Studies toward the synthesis of salinosporamide A, a potent proteasome inhibitor

Article abstract of DOI:10.1016/j.tetlet.2006.04.070

An α-methylenepyrrolidinone bearing all the functionalities and relative configurations of an advanced intermediate in the synthesis of salinosporamide A and analogues has been synthesized from methyl pyroglutamate through regio- and stereoselective N-methylnitrone cycloaddition.

Full text of DOI:10.1016/j.tetlet.2006.04.070

Tetrahedron Letters 47 (2006) 4473–4475
Studies toward the synthesis of salinosporamide A, a potent
proteasome inhibitor
Virginie Caubert and Nicole Langlois*
Institut de Chimie des Substances Naturelles, CNRS, Avenue de la Terrasse, 91198 Gif-sur-Yvette, France
Received 16 March 2006; revised 5 April 2006; accepted 7 April 2006
Available online 12 May 2006
Dedicated to the memory of Pierre Potier, a friend, deceased February 3 2006
Abstract—An a-methylenepyrrolidinone bearing all the functionalities and relative configurations of an advanced intermediate in
the synthesis of salinosporamide A and analogues has been synthesized from methyl pyroglutamate through regio- and stereoselec-
tive N-methylnitrone cycloaddition.
Ó 2006 Elsevier Ltd. All rights reserved.
The examination of numerous Salinispora strains from a
marine actinomycete Salinispora tropica showed that
most of these organisms produce culture extracts dis-
playing potent biological activity, and, particularly,
growth inhibition of tumor cells such as human colon
carcinoma HCT-116. S. tropica strain CNB-392 culture
broth produces salinosporamide A (NPI 0052) 1, which
is structurally related to omuralide, a b-lactone derived
from lactacystin.1,2 Omuralide is known to specifically
inhibit the proteolytic activity of the proteasome 20S
subunit without affecting other proteasome activities.3
Salinosporamide A (1) is a more effective proteasome
inhibitor than omuralide, and exhibits cytotoxicity
against many tumor cell lines,4 and particularly, against
VelcadeÒ resistant multiple myeloma cells.2,5 Two of the
synthetic routes to salinosporamide A developed so
far4,6,7 involve the multifunctionalized a-methylene-lac-
tam 2 as a key intermediate. This intermediate 2 was
synthesized by Corey et al., through a cyclization step
performed under Baylis–Hillman conditions,4 or better,
by treatment with Kulinkovich reagent,6 and structural
analogues have also been prepared in a similar way.8,9
We describe here a novel diastereoselective route to a
closely related a-methylene-lactam 3, which possesses
all the required functionalities, starting from inexpensive
methyl pyroglutamate 4. This diastereocontrolled syn-
thesis, as outlined in Scheme 1, was based on a selective
1,3-dipolar cycloaddition of N-methylnitrone to intro-
duce, in one step, the C-3 hydroxyl group and the pre-
cursor of C-4 exo-methylene group.
Our initial efforts were devoted to the preparation of an
unsaturated 3-methylated intermediate 8. The benzyl-
oxymethyl group was directly introduced at C-2 of
methyl pyroglutamate 4. Regioselective deprotonation
of 4 with LiHMDS (2.2 equiv),10 and a-alkylation with
chloromethylbenzylether gave rise to 5 (53%). Then,
N-protection as tert-butyl carbamate 6 (100%), improv-
ing the reactivity of lactam carbonyl, allowed efficient
classical introduction of the D3 double bond to give 7
(89%, Scheme 1).
Cl
OH
OH
O
O
CO2Me
OBn
CO2Me
OBn
OH
O
O
O
N
N
N
H
H
PMB
Boc
2
3
1
At this stage, two ways were investigated to convert pyr-
rolidinone 7 into its 3-methylated counterpart 8 (Scheme
2). The first one involved a 1,3-dipolar diazomethane
cycloaddition followed by thermolysis. Literature
surveys indicated that thermolysis of the adducts
between diazomethane and a,b-unsaturated lactam
derivatives received little attention for the preparation
Keywords: Protease inhibitor; Salinosporamide A; Pyroglutamate; 1,3-
dipolar cycloadditions; Diazomethane; N-Methylnitrone; Samarium
diiodide.
*
Corresponding author. Fax: +33 1 69077247; e-mail: nicole.langlois
@icsn.cnrs-gif.fr
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.04.070

4474
V. Caubert, N. Langlois / Tetrahedron Letters 47 (2006) 4473–4475
a
c
see scheme 2
CO2Me
OBn
CO2Me
OBn
O
O
O
O
CO2Me
N
R
N
N
H
4
Boc
7
5 R = H
6 R = Boc
b
d
N
O
OH
see scheme 3
CO2Me
OBn
CO2Me
OBn
CO2Me
OBn
O
N
O
N
N
Boc
Boc
Boc
8
3
9
Scheme 1. Reagents: (a) LiHMDS, THF, BnOCH2Cl (53%); (b) (Boc)2O, DMAP, CH3CN (100%); (c) (i) LiHMDS, THF, PhSeCl; (ii) H2O2,
CH2Cl2, py (89%); (d) N-methylnitrone, tol, D (57%).
N
N
to 13 in high yield (83%) as a sole detected diastereomer,
the relative configuration of which was deduced from
steric factors only. The introduction of D3 double bond,
as described for 7, afforded 8 in 75% yield (Scheme 2).
These nucleophilic and electrophilic additions of methyl
and phenylselenyl groups, respectively, could be realized
in one step, but the yield of 8 was dramatically lowered
by the use of this procedure. Nevertheless, the compar-
ison of the results in terms of efficiency is in favor of this
second route (overall yield from 7: 63% vs 35%).
N
N
a
CO2Me
OBn
+
CO2Me
OBn
CO2Me
OBn
O
O
O
N
N
N
Boc
Boc
Boc
7
10
11
c
b
CO2Me
OBn
CO2Me
OBn
CO2Me
OBn
d
O
O
O
N
N
H
N
Boc
Boc
13
8
12
With compound 8 in hand, we next investigated the 1,3-
dipolar cycloaddition of N-methylnitrone to its trisubsti-
tuted double bond. We have shown that N-methylnit-
rone cycloadditions to more simple N-alkoxycarbonyl
pyrrolinones proceed with good regio- and stereoselec-
tivities.16 Still higher selectivities were observed with 8,
although this compound is less reactive. It gave rise to
9 in 57% yield (96% based on recovered 8) after being
heated for 19 h at 110 °C. The structure of 9 was attrib-
uted by NMR spectral analysis, a NOESY experiment
indicating a weak correlation between the C-methyl
group and one of the two protons H-7. These attribu-
tions were confirmed by X-ray analysis of its hydro-
chloride, crystallized in a mixture of MeOH–Et2O–
pentane.17
Scheme 2. Reagents: (a) CH2N2, Et2O (11 + 12: 50%); (b) toluene, D
(70%); (c) Me2CuLi, THF, TMSCl (83%); (d) (i) LiHMDS, THF,
PhSeCl; (ii) H2O2, CH2Cl2, py (75%).
of methylated derivatives. The few examples using these
dipolarophiles occurred however with high regioselectiv-
ity,11–13 and we recently applied this methodology to a
very short synthesis of pulchellalactam.14 In order to
prepare 8, compound 7 was treated with an excess of
diazomethane in Et2O at rt to give pyrazolines 10 and
11, isolated together in 50% yield (76% based on recov-
ered 7) and separated in a ratio of ca 3:1. The structures
of these adducts, principally based on the chemical shifts
in H and 13C NMR, indicated the formation of the ex-
1
pected regioisomers and the relative configurations were
deduced from steric factors and NOESY (Scheme 2).
To cleanly cleave isoxazoline N–O bonds, samarium dii-
odide is known to be efficient at rt,18 and we used this
reagent to keep intact the benzyloxymethyl group.
However, the N-protecting group of 9 was first removed
under these conditions. This result could be explained by
chelation between samarium and the two carbonyl oxy-
gens of lactam and carbamate functions, as observed
with magnesium salts such as magnesium dichlo-
ride.16a,19 Starting with previously N-deprotected com-
pound 14 (100%), treatment with SmI2 led to the 3-
hydroxy-4-N-methylaminomethyl derivative 15 in 45%
not optimized yield. Fortunately, the cleavage of 9 by
hydrogenolysis under H2 using Pearlman’s catalyst pro-
ceeded chemoselectively, and did not affect the O-benzyl
group, affording 16 in 72% yield. Methylation of 16 with
excess iodomethane, followed by treatment with triethyl-
amine, provided the target molecule 3 (65%, Scheme 3).20
The thermolysis of D1-pyrazolines 10 and 11 led to the
same expected b-methyl unsaturated lactam 8 as the ma-
jor product. However, under the same conditions (tolu-
ene, reflux), the results obtained from 10 and 11 were
rather different: whereas 10 cleanly led to 8 (>85% deter-
mined by 1H NMR), the rearrangement of 11 proceeded
more slowly and also gave rise to the loss of N-tert-
butoxycarbonyl protecting group with the formation
of 12 (25%), as already observed in the thermolysis of
N-Boc indoles, pyrroles, and pyrrolidinones.15 It is also
interesting to note the absence of cyclopropane forma-
tion during these thermolyses. Using a mixture of 10
and 11, the b-methyl unsaturated lactam 8 was obtained
in 70% yield.
According to the second, more classical route investi-
gated to prepare 8, the methylcuprate stereoselective
addition to the unsaturated pyrrolidinone 7 gave rise
In conclusion, pyrrolidinone 3 bearing all the function-
alities and relative configurations of an advanced inter-
mediate in the synthesis of salinosporamide A and

V. Caubert, N. Langlois / Tetrahedron Letters 47 (2006) 4473–4475
4475
N
OH
O
a
HN
c
CO2Me
OBn
CO2Me
OBn
CO2Me
O
O
O
b
N
H
N
N
7
OBn
Boc
R
8
9 R = Boc
14 R = H
15
d
_
I
OH
OH
OH
HN
N
+
e
CO2Me
OBn
CO2Me
OBn
CO2Me
OBn
O
O
O
N
N
N
Boc
Boc
Boc
3
16
Scheme 3. Reagents: (a) N-methylnitrone, tol, D (57%); (b) CF3CO2H, CH2Cl2 (100%); (c) SmI2, THF (45%); (d) H2, Pd(OH)2, EtOAc–MeOH,
(72%); (e) MeI, THF, Et3N (65%).
10. Le Nguyen, B. K.; Langlois, N. Tetrahedron Lett. 2003,
44, 5961–5963.
11. Awad, W. I.; Abdel, S. M.; Omran, R.; Sobhy, M. J. Org.
Chem. 1961, 26, 4126–4128.
12. Romo, D.; Romine, J. L.; Midura, W.; Meyers, A. I.
Tetrahedron 1990, 46, 4951–4994.
analogues has been elaborated from methyl pyrogluta-
mate through regio- and stereoselective N-methylnitrone
cycloaddition to a suitable substituted a,b-unsaturated
lactam. An asymmetric version of this route is currently
under investigation.
13. Cuiper, A. D.; Kouwijzer, M. L. C. E.; Grootenhuis, P. D.
J.; Kellogg, R. M.; Feringa, B. L. J. Org. Chem. 1999, 64,
9529–9537.
Acknowledgements
14. Hermet, J.-P.; Caubert, V.; Langlois, N. Synth. Commun.
2006, 36, in press.
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26, 6141–6142; (b) Siro, J. G.; Martin, J.; Garcia-Navio, J.
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We are grateful to Professor J.-Y. Lallemand, Director
of ICSN, for a Grant (V.C.) and to P. Retailleau for
X-ray analysis of 9.
16. (a) Langlois, N.; Van Bac, N.; Dahuron, N.; Delcroix, J.-
M.; Deyine, A.; Griffart-Brunet, D.; Chiaroni, A.; Riche,
C. Tetrahedron 1995, 51, 3571–3586; (b) Langlois, N.;
Rakotondradany, F. Tetrahedron 2000, 56, 2437–2448.
17. Retailleau, P., Cambridge Crystallographic Data centre
(CCDC), CCDC 603586, 2006.
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20. Spectral data of 3: IR: 3448, 2929, 1777, 1726, 1454, 1368,
1299, 1253 cmÀ1. MS (ESI, MeOH) m/z: 428 (MNa)+, 328
[(MNaÀBoc)+, 100%]. 1H NMR (500 MHz, CDCl3:
7.27 ppm): 7.29, 7.18 (5H, H–Ar), 6.27 and 5.68 (2s, 2H,
H2C@C), 4.45 (apparent s, 2H, CH2Ph), 4.15 (d, 1H,
J = 10.4 Hz, Ha-7), 4.03 (d, 1H, J = 10.4 Hz, Hb-7), 3.77
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(s, 3H, OCH3), 1.59 (s, 3H, CH3-3), 1.49 (s, 9H, t-Bu). 13
C
(125 MHz, CDCl3: 77.14 ppm): 168.30 (CO, ester), 165.20
(NCO), 150.41 (NCO2), 145.61 (qC, C-4), 137.52 (qC, Ar),
128.60, 127.96, 127.66 (CH, Ar), 120.38 (CH2@), 84.07
(qC, t-Bu), 74.51, 74.04 (C-2, C-3), 73.62 (OCH2Ph), 67.87
(C-7), 52.68 (OCH3), 28.11 (CH3, t-Bu), 21.06 (CH3-3).
HRMS: calcd for C21H27NO7Na: 428.1685, found:
428.1682.