Stereoselective synthesis of functionalized pyroglutamates

Article abstract of DOI:10.1016/j.tetlet.2011.08.029

Novel stereoselective synthesis of α-methylene-β-substituted pyroglutamates, and α-alkylidene-pyroglutamates has been achieved via substrate controlled asymmetric alkylation of l-threonine derived oxazole with Baylis-Hillman reaction based allyl bromides

Full text of DOI:10.1016/j.tetlet.2011.08.029

Tetrahedron Letters 52 (2011) 5349–5351
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Stereoselective synthesis of functionalized pyroglutamates
Matthew J. Just a,c, Srinivas Tekkam a, Mohammad A. Alam a, Subash C. Jonnalagadda b, Joseph L. Johnson a,
Venkatram R. Mereddy a,c,
⇑
a Department of Chemistry and Biochemistry, University of Minnesota Duluth, Duluth, MN 55812, USA
b Department of Chemistry and Biochemistry, Rowan University, Glassboro, NJ 08028, USA
c Department of Pharmacy Practice and Pharmaceutical Sciences, College of Pharmacy, University of Minnesota, Duluth, MN 55812, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel stereoselective synthesis of
lutamates has been achieved via substrate controlled asymmetric alkylation of
zole with Baylis–Hillman reaction based allyl bromides and acetates, respectively. The synthesized
compounds were evaluated for their proteasome inhibition and cytotoxicity on multiple myeloma cells.
Ó 2011 Elsevier Ltd. All rights reserved.
a
-methylene-b-substituted pyroglutamates, and
a-alkylidene-pyrog-
Received 7 July 2011
Revised 1 August 2011
Accepted 3 August 2011
Available online 11 August 2011
L
-threonine derived oxa-
Keywords:
Pyroglutamates (c-carboxy-c-lactams)
Threonine
Oxazoles
Substrate controlled alkylation
Baylis–Hillman bromides/acetates
1. Introduction
O
Cl
O
O
O
O
C5H11
HN
O
HO
Five-membered nitrogen containing heterocyclic structural
units such as pyroglutamates (c-carboxy-c-lactams) are important
HN
HN
HN
OH HO
OH
HO
H
HO
H
S
O
structural motifs that serve as valuable synthons in the preparation
of several types of complex natural products and heterocycles.1 Re-
cently, several pyroglutamate containing natural products such as
lactacystin 1, omuralide 2, salinosporamides 3, cinnabaramides A–
G 4, etc. (Fig. 1) have been isolated from terrestrial strain of Strep-
tomyces sp. and marine actinomycete Salinispora tropica.2 Several
of these molecules were found to exhibit potent anti-cancer,
anti-microbial, and other important medicinal properties.2 Owing
to the importance of pyroglutamates in medicinal and materials
chemistry, a general methodology for facile synthesis of highly
substituted chiral pyroglutamates is highly desirable. Recently,
we reported a diastereoselective methodology for the synthesis
of pyroglutamates starting from chlorobenzaldehyde imines of
O
O
O
O
Ac
1
4
2
N
3
Omuralide
H
Cinnabaramide
O
Salinosporamide
Lactacystin
Figure 1. Pyroglutamate based natural products.
O
NH.HCl
OH
O
R
OMe
OMe
OBn
LDA
THF, -78oC
OBn
8a, R = H
6
8a-c
O
N
O
8b
, R = Me
, R = Ph
5
Br
NH2
8c
CH2Cl2, reflux
7
racemic a
-amino acids.3 In this Letter, we report the enantioselec-
tive synthesis of functionalized pyroglutamates starting from
threonine derived oxazole.
O
L-
O
Ph
O
OMe
O
OMe
O
HN
O
O
O
N
Ph
NH2
HCl
R
O
reflux
R
65-69%
R
OBn
11a-c
2. Results and discussion
COOBn
9a-c
COOBn
de > 90%
10a-c
We envisaged that the alkylation4 of threonine oxazole 7 with
ester containing allylic bromides or acetates obtained via Baylis–
Scheme 1. Preparation of a-methylene-b-substituted pyroglutamates.
Hillman (BH) alcohols should provide functionalized pyrogluta-
mates in one step. The required oxazole 7 was synthesized by
⇑
Corresponding author.
E-mail address: vmereddy@d.umn.edu (V.R. Mereddy).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.08.029

5350
M. J. Just et al. / Tetrahedron Letters 52 (2011) 5349–5351
Table 1
Alkylation of oxazole 7 with BH bromides/acetates8
NH.HCl
OH
O
OAc O
O
OMe
OBn
LDA
R
OMe
O
O
6
12a-b
(a) LDA
O
N
O
5
NH2
HN
O
THF, -78oC
OBn
N
O
OBn
R1
12a
, R = Me
CH2Cl2, reflux
8a-c
BH-Bromide
or
Ph
12b, R = Ph
R2
7
7
BH-Acetate 12a-b
OBn
O
O
(b) HCl
R1, R2 = H, Me, or Ph
O
Ph
O
11 or 15
OMe
R
O
HN
O
OMe
O
R
O
NH2
Ph
HCl
#
1
Bromide/acetate
Product
Yield
65
O
N
de > 90%
reflux
OBn
O
O
O
72-78%
15a-b
R
COOBn
14a-b
COOBn
13a-b
R = Me, Ph
O
O
Ph
OMe
HN
Br
8a
Scheme 2. Preparation of a-alkylidene pyroglutamates.
OBn
O
R1 = R2 = H, 11a
O
O
the reaction of threonine benzyl ester 5 with methyl benzimidate 6
in refluxing CH2Cl2. We chose bromides and acetates derived from
BH alcohols obtained via the reaction of formaldehyde, acetalde-
hyde, and benzaldehyde with methyl acrylate.5 Alkylation of oxa-
zole 7 with methyl bromomethylacrylate 8a followed by acidic
work up provided pyroglutamate 11a in good yield (Scheme 1).
The proton NMR analysis of the crude material revealed the forma-
tion of predominantly one diastereomer. The reaction took place in
highly stereoselective fashion and the alkylation proceeded from
the less hindered face of the enolate to provide pyroglutamate
11a as the major diastereomer (>90% de). Similarly, the alkylation
of 7 with BH bromides 8b–c obtained from acetaldehyde and benz-
aldehyde, respectively, proceeded in highly selective manner and
b-methyl/phenyl pyroglutamates 11b–c were obtained as the ma-
jor isomers (Table 1). The relative stereochemistry was ascertained
via single crystal X-ray analysis of the b-phenylpyroglutamate 11c
(Fig. 2).
O
OMe
HN
O
Ph
2
3
4
5
67
69
72a
78
Br
8b
OBn
O
R1 = H, R2 = Me, 11b
O
O
O
Ph
Ph
OMe
HN
O
Br
8c
Ph
OBn
O
R1 = H, R2 = Ph, 11c
OAc
O
O
O
Ph
OMe
HN
O
12a
OBn
O
R1 = Me, R2 = H, 15a
OAc
O
O
O
Ph
OMe
HN
We then extended the methodology towards the synthesis of a-
O
Ph
Ph
alkylidine/arylidene pyroglutamates via the alkylation of 7 with
allylic acetates 12a–b. The reaction took place smoothly in SN20
fashion, and the products 15a–b were obtained in good yield and
diastereoselectivity (Scheme 2 and Table 1).
All of the pyroglutamate natural products described in Figure 1
are potent proteasome inhibiting anti-cancer agents. Since the syn-
thesized molecules described in Schemes 1 and 2 have the core
pyroglutamate moiety, we evaluated the biological efficacy of
12b
OBn
R1 = Ph, R2 = H, 15b
O
a
The (Z)-diastereomer was also obtained as the minor isomer (ꢀ10%) for pyro-
glutamate 15a.
Figure 2. X-ray crystal structure of 11c.

M. J. Just et al. / Tetrahedron Letters 52 (2011) 5349–5351
5351
155–204; (d) Poulsen, T. B.; Dickmeiss, G.; Overgaard, J.; Jørgensen, K. A. Angew.
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Y.; Chang, N. C. Org. Lett. 2003, 5, 1761–1763.
these analogs as potential proteasome inhibitors. Unfortunately,
none of these molecules showed any significant enzyme inhibition
activity6 or cytotoxicity against multiple myeloma (RPMI-8226)
cancer cell lines7 even at 50
lM concentration.
2. (a) Guilder, T. A. M.; Moore, B. S. Angew. Chem., Int. Ed. 2010, 49, 9346–9367; (b)
Shibasaki, M.; Kanai, M.; Fukuda, N. Chem. Asian J. 2007, 2, 20–38; (c) Stadler, M.;
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Morgan, A. J.; Adams, J.; Panek, J. S. Eur. J. Org. Chem. 2000, 2513–2528.
3. Tekkam, S.; Alam, M. A.; Jonnalagadda, S. C.; Mereddy, V. R. Chem. Commun.
2011, 3219–3221.
4. (a) Seebach, D.; Aebi, J. D. Tetrahedron Lett. 1983, 24, 3311–3314; (b) Calderari,
G.; Seebach, D. Helv. Chim. Acta 1985, 68, 1592–1604; (c) Corey, E. J.; Choi, S.
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Douglas, C. J.; Garg, N. K.; Hiebert, S.; Jaroch, S.; Matsuoka, R. T.; Middleton, J. A.;
Ng, F. W.; Overman, L. E. J. Am. Chem. Soc. 2007, 129, 11987–12002.
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(b) Basavaiah, D.; Rao, A. J.; Satyanarayana, T. Chem. Rev 2003, 103, 811.
6. All the synthesized pyroglutamates 11a–c and 15a–b were tested for their
ability to inhibit human erythrocyte 20S proteasome (Enzo Life Sciences) in
3. Conclusions
In conclusion, we have carried out a highly diastereoselective
alkylation of threonine based oxazoline with Baylis–Hillman-de-
rived allyl bromides. Upon acidic hydrolysis,
a-methylene-b-al-
kyl/aryl- -carboxy- -lactams with -hydroxyethyl side chain
c
c
a
were obtained in enantiomerically pure form. The oxazoline upon
alkylation with Baylis–Hillman reaction derived allylic acetates
proceeded smoothly with allylic rearrangement to provide
alkylidine/arylidene- -carboxy- -lactams in highly stereoselective
fashion. Owing to the importance of pyroglutamates in various
fields and the scarcity of synthetic procedures coupled with the
versatility of BH chemistry make the current methodologies highly
important.
a-
c
c
100 ll real-time FRET assays employing the Suc-LLVY-AMC fluorogenic peptide
substrate in ½ volume white 96-well plates. Release of free AMC by proteolytic
activity was monitored on Molecular Devices M5 plate reader with excitation
and emission wavelengths of 360 and 460 nm, respectively. Inhibition assays at
multiple compound concentrations were done in triplicate and averaged.
Acknowledgments
However, none of the compounds showed significant inhibition at 50
concentration.
lM
7. RPMI-8226 multiple myeloma cancer cells were plated in 96 well plates and
allowed to adhere for 72 h. Cells were then treated with each compound
We thank the Departments of Chemistry and Biochemistry, Uni-
versity of Minnesota Duluth and Rowan University for the funding.
Partial support for this work was also provided by research grants
from the University of Minnesota Academic Health Center Faculty
Development Grant (V.R.M.), Whiteside Institute for Clinical Re-
search (V.R.M.), and Rowan University Non-Salary Financial Sup-
port Grants (NSFSG) (S.C.J.). We thank Dr. Victor G. Young, Jr.
(University of Minnesota X-ray Crystallographic Laboratory) for
providing the crystal structure.
(50
number of remaining viable cells after exposure to compounds. 20
was added to 100 l culture medium in each well. After incubation at 37 °C for
3 h, absorbance was measured using an ELISA plate reader. All the compounds
tested under these conditions proved to be non-toxic at 50 M.
l
M) or with DMSO alone for 24 h. MTS assay was used for determining the
ll of MTS
l
l
8. Representative procedure for the preparation of functionalized pyroglutamates: To a
solution of LDA (30 ml, 2 M in THF/heptane/ethyl benzene) in THF (140 ml) was
added threonine oxazoline (10 g, 33.9 mmol) drop wise at ꢁ78 °C. After 1 h,
bromide 8a (8.9 g, 50 mmol) was added and stirred for 6 h at ꢁ78 °C. The
reaction was quenched with 3 M HCl and extracted with ethyl acetate, washed
with brine, and concentrated in vacuo. The crude product was refluxed in THF
and 3 M HCl for 1 h. The reaction was worked up with NaHCO3 and ethyl acetate,
dried (MgSO4), concentrated in vacuo, and purified by silica gel column
chromatography to get the lactam 11a (8.36 g, 65% yield). 1HNMR (500 MHz,
CDCl3) d 7.89–7.87 (m, 2H), 7.55 (m, 1H), 7.39–7.36 (m, 7H), 7.25 (br s, 1H), 6.00
(dd, J = 2.5, 3.0 Hz, 1H), 5.45 (q, J = 6 Hz, 1H), 5.31–5.33 (m, 1H), 5.29 (d,
J = 12.5 Hz, 1H), 5.21 (d, J = 12.5 Hz, 1H), 3.19 (ddd, J = 2.5, 3.0, 17.5 Hz, 1H), 3.00
(ddd, J = 2.5, 3.0, 17.5 Hz, 1H), 1.31 (d, J = 6.5 Hz, 3H); 13C NMR (125 MHz, CDCl3)
d 170.9, 169.7, 165.7, 137.4, 134.9, 133.6, 130.0, 129.5, 129.0, 128.9, 128.7,
128.6, 117.4, 73.8, 68.4, 65.7, 34.2, 15.2; ESI-MS: 402 [M+Na]+, 380 [(M+H)+,
100%], 362 [(M+HꢁH2O)+]; HRMS (ESI) m/z: calcd for C22H21NO5 [M+Na]+:
402.1334, found 402.1318.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.08.029.
References and notes
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Benoit, R.; Pascal, C.; Dominique, F.; Francois, S. Trends Heterocycl. Chem. 1991, 2,