Stereoselective formal synthesis of the potent proteasome inhibitor: salinosporamide A

Article abstract of DOI:10.1016/j.tetlet.2006.11.087

(2R,3S)-α-Methylenelactam 3, the key intermediate in Corey's syntheses of salinosporamide A, has been synthesized from (S)-methyl 2-hydroxymethylpyroglutamate through chemoselective O-protection, regio- and stereoselective N-methylnitrone cycloaddition an

Full text of DOI:10.1016/j.tetlet.2006.11.087

Tetrahedron Letters 48 (2007) 381–384
Stereoselective formal synthesis of the potent proteasome
inhibitor: salinosporamide AI
*
´
Virginie Caubert, Julien Masse, Pascal Retailleau and Nicole Langlois
Institut de Chimie des Substances Naturelles, CNRS, Avenue de la Terrasse, 91198 Gif-sur-Yvette, France
Received 6 October 2006; revised 10 November 2006; accepted 14 November 2006
Available online 4 December 2006
Dedicated to the memory of Guy Ourisson, deceased November 3 2006
Abstract—(2R,3S)-a-Methylenelactam 3, the key intermediate in Corey’s syntheses of salinosporamide A, has been synthesized from
(S)-methyl 2-hydroxymethylpyroglutamate through chemoselective O-protection, regio- and stereoselective N-methylnitrone cyclo-
addition and quaternarization–elimination reactions as the main steps.
Ó 2006 Elsevier Ltd. All rights reserved.
The microbial secondary metabolite salinosporamide A
(1) was isolated in 2003 by Fenical and co-workers from
Salinispora tropica, a unique marine microorganism
belonging to the MAR 1 phylotype of these actino-
mycetes.2 Produced by the CNB-392 strain, salinospor-
amide A is of considerable interest owing to its
These properties initiated several synthetic studies
which, so far, led to the publication of four total synthe-
ses by the groups of Corey, Danishefsky and Patten-
den.6–9 Both syntheses developed by Corey et al.
involved the highly functionalized a-methylene-lactam
3 as a key intermediate.6,7
Cl
OH
OH
O
O
O
O
CO2Me
OBn
CO2Me
OBn
OH
OH
O
O
O
O
N
H
N
H
N
N
H
Boc
PMB
1
2
3
4
remarkable biological properties and potentialities. It
indeed inhibits the three proteolytic sites of the protea-
some 20S subunit in a particularly effective and selective
way,3 being even more potent than the structurally
related b-lactone-c-lactam omuralide (2) itself. More-
over, 1 was found to display high cytotoxicity towards
many tumour cell lines, namely HCT-116 human colon
carcinoma and thalomid- and bortezomib-resistant mul-
tiple myeloma cells,4,5 and the molecule (as NPI-0052) is
currently in clinical development for treatment of cancer.
Recent efforts in our laboratory focused on the highly
diastereoselective synthesis of 4, as a closely related scaf-
fold that contains the required functionalities and can
provide access to the racemic target molecule. Our syn-
thetic strategy relied on the practical preparation of
pyrrolinone 5 as a suitable electron-deficient dipolaro-
phile and was based on the regio- and stereo-selective
1,3-dipolar cycloaddition of N-methylnitrone leading
to 6 (Scheme 1). Thus, 4 was obtained in two steps from
the cycloadduct 6, through hydrogenolysis with Pearl-
man’s catalyst and subsequent quaternarization–elimi-
nation step.10,11
Keywords: Proteasome inhibitor; Salinosporamide A; Pyroglutamate;
1,3-Dipolar cycloadditions; N-Methylnitrone; Selective O-benzylation.
As an extension of this work, we next turned our atten-
tion towards an asymmetric route to 4 and to Corey’s
key intermediate 3 itself, and we report here our
results in both directions.1 To obtain the corresponding
q See Ref. 1.
*
Corresponding author. Fax: +33
langlois@icsn.cnrs-gif.fr
1 69077247; e-mail: nicole.
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.11.087

382
V. Caubert et al. / Tetrahedron Letters 48 (2007) 381–384
N
O
OH
a
b
CO2Me
OBn
CO2Me
OBn
CO2Me
OBn
O
O
O
N
N
N
Boc
Boc
Boc
rac-5
rac-4
rac-6
Scheme 1. Reagents and conditions: (a) N-methylnitrone, toluene, D, 57% and (b) (i) H2, Pd(OH)2, EtOAc–MeOH, 72%; (ii) MeI, THF, Et3N, 65%.
enantiopure 3 and 4 with ‘natural’ 2R,3S configurations,
it was assumed that (S)-methyl 2-hydroxymethylpyro-
glutamate 8 could be the precursor of choice. This com-
pound has been prepared for the first time from
bicyclic nitrile 7 as an intermediate in the synthesis of
deoxydysibetaine.12
validity of this pathway was demonstrated from racemic
9 (Scheme 3, relative configurations shown).
Accordingly, 2-benzyloxymethylpyroglutamate 9 was
treated with KHMDS and 4-methoxybenzylbromide to
give the N-PMB derivative 10 in high yield (93%). How-
ever, starting from 10, the conjugate double bond could
only be introduced in poor yield (17%) under classical
conditions of phenylselenylation, using LDA as the
base, followed by selenoxide elimination. This could be
explained, in part, by lower reactivity of 10 compared
to its N-Boc analogue, due to electronic factors. Conse-
quently, the PMB group was introduced at a later stage
of the synthesis. Pyrrolinone 11 was quantitatively
obtained from 5 by removal of the N-Boc group and
subsequent N-4-methoxybenzylation furnished the pro-
tected derivative 12 (57%).
Therefore, selective O-benzylation of (S)-8 into (S)-9
was investigated under varied conditions and shown to
be rather difficult, due to the sensitivity of the methoxy-
carbonyl group towards hydrolysis or transesterifica-
tion, giving rise to the corresponding carboxylic acid
and benzyl ester.13 N,O-Dibenzylation could not be
avoided with benzylbromide and Cs2CO3 as the base,
and experiments using benzyl 2,2,2-trichloroimidate
failed.14 The best results were reached with 2-benzyl-
oxy-1-methylpyridinium triflate as a selective, mild and
nearly neutral benzylating agent in the presence of
MgO,15 leading to (S)-9 in 75% yield (Scheme 2).16
N-Methylnitrone cycloaddition to pyrrolinone 12 was
performed by heating in toluene and gave essentially a
major product 14A (54%; 79% based on recovered start-
ing material) and small amounts of another cycloadduct
14B (14%). The structures of these adducts were de-
duced from spectroscopic and particularly NMR data
and comparison with related cycloadducts such as 6,
This result constitutes an access to enantiopure (2R,3S)-
4, since the later steps to 4, according to Scheme 1,10
cannot lead to racemization. (S)-Methyl 2-benzyl-
oxymethylpyroglutamate (S)-9 could also lead to
(2R,3S)-3 following the route depicted below, and the
BnO
TfO–
N
+
MgO
CN
CO2Me
OH
CO2Me
OBn
see ref. 12
O
N
O
N
H
O
N
H
PhCF3, Δ, 75%
O
Ph
(S)-8
7
(S)-9
Scheme 2.
N
N
N
3
O
O
CO2Me
OBn
CO2Me
OBn
b
3a
6a
d
CO2Me
OBn
O
O
+
N
CO2Me
OBn
N
6
O
N
R
R
O
PMB
PMB
9 : R = H
11 : R = H
a
14B
c
14A
10 : R = PMB
12 : R = PMB
OH
OH
HN
f
e
CO2Me
OBn
CO2Me
OBn
O
N
O
N
PMB
PMB
3
17
Scheme 3. Reagents and conditions: (a) KHMDS, PMBBr, THF, 93%; (b) (i) to 5, see Ref.10; (ii) CF3CO2H, CH2Cl2, 100%; (c) Cs2CO3, PMBBr,
DMF, 57%; (d) N-methylnitrone, toluene, D, 68%; (e) H2, Pd(OH)2, EtOAc–MeOH, 64% and (f) (i) MeI, MeOH; (ii) Na2CO3, CH2Cl2, 90%.

V. Caubert et al. / Tetrahedron Letters 48 (2007) 381–384
383
Table 1. Chemical shifts for characteristic protons and carbons of cycloadducts 6, 14–16 (300 MHz, CDCl3)
H-6a
H-3
H-3a
3.14
C-6a
C-3
C-3a
N
N
3
O
3a
6a
CO2Me
—
3.64
2.60
83.60
60.67
58.01
56.80
O
OBn
Boc
6
N
O
3
3a
6a
—
3.71
2.59
3.07
84.61
59.94
CO2Me
OBn
O
N
PMB
14A
N
O
3
3a
6a
—
3.69
2.64
3.04
3.59
3.55
83.58
78.66
77.34
61.03
60.63
60.11
56.71
53.20
51.82
CO2Me
OBn
O
N
PMB
14B
15
N
O
3
3a
6a
4.66
4.65
3.64
2.62
CO2Me
OBn
O
N
Boc
N
O
3
3a
6a
3.64
2.57
CO2Me
OBn
O
N
PMB
16
15 and 16, previously prepared in our group. The
informative chemical shifts of isoxazolidine protons
and carbons are collected in Table 1. Similar values were
observed for 14A and 14B, indicating that these two
compounds result from the same regioselectivity in the
1,3-dipolar cycloaddition. Indeed, the protons at the
ring junction (H-3a) give rise to signals at 3.07 and
3.04 ppm, whereas the corresponding methine carbons
generate signals at 56.80 and 56.71 ppm, respectively.
These chemical shifts are close to those of structural
analogue 6, in agreement with a position a to the N-
methylene. A NOESY experiment performed on 14A
showed a correlation between 6a-CH3 and one of the
6-CH2 protons, consistent with these two groups being
on the same side of the pyrrolidinone ring. This, conse-
quently, allowed the assignment of the relative configu-
rations of the stereoisomer 14B. A X-ray analysis
carried on 14B confirmed these conclusions (Fig. 1).17
With compound 14A in hand, the synthesis was pursued
towards intermediate 3. The isoxazolidine ring was
hydrogenolyzed in the presence of Pd(OH)2 (Scheme
3). Fortunately, these conditions did not affect the O-
benzyl protecting group and compound 17 was obtained
in 64% yield. N-Methylation with iodomethane in
methanol led to the corresponding trimethyl ammonium
salt, which was treated straight away by a biphasic
mixture of aqueous sodium carbonate and dichloro-
methane. This elimination step required a long reaction
time to afford, after stirring for 4 days at room temper-
ature, the target a-methylenelactam 3 in high yield
Therefore, the N-methylnitrone cycloaddition to 12
occurred with expected regioselectivity. As generally
agreed, 1,3-dipolar cycloadditions are concerted and
the regiochemistry appears to be controlled by frontier
orbital interactions. The stereoselectivity (ratio 4:1)
however was somewhat lower than those observed in
the case of 5 when only one diastereoisomer was de-
tected. This selectivity depends on a subtle interplay of
several steric and electronic factors, and the nature of
the protecting group at the lactam nitrogen seemed, in-
deed, to have a sensitive impact on the transition state,
and therefore on the outcome of the reaction.18–20
Figure 1. ORTEP diagram from the X-ray crystallographic analysis of
cycloadduct 14B.

384
V. Caubert et al. / Tetrahedron Letters 48 (2007) 381–384
(90%).10,11 The whole analytical data of 3 are in full
agreement with those described,6,7 and these results
complete the formal synthesis of salinosporamide A.
8. Endo, A.; Danishefsky, S. J. J. Am. Chem. Soc. 2005, 127,
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In conclusion, N-methylnitrone cycloaddition to a
conveniently polysubstituted pyrrolinone derived from
(S)-pyroglutaminol provides a valuable pathway to the
key intermediate 3, which has been converted to salinos-
poramide A by Corey and co-workers. The cyclo-
addition reaction occurred with good regio- and
stereoselectivities. Extension of this methodology to
the synthesis of various analogues, which could be
designed by the binding mode of salinosporamide A,21
is currently under investigation.
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Acknowledgement
We are grateful to ICSN (CNRS, Gif-sur-Yvette) for
financial assistance.
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16. Spectral data of (S)-9: Mp = 79–80 °C. ½aꢀD +25.7 (c 1.72,
´
1. Caubert, V.; Masse, J.; Retailleau, P.; Langlois, N.
CHCl3). IR: 3194, 3079, 2956, 1731, 1692, 1432, 1351. MS
(ESI, MeOH) m/z: 287, 286 [(MNa)+ 100%], 264. 1H
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