Enantiospecific total synthesis of the squalene synthase inhibitors (-)-CJ-13,982 and its enantiomer from a common intermediate

Article abstract of DOI:10.1038/ja.2017.127

The total syntheses of both the natural and unnatural enantiomers of the alkyl citrate natural product CJ-13,982 (1) from the common d-ribose-derived acid 6 are described.

Full text of DOI:10.1038/ja.2017.127

The Journal of Antibiotics (2017), 1–6
2017 Japan Antibiotics Research Association All rights reserved 0021-8820/17
www.nature.com/ja
&
ORIGINAL ARTICLE
Enantiospecific total synthesis of the squalene
synthase inhibitors (–)-CJ-13,982 and its enantiomer
from a common intermediate
Dayna Sturgess, Zongjia Chen, Jonathan M White and Mark A Rizzacasa
The total syntheses of both the natural and unnatural enantiomers of the alkyl citrate natural product CJ-13,982 (1) from the
common D-ribose-derived acid 6 are described.
The Journal of Antibiotics advance online publication, 25 October 2017; doi:10.1038/ja.2017.127
INTRODUCTION
the β-OPMB group in high diastereoselectivity. Ester 8 was then
The alkyl citrate family of natural products contain a common 2-alkyl converted into the lactone 5 in a seven-step sequence.11
citrate moiety with a wide variety of side chains and oxidation levels.1
These metabolites are inhibitors of squalene synthase, the enzyme
responsible for the first pathway-specific step in cholesterol biosynth-
esis. Some of the simplest examples include CJ-13,982 (1) and
CJ-13,981 (2) (Figure 1), which contain an unsubstituted alkyl or
alkenyl group, and were isolated from an unidentified fungus CL15036
in 2001.2 A more complex example is the alkyl citrate L-731,120 (3),3
which was isolated from the same fungal species as zaragozic acid A
(4),4–8 a picomolar inhibitor of squalene synthase. Recently, L-731,120
(3) has also been identified as a key intermediate in the biosynthesis
zaragozic acid A (4).9 The absolute configuration at C12 in L-731,120
(3) is inferred from its biosynthetic relationship to 4.
The synthesis of alkyl citrate natural products is challenging due to
the high oxidation level and the need for stereocontrol of the two
contiguous asymmetric centres.1 To date, only total syntheses of the
unnatural enantiomers (+)-CJ-13,982 (ent-1) and (+)-CJ12,981 (ent-
2) have been reported utilising an anti-Evans aldol reaction followed
by a stereoselective Seebach self retention of a stereocentre (SRS)
alkylation as the key steps to secure the alkyl citrate fragment.10
For the total synthesis of (+)-(2R, 3R)-CJ-13,982 (ent-1), lactone 5
was reduced with DiBALH to give a mixture of the corresponding
lactol and a small amount of diol, which could be oxidised to the lactol
with DMP. Wittig extension of the lactol afforded alkene 9 in 46%
overall yield. Cross metathesis14 with undecene using Grubbs second-
generation catalyst provided the E-alkene 10 as the only stereoisomer.
Hydrogenation then afforded the saturated triester 11, which on
treatment with trifluoroacetic acid (TFA) gave (+)-CJ-13,982 (ent-1),
[α]D25 = +14.1 (c 0.35, acetone) in high yield. This material was
identical to that reported by Barrett.10
Whilst the above route could provide the (–)-1 using the known
enantiomer of 6,11 it is inefficient especially with respect to construct-
ing the contiguous asymmetric centres. Therefore, we envisaged a
more convergent approach which would provide the natural enantio-
mer (–)-(2S,3S)-CJ-13,982 (1) as well as related alkyl citrate natural
products.
As shown in Scheme 2, an alternative approach would be the [3,3]-
sigmatropic rearrangement of the Z-silylketene acetal 12 (also derived
from D-deoxyribose) with the correct side chain attached via the chair-
like transition state shown to afford the alkene 13 with the two new
stereocenters introduced in a steroselective manner in a single C–C
bond-forming step. This is in analogy with our approach to the
bicyclic core of the zaragozic acid C.15 Oxidative cleavage of the alkene
and degradation of the THF ring to introduce the final carboxylic acid
would then secure the alkyl citrate moiety 14.
DISCUSSION
We envisaged an approach to the unnatural enantiomer of CJ-13,982
(1) starting from the known γ-lactone 5, which was utilised by us for
the total synthesis of trachyspic acid11,12 and the proposed structure
for citrafungin A.13 The key step in the synthesis of 5 was the Ireland–
Claisen rearrangement of allyl ester 7, synthesised from D-deoxyribose-
derived acid 6 (Scheme 1). This reaction proceeds without any
observed β-elimination under the standard conditions12 shown to
afford the ester 8. The [3,3]-sigmatropic rearrangement occurs to form
This proposal was tested first using the ester 16 derived from a
coupling between acid 6 and allylic alcohol 15 as shown in Scheme 3.
Exposure of 16 to the standard conditions11,12 (Scheme 1) gave the
alkene 17 as an inseparable ~ 3:1 mixture of diastereoisomers as
1
judged by H NMR integration, presumably differing at the indicated
the new C–C bond on face of the tetrahydrofuran (THF) ring opposite stereocenter. Cleavage of the t-butyldimethylsilyl (TBS) ether induced
School of Chemistry, The Bio21 Institute, The University of Melbourne, Parkville, Victoria, Australia
Correspondence: Professor MA Rizzacasa, School of Chemistry, The University of Melbourne, Bio21 Institute, 30 Flemington Rd, Parkville, Victoria 3010, Australia.
E-mail: masr@unimelb.edu.au
Dedicated to Professor KC Nicolaou.
Received 4 August 2017; revised 11 September 2017; accepted 18 September 2017

Synthesis of a squalene synthase inhibitor
D Sturgess et al
2
HO
HO
HO
lactonisation and subsequent PMB group deprotection gave the
spirolactones 18 and 19 in a 2.9:1 ratio. These compounds were
easily separated by flash chromatography and both were crystalline.
X-ray analysis of each revealed the stereochemistry was as predicted.
Thus, the rearrangement proceeded via the chair-like transition state
as shown in Scheme 2 to afford the desired isomer 18 as the major
product. The minor diaseteroisomer 19, with the incorrect stereo-
chemistry at the stereocenter marked (Scheme 3) probably results
from [3,3]-sigmatropic rearrangement of the corresponding E-silylk-
etene acetal, which is formed as the minor geometric isomer during
the enolisation/silylation reaction.16
CO2H
HO2C
CJ-13,982 (1)
IC50 1.1 µM
CO2H
CO2H
HO2C
HO2C
CJ-13,981 (2)
IC50 2.8 µM
CO2H
CO2H
12
CO2H
Me Me
L-731,120 (3)
The synthesis of the (–)-CJ-13,982 precursor began as shown in
Scheme 4. Ester 21 was formed by dicylohexylcarbodiimide-mediated
coupling of acid 6 and alcohol 20. Ireland–Claisen rearrangement
under the standard conditions11 gave the desired isomer 22 as the
major product along with the minor isomer 23 in a 2.5:1 ratio that
were separable by flash chromatography. The stereochemistry of the
major isomer was assigned in analogy with the initial study (Scheme
3), as well as its subsequent conversion into the target compound.
The total synthesis of (–)-CJ-13,982 (1) is detailed in Scheme 5 and
begins with the conversion of acetal 22 into lactone 24 by hydrolysis
and oxidation. Removal of the PMB ether and mesylation with
concomitant elimination gave the α,β-unsaturated lactone 25 in good
yield. DiBAlH reduction afforded the allylic alcohol 26, which was
protected on the tertiary alcohol via bis-silylation and desilylation of
the primary alcohol to give trimethylsilyl (TMS) ether 27. Alcohol
reduction with alkene transposition using the conditions reported by
Movassaghi and Ahmad17 with the reagent N-isopropylidene-N0-2-
nitrobenzenesulfonyl hydrazine (IPNBSH) gave the diene 28. Protec-
tion of the tertiary alcohol was required for this transformation to
avoid formation of a cyclic ether in the Mitsunobu reaction.
Ozonolysis, oxidation and formation of the t-butyl esters using
dicyclohexyl-t-butylisourea18 gave tri-t-butyl ester ent-11 (the TMS
ether was also removed in this sequence) and a final deprotection
IC50 0.26 µM in vivo ED50 15 mg/kg
O
O
OH
O
OAc
Me Me
HO2C
HO2C
O
Ph
Me
CO2H
OH
Zaragozic Acid A (4)
IC50 0.5 nM
Figure 1 Alkyl citrate natural products 1–4. A full colour version of this
figure is available at the Journal of Antibiotics journal online.
Standard
PMBO
RO2C
conditions
LDA,TMSCl/NEt3
HMPA/THF
PMBO
tBuO2C
OMe
O
–100°C-RT
then NaOH
DTBI
OMe
O
6 R = H
7 R = allyl
tBuO2C
OH
DCC
8
72%
1) DiBALH
74%
ds > 95:5
t
HO
CO2 Bu
ref. 9a
tBuO2C
tBuO2C
tBuO2C
7 steps
2) CH2=PPh3
O
O
t
9
CO2 Bu
5
64%
25
t
HO
CO2 Bu
using TFA afforded (–)-CJ-13,981 (1), [α]D –14.1 (c 0.25, acetone),
which was identical to the natural product.
tBuO2C
(CH2)8CH3
Grubbs II
74%
t
10
CO2 Bu
CONCLUSION
HO
2R
CO2R
3R
H2, Pd/C
92%
We have completed a total synthesis of (+)-CJ-13,981 (ent-1) and the
first synthesis of natural (–)-CJ-13,981 (1) from the common acid 6.
Each approach utilised an Ireland–Claisen rearrangement in the
presence of a β-leaving group. The first involved a sequential
formation of the two stereocenters present in the alkyl citrate whilst
the second more convergent shorter approach introduced both
asymmetric centres in a single C–C bond-forming reaction with good
stereocontrol for the desired configuration. This methodology could
supply all the simple alkyl citrates in a highly convergent manner.
RO2C
CO2R
11 R = tBu
TFA
99%
ent-1 R = H; (+)-CJ-13,982
Scheme 1 Synthesis of (+)-CJ-13,982 (ent-1). A full colour version of this
scheme is available at the Journal of Antibiotics journal online.
‡
OPMB
OPMB
R
EXPERIMENTAL PROCEDURES
General
Standard
Z
conditions
MeO
O
O
1H NMR spectra (600, 500 or 400 MHz) and proton decoupled carbon NMR
spectra (13C NMR, 150, 125 or 100 MHz) were obtained in deuterochloroform
with residual chloroform as internal standard unless otherwise noted. Chemical
shifts are followed by multiplicity, coupling constant(s) (J, Hz), integration and
assignments where possible. Flash chromatography was carried out on silica gel
60. Analytical TLC was conducted on aluminium-backed 2 mm-thick silica gel
60 GF254 and chromatograms were visualised with 20% w/w phosphomolybdic
acid in ethanol or aq. KMnO4. High-resolution MS were obtained by ionising
samples via ESI. Anhydrous THF, Et2O and CH2Cl2 were dried using a solvent
cartridge system. Dry methanol was distilled from magnesium methoxide. All
other solvents were purified by standard methods. Petrol used refers to
O
MeO
O
R = Alkyl
R
OTMS
OTMS
12
H
OPMB
HO
CO2H
3S
[3,3]
CO2TMS
R
HO2C
R
2S
MeO
O
CO2H
14
13
Scheme 2 Convergent approach to simple alkyl citrates. A full colour version
of this scheme is available at the Journal of Antibiotics journal online.
The Journal of Antibiotics

Synthesis of a squalene synthase inhibitor
D Sturgess et al
3
petroleum ether 40–60 °C boiling range. All other commercially available
reagents were used as received.
Tri-tert-butyl ester 9
A solution of DiBAlH in hexanes (0.55 ml, 1 M, 0.55 mmol) was added to
lactone 5 (74 mg, 0.19 mmol) in dry THF (2 ml) at −78 °C and the resulting
solution was allowed to stir at − 78 °C for 5 h. The reaction was quenched with
10% HCl and water followed by the usual workup with EtOAc. Purification by
flash chromatography with 20% EtOAc/petrol yielded lactol (43 mg, 58%) as a
pale yellow oil. Further elution with 20% EtOAc/petrol provided diol (12 mg,
16 %) as a pale yellow oil. [α]D23 –2.74 (c 1.20, CH2Cl2); IR (ATR) νmax 3486,
;
2978, 1730, 1369, 1251, 1153 and 846 cm− 1 1H NMR (500 MHz, CDCl3) δ
1.44 (s, 9H, C(CH3)3), 1.46 (s, 9H, C(CH3)3), 1.48 (s, 9H, C(CH3)3), 1.94 (m,
t
t
t
2H, CH2CHCO2 Bu), 2.79 (m, 3H, CHCO2 Bu and CH2CO2 Bu), 3.62 (m, 1H,
OHCHAHB), 3.70 (dd, J = 11.25 and 5.61 Hz, 1H, OHCHAHB) and 4.19 (s, 1H,
OH); 13C NMR (125 MHz, CDCl3) δ 28.0, 28.2, 28.2, 30.1, 42.0, 50.5, 61.0,
75.4, 81.5, 81.9, 83.1, 169.9, 171.9 and 172.6 ; HRMS (ESI): calculated for
C20H36O8Na [M+Na]+ 427.23024; found 427.23015. Diol (12 mg, 0.03 mmol)
was dissolved in CH2Cl2 (2 ml) and treated with Dess-Martin periodinane
(12.5 mg, 0.03 mmol) for 30 min. Sat. aqueous Na2S2O3 and NaHCO3 were
added and the biphasic mixture stirred for 30 min followed by the usual
workup with EtO2. Column chromatography with 20% EtOAc/petrol eluent
yielded further (10 mg, 83%, total of 53 mg, 72% over two steps) as a pale
yellow oil.
Scheme 3 Ireland–Claisen rearrangement of ester 16. A full colour version
of this scheme is available at the Journal of Antibiotics journal online.
tBuOK (62 mg, 0.549 mmol) was added to methyltriphenylphosphonium
bromide (209 mg, 0.585 mmol) in dry THF (5 ml) at 0 °C. The resulting
solution was stirred at room temperature (RT) for 1 h before cooling to −78 °
C, lactol (47 mg, 0.117 mmol) in dry THF (2 ml) was added and the reaction
stirred at −78 °C for 1 h. The reaction was warmed to −10 °C and stirred for
an additional 2.5 h before quenching with sat. NH4Cl and the usual workup
with Et2O. Flash chromatography with 5% EtOAc/petrol as eluent gave alkene 9
[3,3]
Standard
conditions
OPMB
(CH2)11CH3
HO
(CH2)11CH3
MeO
20
6
O
DCC
DMAP
63%
O
21
74%
20
O
(30 mg, 64%) as a yellow oil. [α]D –4.80 (c 1.05, CH2Cl2); IR (ATR) νmax
2926, 1731, 1369 and 1152 cm− 1; 1H NMR (500 MHz, CDCl3) δ 1.43 (s, 9H, C
(CH3)3), 1.46 (s, 9H, C(CH3)3), 1.51 (s, 9H, C(CH3)3), 2.19 (m, 1H,
CH2 = CHCHAHB), 2.51 (m, 1H, CH2 = CHCHAHB), 2.60 (dd J = 11.80 and
OPMB
OPMB
t
t
CO2 Bu
CO2 Bu
(CH2)11CH3
(CH2)11CH3
t
t
MeO
MeO
O
O
2.93 Hz, 1H, CH2CHCO2 Bu), 2.79 (ABq, J = 16.72 Hz, 2H, CH2CO2 Bu), 3.93
(s, 1H, OH), 5.01 (apparent d, J = 10.04 Hz, 1H, CHAHB = CH), 5.06 (ddd,
J = 17.06, 3.18 and 1.42 Hz, 1H, CHAHB = CH) and 5.71 (m, 1H, CH2 = CH);
13C NMR (125 MHz, CDCl3) δ 28.1, 28.2, 28.3, 31.9, 42.6, 54.0, 75.5, 81.5,
81.6, 83.1, 117.0, 135.4, 170.1, 170.9 and 172.7; HRMS (ESI): calculated for
C21H36O7Na [M+Na]+ 423.23532; found 123.23525.
22
Major
23
2.5:1 ratio
Scheme 4 Ireland–Claisen rearrangement of ester 21. A full colour version
of this scheme is available at the Journal of Antibiotics journal online.
OPMB
1) DDQ; 90%
2) MsCl, NEt3; 78%
1) HCl; 60%
2) PCC; 55%
E-Alkene 10
t
CO2 Bu
(CH2)11CH3
Grubbs second-generation catalyst (3.2 mg, 0.0038 mmol) was added to a
solution of triester
22
O
9 (15.4 mg, 0.038 mmol) and 1-undecene (78 μl,
O
24
0.38 mmol) in CH2Cl2 (2 ml). The resulting solution was refluxed for 16 h
before being condensed and purified by flash chromatography with 5% EtOAc/
petrol as eluent to yield alkene 10 (15 mg, 74%) as a colourless oil. [α]D25 –5.3
(c 0.27, CH2Cl2); IR (ATR) νmax 3446, 2925, 1738, 1366, 1229, 1217 and
912 cm− 1; 1H NMR (500 MHz, CDCl3) δ 0.88 (t, J = 7.26 Hz, 3H, CH3C8H16),
1.21–1.34 (m, 14H, CH3C7H14CH2), 1.42 (s, 9H, C(CH3)3), 1.45 (s, 9H, C
(CH3)3), 1.50 (s, 9H, C(CH3)3), 1.94 (q, J =6.96 Hz, 2H, C8H17CH2), 2.12 (m,
1H, C = CHCHAHB), 2.43 (m, 1H, C = CHCHAHB), 2.55 (m, 1H, CH2CH),
HO
t
t
RO
CO2 Bu
CO2 Bu
DiBALH
(CH2)11CH3
(CH2)11CH3
O
O
25
70%
26 R = H
TMSCl
K2CO3/MeOH
H
27 R = TMS; 88%
N
ArSO2
N
t
t
2.61 (d, J = 16.5 Hz, 1H, CHAHBCO2 Bu), 2.94 (d, J = 16.5 Hz, 1H,
TMSO
HO
CO2 Bu
PPh3, DEAD
t
CHAHBCO2 Bu), 3.90 (s, 1H, OH), 5.29 (m, 1H, CH= CH) and 5.46 (m,
Then
CF3CH2OH
1H, CH= CH); 13C NMR (125 MHz, CDCl3) δ 14.3, 22.8, 28.1, 28.2, 28.3,
29.4, 29.5, 29.7, 30.7, 32.1, 32.7, 42.7, 54.7, 75.6, 81.4, 81.5, 83.0, 126.4, 133.3,
170.1, 171.1 and 172.7; HRMS (ESI): calculated for C30H54O7Na [M+Na]+
549.37618; found 549.37633.
28
58%
CO2R
1) O3, DMS
3S
RO2C
2) NaClO2
3) DTBU
2S
Tri-tert-butyl ester 11
CO2R
27%
A suspension of palladium on carbon (10%, 15 mg) and alkene 10 (15 mg,
0.028 mmol) in dry THF (4 ml) under a H2 atmosphere was stirred for 5 h.
The suspension was filtered through Filteraid (medium, Ajax Chemicals,
Victoria, Australia) and washed with EtOAc and the filtrate was condensed
and purified by flash chromatography with 5% EtOAc/petrol as eluent to give
ent-11 R = t-Bu
1 R = H: (–)-CJ-13,982
TFA
80%
Scheme 5 Synthesis of (–)-CJ-13,982 (1). A full colour version of this
scheme is available at the Journal of Antibiotics journal online.
The Journal of Antibiotics

Synthesis of a squalene synthase inhibitor
D Sturgess et al
4
25
alkane 11 (13.8 mg, 92%). [α]D +3.2 (c 0.46, CH2Cl2); IR (ATR) νmax 2528,
2926, 1731, 1368 and 1151 cm− 1; 1H NMR (500 MHz, CDCl3) δ 0.88 (m, 3H,
CH3C11H22), 1.24–1.31 (m, 22H, CH3C11H22), 1.42 (s, 9H, CCH3), 1.47 (s, 9H,
J = 9.97, 3.57 Hz, 0.8H, CHCHAHBOTBS), 3.79 (s, 3H, OCH3), 3.94 (m, 0.2H,
CHCHAHBOTBS), 4.12 (dq, J = 7.14, 1.08 Hz, 0.2H, CHCHAHBOTBS), 4.37–
4.56 (m, 3H, CHOPMB and OCH2Ar), 5.13–5.25 (m, 3H,CHOMe and
CH = CH2), 5.65 (dt, J = 17.16 and 10.15 Hz, 0.8H, CH = CH2), 5.90 (dt,
t
CCH3), 1.50 (s, 9H, CCH3), 2.50 (dd, J = 11.88 and 2.77 Hz, 1H, CHCO2 Bu),
2.77 (ABq, J = 16.45 Hz, 2H, CCH2CO2 Bu), 3.89 (s, 1H, OH); 13C NMR J = 17.02 and 10.52 Hz, 0.2H, CH = CH2), 6.85 (m, 2H, ArH), 7.21 (d,
t
(125 MHz, CDCl3) δ 14.3, 22.8, 27.4, 27.7, 28.1, 28.2, 28.3, 29.5, 29.5, 29.5,
29.7, 29.8, 29.8, 29.8, 32.1, 42.8, 54.5, 75.8, 81.3, 81.4, 82.9, 170.1, 171.8 and
172.8; HRMS (ESI): calculated for C30H56O7Na [M+Na]+551.39183; found
551.39185
J = 8.37 Hz, 2H, ArH).
The ester 17 (~1:3 mixture, 0.29 g, 0.55 mmol) was dissolved in THF
(30 ml) and treated with TBAF (175 mg, 0.67 mmol) for 2 days. The usual
workup with EtOAc gave the crude lactones, which were dissolved in CH2Cl2/
pH 7 buffer (20/2.5 ml), DDQ (0.2 g, 0.88 mmol) was added and the biphasic
mixture was stirred for 16 h. The resulting mixture was filtered through Celite
and the filtrate was concentrated and purified by flash chromatography with
20% EtOAc/petrol yielded alcohol 19 (15 mg, 13 %) as colourless needle-
shaped crystals. m.p. 73–77 °C; [α]D23 –155.0 (c 0.47, CH2Cl2); IR (ATR) νmax
(+)-CJ-13,982 (ent-1)
Triester 11 (9 mg, 0.017 mmol) in CH2Cl2 (3 ml) was cooled to 0 °C and
treated with TFA (360 μl). The reaction was stirred at 0 °C for 2 h before
warming to RT and stirring for a further 16 h. The volatile organics were
removed under reduced pressure to give (+)-CJ-13,982 (ent-1) (6.1 mg, 99%)
3473, 1771, 1079 and 922 cm− 1 1H NMR (500 MHz, CDCl3) δ 2.12 (brd,
;
as a white amorphous powder. [α]D +14.1 (c 0.35, Acetone); lit.10 [α]D
+16.6 (c 0.5, Acetone); IR (ATR) νmax 3508, 2918, 2850, 1699, 1463, 1415,
1248, 1226 and 1116 cm− 1 1H NMR (500 MHz, CD3OD)
0.90
25
25
J = 7.79 Hz, 1H, OH), 2.27 (dd, J = 12.56 and 7.04 Hz, 1H, CHAHB), 2.36
(ddd, J = 12.60, 9.34 and 5.20 Hz, 1H, CHAHB), 3.34 (s, 3H, OCH3), 4.22
(t, J = 9.33 Hz, 1H, CHCHAHBO), 4.47 (dd, J = 8.84 and 7.83 Hz, 1H,
CHCHAHBO), 4.71 (dd, J = 16.74 and 7.64 Hz, 1H, CHOMe), 5.19 (dd,
J = 5.09 and 0.49 Hz, 1H, CHOH), 5.25 (dt, J = 17.35 and 1.21 Hz, 1H,
CH = CHAHB), 5.36 (dt, J = 10.57 and 1.10 Hz, 1H, CH=CHAHB), 5.88
(ddd, J = 17.46, 10.45 and 7.11 Hz, 1H, CH = CH2); 13C NMR (125 MHz,
CDCl3) δ 40.8, 47.8, 55.3, 68.3, 71.8, 87.2, 104.6, 120.2, 130.8 and 175.7; HRMS
(ESI): calculated for C10H14O5Na [M+Na]+ 237.07334; found 237.07318.
Further elution with 20% EtOAc/petrol gave 18 (45 mg, 38 %) as colourless
;
δ
(t, J = 6.90 Hz, 3H, CH3C11H22), 1.17–1.39 (m, 20H, CH3C10H20CH2), 1.49
(m, 1H, C11H23CHAHBCH), 1.81 (m, 1H, C11H23CHAHBCH), 2.65 (dd,
J = 11.90, 2.65 Hz, 1H, CHCO2H), 2.69 (d, J = 16.28 Hz, 1H, CHAHBCO2H),
3.03 (d, J = 6.44 Hz, 1H, CHAHBCO2H); 13C NMR (125 MHz, CD3OD) δ 14.4,
23.7, 28.3, 28.8, 30.4, 30.5, 30.6, 30.7, 30.7, 30.7, 33.0, 43.1, 54.5, 77.0, 174.5,
177.0 and 177.5; HRMS (ESI): calculated for C18H32O7Na [M+Na]+ 383.20412;
found 383.20402.
24
needle-shaped crystals. m.p. 66–69 °C; [α]D –156.1° (c 1.20, CH2Cl2); IR
;
(ATR) νmax 3447, 2921, 1773, 1105 and 1056 cm− 1 1H NMR (500 MHz,
Ester 16
CDCl3) δ 2.28–2.43 (m, 3H, CHCH2CH and OH), 3.16 (q, J = 8.10 Hz, 1H,
CHCH = CH2), 3.36 (s, 3H, OCH3), 4.23 (t, J = 8.71 Hz, 1H, CHCHAHBO),
4.44 (dd, J = 8.88, 7.44 Hz, 1H, CHCHAHBO), 4.49 (m, 1H, CHOMe), 5.12
(d, J = 5.32 Hz, 1H, CHOH), 5.26–5.33 (m, 2H, CH = CH2), 5.83 (m, 1H,
CH = CH2); 13C NMR (125 MHz, CDCl3) δ 40.5, 47.2, 55.7, 70.4, 73.5, 85.3,
105.1, 120.6, 130.5 and 175.1; HRMS (ESI): calculated for C10H14O5Na [M
+Na]+ 237.07334; found 237.07319. Crystallographic data have been deposited
with the Cambridge Crystallographic Centre deposit codes: CCDC-1566533
and 1566534.
DMAP (53 mg, 0.43 mmol) and (E)-4-(tert-butyldimethylsilyloxy)but-2-en-1-
ol (0.68 g, 3.38 mmol) were added to a solution of acid 6 (0.78 g, 2.75 mmol)
in CH2Cl2 (15 ml). The solution was stirred at 0 °C for 5 min before DCC
(1.74 g, 8.4 mmol) was added. The resulting suspension was stirred at 0 °C for
2 h before warming to RT for a further 16 h before filtering through Celite. The
condensed filtrate was purified by flash chromatography with 15% EtOAc/
petrol as eluent yielded ester 16 (0.81 g, 62%) as a pale yellow oil. [α]D24 –43.3
(c 1.10, CH2Cl2); IR (ATR) νmax 2929, 1757, 1730, 1514, 1248, 1060 and
834 cm− 1; 1H NMR (500 MHz, CDCl3) δ 0.06 (s, 6H, SiCH3), 0.90 (s, 9H, SiC
(CH3)3), 2.07 (m, 1H, CHAHB), 2.23 (m, 1H, CHAHB), 3.38 (s, 3H, OCH3),
3.79 (s, 3H, OCH3), 4.18 (m, 2H, CH=CHCH2OTBS), 4.52 (ABq, J = 39.99,
11.42 Hz, 2H, OCH2Ar), 4.54–4.58 (m, 2H, CHOMe and CHOPMB), 4.67
(m, 2H, OCH2CH= CH), 5.15 (dd, J = 5.12, 1.61 Hz, 1H, CHCO2), 5.80–5.91
(m, 2H, CH = CH), 6.87 (m, 2H, ArH), 7.26 (m, 2H, ArH); 13C NMR
(125 MHz, CDCl3) δ −5.2, 18.5, 26.1, 39.9, 55.4, 55.5, 62.9, 65.4, 71.9, 80.9,
82.2, 106.4, 114.0, 123.1, 129.6, 129.9, 134.9, 159.5 and 171.5; HRMS (ESI):
calculated for C24H38O7SiNa [M+Na]+ 489.22790; found 489.22760.
Allylic alcohol 20
Tridecan-1-ol (2 g, 9.98 mmol) was dissolved in CH2Cl2 (75 ml) and Dess-
Martin periodinane (6.39 g, 14.96 mmol) was added to the solution. After
stirring at RT for 45 min, sat. NaHCO3, 1.5 M NaS2O3 and CH2Cl2 were added
and resulting mixture stirred until both layers became clear. The normal
workup with CH2Cl2gave a residue, which was directly used in next step. To a
solution of the crude aldehyde in CH2Cl2 (100 ml) was added methyl 2-
(triphenylphosphoranylidene)acetate (4.01 g, 12 mmol) and the resultant
solution was stirred at RT for 2 h. The normal workup with Et2O followed
by purification of the crude product by flash chromatography (5% EtOAc/
petrol) to give methyl ester (1.9 g, 8.65 mmol, which was dissolved in THF
(86 ml) cooled to − 78 °C and treated with DiBAlH (1 M in THF, 37 ml,
37 mmol). After stirring at −78 °C for 2 h, the solution was warmed to 0 °C
and 10% HCl was added dropwise to quenched the reaction. The normal
workup with EtOAc and flash chromatographic purification of the crude
product (10% EtOAc/petrol) gave allylic alcohol 20 (1.52 g, 72%) as white
solid. m.p. 27–29 °C IR (ATR) νmax 3314, 2922, 2853, 1466, 1002, 968 and
720 cm− 1; 1H NMR (400 MHz, CDCl3) δ 0.88 (t, J = 6.7 Hz, 3H, C10H20CH3),
1.19–1.42 (m, 20H, C10H20CH3), 2.04 (dd, J = 13.8 and 6.7 Hz, 2H,
CH2C11H23), 4.08 (s, 1H, CH2OH), 5.74–5.59 (m, 2H, CH = CH) ; 13C
NMR (101 MHz, CDCl3) δ 14.10, 22.67, 29.12, 29.17, 29.34, 29.48, 29.59,
29.65, 31.91, 32.20, 63.87, 128.75 and 133.65.
Spirolactones 18 and 19
A solution of nBuLi in hexanes (2.06 ml, 1.88 M, 3.88 mmol) was added
dropwise to a solution of iPr2NH (0.5 ml, 3.3 mmol) in THF (6 ml) at − 78 °C.
The resulting solution was stirred at 0 °C for 10 min before cooling to − 78 °C
and being added dropwise to a solution of ester 16 (0.8 g, 1.73 mmol) and the
supernatant of a centrifuged mixture of freshly distilled TMSCl (1.1 ml,
8.8 mmol) and NEt3 (1.1 ml, 7.7 mmol) in THF/HMPA (15/3 ml) at
− 100 °C. The resulting solution was stirred at − 100 °C for 10 min before
warming to RT and stirring for a further 16 h. The reaction was quenched with
1 M NaOH (5 ml) and the resulting layers separated, the aqueous layer was
acidified with HCl (10%) and underwent the usual workup with Et2O. The
crude acid was dissolved in CH2Cl2 (7 ml) and N,N′-diisopropyl-O-tert-
butylisourea18 (2.7 g, 13.3 mmol) was added, the solution was allowed to stir
for 18 h at RT. The resulting suspension was filtered through Celite and the
filtrate condensed. Flash column chromatography with 10% EtOAc/petrol as
eluent gave ester 17 (~1:3 mixture, 594 mg, 65%) as a colourless oil. 1H NMR
Ester 21
(500 MHz, CDCl3) δ 0.03 (s, 1.5H, SiCH3), 0.04 (s, 4.5H, SiCH3), 0.88 A solution of alcohol 20 (1.0 g, 4.4 mmol) and acid 6 (1.4 g, 4.8 mmol) in
(s, 9H, SiC(CH3)3), 1.40 (s, 2.25H, C(CH3)3), 1.45 (s, 6.75H, C(CH3)3), 2.07 CH2Cl2 (11 ml) was cooled to 0 °C and DMAP (58 mg, 0.48 mmol) was added.
(m, 1H, CHAHB), 2.18 (m, 1H, CHABB), 2.78 (m, 0.2H, CHCH= CH2), 3.00 The resulting solution was stirred at 0 °C for 10 min then treated with DCC
(td, J = 9.43, 3.49 Hz, 0.8H, CHCH = CH2), 3.37 (s, 1.8H, OCH3), 3.39 (1.85 g, 8.96 mmol) and the reaction was slowly warmed up to RT and further
(s, 0.6H, OCH3), 3.60 (t, J = 9.64 Hz, 0.8H, CHCHAHBOTBS), 3.74 (dd,
stirred for 36 h. The suspension was filtered through Celite and evaporation of
The Journal of Antibiotics

Synthesis of a squalene synthase inhibitor
D Sturgess et al
5
the solvent followed by purification by flash chromatography (20% Et2O/ molecular sieves (370 mg) and PCC (280 mg, 1.30 mmol) were added. The
petrol) gave the ester 21 (1.7 g, 63%) as a colourless oil. [α]D24–39.9 (c 1.00, dark brown suspension was stirred at 0 °C for 2 h before warming to RT over
CH2Cl2); IR (ATR) νmax 2924, 2857, 1758, 1729, 1614, 1514, 1465, 1110, 1065 16 h. The solution was filtered through fluorosil, washed with Et2O and
and 971 cm− 1
;
1H NMR (600 MHz, CDCl3) δ 0.88 (t, J = 7.0 Hz, 3H, concentrated. Purification of the crude product by flash chromatography with
25
C11H22CH3), 1.26 (m, 18H, C9H18CH3), 1.36 (m, 2H, CH2C10H21), 2.07 5% EtOAc/petrol yielded lactone 24 (318 mg, 55%) as a pale yellow oil. [α]D
+0.55 (c 1.59, CH2Cl2); IR (ATR) νmax 2925, 2854, 1794, 1734, 1614, 1515,
1466, 1369, 1247, 1156 and 1035 cm− 1 1H NMR (600 MHz, CDCl3) δ 0.88
(m, 3H, CH = CHCH2 and CH3OCHCHAHB), 2.22 (ddd, J =13.27, 6.58 and
1.58 Hz, 1H, CH3OCHCHAHB), 3.39 (s, 3H, OCH3), 3.80 (s, 3H, ArOCH3),
4.48 (d, J = 11.45 Hz, 1H, OCHAHBAr), 4.59 (m, 5H, OCHAHBAr,
CO2CH2CH= , CHOCH2Ar and CHCO2), 5.15 (dd, J = 5.10 and 1.59 Hz,
1H, CHOCH3 ), 5.58 (dtt, J = 5.10, 6.68 and 1.54 Hz, 1H, CH = CHC12H25),
5.80 (dtt, J =5.10, 6.68 and 1.54 Hz, 1H, CH= CHC12H25), 6.87 (m, 2H, ArH),
7.26 (m, 2H, ArH); 13C NMR (150 MHz, CDCl3) 14.1, 22.7, 28.9, 29.2, 29.3,
29.5, 29.6, 29.6, 29.7, 29.7, 31.9, 32.3, 39.7, 55.3, 55.3, 66.0, 71.7, 80.7, 82.1,
106.2, 113.8, 123.8, 123.2, 129.4, 129.7, 137.3, 159.3 and 171.4; HRMS (ESI):
calculated for C29H46O6Na [M+Na]+ 513.3180; found 513.3181.
;
(t, J = 6.98 Hz, 3H, C11H22CH3), 1.15–1.44 (m, 22H, C11H22CH3), 1.46 (s, 9H,
C(CH3)3), 2.59 (dd, J = 17.89 and 6.29 Hz, 1H, CHACHB), 2.68 (dd, J = 17.89
and 7.83 Hz, 1H, CHACHB), 2.75 (td, J = 9.87 and 3.36 Hz, 1H, CHCH=CH2),
3.81 (s, 3H, ArOCH3), 4.29 (t, J =7.82 and 6.29 Hz, 1H, CHOCH2Ar), 4.42
(d, J = 11.40 Hz, 1H, OCHAHBAr), 4.51 (d, J =11.58 Hz, 1H, OCHAHBAr),
5.11 (dd, J = 17.16 and 1.78 Hz, 1H, CH = CHAHB), 5.22 (dd, J = 10.20 and
1.87 Hz, 1H, CH = CHAHB ), 5.43 (dd, J = 17.1 and 10.1 Hz, 1H, CH =CH2),
6.86 (m, 2H, ArH) and 7.21 (m, 2H, ArH); 13C NMR (101 MHz, CDCl3) δ
14.1, 22.7, 27.3, 27.9, 29.2, 29.3, 29.4, 29.6, 29.6, 29.7, 31.9, 35.1, 49.1, 55.3,
72.3, 83.1, 91.9, 113.8, 120.4, 129.0, 129.4, 135.8, 159.5, 167.5 and 173.9;
HRMS (ESI): calculated for C32H50O6Na [M+Na]+ 553.3505; found 553.3503.
Esters 22 and 23
To a solution of the ester 21 (1.1 g, 2.3 mmol) in THF/HMPA (20 ml, 5:1) was
added the supernatant of a centrifuged mixture of freshly distilled TMSCl
(1.25 g, 11.24 mmol, 1.47 ml) and NEt3 (0.97 g, 10.39 mmol, 1.45 ml) via
cannula. The resulting solution was cooled to 100 °C and a freshly prepared
solution of LDA (4.5 mmol, 0.3 M in THF, 15 ml) was added dropwise. After
stirring at − 100 °C for 10 min, the reaction was slowly warmed to RT over
16 h. The resulting solution was extracted with 1 M NaOH three time and the
aqueous layer was acidified with 10% HCl and a normal workup with EtOAc
gave the crude acid, which was dissolved in CH2Cl2 and N,N′-diisopropyl-O-
tert-butylisourea (5 ml, 21 mmol) was added. After stirring for 24 h, the
suspension was filtered through Celite and rinsed with CH2Cl2. Rotary
evaporation and flash chromatography (5% EtOAc/petrol) purified the result-
α,β-Unsaturated lactone 25
To a biphasic solution of lactone 24 (150 mg, 0.28 mmol) in CH2Cl2 and pH 7
buffer solution (2.7/0.4 ml) was added DDQ (130 mg, 0.57 mmol) and the
mixture was allowed to stir for 16 h before being filtered through Celite and
the was filtrate concentrated. Flash chromatography (10% EtOAc/petrol) of the
resulting residue and gave alcohol (103mg, 90%), which was dissolved in
pyridine (1 ml) and cooled to 0 °C and MsCl (60 μl, 0.77 mmol) was added and
the solution was stirred at 0 °C for 2 h before warming to RT and stirring for an
additional 16 h. Water and Et2O were added and the combined organic extracts
were washed with sat. CuSO4, water and dried over MgSO4 and concentrated.
Purification by flash chromatography with 10% EtOAc/petrol as eluent gave α,
β-unsaturated lactone 25 (86 mg, 78%) as a colourless oil. [α]D25 +50.0 (c 1.45,
CH2Cl2); IR (ATR) νmax 2924, 2854, 1789, 1728, 1370, 1252, 1139, 921 and
24
ing residue and gave ester 22 (670 mg, 53%) as a colourless oil. [α]D –61.7
(c 1.19, CH2Cl2); IR (ATR) νmax 2926, 2854, 1735, 1615, 1515, 1466, 1249, 1104
and 1037 cm− 1 1H NMR (600 MHz, CDCl3) δ 0.88 (t, J =6.99 Hz, 3H,
;
; δ 0.88 (t, J = 6.96 Hz, 3H,
825 cm− 1 1H NMR (600 MHz, CDCl3)
C11H22CH3), 1.25 (m, 20H, C10H20CH3), 1.38 (m, 2H, CH2C11H23), 1.45
(s, 9H, C(CH3)3), 2.09 (ddd, J = 12.86, 7.10 and 1.64 Hz, 1H, CHACHB), 2.22
(ddd, J = 12.86, 8.87 and 5.56 Hz, 1H, CHACHB), 2.71 (td, J =9.69 and
2.76 Hz, 1H, CHCH=CH2), 3.39 (s, 3H, OCH3), 3.80 (s, 3H, ArOCH3), 4.30
(dd, J = 8.87 and 7.09 Hz, 1H, CHOCH2Ar), 4.39 (d, J =11.47 Hz,1H,
OCHAHBAr), 4.54 (d, J = 11.47 Hz,1H, OCHAHBAr), 5.04 (dd, J =17.26 and
2.15 Hz, 1H, CH= CHAHB ), 5.16–5.18 (m, 2H, CHOCH3 and CH = CHAHB ),
5.56 (dt, J = 17.24 and 10.14 Hz, 1H, CH = CH2), 6.85 (m, 2H, ArH) and 7.21
(m, 2H, ArH); 13C NMR (150 MHz, CDCl3) 14.1, 22.7, 27.7, 28.0, 29.3, 29.5,
29.6, 29.6, 29.6, 29.7, 30.7, 31.9, 37.9, 49.3, 55.2, 55.3, 71.9, 80.3, 81.5, 91.2,
104.1, 113.6, 118.4, 129.0, 130.2, 137.9, 159.1 and 171.0; HRMS (ESI):
calculated for C33H54O6Na [M+Na]+ 569.38043; found 569.38043. Further
elution with 10% EtOAc/petrol gave 23 (265 mg, 21%) as a colourless oil.
C11H22CH3), 1.24–1.30 (m, 20H, C10H20CH3), 1.45 (m, 2H, CH2C11H23),
1.48 (s, 9H, C(CH3)3), 2.84 (td, J = 9.97 and 3.40 Hz, 1H, CHCH= CH2), 5.09
(d, J = 17.10 Hz, 1H, CH = CHAHB ), 5.15 (d, J = 10.19 Hz, 1H, CH=CHAHB),
5.35 (dt, J = 17.18 and 9.76 Hz, 1H, CH =CH2), 6.07 (d, J = 5.56 Hz, 1H,
COCH= CH), 7.29 (d, J = 5.59 Hz, 1H, COCH = CH) ; 13C NMR (101 MHz,
CDCl3) δ 14.1, 22.7, 27.0, 27.9, 28.9, 29.2, 29.3, 29.4, 29.6, 29.6, 31.9, 48.7, 84.0,
92.4, 120.0, 121.7, 134.1, 155.0, 166.3 and 171.9; HRMS (ESI): calculated for
C24H40O4 Na [M+Na]+ 415.2822; found 415.2818.
Allylic alcohol 26
A solution of DiBAlH in CH2Cl2 (0.9 ml, 1 M, 0.9 mmol) was added to a
solution of the lactone 25 (70 mg, 0.18 mmol) in dry CH2Cl2 (1 ml) at − 78 °C
and the resulting solution was allowed to stir at − 78 °C for 30 min. The
reaction was quenched with 10% HCl and water and the usual workup with
EtOAc followed by purification of the crude product by flash chromatography
with 15% EtOAc/petrol as eluent yielded diol 26 (50 mg, 70%) as a pale yellow
oil. [α]D25 +32.1 (c 0.85, CH2Cl2); IR (ATR) νmax 3500, 2925, 2854, 1718, 1458,
25
[α]D –29.4 (c 1.21, CH2Cl2); IR (ATR) νmax 2925, 2854, 1735, 1614, 1515,
1466, 1249, 1103 and 1037 cm− 1 1H NMR (600 MHz, CDCl3) δ 0.88 (t,
;
J = 6.99 Hz, 3H, C11H22CH3), 1.32–1.19 (m, 20H, C10H20CH3), 1.42 (s, 9H, C
(CH3)3), 1.63–1.71 (m, 2H, CH2C11H23), 2.06 (ddd, J = 13.27, 6.36 and 2.50 z,
1H, CHACHB), 2.22 (ddd, J = 13.27, 6.36 and 2.50 Hz, 1H, CHACHB), 2.34
(td, J = 10.36 and 2.56 Hz, 1H, CHCH= CH2), 3.38 (s, 3H, OCH3), 3.80 (s, 3H,
ArOCH3), 4.30 (t, J = 6.33 Hz, 1H, CHOCH2Ar), 4.42 (ABq, J = 14.95 Hz, 2H,
OCH2Ar), 5.03 (dd, J = 17.17 and 2.22 Hz, 1H, CH = CHAHB), 5.09
(dd, J = 10.17 and 2.25 Hz, 1H, CH = CHAHB ), 5.22 (dd, J = 5.59 and
2.50 Hz, 1H, CHOCH3), 5.91 (dt, J = 17.16 and 10.07 Hz, 1H, CH = CH2),
6.84 (m, 2H, ArH), 7.21 (m, 2H, ArH); 13C NMR (125 MHz, CDCl3) 14.1,
22.7, 27.6, 28.1, 29.3, 29.4, 29.6, 29.6, 29.6, 29.7, 30.7, 31.9, 38.2, 52.1, 55.2,
55.4, 71.7, 81.4, 81.4, 91.2, 104.8, 113.6, 117.3, 129.2, 130.1, 139.1, 159.1
and 169.5.
1370, 1254, 1139 and 1032 cm− 1 1H NMR (600 MHz, CDCl3) δ 0.88
;
(t, J = 7.06 Hz, 3H, C11H22CH3), 1.13–1.36 (m, 22H, C11H22CH3), 1.49 (s,
9H, C(CH3)3), 2.33 (td, J = 9.29 and 2.43 Hz, 1H, CHCH= CH2), 2.40 (brs,
1H, CH2OH), 3.82 (s, 1H, COH), 4.14 (dt, J = 12.83 and 6.08 Hz, 1H,
CHAHBOH), 4.31 (dt, J = 13.64 and 6.19 Hz, 1H, CHAHBOH), 5.05 (dd,
J = 17.20 and 1.41 Hz, 1H, CH=CHAHB ), 5.17 (dd, J = 10.20 and 1.81 Hz, 1H,
CH= CHAHB ), 5.51 (d, J = 12.00 Hz, 1H, CH2CH= CH), 5.59 (dt, J = 17.14
and 9.94 Hz, 1H, CH = CH2), 5.75 (dt, J = 12.01 and 6.36 Hz, 1H, CH2CH =
CH); 13C NMR (101 MHz, CDCl3) δ 14.1, 22.7, 27.2, 27.9, 28.2, 29.3, 29.4,
29.5, 29.6, 29.6, 31.9, 52.4, 58.8, 80.2, 83.3, 118.4, 131.8, 132.2, 136.6 and 174.0;
HRMS (ESI): calculated for C24H44O4Na [M+Na]+ 419.3126; found 419.3127.
Lactone 24
The solution of ester 22 (600 mg, 1.09 mmol) in THF (22 ml) was treated with
10% HCl (20 ml) at RT and the solution was stirred for 5 days. Sat. NaHCO3
Diene 28
was added followed by a normal workup with EtOAc and purification of the To a solution of diol 26 (25 mg, 0.063 mmol) in CH2Cl2 (1.6 ml) was added
residue with 15% EtOAc/petrol gave the lactols (348 mg, 60%). The mixture 2,6-lutidine (87 mg, 1.27 mmol) and freshly distilled TMSCl (163 μl,
lactols was dissolved in dry CH2Cl2 (12 ml) and cooled to 0 °C and 4 Å
1.27 mmol) and the resulting suspension was stirred at RT for further 24 h.
The Journal of Antibiotics

Synthesis of a squalene synthase inhibitor
D Sturgess et al
6
Sat. NaHCO3 was added to adjust pH then followed by normal workup with acetone). The IR and NMR spectra were identical to those for the (+)-enan-
tiomer. HRMS (ESI): calculated for C30H56O7Na [M+Na]+ 383.2041; found
Et2O. The crude bis-silyl ether was directly dissolved in anhydrous methanol
(0.5 ml) at 0 °C. A catalytic amount of K2CO3 (~10 mg) then was added to the
solution and this was stirred for 45 min and a small amount of sat. NH4Cl was
added to adjust pH and followed by normal workup with EtOAc. Purification
by flash chromatography with 15% EtOAc/petrol as eluent yielded the allylic
alcohol 27 (26 mg, 88%) as a colourless oil, which was dissolved in THF/1-
hexene (10:1, 2 ml), and PPh3 (57 mg, 0.22 mmol) and IPNBSH (58 mg,
0.22 mmol) were added. The reaction was cooled to 0 °C and diethyl
azodicarboxylate (35 μl, 0.22 mmol) was added dropwise to the solution. After
stirring at 0 °C for 1 h, a solution of TFE in water (650 μl, 1:1) was added and
the reaction was warmed up to RT and stirred further 16 h. The usual workup
with Et2O and purification by flash chromatography (100% petrol) yield diene
383.2041.
CONFLICT OF INTEREST
The authors declare no conflict of interest.
ACKNOWLEDGEMENTS
We thank Australian Research Council for funding.
25
28 (16 mg, 58%) as a colourless oil. [α]D –19.5 (c 0.60, CH2Cl2); IR (ATR)
1
νmax 2925, 2854, 1738, 1458, 1368, 1258, 1155, 1096 and 840 cm−1; H NMR
1
2
3
Rizzacasa, M. A. & Sturgess, D. Total synthesis of alkyl citrate natural products. Org.
Biomol. Chem. 12, 1367–1382 (2014).
Watanabe, S. et al. CJ-13,981 and CJ-13,982, new squalene synthase inhibitors. J.
Antibiot. 54, 1025–1030 (2001).
Harris, G. H. et al. Isolation, structure determination and squalene synthase activity of
L-731,120 and L-731,128, alkyl citrate analogs of zaragozic acids A and B. Bioorg.
Med. Chem. Lett. 5, 2403–2408 (1995).
(600 MHz, CDCl3)
δ 0.12 (s, 9H, Si(CH3)3), 0.87 (t, J = 7.1 Hz, 3H,
C11H22CH3), 1.30–1.22 (m, 22H, C11H22CH3), 1.44 (s, 9H, C(CH3)), 2.23
(td, J = 10.6 and 2.3 Hz, 1H, CHCH= CHAHB), 2.33 (ddd, J = 21.6, 14.0 and
7.2 Hz, 2H, CH2CH= CH2), 5.03–4.95 (m, 3H, CH2CH= CH2 and
CHCH= CHAHB), 5.11 (dd, J = 10.2 and 2.3 Hz, 1H, CHCH = CHAHB), 5.56
(dt, J = 17.3 and 10.0 Hz, 1H, CHCH =CH2), 5.77–5.65 (m, 1H, CH2CH =
CH2); 13C NMR (101 MHz, CDCl3) δ 2.9, 14.1, 22.7, 27.2, 28.1, 29.3, 29.4,
29.6, 29.6, 29.6, 29.6, 31.9, 44.3, 52.4, 81.1, 82.9, 117.5, 117.5, 133.9, 138.2 and
173.2; HRMS (ESI): calculated for C27H52O3SiNa [M+Na]+ 453.3750; found
453.3757.
4
5
6
7
Bergstrom, J. D. et al. Zaragozic acids: a family of fungal metabolites that are picomolar
competitive inhibitors of squalene synthase. Proc. Natl Acad. Sci. USA 90,
80–84 (1993).
Hensens, O. D., Dufresne, C., Liesch, J. M. & Zink, D. L. The zaragozic acids: structure
elucidation of a new class of squalene synthase inhibitors. Tetrahedron Lett. 34,
399–402 (1993).
Dawson, M. J. et al. The squalestatins, novel inhibitors of squalene synthase produced
by a species of Phoma. I. Taxonomy, fermentation, isolation, physico-chemical proper-
ties and biological activity. J. Antibiot. 45, 639–647 (1992).
Sidebottom, P. J., Highcock, R. M., Lane, S. J., Procopiou, P. A. & Watson, N. S. The
squalestatins, novel inhibitors of squalene synthase produced by a species of Phoma. II.
Structure elucidation. J. Antibiot. 45, 648–658 (1992).
Tri-t-butyl ester ent-11
A solution of the diene 28 (40 mg, 0.09 mmol) in CH2Cl2 (2 ml) and MeOH
(17 μl) was cooled to − 78 °C and ozone was bubbled through the solution until
a blue colour persisted. Me2S (105 mg, 1.7 mmol) was added and the solution
allowed to warm to RT for 30 min and then concentrated under reduced
pressure. The crude dialdehyde was dissolved in tBuOH (2.1 ml) and 2-methyl-
2-butene (420 μl) and a solution of 80% NaClO2 (150 mg, 1.65 mmol) and
NaH2PO4 (91 mg, 0.75 mmol) in water (0.7 ml) was added and the biphasic
solution was stirred for 16 h. Water and EtOAc were added and the phases were
separated and the aqueous phase was adjusted to pH 2 with HCl and further
extracted with EtOAc. The combined organic extracts were washed with brine,
dried and concentrated and the crude diacid was dissolved in CH2Cl2 (1.5 ml)
and treated with N,N′-diisopropyl-O-tert-butylisourea (400 mg, 0.5 ml) for
16 h. The resulting suspension was filtered through a pad of Celite and the
filtrate was concentrated and the crude product was purified by flash
chromatography (5% EtOAc/petrol) to give triester ent-11 (13 mg, 27%) as a
8
9
Nadin, A. & Nicolaou, K. C. Chemistry and biology of the zaragozic acids. Angew. Chem.
Int. Ed. 35, 1622 (1996).
Liu, N. et al. Identification and heterologous production of
a
benzoyl-primed
tricarboxylic acid polyketide intermediate from the zaragozic acid
A
biosynthetic
pathway. Org. Lett. 19, 3560–3563 (2017).
10 Calo, F., Bondke, A., Richardson, J., White, A. J. P. & Barrett, A. G. M. Total synthesis
and determination of the absolute stereochemistry of the squalene synthase inhibitors
CJ-13,981 and CJ-13,982. Tetrahedron Lett. 50, 3388–3390 (2009).
11 Zammit, S., Ferro, V., Hammond, E. & Rizzacasa, M. Enantiospecific synthesis of the
heparanase inhibitor (+)-trachyspic acid and stereoisomers from a common precursor.
Org. Biomol. Chem. 5, 2826–2834 (2007).
12 Zammit, S., White, J. M.
& Rizzacasa, M. A. Enantiospecific Synthesis of
(–)-Trachyspic Acid. Org. Biomol. Chem. 3, 2073 (2005).
13 Tsegay, S., Hügel, A. H. & Rizzacasa, M. A. Formal total synthesis of citrafungin A.
Aust. J. Chem. 72, 676–682 (2009).
14 Chatterjee, A. K., Choi, T.-L., Sanders, D. P. & Grubbs, R. H. A general model for
selectivity in olefin cross metathesis. J. Am. Chem. Soc. 125, 11360–11370 (2003).
25
colourless oil. [α]D –3.2 (c 0.42, CH2Cl2); the IR and NMR spectra were
identical to those for the (+)-enantiomer; HRMS (ESI): calculated for
C30H56O7Na [M+Na]+ 551.39183; found 551.39178.
15 Bunte, J., Cuzzupe, A., Daly, A.
& Rizzacasa, M. A. Formal Total Synthesis of
(+)-Zaragozic Acid C through an Ireland–Claisen Rearrangement. Angew. Chem. Int.
Ed. 45, 6376–6380 (2006).
16 Ireland, R. E., Wipf, P. & Armstrong, J. D. III. Stereochemical control in the ester
enolate Claisen rearrangement. 1. Stereoselectivity in silyl ketene acetal formation. J.
Org. Chem. 56, 650–657 (1991).
17 Movassaghi, M. & Ahmad, O. K. N-isopropylidene-N‘-2-nitrobenzenesulfonyl hydrazine,
a reagent for reduction of alcohols via the corresponding monoalkyl diazenes. J. Org.
Chem. 72, 1838–1841 (2007).
(–)-CJ-13,982 (1)
A solution of triester ent-11 (8 mg, 0.015 mmol) in CH2Cl2 (3 ml) was cooled
to 0 °C and treated with TFA (340 μl) then was stirred at 0 °C for 2 h before
warming to RT and stirring for a further 16 h. The volatile organics were
removed under reduced pressure gave (–)-CJ-13,982 (1) (6 mg, 80%) as a white
18 Mathias, L. J. Esterification and alkylation reactions employing isoureas. Synthesis 11,
561–576 (1979).
25
25
amorphous powder. [α]D –14.1 (c 0.25, acetone); lit.2 [α]D –18.3 (c 2.6,
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