1.0 equiv) was dissolved in
toluene (229 mL, 0.1 M) at ꢂ78 ꢀC. A solution of freshly prepared
Grignard reagent in Et2O (0.7 M, 49 mL, 1.5 equiv) was added over
5 min. The resulting solution was stirred at ꢂ78 ꢀC for 30 min and
then the dry-ice bath was replaced with an ice bath. Stirring was
continued for an additional 30 min at 0 ꢀC before saturated aqueous
NH4Cl (300 mL) was added. The layers were separated and the
aqueous layer was extracted with Et2O (2ꢁ100 mL). The combined
organic layers were washed with brine (50 mL), dried over MgSO4,
filtered, and concentrated in vacuo. The crude alcohol thus
obtained (ca. 22 mmol) was then dissolved in DCM (110 mL, 0.2 M).
Molecular sieves (3 Å, 20 g) and PCC (5.90 g, 27.5 mmol, 1.2 equiv)
were added and the resulting brown mixture was stirred at room
4.1.2. (S)-2-Isopropyl-5-oxohexanal (14). In a flame-dried round-
bottom flask, a mixture of freshly distilled isovaleraldehyde (8.1 mL,
74.8 mmol, 1.0 equiv), prolinol catalyst (S)-1927 (1.01 g, 3.74 mmol,
0.05 equiv), and co-catalyst ethyl 3,4-dihydroxybenzoate (2.73 g,
15.0 mmol, 0.20 equiv) was cooled to 0 ꢀC. Freshly distilled methyl
vinyl ketone (9.1 mL, 112.2 mmol, 1.5 equiv) was then added, and
the resulting brown, homogeneous solution was stirred under ni-
trogen at 4 ꢀC for 36 h. The crude material was purified by column
chromatography (silica gel, gradient from 10:1 to 1:1 hexanes/
EtOAc) to provide (S)-14 (10.3 g, 89%), whose spectroscopic data
was identical to the previous report.15a,16 [Note: This compound
was prepared by following the reported procedure without further

K. Chen et al. / Tetrahedron 66 (2010) 4738–4744
4743
temperature for 6 h. The dark reaction mixture was then filtered
through a short silica plug eluting with DCM to provide 11 as
58.6, 38.5, 38.2, 36.0, 33.0, 27.5, 26.3, 23.1, 21.7, 21.1, 20.1, 18.5;
HRMS (ESI) calcd for C15H24O [MþNa]þ 243.1725, found 243.1719.
a colorless oil (5.64 g, 74% over two steps). Rf¼0.4 (silica gel, 1:1
23
hexanes/DCM); [
a]
D þ26.3 (c 1.14, DCM); IR (film) nmax 2958, 2356,
4.1.9. Dihydrojunenol (1). Under a blanket of hydrogen gas (1 atm),
a solution of 17 (1.21 g, 5.5 mmol, 1.0 equiv) in EtOAc (55 mL, 0.1 M)
was stirred with palladium on carbon (0.51 g, 0.55 mmol, 0.1 equiv)
at room temperature for 30 min. The reaction mixture was filtered
through a plug of silica gel topped with Celite (EtOAc elution) to
provide a saturated ketone (ca. 1.4 g). Without further purification,
this ketone was dissolved in anhydrous ethanol (55 mL, 0.1 M)
under argon. Sodium (0.64 g, 27.5 mmol, 5.0 equiv) was added in
five portions and the reaction mixture was stirred at room tem-
perature for 30 min before being quenched with saturated aqueous
NH4Cl solution (50 mL). The quenched mixture was then extracted
with Et2O (3ꢁ100 mL), and the combined organic layers were
washed with brine (50 mL), dried over MgSO4, filtered, and con-
centrated in vacuo. The crude material was purified by filtering
through a plug of silica gel (Et2O elution) to provide dihydrojunenol
(1) as a colorless oil that slowly solidified at room temperature
(1.07 g, 87% over two steps). This natural product had spectroscopic
1829, 1675, 1587, 1559, 1458, 1158, 911 cmꢂ1
CDCl3)
;
1H NMR (500 MHz,
d
5.80–5.86 (m, 1H), 5.05 (dd, J¼17.1, 1.5 Hz, 1H), 5.02 (d,
J¼10.2 Hz, 1H), 2.58–2.63 (m, 1H), 2.45–2.52 (m, 3H), 2.35–2.41 (m,
1H), 2.25–2.28 (m, 1H), 2.14–2.17 (m, 2H), 1.94–1.99 (m, 1H), 1.76–
1.82 (m, 1H), 1.58–1.64 (m, 2H), 0.94 (d, J¼7.0 Hz, 3H), 0.85 (d,
J¼6.8 Hz, 3H); 13C NMR (125 MHz, CDCl3)
d 194.2,168.2,137.9,115.6,
107.8, 52.1, 44.1, 33.7, 32.0, 27.0, 26.4, 22.9, 20.8, 18.6; HRMS (ESI)
calcd for C14H21IO [MþNa]þ 355.0535, found 355.0532.
4.1.7. (S)-2-Isopropyl-8-methylene-3,4,5,6,7,8-hexahydronaph-
thalen-1(2H)-one (10). Monocyclic enone 11 (3.43 g, 10.3 mmol,
1.0 equiv), triphenyl phosphine (0.81 g, 3.1 mmol, 0.3 equiv), sil-
ver carbonate (2.84 g, 10.3 mmol, 1.0 equiv), and Et3N (1.71 mL,
12.3 mmol, 1.2 equiv) were added to MeCN (100 mL, 0.1 M) and
the resulting mixture was thoroughly degassed with argon. Pal-
ladium (II) acetate (0.23 g, 1.03 mmol, 0.1 equiv) was then added.
The reaction mixture was heated to 70 ꢀC, and the homogenous,
brown solution was stirred for 3 h under argon before being
cooled back to room temperature. Aqueous HCl solution (1 N,
300 mL) was then added and the mixture was extracted with Et2O
(3ꢁ100 mL). The combined organic layers were washed with brine
(100 mL), dried over MgSO4, filtered, and concentrated in vacuo.
The crude mixture was purified by column chromatography (silica
gel, gradient from 4:1 to 1:2 hexanes/DCM) to provide compound
23
data identical to the previous report.5 Rf¼0.50 (silica gel, DCM); [
a]
D
0 (c 0.70, DCM); lit.5 0 (c 1.01, CHCl3); IR (film) nmax 3261, 2930,1472,
1345, 1160 cmꢂ1
;
1H NMR (500 MHz, CDCl3)
d
3.52 (dt, J¼5.4,
10.1 Hz, 1H), 2.18–2.25 (m, 2H), 1.60–1.66 (m, 2H), 1.41–1.49 (m,
2H), 1.34–1.39 (m, 2H), 1.20–1.32 (m, 3H), 1.03–1.16 (m, 4H), 0.97 (d,
J¼7.5 Hz, 3H), 0.93 (d, J¼7.0 Hz, 3H), 0.89 (s, 3H), 0.86 (d, J¼6.9 Hz,
3H); 13C NMR (125 MHz, CDCl3)
d 69.4, 53.7, 51.9, 44.2, 42.2, 35.0,
33.5, 27.0, 26.5, 21.3, 20.8, 19.0, 17.3, 16.4, 14.7; HRMS (ESI) calcd for
23
10 as a colorless oil (2.02 g, 95%). Rf¼0.5 (silica gel, DCM); [
a
]
C15H28O [MþNa]þ 247.2038, found 247.2038.
D
þ9.4 (c 0.32, DCM); IR (film) nmax 2932, 1675, 1384, 1192 cmꢂ1; 1H
NMR (500 MHz, CDCl3)
d 5.77 (br s, 1H), 5.04 (br s, 1H), 2.28–2.36
4.2. X-ray crystallographic data
(m, 7H), 2.08–2.12 (m, 1H), 1.96–1.98 (m, 1H), 1.73–1.83 (m, 3H),
0.93 (d, J¼6.9 Hz, 3H), 0.88 (d, J¼6.8 Hz, 3H); 13C NMR (125 MHz,
Crystallographic data for 1, and for the p-nitrobenzoyl ester de-
rivatives of 7 and 8 have been deposited with the Cambridge Crys-
tallographic Data Centre. Copies of the data can be obtained free of
charge from http://www.ccdc.cam.ac.uk/products/csd/request/
(CCDC # 743414 for 1, 766068 for the p-nitrobenzoyl ester derivative
of 7, and 766069 for the p-nitrobenzoyl ester derivative of 8).
CDCl3) d 201.2, 156.7, 138.3, 130.9, 113.5, 54.2, 34.5, 32.9, 30.8, 26.6,
23.0, 22.9, 21.1, 19.0; HRMS (ESI) calcd for C14H20O [MþNa]þ
227.1412, found 227.1408.
4.1.8. (2S,4aR)-2-Isopropyl-4a,8-dimethyl-3,4,4a,5,6,7-hexahy-
dronaphthalen-1(2H)-one (17). In
a flame-dried round-bottom
flask charged with argon, a solution of 10 (1.97 g, 9.6 mmol,
1.0 equiv) in DCM (96 mL, 0.1 M) was cooled to 0 ꢀC. A freshly
prepared Et2O solution of lithium dimethylcuprate33 (0.50 M,
29 ml, 1.5 equiv) was added in one portion. The reaction mixture
was stirred for 4 h before being quenched with saturated aqueous
NH4Cl (100 mL). The mixture was extracted with Et2O (3ꢁ200 mL),
and then the combined organic layers were washed with brine
(50 mL), dried over MgSO4, filtered, and concentrated in vacuo. The
crude material was purified by column chromatography (silica gel,
gradient from 4:1 to 1:1 hexanes/DCM). The first fraction provided
Acknowledgements
We thank Dr. D.-H. Huang and Dr. L. Pasternack for NMR spec-
troscopic assistance, Dr. G. Siuzdak for mass spectrometric and Dr.
A. Rheingold (UCSD) for X-ray crystallographic assistance. Financial
support for this work was provided by The Scripps Research In-
stitute and Bristol-Myers Squibb.
References and notes
23
the desired compound 17 (1.21 g, 56%). Rf¼0.5 (silica gel, DCM); [
þ106 (c 0.22, DCM); IR (film) nmax 2931, 2358, 2341, 1684, 1653,
1540, 1457, 1106 cmꢂ1 1H NMR (500 MHz, CDCl3)
2.28–2.36 (m,
a]
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1H), 2.08–2.12 (m, 1H), 1.99–2.02 (m, 2H), 1.86–1.91 (m, 1H), 1.68 (s,
3H), 1.54–1.75 (m, 6H), 1.36–1.42 (m, 1H), 0.94 (s, 3H), 0.94 (d,
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d 210.7, 139.1, 138.1,

4744
K. Chen et al. / Tetrahedron 66 (2010) 4738–4744
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