A fast and straightforward route towards the synthesis of the lissoclimide class of anti-tumour agents

Article abstract of DOI:10.1016/j.tet.2010.09.031

The synthesis of the chiral core structure of the lissoclimide class of anti-tumour agents containing three rings, including a chiral succinimide subunit and an exocyclic double bond, has been investigated. The compound 7 was obtained, without the use of

Full text of DOI:10.1016/j.tet.2010.09.031

Tetrahedron 66 (2010) 9270e9276
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A fast and straightforward route towards the synthesis of the lissoclimide class
of anti-tumour agents
Tuan Minh Nguyen a, Nguyen Quang Vu a, Jean-Jacques Youte a, Jacelyn Lau a, Angie Cheong a,
Ying San Ho a, Benjamin S.W. Tan a, Kanagasundaram Yoganathan b, Mark S. Butler b,
,c,
Christina L.L. Chai a
*
a Institute of Chemical and Engineering Sciences (ICES), Agency for Science Technology and Research, 8 Biomedical Grove, #07-01/2/3, Neuros, Singapore 138665
b MerLion Pharmaceuticals Pte Ltd, 1 Science Park Road, #05-01 The Capricorn, Singapore Science Park II, Singapore 117528
c Department of Pharmacy, Faculty of Science, National University of Singapore, Singapore 117543
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 May 2010
Received in revised form 23 August 2010
Accepted 13 September 2010
Available online 17 September 2010
The synthesis of the chiral core structure of the lissoclimide class of anti-tumour agents containing three
rings, including a chiral succinimide subunit and an exocyclic double bond, has been investigated. The
compound 7 was obtained, without the use of any protecting groups for the alcohol at C-12, via an
asymmetric boron-mediated aldol addition as the key step to install the chiral centres of the rare suc-
cinimido methanol moiety. This was followed by a sequence of lactonisation, microwave-assisted ami-
dation and imidation reactions.
Dedicated to the memory of the late
Professor Athel Beckwith, FRS
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Aldol reaction
anti-Tumour agents
Lissoclimides
Natural products
Terpenoids
1. Introduction
The first member of the lissoclimide class of compounds,
dichlorolissoclimide 1 (Fig. 1), was isolated in 1991 from a specimen
of New Caledonian ascidian, Lissoclinumvoeltzkowi.1 This compound,
which hasan unusualvicinal trans-diequatorial dichloride, displayed
extremely potent cytotoxic activity (IC50¼1 ng/mL against P388
leukaemia cell lines; 14 ng/mL against KB human carcinoma cell
lines).2 The isolation of chlorolissoclimide 23 and the absolute con-
figuration of dichlorolissoclimide 14 were subsequently reported. In
2001, related compounds, the haterumaimides AeE, were isolated
from an Okinawan Lissoclinum sp.5 Subsequent papers described the
isolation and biological activity of haterumaimide FeI (3e6),6 hat-
erumaimide JeK,7 haterumaimide NeQ and semi-synthetic
derivatives.8 In addition, haterumaimide LeM and 3
b-hydroxy-
chlorolissoclimide have been isolated from molluscs, Pleurobranchus
albiguttatus,9 and Pleurobranchus forskalii,9,10 which are known to
feed on ascidians. From a structural viewpoint, the skeleton of this
class of labdane diterpene alkaloid consists of a trans-decalin ring
* Corresponding author. Tel.: þ65 6799 8522; fax: þ65 6316 6184; e-mail
address: christina_chai@ices.a-star.edu.sg (C.L.L. Chai).
Fig. 1. Some examples of lissoclimides.
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.09.031

T.M. Nguyen et al. / Tetrahedron 66 (2010) 9270e9276
9271
subunit (rings A and B) and a rare chiral succinimide subunit (ring C)
with seven to eight chiral centres. The biological mode of action of
lissoclimides was unknown until 2006, when Robert and co-workers
showed that dichlorolissoclimide 1 and chlorolissoclimide 2 were
potent inhibitors of eukaryotic translation.10
As part of our ongoing natural product-based drug discovery
activities, we were interested in the synthesis of the core structure 7,
whichconsistsoftheentirecarboskeletonof thelissoclimidealkaloids
(Scheme 1). The aim of the work reported herein is to develop a gen-
eral asymmetric synthetic route for the installation of the chiral suc-
cinimide (ring C), which is applicable to the synthesis of the
aforementioned natural products. To the best of our knowledge, no
total syntheses of members of this class of compounds have been
reported. The closest related study to our work is the recent publica-
tion of González and co-workers.11 In their synthesis, nucleophilic
addition of a succinimide anion to the aldehyde 8 yielded a di-
astereomericmixtureofcompoundswiththecovetedcorestructure7.
Scheme 2. Synthesis of the aldehyde 8 and chiral oxazolidinone 9. (a) n-BuLi, succinic
anhydride, THF, 0 ꢀC to rt; (b) MeI, K2CO3, acetone, 40 ꢀC.
reaction mixture obtained from the aldol reaction with p-TsOH
(0.1 equiv) under reflux in CH2Cl2. This is to ensure complete lac-
tonisation of the intermediate 13 and following this procedure,
lactone 10 could be isolated as a single diastereomer with an
improved yield of 58%.
Scheme 3. Asymmetric aldol addition followed by lactonisation. (a) (i) n-Bu2BOTf,
DIPEA, CH2Cl2, ꢁ78 ꢀC, (ii) pH 7 phosphate buffer, 30% H2O2, MeOH; (b) p-TsOH (cat.),
CH2Cl2, reflux, 2 h.
Scheme 1. Retrosynthetic analysis of the core structure 7 of the lissoclimides alkaloids.
Our efforts to improve the preparation of the lactone 10 revealed
that with prolonged heating of crude 13 in CH2Cl2, the yield of 10
decreased significantly. The 1H NMR spectra of isolated products
suggested that the exocyclic double bond of 10 had migrated to the
endocyclic positions. Indeed, when the pure compound 10 was
treated with p-TsOH (0.1 equiv) in CDCl3 at 90 ꢀC for 3 h, complete
isomerisation occurred (Scheme 4). A similar observation was also
noted by Uddin and co-workers, when haterumaimide G (4) was
treated with PPTS in MeOH at rt to give haterumaimide H (5).6
These observations suggested the propensity of the double bond
isomerisation in this class of compounds under acidic conditions.
2. Results and discussion
Retrosynthetically, the disconnection of the succinimido meth-
anol moiety reveals that the two chiral centres of this rare motif
could be obtained via an asymmetric aldol addition using a chiral
auxiliary. The remaining chiral centres of the decalin moiety could
be obtained fromthe chiralpool (Scheme 1). This synthetic sequence
could then be achieved via an asymmetric aldol addition, in situ
lactonisation followed by amidation and imidation. Interestingly,
a procedure involving an asymmetric aldol addition followed by in
situ lactonisation to give either the syn- or anti-aldol products was
reported by Hajra and co-workers in 2007.12 The diastereoselectivity
was controlled by varying the addition sequence of the base and
aldehyde, when chiral N-acyl-2-oxazolidinones containing addi-
tional g/d-chelating functional groups were used.
The oxazolidinone 9 was prepared by acylation of the chiral
auxiliary 11 via its lithium amide,13 followed by O-methylation
(Scheme 2). After purification, the oxazolidinone 9 was obtained in
89% yield over two steps. Starting from commercially available
(3aR)-(þ)-sclareolide 12, the aldehyde 8 was synthesised in 60%
yield over three steps using reported procedures.14
With oxazolidinone 9 and aldehyde 8 in hand, a syn-selective
aldol condensation was carried out under mild conditions with
n-Bu2BOTf/DIPEA in CH2Cl2 (Scheme 3).12 As expected, after
a standard workup with pH 7 phosphate buffer and 30% H2O2 in
MeOH, the syn-aldol product lactone 10 was obtained as a single
diastereoisomer in 30% isolated yield (for detailed studies related to
the stereochemistry determination, see below). The yield of the
desired lactone 10 could be improved by treatment of the crude
Scheme 4. Migration of the exocyclic double bond into endocyclic positions. (a) p-
TsOH (0.1 equiv), CDCl3, 90 ꢀC, 3 h.
We next investigated possible routes that could lead to the
formation of succinimide ring C (Scheme 5). In route A, a LiOOH-
mediated hydrolysis of compound 10 was carried out and this gave
selectively the monocarboxylic acid 16 in 90% yield.15 However, all
attempts to transform this compound into the corresponding am-
ide 17 with NH3/H2O or NH3 gas followed by imidation in DMF at

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T.M. Nguyen et al. / Tetrahedron 66 (2010) 9270e9276
Scheme 5. Investigation of possible routes leading to the core structure 7. (a) LiOH,
H2O2, THF/H2O, 0 ꢀC to rt, 90%; (b) NH4OH, DMF, 150 ꢀC; (c) (i) NH3 gas, rt, DMF; (ii)
DMF, 150 ꢀC.
high temperature were unsuccessful, giving only complex mixtures
of highly polar products without any traces of compound 7.16 In-
vestigation of route B was initiated by attempting to convert
compound 10 into the corresponding amide 18. The classical pro-
cedure for transamination of Evans’ auxiliary using NH4Cl/Me3Al17
was not suitable in this case, due to the lack of chemoselectivity
between the ester and the ‘imide-like’ functional groups at C-15
and C-16 (as labelled in the Scheme 4), respectively. Fortunately,
treating compound 10 with a large excess of ammonium acetate
under microwave irradiation (MWI) at 150 ꢀC for 15 min gave the
amides 18, 19 and 20 as a mixture of regioisomers in 5.3:3.7:1.0 ratio
(as determined in the following step, see Scheme 6) in a combined
yield of 57%.18 Although the aforementioned isomerisation of the
double bond had occurred, this unprecedented reaction provided
a method for the transamination of chiral lactone 10.
Imidation was achieved by treating the mixture of amides with
sodium hydride giving a mixture of succinimides 7, 21 and 22 in
quantitative yield. Compound 22 was isolated in 10% yield by pre-
parative LC-MS, whereas 7 and 21 were obtained as a 1:0.7 mixture
of compounds in 90% isolated yield. The separation of these two
isomers proved difficult, and only an analytically pure sample of
compound 7 could be obtained. Subsequently, an esterification of
the mixture with (R)-methoxyphenylacetic acid (MPA) was carried
out, which facilitated the separation. Therefore, esters 23 and 24
were obtained in 56% and 35% isolated yields, respectively.
Scheme 6. Microwave-assisted amidation followed by imidation. (a) NH4OAc, micro-
wave irradiation (150 ꢀC, 15 min); (b) (i) NaH, THF, rt, 30 min, (ii) MeOH, 0 ꢀC. (c) EDC/
DMAP, (R)-MPA acid, CHCl3.
In order to determine the absolute stereochemistry of the chiral
centres C-12 and C-13 resulting from the aldol addition step,
compound 22 was derivatized as (R)-MPA ester 25 and (S)-MPA
ester 26 (Fig. 3).21 The 1H NMR signals of these two compounds
were assigned based on 2D NMR, and the obtained Dd
(
dR S, ppm)
d
ꢁ
values allowed us to confirm the 12S configuration of 22. Based on
the relative stereochemistry already deduced from NOE experi-
ments, the R configuration of C-13 was assigned.
3. Conclusion
A series of 1D NOE difference experiments were carried out in
order to establish the relative stereochemistry of the chiral centres
generated during the aldol reaction. The observed NOE correlations
of 10, supported by conformation analysis (Fig. 2a),19 strongly
suggest a syn-aldol addition. Independently, the relative stereo-
In summary, we have developed a useful synthetic strategy to-
wardsthesynthesisofthechiralcorestructureofthelissoclimideclass
of anti-tumour agents. The rapid and straightforward synthetic route
gave the desired core structure 7 in a reasonable yield (17.5% in four
steps) from readily available 8. It should be noted that in this reaction
sequence, no protecting groupwasneeded to access the alcohol group
at C-12 of the core structure. These studies should pave theway for the
total synthesis of dichlorolissoclimide 1 and its analogues.
chemistries of compound 22, 12S
a careful examination of NOE correlations of H-12/H-13, H-12/H-17,
H-12/H-11 , H-12/H-14 , H-13/H-14 , H-11 /H-17, H-11 /H-20, H-
11 /H-1eq, together with vicinal coupling constants of H-12 (ddd,
*
and 13R were proposed by
*
a
a
a
a
b
b
J¼10.8, 4.0, 2.4 Hz) and H-13 (ddd, J¼8.8, 4.8, 2.4 Hz) and confor-
mation analysis (Fig. 2b). In addition, comparison of these data with
those from related compounds described in the literature4,6,7
allowed us to confirm the relative stereochemistry of compounds
22 and 10. This result is in good agreement with stereochemical
outcomes as reported by the Evans group,20 as well as Hajra and co-
workers for aldol reaction using Evans’ auxiliary.12
4. Experimental
4.1. General
1H and 13C NMR spectra were recorded on a Bruker Advance 400
and Bruker Advance DRX-500 NMR instruments in CDCl3 unless

T.M. Nguyen et al. / Tetrahedron 66 (2010) 9270e9276
9273
Fig. 2. Energy minimised conformations and selected spatial NOE correlation of: (a) the lactone 10; (b) the succinimide 22.
otherwise stated. Coupling constants are given in hertz and
chemical shift are expressed as values in parts per million, using
chromatography purification was carried out either manually or by
using a Biotage SP1Ô purification system by gradient elution. Pre-
parative reversed-phase HPLC was performed either on a Gilson
system complete with UniPoint software, 170 DAD detector, dual
306 pumps, 811C dynamic mixer, Gilson 202 fraction collector, and
a Rheodyne 7125 injector with a 5 mL injection loop, or a Waters
2545 Binary Gradient Module with a 2996 PDA detector. Semi-
preparative TLC was done using Merck silica gel 60 (F254) pre-
coated glass plates, 0.25 mm thickness.
d
either TMS or the residual undeuterated solvent signal as reference.
IR spectra were measured on a Bio-Rad FTS 3000MX FT-IR spec-
trometer as liquid film or as an evaporated film. Optical rotations
were measured using a Jasco P-1030 polarimeter. Mass spectra
were run by the electron impact (EI, 70 eV) mode on a Thermo
Finnigan MAT XP95 mass spectrometer or by the electrospray
ionization time-of-flight (ESI-TOF) mode on an Agilent 6210 mass
spectrometer. Microwave reaction was carried out in sealed
reactors in a Biotage InitiatorÔ Microwave Synthesiser with built-
in temperature and pressure detectors. Solvents were taken from
a Glass contour solvent purification system under nitrogen. Com-
mercially available reagents were used as received. Flash column
4.2. Synthesis
4.2.1. (R)-Methyl 4-(4-benzyl-2-oxooxazolidin-3-yl)-4-oxobutanoate
(9). A solution of (R)-4-Benzyloxazolidin-2-one 11 (5.0 g,
28.22 mmol, 1 equiv) in dry THF (100 mL) was stirred at 0 ꢀC for
15 min before n-BuLi (2.5 M, 12.4 mL, 31.04 mmol) was added to
give an orange-coloured solution. Succinic anhydride (3.10 g,
31.04 mmol) was added and stirred for 15 h at rt. Saturated NH4Cl
(30 mL) was added to quench the reaction and a hard block of white
gel was formed. HCl (2 M) solution was added until the pH of
aqueous phase is acidic and the reaction mixture was extracted
with CH2Cl2 (100 mLꢂ3). The combined organic layers were
washed with water (100 mLꢂ2), dried over MgSO4, filtered and
evaporated under reduced pressure. The crude product was directly
submitted to methylation without further purification.
A suspension of K2CO3 (2.50 g, 180 mmol) in acetone (60 mL)
was stirred for 30 min at 40 ꢀC. The above crude oxazolidinone in
acetone (20 mL) was added to the suspension. The reaction mixture
was stirred for an additional 30 min before methyl iodide (25.6 g,
11.2 mL, 180 mmol) was added and stirred for another 15 h at 40 ꢀC.
The reaction mixture was filtered to remove K2CO3 and evaporated
Fig. 3. DdRS values (DdRS
ester 26.
¼
dR S) of selected protons of (R)-MPA esters 25 and (S)-MPA
d
ꢁ

9274
T.M. Nguyen et al. / Tetrahedron 66 (2010) 9270e9276
under reduced pressure. The crude reaction mixture was dissolved
in EtOAc (300 mL) and then washed with H2O (100 mLꢂ3). The
organic phase was dried over MgSO4, filtered and evaporated under
reduced pressure. The residue obtained was purified by silica gel
chromatography and eluted with 10e50% EtOAc in hexane to pro-
vide the title compound 9 as a white solid (7.32 g, 89% after two
steps). Mp 91.7e92.5 ꢀC (lit.2: 90e92 ꢀC). Rf (50% EtOAc/hexane)
38.0, 33.6, 33.5, 31.8, 29.7, 29.4, 24.2, 21.7,19.3,14.4; HRMS (ESIꢁ) m/
z calcd for C20H29O4 [MꢁH]ꢁ 333.2071, found 333.2055.
4.2.4. Microwave-assisted amidation of compound 10. Compound
10 (1.11 g, 2.252 mmol) and a large excess of NH4OAc (7.77 g,
100.9 mmol) were submitted to microwave irradiation at 150 ꢀC for
15 min without the use of any solvent. The reaction mixture was
then cooled to rt, following which EtOAc (10 mL) was added. The
solution was dried over MgSO4, filtered and evaporated under re-
duced pressure. The residue obtained was purified by silica gel
chromatography and eluted with 20e100% EtOAc in hexane to
provide a mixture of 18/19/20 (431 mg, combined yield 57%). An-
alytically pure samples of compounds 18 and 19 were obtained by
0.43; 1H NMR (400 MHz, CDCl3)
d 7.19e7.35 (m, 5H), 4.68 (dddd,
J¼10.8, 9.5, 8.7, 3.1 Hz, 1H), 4.22 (dd, J¼9.1, 8.7 Hz, 1H), 4.18 (dd,
J¼9.1, 3.1 Hz, 1H), 3.72 (s, 3H), 3.31e3.24 (m, 3H), 2.79 (dd, J¼13.4,
9.5 Hz, 1H), 2.74e2.69 (m, 2H).
4.2.2. (R)-4-Benzyl-3-((2S,3R)-5-oxo-2-(((1S,4aS,8aS)-5,5,8a-tri-
methyl-2-methylenedecahydronaphthalen-1-yl)methyl)tetrahydrofu-
ran-3-carbonyl)oxazolidin-2-one (10). To a solution of 9 (1.90 g,
6.55 mmol) in dry CH2Cl2 (30 mL) was added a solution of
n-Bu2BOTf (1.0 M in CH2Cl2, 7.86 mL, 7.86 mmol) and DIPEA (1.70 mL,
9.82 mmol) and stirred for 15 min at 0 ꢀC. The reaction mixture was
cooled to ꢁ78 ꢀC before the aldehyde 814 (1.50 g, 6.41 mmol) in dry
CH2Cl2 (15 mL) was added and stirred for another 1 h at ꢁ78 ꢀC,
followed byanother 72 h at 0 ꢀC. The reaction mixturewas quenched
with the addition of a pH 7 phosphate buffer solution (20 mL), 30%
hydrogen peroxide (60 mL) and MeOH (20 mL) solution at 0 ꢀC and
stirred for 1 h. The reaction mixturewas extracted with Et2O (100 mL
ꢂ3). The combined organic layers was washed with water
(50 mLꢂ3), dried over MgSO4, filtered and evaporated under re-
duced pressure to afford a red-orange oil. The residue obtained,
containing compound 13 as determined by 1H NMR, was lactonised
by refluxing the reaction mixture with p-TsOH (100 mg) in CH2Cl2
(300 mL) at 70 ꢀC with a Soxhlet filled with calcium hydride for 2 h.
The residue was purified by silica gel chromatography and eluted
with 10e20% EtOAc in hexane to provide the title compound 10 as
a viscous pale yellow oil (1.89 g, 58% after two steps). Rf (25% EtOAc/
reverse-phase HPLC (Luna Phenylhexyl C18 column,
5 m,
21.2ꢂ150 mm; flow rate: 15 mL/min; mobile phase: acetonitrile in
water with 0.1% formic acid; gradient: 40e46% (100 min)) to give 18
(rt 41 min) and 19 (rt 44 min).
4.2.5. (2S,3R)-5-Oxo-2-(((1S,4aS,8aS)-5,5,8a-trimethyl-2-methyl-
enedecahydronaphthalen-1-yl)methyl)tetrahydrofuran-3-carbox-
amide (18). Mp 179e179.5 ꢀC; [
a
]
D
20 ꢁ13.60 (c 0.22, CHCl3); IR (film)
nmax 3381, 2928, 2867, 2360, 2237, 1778, 1678, 1458, 1423, 1383,
1365, 1292, 1224, 1149, 1023, 968, 889 cmꢁ1
CDCl3)
5.53 (br s, 2H), 4.93 (s, 1H), 4.77 (td, J¼7.9, 4.0 Hz, 1H), 4.66
(s, 1H), 2.92e2.72 (m, 3H), 2.44e1.12 (m, 14H), 0.89 (s, 3H), 0.81 (s,
3H), 0.70 (s, 3H); 13C NMR (125 MHz, CDCl3)
175.4, 173.3, 149.4,
;
1H NMR (500 MHz,
d
d
108.4, 82.8, 56.7, 52.7, 48.0, 43.0, 40.8, 40.3, 39.1, 34.6, 34.5, 34.4,
29.9, 25.3, 22.6, 20.3, 15.4; HRMS (ESIþ) m/z calcd for C20H31NO3Na
[MþNa]þ 356.2196; found 356.2197.
4.2.6. (2S,3R)-5-Oxo-2-(((1S,4aS,8aS)-2,5,5,8a-tetramethyl-
1,4,4a,5,6,7,8,8a-octahydronaphthalen-1-yl)methyl)tetrahydrofuran-
3-carboxamide (19). Mp 167e168 ꢀC [
a]
20 ꢁ53.85 (c 0.18, CHCl3); IR
D
hexane) 0.31; [
a
]
20 ꢁ35.15 (c 0.52, CHCl3); IR (film) nmax 2937, 2845,
(film) nmax 3382, 2928, 2867, 2361, 2238, 1778, 1678, 1459, 1423,
D
1782, 1697, 1389, 1209, 1200, 1112, 1006, 890 cmꢁ1
;
1H NMR
1383, 1364, 1292, 1224, 1149, 1022, 968, 889 cmꢁ1
(500 MHz, CDCl3)
5.57 (br s, 2H), 5.46 (s, 1H), 4.85 (dd, J¼13.2,
6.5 Hz,1H), 2.92e2.77 (m, 3H), 2.02e0.95 (m,12H),1.72 (s, 3H), 0.89
(s, 3H), 0.87 (s, 3H), 0.78 (s, 3H); 13C NMR (125 MHz, CDCl3)
175.1,
;
1H NMR
(400 MHz, CDCl3)
d
7.36e7.17 (m, 5H), 4.95 (ddd, J¼8.3, 5.6, 4.5 Hz,
d
1H), 4.85 (s, 1H), 4.68 (dddd, J¼9.3, 7.2, 3.7, 3.4 Hz, 1H), 4.59 (s, 1H),
4.29 (dd, J¼9.2, 7.2,1H), 4.25 (dd, J¼9.2, 3.7,1H), 4.15 (ddd, J¼9.3, 7.1,
5.6 Hz,1H), 3.24 (dd, J¼13.4, 3.4 Hz,1H), 3.00 (dd, J¼17.6, 9.3 Hz,1H),
2.81 (dd, J¼13.4, 9.3 Hz, 1H), 2.61 (dd, J¼17.6, 7.1 Hz, 1H), 2.39 (ddd,
J¼12.7, 6.4, 4.1 Hz, 1H), 2.01e0.94 (m, 13H), 0.96 (s, 3H), 0.79 (s, 3H),
d
173.2, 134.3, 124.5, 84.4, 51.2, 51.0, 48.4, 43.1, 40.8, 37.8, 34.6, 34.1,
34.0, 33.7, 24.7, 23.4, 22.8, 19.7, 14.4; MS (ESIþ) m/z [MþNa]þ
356.59; MS (ESIꢁ) m/z [MꢁH]ꢁ 352.52.
0.67 (s, 3H); 13C NMR (100 MHz, CDCl3)
d 174.1, 171.3, 152.9, 148.2,
134.5, 129.3, 129.0, 127.6,106.9, 80.9, 66.7, 55.6, 55.3, 51.5, 44.6, 41.9,
39.6, 39.0, 38.0, 37.7, 33.5, 33.5, 32.9, 28.5, 24.2, 21.6, 19.2, 14.4. MS
(ESIþ) m/z [MH]þ 494; HRMS (ESIþ) m/z calcd for C30H39NO5Na
[MþNa]þ 516.2720, found 516.2714; calcd for C30H39NO5K [MþK]þ
532.2460; found: 532.2459.
4.2.7. Procedure for the formation of succinimides starting from the
isomeric mixture 18/19/20. To a solution of the mixture of com-
pounds 18, 19 and 20 (50 mg, 0.15 mmol) in THF (2 mL) was added
NaH (13.8 mg, 0.345 mmol, 60% in mineral oil) at 0 ꢀC followed by
stirring at rt for 30 min. The reaction mixturewas quenched with the
addition of MeOH (5 mL) and evaporated to dryness under reduced
pressure. The residue was purified by silica gel chromatography
eluted with 3% MeOH in CH2Cl2 to provide a mixture of 7/21/22 as an
inseparable mixture (50 mg, quant.). Purification using reverse
4.2.3. (2S,3R)-5-Oxo-2-(((1S,4aS,8aS)-5,5,8a-trimethyl-2-methyl-
enedecahydronaphthalen-1-yl)methyl)tetrahydrofuran-3-carboxylic
acid (16). A solution of lactone 10 (120 mg, 0.24 mmol) in 4:1 THF/
H2O (5 mL) was treated at 0 ꢀC with 30% H2O2 (198
mL, 1.92 mmol)
phase HPLC-MS (Luna Phenylhexyl C18 column, 5
m
, 21.2ꢂ250 mm;
followed by LiOH (7.2 mg, 0.3 mmol). The resulting mixture was
stirred at rt for 3 h, and the excess of peroxide was quenched at 0 ꢀC
with 1.5 M aqueous Na2SO3. The mixture was then extracted with
EtOAc (50 mLꢂ3) and the organic layers were combined, dried over
MgSO4 and evaporated under reduced pressure. The residue
obtained was purified by silica gel chromatography and eluted with
flow rate: 20 mL/min; mobile phase: acetonitrile in water with 0.1%
formic acid; gradient: 40% (10 min), 40e60% (30 min), 80% (5 min))
gave the mixture 7/21 (45 mg, rt 28.1 min) in 1:0.7 ratio (determined
by 1H NMR spectroscopy) and the pure compound 22 (5 mg, 10%, rt
29.9) as a white amorphous solid. The mixture of compounds 7 and
21 was further purified by using normal phase HPLC (IA column, 5 m,
2e5% MeOH in CH2Cl2 to provide the title compound 16 as a white
20ꢂ250 mm; flow rate: 20 mL/min; mobile phase: isopropanol in
hexane; gradient: 5% (2 min), 5e20% (30 min), 20% (20 min)) to give
theanalyticallypurecompound 7(4 mg)as awhiteamorphous solid.
24
amorphous solid (74 mg, 90%). Rf (10% MeOH/CH2Cl2) 0.36; [a]
D
ꢁ7.37 (c. 2.35, CHCl3). IR (film) nmax 3435, 2928, 2850, 1777, 1766,
1641, 1585, 1414, 1385, 1262, 1202, 1018, 890, 803 cmꢁ1 1H NMR
(400 MHz, CDCl3) 4.87 (s, 1H), 4.72 (m, 1H), 4.63 (s, 1H), 2.90 (m,
;
d
4.2.8. (R)-3-((S)-1-Hydroxy-2-((1S,4aS,8aS)-5,5,8a-trimethyl-2-
methylenedecahydronaphthalen-1-yl)ethyl)pyrrolidine-2,5-dione
1H), 2.81 (m, 1H), 2.71 (m, 1H), 2.39 (m, 1H), 1.99 (m, 2H), 1.79e0.98
(m, 11H), 0.88 (s, 3H), 0.80 (s, 3H), 0.68 (s, 3H); 13C NMR (100 MHz,
(7). [
a
]
20 þ40.45 (c 0.71, CHCl3); IR (film) nmax 3470, 3193, 2928, 2845,
D
CDCl3)
d
177.7, 176.6, 148.1, 107.0, 82.6, 55.3, 51.4, 42.0, 39.6, 39.0,
1775, 1713, 1456, 1363, 1261, 1186, 1090, 1037 cmꢁ1
;
1H NMR

T.M. Nguyen et al. / Tetrahedron 66 (2010) 9270e9276
9275
(400 MHz, CDCl3)
d
7.73(brs,1H), 4.95(s,1H),4.68(s,1H), 4.37(m,1H),
d 176.6, 175.4, 169.3, 136.0, 133.4, 128.8, 128.7 (2C), 126.8 (2C), 123.3,
2.95e2.85 (m, 2H), 2.69 (dd, J¼17.6, 8.0 Hz, 1H), 2.43 (ddd, J¼12.8, 4.4,
2.8 Hz, 1H), 2.00 (dt, J¼12.8, 4.8 Hz, 1H), 1.92 (br s, 1H), 1.79e1.07 (m,
12H), 0.89 (s, 3H), 0.81 (s, 3H), 0.70 (s, 3H); 13C NMR (100 MHz, CDCl3)
82.6, 73.5, 57.5, 49.92, 49.87, 43.9, 42.0, 39.6, 36.7, 33.0, 32.9, 30.3,
29.3, 23.7, 22.2, 21.7, 18.7, 13.4; HRMS (ESIþ) m/z calcd for
C29H39NO5Na [MþNa]þ 504.2720; found 504.2714.
d
178.7, 176.3, 149.0, 107.3, 69.5, 55.6, 53.9, 46.8, 42.0, 39.2, 38.2, 33.6,
33.5, 29.7, 29.4, 29.1, 24.3, 21.6, 19.3, 14.4; HRMS (ESIþ) m/z calcd for
C20H31NO3Na [MþNa]þ 356.2196; found 356.2195.
4.3.2. Synthesis of (R)- and (S)-MPA esters of compound
22. 4.3.2.1. (R)-((S)-1-((R)-2,5-Dioxopyrrolidin-3-yl)-2-((4aS,8aS)-
2,5,5,8a-tetramethyl-3,4,4a,5,6,7,8,8a-octahydronaphthalen-1-yl)
ethyl) 2-methoxy-2-phenylacetate (25). Following the general
procedure for MPA esters, compound 22 (2.0 mg, 0.006 mmol),
(R)-MPA (2 mg, 0.012 mmol), DMAP (0.05 mg, 0.0004 mmol) in
CDCl3 (0.5 mL) were reacted with EDC$HCl (7.8 mg, 0.04 mmol).
The crude mixture was purified first by silica gel chromatography
eluted with 20% EtOAc in hexane then by preparative TLC eluted
with 25% EtOAc in hexane to provide the (R)-MPA ester 25 as
4.2.9. (R)-3-((S)-1-Hydroxy-2-((4aS,8aS)-2,5,5,8a-tetramethyl-
3,4,4a,5,6,7,8,8a-octahydronaphthalen-1-yl)ethyl)pyrrolidine-2,5-di-
20
one (22). [
a
]
D
þ9.90 (c 0.31, CHCl3); IR (film) nmax 3464, 3229,
2927, 2867, 1774, 1709, 1707, 1459, 1359, 1264, 1190, 1097, 1039,
947 cmꢁ1 1H NMR (400 MHz, CDCl3)
;
d
7.89 (br s, 1H), 4.38 (ddd,
J¼10.8, 4.0, 2.4 Hz, 1H), 2.98 (dd, J¼17.2, 4.8 Hz, 1H), 2.92 (ddd,
J¼8.8, 4.8, 2.4 Hz, 1H), 2.73 (dd, J¼17.2, 8.8 Hz, 1H), 2.32 (dd, J¼14.0,
10.8 Hz,1H), 2.18 (dd, J¼14.0, 4.0 Hz,1H), 2.00e2.15 (m, 2H),1.89 (br
s,1H),1.80 (m,1H),1.41e1.73 (m, 5H),1.65 (s, 3H),1.23e1.12 (m, 3H),
0.98 (s, 3H), 0.90 (s, 3H), 0.85 (s, 3H); 13C NMR (100 MHz, CDCl3)
a white foam in 75% yield (2.7 mg). Rf (50% EtOAc/hexane) 0.62;
20
[
a
]
59.75 (c 1.05, CHCl3); IR (film) nmax 3270, 2925, 2848, 1748,
D
1715, 1712, 1636, 1456, 1352, 1262, 1174, 1099, 1028, 800, 737, 699,
d
178.8, 176.9, 135.4, 131.9, 69.0, 51.4, 47.4, 41.5, 39.1, 37.7, 33.7, 33.3,
607; 1H NMR (400 MHz, CDCl3)
d
7.52 (br s, 1H), 7.38e7.34 (m, 5H),
33.24, 33.20, 29.9, 21.7, 20.9, 20.3, 18.9, 18.8; HRMS (ESIþ) m/z calcd
5.68 (ddd, J¼8.8, 7.2, 2.4, 1H), 4.67 (s, 1H), 3.37 (s, 3H), 3.05 (ddd,
J¼8.4, 6.0, 2.4, 1H), 2.76 (d, J¼6.0 Hz, 1H), 2.75 (d, J¼8.4 Hz, 1H),
2.42 (dd, J¼14.4, 8.8 Hz, 1H), 2.13 (dd, J¼14.4, 7.2 Hz, 1H), 1.91e1.73
(m, 3H), 1.40 (s, 3H), 1.68e1.26 (m, 7H), 1.19e1.11 (m, 1H), 0.94 (s,
for C20H31NO3Na [MþNa]þ 356.2196; found 356.2193.
4.3. General procedure for the MPA esters
3H), 0.87 (s, 3H), 0.82 (s, 3H); 13C NMR (100 MHz, CDCl3)
d 176.6,
To a solution of the appropriate succinimides, methoxyphenyl-
acetic acid (MPA) and 4-dimethylamino pyridine (DMAP) in either
CDCl3 or CHCl3, was added N-(3-Dimethylaminopropyl)-N0-ethyl-
carbodiimide hydrochloride (EDC$HCl) at rt. The solution was then
stirredfor24h andthesolvents evaporatedto givethe crudemixture.
175.4, 169.4, 135.9, 134.3, 130.9, 128.8, 128.7 (2C), 127.1 (2C), 82.7,
72.6, 57.4, 50.8, 45.2, 41.5, 38.5, 37.2, 33.3, 30.9, 30.6, 30.3, 29.7,
21.8, 20.9, 20.4, 18.9, 18.8; HRMS (ESIþ) m/z calcd for C29H39NO5Na
[MþNa]þ 504.2720; found 504.2737.
4.3.2.2. (S)-((S)-1-((R)-2,5-Dioxopyrrolidin-3-yl)-2-((4aS,8aS)-
2,5,5,8a-tetramethyl-3,4,4a,5,6,7,8,8a-octahydronaphthalen-1-yl)
ethyl) 2-methoxy-2-phenylacetate (26). Following the general pro-
cedure for MPA esters, 22 (1.0 mg, 0.003 mmol), (S)-MPA (2 mg,
0.012 mmol), DMAP (0.05 mg, 0.0004 mmol) in CDCl3 (0.5 mL) were
reacted with EDC (7.8 mg, 0.04 mmol). The crude mixture was
purified first by silica gel chromatography, eluted with 20% EtOAc in
hexane then by preparative TLC eluted with 25% EtOAc in hexane to
provide (S)-MPA ester 26 was obtained in 44% yield (0.8 mg). Rf
4.3.1. Synthesis of (R)-MPA esters 23 and 24. Following the gen-
eral procedure for MPA esters, the mixture of compounds 7 and 21
(in 1/0.7 ratio, 10 mg, 0.03 mmol), (R)-MPA (10 mg, 0.06 mmol),
DMAP (0.5 mg, 0.004 mmol) in CHCl3 (3 mL) were reacted with
EDC$HCl (19.5 mg, 0.1 mmol). The crude mixture was purified first
by silica gel chromatography eluted with 20% EtOAc in hexane to
give the mixture of (R)-esters (in 1:0.7 ratio, 14.4 mg, quant.). This
mixture was further purified by using normal phase preparative
HPLC (OD column, 5
m
, 20ꢂ250 mm; flow rate: 20 mL/min; mobile
(50% EtOAc/hexane) 0.65; [
a]
20 þ7.02 (c 1.05, CHCl3); IR (film) nmax
D
phase: isopropanol in hexane; gradient: 5% (2 min), 5e20%
(30 min), 20% (20 min)) to provide compound 23 (8 mg, 56%) and
compound 24 (5 mg, 35%) as colourless foams.
2960, 2925, 1755, 1715, 1713, 1659, 1462, 1282, 1177, 1097, 1022, 866,
801, 700, 615 cmꢁ1; 1H NMR (400 MHz, CDCl3)
d 7.69e7.32 (m, 5H),
6.72 (br s, 1H), 5.66 (ddd, J¼8.0, 5.2, 2.0 Hz, 1H), 4.60 (s, 1H), 3.38 (s,
3H), 2.89 (ddd, J¼8.5, 5.8, 2.6 Hz, 1H), 2.62 (d, J¼5.8 Hz, 1H), 2.61 (d,
J¼8.5 Hz, 1H), 2.48 (dd, J¼14.4, 9.6 Hz, 1H), 2.21 (dd, J¼14.4, 5.2 Hz,
1H), 2.00 (m, 2H), 1.78 (m, 1H), 1.67e1.15 (m, 11H), 0.97 (s, 3H), 0.89
(s, 3H), 0.84 (s, 3H); HRMS (ESIþ) m/z calcd for C29H39NO5Na
[MþNa]þ 504.2720; found 504.2716. The small quantity compound
26 obtained (0.8 mg) did not allow us to record a full 13C NMR
spectrum.
4.3.1.1. (R)-((S)-1-((R)-2,5-Dioxopyrrolidin-3-yl)-2-((1S,4aS,8aS)-
5,5,8a-trimethyl-2-methylenedecahydronaphthalen-1-yl)ethyl) 2-me-
20
thoxy-2-phenylacetate (23). [
a
]
29.52 (c 2.0, CHCl3); IR (film) nmax
D
3169, 2928, 2850, 1748, 1719, 1643, 1456, 1352, 1177, 1111, 1012, 903,
755, 696 cmꢁ1; 1H NMR (400 MHz, CDCl3)
d
7.70 (br s, 1H), 7.36 (m,
5H), 5.52 (m, 1H), 4.92 (s, 1H), 4.90 (s, 1H), 3.40 (s, 3H), 3.03 (ddd,
J¼8.4, 6.4, 2.4 Hz, 1H), 2.69 (d, J¼8.4 Hz, 1H), 2.67 (d, J¼6.4 Hz, 1H),
2.37 (ddd, J¼12.8, 3.6, 2.0 Hz, 1H), 1.92e1.67 (m, 4H), 1.63e0.98 (m,
10H), 0.90 (s, 3H), 0.78 (s, 3H), 0.61 (s, 3H); 13C NMR (100 MHz,
Acknowledgements
CDCl3)
d
176.7, 175.4, 169.2, 146.3, 136.1, 128.74, 128.67 (2C), 126.8
We thank the Agency for Science, Technology and Research
(2C), 108.5, 82.6, 72.3, 57.5, 55.7, 55.6, 44.0, 41.9, 39.6, 39.1, 38.0,
33.55, 33.51, 29.9, 25.9, 24.1, 21.6, 19.2, 14.2; HRMS (ESIþ) m/z calcd
for C29H39NO5Na [MþNa]þ 504.2720; found 504.2713.
(A STAR) of Singapore and MerLion Pharmaceuticals Pte Ltd for
*
funding this project, Prof. Brian Cox (AstraZeneca, Macclesfield, UK)
and Prof. Richard J. K. Taylor (Department of Chemistry, University
of York) for helpful discussions. We also thank Dr. Paul Bernardo
(ICES) for preliminary studies, Ms. Xu Jin (ICES) and the Chemical
Synthesis Laboratory (ICES) for HPLC support.
4.3.1.2. (R)-((S)-1-((R)-2,5-Dioxopyrrolidin-3-yl)-2-
((1S,4aS,8aS)-2,5,5,8a-tetramethyl-1,4,4a,5,6,7,8,8a-octahydronaph-
20
thalen-1-yl)ethyl) 2-methoxy-2-phenylacetate (24). [
a]
ꢁ2.33 (c
D
2.3, CHCl3); IR (film) nmax 3249, 2924, 2849, 1747, 1714, 1712, 1456,
Supplementary data
1349, 1178, 1104, 754, 699 cmꢁ1 1H NMR (400 MHz, CDCl3)
;
d
7.59
(br s, 1H), 7.36 (m, 5H), 5.56 (m, 1H), 5.41 (br s, 1H), 4.71 (s, 1H), 3.39
(s, 3H), 3.15 (ddd, J¼7.2, 5.1, 2.1 Hz, 1H), 2.70 (d, J¼7.2 Hz, 2H), 1.96
(m, 1H), 1.82 (m, 1H), 1.74 (s, 3H), 1.60e1.36 (m, 8H), 1.13 (m, 2H),
0.86 (s, 3H), 0.85 (s, 3H), 0.67 (s, 3H); 13C NMR (100 MHz, CDCl3)
1H and 13C NMR spectra for compounds 10, 16, 18, 19, 7, 22e25
and 1H NMR spectrum of 26 are provided. Supplementary data
associated with this article can be found in the online version at
doi:10.1016/j.tet.2010.09.031. These data include MOL files and

9276
T.M. Nguyen et al. / Tetrahedron 66 (2010) 9270e9276
10. Robert, F.; Gao, H. Q.; Donia, M.; Merrick, W. C.; Hamann, M. T.; Pelletier, J. RNA
2006, 12, 717.
11. González, M. A.; Romero, D.; Zapata, B.; Betancur-Galvis, L. Lett. Org. Chem.
InChiKeys of the most important compounds described in this
article.
2009, 6, 289.
12. Hajra, S.; Giri, A. K.; Karmakar, A.; Khatua, S. Chem. Commun. 2007, 2408.
13. Stoncius, A.; Nahrwold, M.; Sewald, N. Synthesis 2005, 11, 1829.
14. Boukouvalas, J.; Wang, J.-X.; Marion, O.; Ndzi, B. J. Org. Chem. 2006, 71, 6670.
15. (a) Kim, H.-C.; Park, O.-S. Tetrahedron: Asymmetry 2008, 19, 896; (b) Evans, D.
A.; Briton, T. G.; Ellman, J. A. Tetrahedron Lett. 1987, 28, 6141.
16. For examples of succinimide formation starting from succinic acid derivatives
with ammonia, see: (a) Obniska, J.; Kaminski, K.; Skrzynska, D.; Pichor, J. Eur. J.
Med. Chem. 2009, 44, 2224; (b) Miller, C. A.; Long, M. L. J. Am. Chem. Soc. 1953,
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18. Attempts to recrystallise either 18 or 19 obtained in small quantities after HPLC
separation for X-ray analysis in order to determine the stereochemistry, were
unsuccessful.
References and notes
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19. The energy minimised conformations were obtained using the MMFF94 force
field, Spartan 04 version for Windows; Wavefunction, Inc.
20. (a) Evans, D. A.; Bartrolo, J.; Shih, T. L. J. Am. Chem. Soc. 1981, 103, 2127; (b)
Evans, D. A.; Rieger, D. L.; Bilodeau, M. T.; Urpi, F. J. Am. Chem. Soc. 1991, 113,
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