Total synthesis and determination of the absolute stereochemistry of the squalene synthase inhibitors CJ-13,981 and CJ-13,982

Article abstract of DOI:10.1016/j.tetlet.2009.02.121

The absolute and relative stereochemistries of the potent squalene synthase inhibitors CJ-13,981 and CJ-13,982 were determined to be 3S,4S by total synthesis of their antipodes using, as a key step, the diastereoselective alkylation of a chiral dioxolanon

Full text of DOI:10.1016/j.tetlet.2009.02.121

Tetrahedron Letters 50 (2009) 3388–3390
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Total synthesis and determination of the absolute stereochemistry
of the squalene synthase inhibitors CJ-13,981 and CJ-13,982
Frederick Calo a, Alexander Bondke a, Jeffery Richardson b, Andrew J. P. White a, Anthony G. M. Barrett a,
*
a Department of Chemistry, Imperial College, London SW7 2AZ, England, UK
b Eli Lilly and Company Limited, Erl Wood Manor, Windlesham, Surrey GU20 6PH, England, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
The absolute and relative stereochemistries of the potent squalene synthase inhibitors CJ-13,981 and CJ-
13,982 were determined to be 3S,4S by total synthesis of their antipodes using, as a key step, the diaste-
reoselective alkylation of a chiral dioxolanone.
Received 13 January 2009
Revised 4 February 2009
Accepted 18 February 2009
Available online 21 February 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Squalene synthase (SSase) is a key enzyme in the isoprenoid
pathway, which catalyzes the biosynthesis of squalene, a key cho-
lesterol precursor, by the reductive dimerization of two molecules
of farnesyl pyrophosphate (FPP) via the intermediate presqualene
pyrophosphate. Specific inhibition of SSase would suppress choles-
terol biosynthesis but not prevent the formation of other essential
non-sterol products such as ubiquinone, dolichol, isopentenyl t-
RNA and prenylated proteins and therefore represents an attractive
target for pharmaceutical discovery.1
orate its diastereoisomer 12 starting from 13 by using an anti-aldol
reaction. Contrary to our expectation, attempted anti-aldol reac-
tions of 13 with propenal in the presence of the Lewis acids n-Bu2-
BOTf-Et2AlCl,5 MgCl2,6 c-(C6H11)2BCl,7 or MgBr2 OEt2 either failed
or gave intractable mixtures of diastereoisomers. However, by
changing the electrophile to cinnamaldehyde, the Evans
.
6
.
MgBr2 OEt2 catalyzed anti-aldol reaction proceeded in 84% yield
after TFA-mediated desilyation (Scheme 2).
The resultant secondary alcohol 14 was protected8 as the ace-
tate 15 (96%), the structure of which was confirmed by X-ray crys-
tallography. Ozonolysis using a dimethyl sulfide work-up9 and
subsequent Pinnick oxidation gave the corresponding carboxylic
acid which was esterified using diazomethane10 to give the methyl
ester 16. Triple oxazolidinone, methyl ester and acetate hydrolysis
using lithium hydroxide and hydrogen peroxide gave diacid 17,
which was converted into the cis-dioxolanone 12 using Hoye ace-
talization.11 The structure of 12 was confirmed by NOESY NMR.
Attempted acetalization of 17 using a 4-toluenesulfonic acid-
catalyzed condensation reaction12 gave an inseparable 1:1 mixture
of 12 and its trans-isomer. Seebach SRS alkylation4 of dioxolanone
12 by double deprotonation using lithium hexamethyldisilazide in
DMF at ꢀ70 °C followed by addition of t-butyl bromoacetate gave
exclusively the dioxolanone 18 in 65% yield.
In 2001, Pfizer scientists in Nagoya isolated two new SSase
inhibitors, CJ-13,981 (1) and CJ-13,982 (2), from the fermentation
broth of an unidentified fungus (CL15036) and identified their
structures by FAB-MS and NMR analyses (Fig. 1). CJ-13,981 (1)
and CJ-13,982 (2) inhibited human liver microsomal SSase with
IC50 values of 2.8 and 1.1 l
M, respectively.2 However, their relative
and absolute stereochemistries were not determined although
their optical rotations were reported.3
As part of our research on the total synthesis of alkyl citrate nat-
ural products,4 we sought to determine the full stereochemistries
of 1 and 2 by the total synthesis of two of the four possible stereo-
isomers. Arbitrarily, we set the C-3 stereochemistry as R. The retro-
synthetic analysis for both the 3R,4S and 3R,4R 15-alkene
stereoisomers is shown in Scheme 1.
The triacids 3 and 4 should be available from the precursors 5
and 6 by saponification. In turn, the dioxolanones 5 and 6 should
be available from aldehydes 9 and 10 by Wittig reaction, hydroge-
nation and elimination of the benzyloxy substituent. The key alde-
hydes 9 and 10 should be synthesized from oxazolidinone 13,
respectively, by either a syn- or an anti-aldol reaction, dioxolanon-
es 11 and 12 formation and Seebach Self Retention of Stereocentre
(SRS) alkylation using tert-butyl bromoacetate.
Diazomethane esterification of carboxylic acids 18 and 194b
gave the corresponding methyl esters, which were subjected to
benzyl ether hydrogenolysis and Dess–Martin oxidation13 to give
aldehydes 9 and 10, both in 91% yield over the three steps. Wittig
olefination using n-BuLi and BrPh3PCH2(CH2)8OBn14 (20), respec-
HO2C OH
15
CO2H
16
Dioxolanone 11 was recently applied in the total synthesis of
CO2H
citrafungin A4b and we decided to apply the same strategy to elab-
CJ-13,981 (1) (15,16-olefin)
CJ-13,982 (2) (15,16-dihydro)
* Corresponding author. Tel.: +44 0 207 594 5767; fax: +44 0 207 594 5805.
E-mail address: agmb.office@imperial.ac.uk (A.G.M. Barrett).
Figure 1. Structures of CJ-13,981 (1) and CJ-13,982 (2).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.02.121

F. Calo et al. / Tetrahedron Letters 50 (2009) 3388–3390
3389
tBu
tBu
i. CH2N2
ii. H2, Pd/C
iii. DMP
O
O
HO2C OH
O
O
O
O
CO2H
4
n-BuLi, 20
t
t
(3R, 4R)-9
3
CO2 Bu
CO2 Bu
10
(3R, 4S)-10
10
91%
CO2H
CO2Me
CO2H
OBn
(3R, 4R)-3
(3R, 4S)-4
(3R, 4R)-5
(3R, 4S)-6
(3R, 4S)-18
(3R, 4R)-19
tBu
O
O
tBu
O
tBu
i. DMP
O
ii. Ph3P=CH2
H2, Pd/C
t
O
O
BnO
(3R, 4R)-7 (72%)
(3R, 4S)-8 (70%)
CO2 Bu
10
O
O
O
8
t
CO2 Bu
OH CO2Me
t
(3R, 4R)-21 (92%)
(3R, 4S)-22 (96%)
MeO2C
CO2 Bu
CO2Me
O
(3R, 4R)-9
(3R, 4R)-7
(3R, 4S)-8
MeO2C OH
BF3.Et2O,
MeOH
(3R, 4S)-10
CO2Me
H2, Pd/C
(3R, 4R)-5 (62%)
(3R, 4S)-6 (59%)
tBu
10
84%
O
O
O
CO2Me
O
O
O
H
(3R, 4R)-23
(3R, 4S)-24
N
2
1
MeO2C OH
OBn
CO2Me
CO2H
OBn
Bn
10
(1R, 2R)-11
(1R, 2S)-12
13
CO2Me
(3R, 4R)-25 (98%)
(3R, 4S)-26 (99%)
Scheme 1. Retrosynthetic analysis.
Scheme 3. Synthesis of triesters 23–26.
i. MgBr2.OEt2,
KOH, H2O
80 oC
PhCH=CHCHO,
Me3SiCl, EtOAc
ii. TFA
OH
Ac2O,
4-DMAP
(3R, 4R)-23
(3R, 4R)-25
ent-1
ent-2
Ph
13
72%
COXp
84%
96%
OBn
14
KOH, H2O
80 oC
Xp = (R)-4-Benzyl-oxazolidinone
i. O3; Me2S
ii. NaClO2
iii. CH2N2
OAc
OAc
MeO2C
LiOH,
H2O2
70%
H
Ph
Scheme 4. Synthesis of ent-1 and ent-2.
66%
98%
COXp
OBn
COXp
OBn
15
16
tBu
and ent-2 (Scheme 4). In contrast, attempted saponification of 24
and 26 resulted in extensive decomposition. All the analytical data
for the synthetic samples of ent-CJ-13,981 (1) and ent-CJ-13,982 (2)
matched with those reported for the natural products except for
the signs of the optical rotations.
O
i. Me3SiCl
ii. Me3SiOTf,
t-BuCHO
LiN(SiMe3)2,
OH
H
HO2C
O
O
BrCH2CO2tBu
t
CO2 Bu
12
70%
65%
CO2H
OBn
CO2H
OBn
In conclusion, we report the enantioselective syntheses of ent-1
and ent-2 and the determination of the absolute and relative ste-
reochemistries of both natural products as 3S, 4S.
17
(3R, 4S)-18
Scheme 2. Synthesis of dioxolanone 18.
Acknowledgements
tively, gave cis-alkenes 7 and 8, which were hydrogenated over
palladium on carbon in 66% and 67% yields (two steps) for each
diastereoisomer (Scheme 3). Sequential Dess–Martin oxidation
and Wittig olefination gave alkenes (3R,4R)-5 and (3R,4S)-6 in
62% and 59% yields, respectively, over the two steps.
We thank Glaxo Smith Kline for the generous endowment (to
A.G.M.B.), Eli Lilly and Company and the Engineering and Physical
Sciences Research Council for grant support and Peter R. Haycock
and Richard N. Sheppard, both at Imperial College, London, for
high-resolution NMR spectroscopy.
It is noteworthy in this sequence that the dioxolanone was used
as a protecting group for the a-hydroxy acid as well as the stereo-
Supplementary data
control element in the Seebach alkylation reaction. Dioxolanone
ring opening was effected by reflux in methanol containing boron
trifluoride etherate15 in a sealed tube giving the trimethyl esters 23
and 24 (84%) which were hydrogenated over palladium on carbon
to give esters, 25 and 26, respectively.
Procedures for the synthesis of new compounds, along with
characterization data, spectra and the X-ray crystallographic struc-
ture for 15 are available. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/
j.tetlet.2009.02.121.
At this stage, the 1H NMR spectra of 23 and 25 exhibited high
similarity to those of the natural products 1 and 2 having three ex-
tra methyl ester singlets and are distinct from the corresponding
diastereoisomers 24 and 26.
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