A straightforward diastereoselective synthesis and evaluation of climacostol, a natural product with anticancer activities

Article abstract of DOI:10.1055/S-0029-1218695

On the basis of continued interest in plant-derived natural products as anticancer agents, a shorter and more efficient synthesis of climacostol is reported. This compound showed an anticancer activity better than that of the natural product. The improved

Full text of DOI:10.1055/S-0029-1218695

1550
PAPER
A Straightforward Diastereoselective Synthesis and Evaluation of Climacostol,
A Natural Product with Anticancer Activities
D
iastereoselective
e
Synthesis
o
n
f
Climacosto
n
l
is Fiorini,a Sandra Giuli,a Enrico Marcantoni,*a Luana Quassinti,b Massimo Bramucci,b Consuelo Amantini,c
Giorgio Santoni,c Federico Buonanno,d Claudio Ortenzi*d
a
School of Sciences and Technologies, Section of Chemistry, University of Camerino, via S. Agostino 1, 62032 Camerino (MC), Italy
Fax +39(0737)402297; E-mail: enrico.marcantoni@unicam.it
School of Pharmacy, Physiology, University of Camerino, via Gentile III da Varano, 62032 Camerino (MC), Italy
School of Pharmacy, Anatomy Unit, University of Camerino, via Scalzino 3, 62032 Camerino (MC), Italy
b
c
d
Department of Educational Sciences and Training, University of Macerata, P. le Bertelli 1, 62100 Macerata, Italy
E-mail: claudio.ortenzi@unimc.it
Received 28 December 2009; revised 30 December 2009
Dedicated to Professor Saverio Florio, University of Bari, on the occasion of his 70th birthday
have been reported.2 The chemical structure of these com-
Abstract: On the basis of continued interest in plant-derived natu-
pounds contains resorcinol (1,3-dihydroxybenzene) as the
ral products as anticancer agents, a shorter and more efficient syn-
core, equipped with an odd-numbered carbon chain sub-
stituent, with various degrees of unsaturation (1,
thesis of climacostol is reported. This compound showed an
anticancer activity better than that of the natural product. The im-
proved potency and selectivity can be due to the absence of traces Figure 1).
1,3-Dihydroxy-5-[(Z)-non-2¢-enyl]benzene
of the undesired E-isomer present in the natural climacostol. Fur-
[climacostol (2), Figure 1] is a rare example of a 5-alke-
nylresorcinol having a Z-configured carbon–carbon dou-
ble bond5 at the 2¢-position.
thermore, the versatile strategy developed for this simple molecule
such as climacostol could aid in the synthesis of other more com-
plex natural products.
Key words: bioorganic chemistry, C–C bond formation, diastereo-
selectivity, natural products, protecting groups, Wittig reaction
OH
OH
HO
R
HO
climacostol (2)
To assure the quality of drugs, impurities must be moni-
tored carefully. Impurities in an API (Active Pharmaceu-
tical Ingredient) that include synthetic by-products must
be limited to very small amounts, and are preferably sub-
stantially absent. Availability by synthesis of an authentic
material can allow toxicological studies and provide a
standard for routine monitoring of the drug product.1
5-alkenylresorcinols (1)
R = alkenyl
Figure 1 Structure of 5-alkenylresorcinols
Climacostol was isolated from the freshwater ciliated pro-
tozoan Climacostomum virens, which uses it as chemical
weapon against unicellular and/or multicellular preda-
tors.6 To date, this toxin represents the first example of a
resorcinol isolated from protozoa, and its biological activ-
ity was investigated on free-living ciliates and inverte-
brates, and more recently, on cancer cell lines.7 In
particular, climacostol was preliminarily evaluated for its
effect on human squamous carcinoma (A431) and human
promyelocytic leukemia (HL60) cell lines. In this study,
we extend the screening of the cytotoxic and pro-apoptot-
ic effect of climacostol to additional cancer cell lines, in
order to obtain a more exhaustive picture of the toxin glo-
bal activity spectrum and, consequently, to better study its
use in vivo research, and to evaluate its potential in cancer
chemotherapy. Therefore, since large amounts of the tox-
in are necessary for biological test, and considering that
the extant available synthetic methods have been poorly
amenable to a large scale preparation of this compound,
new synthetic strategies to the preparation of climacostol
were required. To maintain the structural integrity of the
compounds that display anticancer biological activities
such as alkenylresorcinol 2, prevention of isomerization
of the Z-configuration is mandatory. To achieve this goal,
The 5-alkenylresorcinols are a family of resorcinol lipids
of remarkable interest because of their structure, biogene-
sis, mechanism of action, and potent biological activities,2
which include antiparasitic, anticancer, anti-inflammato-
ry,3 and antioxidant properties.4 Although resorcinolic lip-
ids are being found in an increasing number of organisms,
the study of their biological activity, physiological role,
and participation in the regulation of metabolic processes
has not resulted in a complete understanding of their bio-
logical importance.
Because of the ubiquitous occurrence of these alkenylre-
sorcinols and their potential practical applications, there is
an intrinsic interest in determining the role of these mole-
cules in human life as well as in the environment. From
the initial discovery of alkenylresorcinols and the subse-
quent determination of their alkenyl configuration, a
plethora of studies exploring the structure-activity rela-
tionship (SAR) of alkenylresorcinol-based compounds
SYNTHESIS 2010, No. 9, pp 1550–1556
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Advanced online publication: 09.03.2010
DOI: 10.1055/s-0029-1218695; Art ID: Z28309SS
© Georg Thieme Verlag Stuttgart · New York

PAPER
Diastereoselective Synthesis of Climacostol
1551
various methodologies have been adopted for furnishing tecting group was an important objective for a convergent
predominantly the Z-isomer of the final climacostol. synthetic strategy of the climacostol. Thus, as a continua-
However, none of the procedures displayed to date furnish tion of our work on the synthesis of molecule targets with
exclusively the desired Z-isomer.
biological activity,11 we report herein a simple and con-
vergent synthetic approach for climacostol (2).
The carbon–carbon double bond is a structural feature on
the border of framework and functionality, and the Wittig A very broadly applicable strategy for the carbon–carbon
olefination along with the Diels–Alder and the aldol reac- double bond formation involves b-substituted sulfones,
tions is one of the most important reactions of all time. and the Julia–Kocienski olefination is operationally sim-
Generally, in Wittig reaction there is an equilibrium be- ple and enables for the straightforward assembly of ad-
tween reactants and products so that only the more stable vanced and functionalized intermediates in the course of
of the two alkenes is produced. Thus, selectivity in the total synthesis.12 Thus, as an alternative route to acetylen-
course of the construction of an organic molecule target ic methods for deriving 5-alkenyl resocinol double bonds
can be achieved either by the choice of reagents (reagent in the Z-configuration, we turned to the Julia–Kocienski
control) or, less frequently, by the substrate (substrate protocol13 for installing the Z-double bond in the alkenyl
control). One way to enforce the substrate–reagent inter- chain (Scheme 1). Sulfone 5, required for coupling with
actions of a chemical transformation8 is to employ sub- heptanal was easily prepared by a one-pot reaction from
strate-bound reagent-directing groups.9 Herein we report the corresponding primary alcohol. Commercially avail-
the use of this concept in the completion of a diastereo- able phenylacetic acid 3 was reduced to the corresponding
selective total synthesis of climacostol (2).
alcohol 4 using BH3·SMe2,14 and then converted to the pri-
mary sulfone 5 by treatment with N-bromosuccinimide,
triphenylphosphine, and sodium phenylsulfinate under
mild reaction conditions.15 Addition of the lithium deriva-
tive of 5 to heptanal gave an adduct, which was acetylated
in situ to afford a mixture of diastereomeric b-acetoxysul-
fone 6 in good yield. Compound 6 readily underwent b-
elimination upon exposure to powdered NaOH in 1,4-di-
oxane at room temperature delivering vinyl sulfone 7 as a
single stereoisomer.16 Unfortunately, the last step of the
Julia sequence, that is, the desulfination of compound 7 by
exposure to Na2S2O4 and NaHCO3 in a mixture of water–
Despite there being a known procedure for the synthesis
of 2,10 from a practical context there remained consider-
able room for improvement in the synthetic methodology.
Drawbacks included in particular the length of the syn-
thetic route, and the isomerization of the double bond
(D2¢,3¢ → D1¢,2¢) during the removal of hydroxy protecting
group of the 1,3-dihydroxybenzene moiety. Therefore, a
new strategy for obtaining 2 was needed, that would in-
corporate an improved stereoselectivity. Within such a
scheme, the development of an efficient choice of O-pro-
OMe
OMe
O
Me2S•BH3
1. NBS, Ph3P
2. PhSO2Na
THF, 83%
MeO
OH
MeO
OH
NaI, 78%
3
4
OMe
OMe
SO2Ph
SO2Ph
1. n-BuLi, THF, –78 °C
C6H13CHO, –40 °C
MeO
MeO
2. Ac2O, DMAP, 83%
OAc
88%
6
5
NaOH, r.t.
1,4-dioxane
OMe
SO2Ph
MeO
7
Na2S2O4
NaHCO3
H2O–EtOH (1:1)
reflux
OMe
OMe
MeO
MeO
9
8
Scheme 1 Julia–Kocienski method for climacostol (2)
Synthesis 2010, No. 9, 1550–1556 © Thieme Stuttgart · New York

1552
D. Fiorini et al.
PAPER
OH
ethanol (1:1), gave a mixture of isomeric O-protected alk-
enylresorcinols 8 and 9, which were inseparable by com-
mon chromatographic methods.10d After numerous
unsuccessful attempts to selectively replace the phenyl-
sulfonyl group with hydrogen, it became clear that this
procedure would not be effective for producing climacos-
tol. Thus, we proceeded to modify our synthetic strategy
in order to furnish the molecular target without the un-
wanted isomerization of the carbon–carbon double bond.
HO
2
OP
OP
O
O
Retrosynthetic analysis revealed a route based on the con-
vergent approach outlined in Scheme 2. We surmised that
the Z-configured carbon–carbon double bond could be in-
troduced by a Wittig olefination from an aldehyde and an
alkylphosphonium salt. Aldehyde 10 could be derived
from commercially available methyl O-protected (3,5-di-
hydroxyphenyl)acetate 11. Several years ago, Mori et al.
reported a four-step synthesis of climacostol (2) based on
a Wittig reaction using a tert-butyldimethylsilyl-protected
3,5-dihydroxyphenylacetaldehyde.10b However, our tenta-
tive silylalkyl deprotection furnished a mixture richer in
the undesired E-isomer than the 5% reported by the au-
thors, and indeed, it is known that tetrabutylammonium
fluoride causes isomerization of the double bond geome-
try.17 For this reason, we decided to utilize the knowledge
acquired during our study of protecting groups in the
chemoselective transformations of polyhydroxylated
compounds,18 toward the preparation of climacostol (2).
After a number of unsuccessful attempts,19 the meth-
oxymethyl (MOM) group was found to be suitable for the
synthetic problem in hand, and climacostol (2) was syn-
thesized in four steps from commercially available methyl
(3,5-dihydroxyphenyl)acetate (12). As outlined in
Scheme 3, treatment of 12 with an excess of chloromethyl
methyl ether,20 afforded the bis-MOM protected 3,5-dihy-
droxyacetate 13 in 86% of yield after chromatography.21
Subsequent reduction of the ester with diisobutylalumi-
num hydride in toluene at –78 °C released the correspond-
ing deprotected aldehyde 14 in high yield. Next, the high
reactivity of the aldehyde function was exploited in a Wit-
tig olefination with n-heptylidenetriphenylphosphorane,
giving the Z-alkene 15 as the only diastereoisomer (NMR
spectra and direct HPLC analysis) in 79% yield. The ylide
used in the Wittig reaction was obtained by reaction of so-
dium hexamethyldisilazide in THF with n-heptyltriphe-
nylphosphonium bromide, which is not commercially
available, but easily prepared by a known literature proce-
dure.22
PO
H
PO
OR
10
11
Scheme 2 Retrosynthesis of climacostol (2)
OH
MOMO
MeOCH2Cl
i-Pr2NEt, CH2Cl2
O
O
86%
HO
OMe
MOMO
OMe
13
12
DIBAL-H
toluene 74%
–78 °C
MOMO
O
H
MOMO
14
[n-C7H15PPh3]Br
NaHDMS
THF, 0 °C
OMOM
79%
MOMO
15
PTSA, r.t.
CH2Cl2–EtOH (1:1)
98%
OH
HO
2
Scheme 3 Synthesis of climacostol (2)
(aq 6 M HCl)24 led to decomposition of the resorcinol
framework, while other methods commonly used for this
deprotection were also unsuccessful (LiBF4,25 CBr4/i-
PrOH,26 PPTS/t-BuOH,27 20% aq AcOH28). To overcome
the problems encountered in the attempted MOM-depro-
tection,20 we reasoned that the substrate could be unstable
in hydrolytic conditions, and that p-toluenesulfonic acid
(PTSA) in CH2Cl2–methanol solution29 could represent an
effective alternative to the commonly used trifluoroacetic
acid (TFA) in CH2Cl2. It is known30 that residual TFA in
the deprotected product is rather difficult to completely
remove, and its presence in a biology essay could interfere
with the results, because of its toxicity against various
cells. Therefore, nonaqueous conditions [excess PTSA,
CH2Cl2–methanol (1:1), r.t.] successfully afforded target
resorcinol-based climacostol (2) in quantitative yield. Pu-
Having solved the olefin selectivity problem, and with the
Z-isomer in hand, we turned our attention to the removal
of the MOM-protecting group in order to obtain the alke-
nylresorcinol target 2. Following the literature for selec-
tive MOM-deprotection in the presence of a Z-olefin
moiety, adduct 15 was treated with CeCl3·7H2O/NaI com-
bination,11d and with TMSCl and Bu4NBr in CH2Cl2.23
However, in both instances the expected product was ob-
served in low yield in addition to a mixture of products in-
cluding some containing an isomerized double bond.
Standard cleavage conditions of the MOM-protections
Synthesis 2010, No. 9, 1550–1556 © Thieme Stuttgart · New York

PAPER
Diastereoselective Synthesis of Climacostol
1553
rification of the crude product by normal phase column sis of alkenylresorcinols, and this strategy has been effi-
chromatography was not feasible, since the climacostol ciently applied to the preparation of climacostol (2), a
molecule tends to adsorb irreversibly to silica gel, and it natural and potential therapeutic agent for several types of
could not be recovered even when using strong eluents.31 cancers. The strategy introduced herein has three key
Fortunately, our product did not require further purifica- points which make it considerably more useful than the
tion, and the structure of 1,3-dihydroxy-5-[(Z)-non-2¢- previously reported sequences: (i) the synthetic route is
enyl]benzene (2) was established by spectral means. In brief and conveniently produces the target compound
particular, 1H NMR spectrum showed that the proton sig- from easily accessible starting materials, (ii) the desired
nals H-2¢ and H-3¢ were not well separated. Thus, it was Z-configured natural product has been prepared in up to
impossible to unambiguously assess the structure of 2 by 63% overall yield, and (iii) without the presence of the E-
conventional 2D NMR methods such as COSY and NOE- isomer or the rearranged D1¢,2¢-isomer.
SY because it is known that they fail when key proton sig-
In summary, a novel and significantly improved method
for the convergent synthesis of climacostol (2) has been
developed. An alternative, robust linear approach allow-
nals are poorly resolved.32 The following stereochemical
study of 2 was directed to confirm the stereochemistry at
C-2¢–C-3¢ double bond. The absence of the strong 970
ing selective formation of the desired molecule target 2
cm–1 band for the E-geometry in IR spectrum suggested
via aldehyde 14 has been described. Evaluation of these
the Z-geometry for the disubstituted double bond.33
A
methods for the general synthesis of alkenylresorcinols is
under way and will be presented elsewhere.
characteristic electron ionization mass spectrum was ob-
served with a notable molecular ion at m/z 234, and a base
fragment at m/z 124, due to McLafferty rearrangement of
the phenolic ring, and another minor fragment at m/z 123,
due to the dihydroxytropylium ion formed by direct b-
cleavage, and at m/z 137 due to g-cleavage.34 The m/z
abundance ratio of 123/124 is ca 1:5, which is in agree-
ment with a meta-dihydroxy substitution of the benzene
ring.35 But, above all, the odd-electron ion m/z 124 in con-
junction with m/z 137 has been a good diagnostic for the
presence of a double bond between carbons C-2¢ and C-
3¢.36 Thus, in our conditions for accomplishing the conver-
gent synthesis of 2, isomerization of the carbon–carbon
double bond (D2¢,3¢ → D1¢,2¢) was not observed.
The cytotoxic effect of synthetic climacostol on prostatic
adenocarcinoma (PC-3), glioblastoma (T98G and
U87MG), and endothelial (EA.hy926) human cell lines
was evaluated by MTT assay.37 As shown in Table 1, a
maximum cytotoxic effect was observed for cancer cell
lines within the 40–60 mM range of climacostol, whereas
no loss of viability was observed for EA.hy926 cells with
toxin concentration of <100 mM. In particular, climacostol
showed significant activity against PC-3, T98G, and
U87MG cells, with IC50 values of 11.47, 15.14, and 19.74
mM, respectively, compared to calculated 213.20 mM for
the EA.hy926 cells. Since climacostol displayed negligi-
ble cytotoxicity against EA.hy926 cells, no further inves-
tigations were performed on this cell line.
All air-sensitive reactions are carried out in flame dried glassware
under an atmosphere of dry N2. Solvents were distilled under N2,
THF, Et2O, and pentane from sodium benzophenone ketyl and
CH2Cl2 from CaH2. Solutions were evaporated under reduced pres-
sure with a rotary evaporator and the residue was chromatographed
on a Baker silica gel (230–400 mesh) column using a 30% EtOAc
in hexane as the eluent. Analytical TLC was performed using pre-
coated glass-backed plates (Merck Kieselgel 60 F254) and visual-
ized by UV light (254 nm) and/or by dipping the plates into Von’s
reagent [1.0 g of Ce(SO4)2 and 24.0 g of (NH4)2MoO4 in 31 mL of
H2SO4 and 470 mL of H2O].
1H NMR spectra were recorded in CDCl3 on Varian Gemini 200 or
Varian 400 spectrometers and are reported as follows: chemical
shift, d (ppm) [multiplicity, coupling constant J (Hz), and number
of protons]. Residual protic solvent CHCl3 (dH = 7.26) was used as
the internal reference. 13C NMR spectra were recorded in CDCl3 at
50 MHz or 100 MHz on Varian Gemini 200 or Varian 400 spec-
trometers, using the central resonance of CDCl3 (dC = 77.0) as the
internal reference. IR spectra were recorded on PerkinElmer FTIR
Paragon 500 spectrometer using thin films on NaCl plates. Only the
characteristic peaks are quoted. Mass spectra were recorded on a
Hewlett-Packard 5988 gas chromatograph with a mass-selective de-
tector MSD HP 5790 MS, utilizing electron ionization (EI) at an
ionizing energy of 70 eV. A fused silica column (30 m × 0.25 mm
HP-5; cross-linked 5% PhMe siloxane, 0.10 HM film thickness)
was used with a helium carrier flow of 30 mL/min. The temperature
of the column was varied, after a delay of 3 min from the injection,
from 65 to 300 °C with a slope of 15 °C min–1.
Human T98G glioblastoma multiforme cell line (catalogue code
CRL-1690) and human U87MG glioblastoma, astrocytoma cell line
(catalogue code HTB-14) were purchased from the American Type
Culture Collection (ATCC) and were maintained in Eagle’s mini-
mum essential medium (EMEM) with 2 mM L-glutamine, 0.1 mM
nonessential amino acids, 1 mM sodium pyruvate, 100 IU/mL pen-
icillin, 100 mg/mL streptomycin, and supplemented with 10% heat
inactivated fetal bovine serum (HI-FBS). Human PC-3 prostatic ad-
enocarcinoma cell line (catalogue number 90112714) was pur-
chased from the European Collection of Animal Cell Culture
(ECACC), maintained in Coon’s modified Ham’s F12 medium with
2 mM L-glutamine, 100 IU/mL penicillin, 100 mg/mL streptomycin,
and supplemented with 7% HI-FBS. Human noncancer EA.hy926
umbilical vein endothelial cell line (ATCC; catalogue code CRL-
2922) was maintained in Dulbecco modified Eagle’s medium sup-
plemented with 10% HI-FBS, 2% hypoxanthineaminopterin-thymi-
Table 1 Cytotoxic Activity Data for Synthetic Climacostol (2)
Cell line
PC-3
IC50 (mM)
11.47
IC100 (mM)
44.67
T98G
15.14
45.71
U87MG
EA.hy926
19.74
56.23
>200.00
400.00
Following the universal convergent scheme described in
this work, we have found an efficient route to the synthe-
Synthesis 2010, No. 9, 1550–1556 © Thieme Stuttgart · New York

1554
D. Fiorini et al.
PAPER
dine, 2 mM HEPES, 2 mM L-glutamine, 100 IU/mL penicillin, and
100 mg/mL streptomycin. Constituents and supplements of growth
media were purchased from Sigma (St. Louis, MO, USA). All cells
were grown in a humidifier of 5% CO2 in air at 37 °C.
indicated that no substrate 5 remained (2 h). The mixture was
quenched by the addition of sat. aq NH4Cl (10 mL). A standard
workup by extraction with Et2O (3 × 20 mL) and washing the com-
bined Et2O extracts with H2O (2 × 20 mL) and brine (2 × 20 mL)
gave a viscous oil in 83% crude yield, which was used directly with-
out purification in the next step.
2-(3,5-Dimethoxyphenyl)-1-ethanol (4)
Caution! Me2S complexed with borane may release excess of Me2S
during the course of the reaction (stench!). This experiment should
be carried out in a well-ventilated hood.
(Z)-1-(3,5-Dimethoxybenzyl)hept-1-enylphenylsulfone (7)
NaOH (35 mg, 0.88 mmol) was added to a solution of compound 6
(0.20 g, 0.44 mmol) in 1,4-dioxane (5 mL). The mixture was stirred
at r.t. for 1 h (until no starting material remained, as monitored by
TLC). The mixture was diluted with Et2O (20 mL) and treated with
H2O (10 mL). The organic layer was separated, and the aqueous lay-
er was extracted with Et2O (3 × 25 mL). The combined organic ex-
tracts were washed with brine (2 × 20 mL), dried (Na2SO4), and
evaporated to give the vinyl sulfone 7, which was purified by flash
chromatography (hexanes–EtOAc, 8:2); yield: 0.31 g (88%); pale
yellow oil.
In a 50 mL, two-necked, round-bottomed flask mounted with a
short reflux condenser was placed a solution of 2-(3,5-dimethoxy-
phenyl)acetic acid (3; 0.47 g, 2.42 mmol) in anhyd THF (15 mL).
To this was added BH3·SMe2 (1.23 mL of a 2 M solution in THF)
dropwise at r.t. The resulting mixture was stirred until the disap-
pearance of acid (4 h) on TLC and GC diagnosis. The mixture was
treated with anhyd MeOH (5.20 mL) and the resulting solution was
concentrated to give a colorless oil, which was purified by column
chromatography on silica gel (eluent: hexanes–EtOAc–MeOH,
6:3.5:0.5) affording alcohol 4 in 83% yield (0.36 g); colorless oil.
IR (neat): 3060, 1635, 1440, 1140, 1032 cm–1.
IR (neat): 3392, 1600, 1144 cm–1.
1H NMR (200 MHz, CDCl3): d = 0.82 (t, J = 6.60 Hz, 3 H), 1.20–
1.28 (m, 8 H), 2.90–2.97 (m, 2 H), 3.15–3.29 (m, 2 H), 3.78 (s, 6 H),
6.90 (t, J = 7.0 Hz, 1 H), 6.30–6.39 (m, 3 Harom), 7.40–7.82 (m, 5
1H NMR (400 MHz, CDCl3): d = 1.80 (br s, 1 H, OH), 2.80 (t,
J = 6.60 Hz, 2 H,), 3.78 (s, 6 H), 3.83 (t, J = 6.60 Hz, 2 H), 6.33–
6.40 (m, 3 Harom).
13C NMR (100 MHz, CDCl3): d = 40.3, 56.2, 64.1, 98.7, 107.3,
143.2, 160.4.
Harom).
13C NMR (50 MHz, CDCl3): d = 13.8, 22.0, 27.9, 28.5, 28.9, 31.4,
33.9, 54.2, 105.4, 113.0, 127.9, 129.3, 134.2, 140.5, 142.9, 145.2,
149.7.
MS (EI): m/z = 164 [M+ – H2O], 151 (100), 137, 112, 69, 51, 43, 39.
Anal. Calcd for C23H30O4S: C, 68.62; H, 7.51; S, 7.97. Found: C,
68.59; H, 7.54; S, 7.95.
Anal. Calcd for C10H14O3: C, 65.91; H, 7.74. Found: C, 65.88; H,
7.70.
Methyl 2-[3,5-Di(methoxymethoxy)phenyl]acetate (13)
1,3-Dimethoxy-5-[2-(phenylsulfonyl)ethyl]benzene (5)
To a stirred solution of methyl (3,5-dihydroxyphenyl)acetate (12;
0.50 g, 2.94 mmol) in CH2Cl2 (60 mL) were added i-Pr2NEt (20 mL,
11.5 mmol) and chloromethyl methyl ether (8.95 mL, 11.76 mmol)
dropwise at 0 °C under N2. The mixture was warmed to r.t. and
stirred overnight. CH2Cl2 was removed in vacuum, the residue tak-
en up in Et2O (45 mL), washed with 10% aq HCl (2 × 30 mL), H2O
(2 × 30 mL), aq 10% NaOH (2 × 30 mL), and brine (45 mL), dried
(MgSO4), and concentrated at reduced pressure. The residue was
purified by silica gel column chromatography (hexanes–EtOAc,
8:2) to give 13 (0.68 g, 86%) as a colorless oil.
IR (neat): 3005, 1730, 1605, 1200, 840 cm–1.
1H NMR (400 MHz, CDCl3): d = 3.40 (s, 6 H), 3.62 (s, 2 H), 3.69
(s, 3 H), 5.15 (s, 4 H), 6.60–6.65 (m, 3 Harom).
13C NMR (100 MHz, CDCl3): d = 41.0, 51.8, 57.2, 92.3, 98.7,
105.5, 139.0, 157.9, 172.6.
To a stirred mixture of alcohol 4 (0.27 g, 1.52 mmol) and Ph3P (0.64
g, 2.43 mmol) in anhyd DMF (7.6 mL) under argon was added at
0 °C N-bromosuccinimide (0.43 g, 2.43 mmol) in small portions
over 10 min. The mixture was stirred for 2 h at r.t. To this mixture
was added a mixture of PhSO2Na (0.5 g, 3.04 mmol) and NaI (0.023
g, 0.152 mmol) in 3 portions over 10 min. The resulting mixture was
heated at 70 °C for 20 h, and then diluted with EtOAc (15.2 mL) and
3% aq Na2S2O3 (15.2 mL). The layers were separated and the aque-
ous phase was extracted with EtOAc (3 × 20 mL). The combined
organic extracts were successively washed with H2O (2 × 20 mL)
and brine (2 × 20 mL), and dried (Na2SO4). After filtration and con-
centration under reduced pressure, the residue was chromato-
graphed on silica gel (hexanes–EtOAc, 6:4) to give 0.36 g (78%) of
sulfone 5; colorless oil.
IR (neat): 3090, 1440, 1150 cm–1.
1H NMR (200 MHz, CDCl3): d = 2.91–3.05 (m, 2 H), 3.29–3.78 (m,
MS (EI): m/z (%) = 270 [M+], 238, 211, 121, 45 (100).
2 H), 3.83 (s, 6 H), 6.22–6.30 (m, 3 Harom), 7.50–7.95 (m, 5 Harom).
Anal. Calcd for C13H18O6: C, 57.77; H, 6.71. Found: C, 57.75; H,
6.68.
13C NMR (50 MHz, CDCl3): d = 32.3, 50.2, 54.7, 100.7, 110.8,
128.4, 129.2, 133.3, 138.3, 140.4, 163.9.
2-[3,5-Di(Methoxymethoxy)phenyl]acetaldehyde (14)
A solution of DIBAL-H in toluene (1.0 M, 2.21 mL, 2.21 mmol)
was added to a stirred and cooled solution of 13 (0.50 g, 1.85 mmol)
in anhyd toluene (10 mL) at –78 °C under N2. The reaction mixture
was allowed to warm to –60 °C while stirring for 3 h, then cooled to
–78 °C again, and afterwards quenched with MeOH (5 mL). The
mixture was filtered through Celite, and the resulting solid was
washed with Et2O (75 mL). The filtrate and washings were succes-
sively washed with H2O (2 × 20 mL) and brine (2 × 20 mL), dried
(MgSO4), and concentrated under reduced pressure. The residue
was purified by column chromatography on silica gel (EtOAc–hex-
anes, 8:2) to give the title compound 14 (0.42 g, 94%) as a pale yel-
low oil.
Anal. Calcd for C16H18O4S: C, 62.72; H, 5.92; S, 10.47. Found: C,
62.70; H, 5.88; S, 10.42.
1-[2-(3,5-Dimethoxyphenyl)-1-(phenylsulfonyl)ethyl]hexyl Ace-
tate (6)
A 1.6 M solution of n-BuLi in hexane (0.72 mL, 1.15 mmol) was
added slowly to a solution of phenyl sulfone 5 (0.29 g, 0.96 mmol)
in anhyd THF (10 mL) at –78 °C. After stirring at the same temper-
ature for 30 min, the reaction mixture was allowed to warm to
–40 °C, while a yellow color developed. To this mixture was trans-
ferred by cannula a THF (5 mL) solution of heptanal (0.13 g, 1.140
mmol), followed after 2 h, by an excess of Ac2O (0.16 mL, 1.71
mmol) and a catalytic amount of DMAP (7.36 mg, 0.06 mmol). The
resulting mixture was warmed at r.t. and then left to stir until TLC
IR (neat): 3006, 2822, 1735, 1598, 1190, 880 cm–1.
Synthesis 2010, No. 9, 1550–1556 © Thieme Stuttgart · New York

PAPER
Diastereoselective Synthesis of Climacostol
1555
1H NMR (200 MHz, CDCl3): d = 3.60 (d, J = 2.1 Hz, 2 H), 5.75 (s,
4 H), 6.30–6.35 (m, 3 Harom), 9.57 (d, J = 2.1 Hz, 1 H).
5.38–5.58 (m, 2 H), 6.11 (br s, 2 H), 6.19 (t, J = 2.2 Hz, 1 H), 6.29
(d, J = 2.1 Hz, 2 H).
13C NMR (50 MHz, CDCl3): d = 52.6, 56.6, 92.7, 99.3, 104.6,
138.7, 158.6, 200.0.
13C NMR (50 MHz, CDCl3): d = 14.3, 22.8, 27.4, 29.2, 29.8, 31.9,
33.5, 100.7, 108.2, 127.57, 131.6, 144.6, 156.9.
MS (EI): m/z (%) = 240 [M+], 180, 152, 123, 45 (100).
MS (EI): m/z (%) = 234 [M+], 163, 137, 124 (100), 123, 107, 91, 69.
Anal. Calcd for C12H16O5: C, 59.99; H, 6.71. Found: C, 59.97; H,
6.70.
Anal. Calcd for C15H22O2: C, 76.88; H, 9.46. Found: C, 76.86; H,
9.45.
n-Heptyltriphenylphosphonium Bromide
Acknowledgment
To a solution of 1-bromoheptane (25 mmol) in toluene (50 mL) was
added Ph3P (7.20 g, 27.5 mmol). After refluxing for 48 h, the reac-
tion mixture was cooled to r.t. and the solvent was removed under
reduced pressure. The crude product was dissolved in CH2Cl2 (15
mL), then added dropwise to Et2O (75 mL). After stirring for 1 h,
the precipitate was filtered and dried under vacuum affording the
title compound in pure form (10.35 g, 96%); white crystals; mp
165 °C.
This work was supported by funds from the Universities of Came-
rino and Macerata, and from the MIUR, Rome (PRIN 2006).
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Synthesis 2010, No. 9, 1550–1556 © Thieme Stuttgart · New York

1556
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Synthesis 2010, No. 9, 1550–1556 © Thieme Stuttgart · New York