An efficient synthesis of [2.2.1] heterobicyclic pyroglutamates

Article abstract of DOI:10.1002/jhet.1097

A novel methodology for the efficient synthesis of [2.2.1] heterobicyclic pyroglutamates has been described. The key synthetic steps involve alkylation of amino acid-derived iminoesters with Baylis-Hillman bromide, RhCl ₃-catalyzed exocyclic olefin isomerization, diastereoselective dihydroxylation, and regioselective lactonization. All the compounds were evaluated for their cytotoxicity using multiple myeloma cancer cell lines RPMI 8226.

Full text of DOI:10.1002/jhet.1097

July 2013
An Efficient Synthesis of [2.2.1] Heterobicyclic Pyroglutamates
969
Srinivas Tekkam,a Joseph L. Johnson,a Subash C. Jonnalagadda,b and Venkatram R. Mereddya,c
*
aDepartment of Chemistry and Biochemistry, University of Minnesota Duluth,
Duluth, Minnesota 55812
bDepartment of Chemistry and Biochemistry, Rowan University, Glassboro, New Jersey 08028
cDepartment of Pharmacy Practice and Pharmaceutical Sciences, College of Pharmacy, University of Minnesota,
Duluth, Minnesota 55812
*
E-mail: vmereddy@d.umn.edu
Received March 14, 2011
DOI 10.1002/jhet.1097
Published online 1 July 2013 in Wiley Online Library (wileyonlinelibrary.com).
A novel methodology for the efficient synthesis of [2.2.1] heterobicyclic pyroglutamates has been
described. The key synthetic steps involve alkylation of amino acid-derived iminoesters with Baylis-Hillman
bromide, RhCl3-catalyzed exocyclic olefin isomerization, diastereoselective dihydroxylation, and
regioselective lactonization. All the compounds were evaluated for their cytotoxicity using multiple
myeloma cancer cell lines RPMI 8226.
J. Heterocyclic Chem., 50, 969 (2013).
INTRODUCTION
the benzyl ester 2. Treatment of this amino ester with
p-chlorobenzaldehyde in the presence of MgSO4 provided
quantitative yield of the imine 3 essentially as a pure
product. Imine 3 was used without further purification in
the next step. Reaction of 3 with lithium hexamethyldisilazide
(LiHDMS) provided the enolate 4. Alkylation of 4 with
Baylis-Hillman reaction [5] derived α-bromomethylacrylate
5, followed by acidic work up afforded the amino ester
7 on imine hydrolysis. The crude reaction mixture was
heated in toluene to affect lactamization to provide
pure α-methylenepyroglutamate 8a in good yield after
work up and silica gel column chromatography
(Scheme 1) [3].
The exocyclic methylene group in lactam 8a was
readily internalized via isomerization [3,6] with RhCl3
in refluxing tetrahydrofuran (THF) in excellent yield.
Upjohn dihydroxylation [7] of 9a with catalytic OsO4
and NMO afforded diol lactam 10a as the major diaste-
reomer (dr 10:1). The benzyl ester was efficiently
deprotected under standard hydrogenolysis conditions
using H2/Pd-C to provide the diol acid 11a in 90%
yield. Treatment of 11a with the condensing agent bis
(2-oxo-3-oxazolidinyl)phosphonic chloride (BOP chlo-
ride) afforded energetically favored [2.2.1] bicyclic
lactam-lactone (12a; Scheme 2). It was interesting to
note that the lactonization proceeded exclusively via
condensation of tertiary hydroxyl group in 11a with
the carboxylic acid unit. The [3.2.0] heterobicyclic
pyroglutamate that would result via the condensation
of β-hydroxy group with the acid was not observed
under the reaction conditions. To show the generality
Pyroglutamates (γ-carboxy-γ-lactams) are azaheterocyclic
compounds that serve as important building blocks for the
preparation of various heterocycles [1]. Derivatization of
the lactam and carboxylate moieties in pyroglutamates
allows the synthesis of various structurally interesting mole-
cules and natural products (Fig. 1). Some of the other uses of
these compounds include their applications in materials
chemistry as biocompatible nanopolymers [2a-c], lumines-
cent probes [2], nonlinear optics [2e], and high-strength or-
ganic–inorganic hybrid gel materials [2f]. They also find
applications as laundry additives to detergents [2g] and as
moisturizing agents in cosmetic applications [2h]. Although
there have been some reports on the synthesis of
pyroglutamates [1], a reliable methodology for the diverse
functionalization of pyroglutamates would be highly desir-
able. In this regard, we have recently described a novel pro-
tocol for the synthesis of functionalized pyroglutamates
using alkylation of amino acid-derived imines with Baylis-
Hillman bromides and acetates [3]. In this article, we report
the application of our methodology for the synthesis of
[2.2.1] heterobicyclic pyroglutamates. These molecules
could serve as useful analogs for biologically important
pyroglutamate containing bicyclic natural products such as
omuralide, salinosporamides, and cinnabaramides [4].
RESULTS AND DISCUSSION
We initiated the synthesis of bicyclic pyroglutamates
with the esterification of racemic amino acid leucine 1 as
© 2013 HeteroCorporation

970
S. Tekkam, J. L. Johnson, S. C. Jonnalagadda, and V. R. Mereddy
Vol 50
Figure 1. Some applications of the pyroglutamate unit.
of the reaction sequence, we prepared the [2.2.1]
heterobicyclic lactams 12b and 12c starting with amino
acids valine and phenyl alanine (Scheme 2). All these
molecules were characterized by proton and carbon
nuclear magnetic spectroscopy (NMR), HRMS, as well
as single crystal X-ray analysis of 12c (Fig. 2).
We evaluated the biological efficacy of all the synthetic
analogs against multiple myeloma cancer cells. Multiple
myeloma cancer cells (RPMI-8226) were plated in 96 well
plates and allowed to adhere for 3 days. Cells were then
treated with 50 μM of each compound or with DMSO
alone for 1 day. MTS assay was used for determining the
number of remaining viable cells after exposure to com-
pounds. 20 μl of MTS was added to 100 μl culture medium
in each well. After incubation at 37°C for 3h, absorbance
was measured using an ELISA plate reader. Unfortunately,
none of these molecules showed any significant cytotoxic-
ity at 50 μM concentration.
CONCLUSIONS
In conclusion, we have developed an efficient and ex-
ceptionally short methodology for the synthesis of
[2.2.1] heterobicyclic pyroglutamates via alkylation of
amino acid-derived iminoesters with Baylis-Hillman re-
action derived allyl bromide, diastereoselective
dihydroxylation, and regioselective lactonization. Owing
to the importance of Baylis-Hillman reaction and the
availability of a variety of natural and unnatural amino
acids, we believe that the present methodology should
provide facile access to this class of biologically impor-
tant pyroglutamates.
EXPERIMENTAL
General methods. All operations were carried out under an
inert nitrogen atmosphere. THF, CH2Cl2, toluene, and ethyl
acetate were purchased as anhydrous solvents and used as such.
Scheme 1. Synthesis of α-methylene pyroglutamates.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

July 2013
An Efficient Synthesis of [2.2.1] Heterobicyclic Pyroglutamates
971
Scheme 2. Synthesis of [2.2.1] heterobicyclic pyroglutamates.
The 1H- and 13C-NMR spectra were plotted on a Varian Gemini-
500 spectrometer. High-resolution mass spectra (HRMS) were
recorded using a Bruker BioTOF II ESI mass spectrometer. The
pyroglutamates 8a–c and 9a–c were synthesized using our
earlier protocol [3].
Valine dihydroxy-γ-carboxy-γ-lactam (10b). Yield (79%).
1H-NMR (500 MHz, DMSO-d6): δ = 8.44 (br s, 1H), 7.43–7.46
(m, 5H), 4.81 (d, J = 8.5 Hz, 1H), 3.76 (d, J = 8.5 Hz, 1H),
3.35 (s, 1H), 2.13 (m, 1H), 1.18 (s, 3H), 0.93 (d, J = 7 Hz, 3H),
0.85 (d, J = 7 Hz, 3H); 13C-NMR (125 MHz, DMSO-d6): δ =
175.6, 172.8, 136.6, 129.0, 128.7, 128.6, 77.4, 72.9, 69.9, 66.7,
Leucine dihydroxy-γ-carboxy-γ-lactam (10a).
NMO (50
wt % in water 6.96 mmol) and OsO4 (0.7 mmol) were added
successively to the lactam 9a (3.48 mmol) dissolved in
acetone (20 mL). The reaction mixture was stirred at room
temperature for 40 h. After completion of the reaction, water
was added, extracted with ethyl acetate (3 × 40 mL), washed
with brine (10 mL), dried over MgSO4, and concentrated
in vacuo. The crude product was purified by column
chromatography (70% EtOAc in hexanes) to afford diol 10a
1
(yield 75%). H-NMR (500 MHz, CDCl3): δ = 7.5 (br s, 1H),
7.40–7.35 (m, 5H), 5.24 (m, 2H), 4.61 (s, 1H), 4.18 (d, J = 6.5
Hz, 1H), 3.86 (d, J = 6.5 Hz, 1H), 2.20 (dd, J = 7 Hz, 14 Hz,
1H), 1.68 (m, 1H), 1.60 (dd, J = 7 Hz, 14 Hz, 1H), 1.46 (s, 3H),
0.94 (d, J = 6.5 Hz, 3H), 0.83 (d, J = 6.5 Hz, 3H); 13C-NMR
(125 MHz, CDCl3): δ = 177.5, 172.4, 135.2, 128.9, 129.1,
129.0, 128.7, 80.9, 73.6, 68.5, 67.9, 45.6, 25.0, 24.2, 23.5, 22.9;
HRMS (ESI) m/z: Calcd for C17H23NO5 [M+Na]+: 344.1468;
found: 344.1469.
Figure 2. X-ray crystal structure of [2.2.1] pyroglutamate (12c).
Journal of Heterocyclic Chemistry
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S. Tekkam, J. L. Johnson, S. C. Jonnalagadda, and V. R. Mereddy
Vol 50
33.3, 22.1, 18.7, 17.4; HRMS (ESI) m/z: Calcd for C16H21NO5
[M+Na]+: 330.1312; found: 330.1312.
Phenylalanine [2.2.1] pyroglutamate (12c).
Yield (55%).
1H-NMR (500 MHz, DMSO-d6): δ = 7.76 (br s, 1H), 7.28–7.26
(m, 2H), 7.19–7.13 (m, 3H), 5.72 (d, J = 4 Hz, 1H), 3.7 (d, J =
4 Hz, 1H), 3.10 (d, J = 14.5 Hz, 1H), 3.04 (d, J = 14.5 Hz,
1H), 1.32 (s, 3H); 13C-NMR (125 MHz, DMSO-d6): δ = 174.4,
171.5, 134.9, 130.2, 128.6, 127.3, 89.3, 82.0, 68.8, 9.9; HRMS
(ESI) m/z: Calcd for C13H13NO4 [M+Na]+: 270.0737; found:
270.0725.
Phenylalanine dihydroxy-γ-carboxy-γ-lactam (10c). Yield
(80%). 1H-NMR (500 MHz, CDCl3): δ = 7.35–7.24 (m, 8H),
7.07–7.05 (m, 2H), 6.89 (br s, 1H), 5.16 (m, 2H), 4.75 (s, 1H),
4.20 (d, J = 6 Hz, 1H), 3.99 (d, J = 6 Hz, 1H), 3.55 (d, J = 13.5
Hz, 1H), 2.92 (d, J = 13.5 Hz, 1H), 1.43 (s, 3H); 13C-NMR
(125 MHz, CDCl3): δ = 176.8, 171.3, 135.2, 134.4, 130.2,
129.1, 129.0, 128.8, 128.7, 127.8, 79.3, 73.5, 69.1, 68.0, 42.5,
23.0; HRMS (ESI) m/z: Calcd for C20H21NO5 [M+Na]+:
378.1312; found: 378.1295.
Acknowledgments. We thank the Departments of Chemistry
and Biochemistry, University of Minnesota Duluth and Rowan
University for the funding. Partial support for this work was
also provided by research grants from University of Minnesota
Academic Health Center Faculty Development Grant (VRM),
Whiteside Institute for Clinical Research (VRM), and Rowan
University Non-Salary Financial Support Grants (NSFSG)
(SCJ). We thank Dr. Victor G. Young, Jr. (University of
Minnesota X-ray Crystallographic Laboratory) for providing the
crystal structure.
Leucine dihydroxy acid (11a). To the diol 10a (1.56 mmol)
dissolved in methanol (8 mL), Pd/C (50 mg) was added and stirred
under hydrogen atmosphere for 1 h. The reaction mixture was
filtered and concentrated in vacuo to give the diol acid (90%
yield), which was used for next step without any further
1
purification. H-NMR (500 MHz, CD3OD): δ = 8.25 (br s, 1H),
3.81 (s, 1H), 2.10 (dd, J = 7, 14.5 Hz, 1H), 1.80 (m, 1H), 1.65 (dd,
J = 7, 14.5 Hz, 1H), 1.41 (s, 3H), 0.98 (d, J = 6.5 Hz, 3H), 0.96
(d, J = 6.5 Hz, 3H); 13C-NMR (125 MHz, CDCl3): δ = 177.7,
174.7, 79.6, 73.2, 67.3, 45.4, 24.5, 23.7, 22.3, 22.1; HRMS (ESI)
m/z: Calcd for C10H17NO5 [M+Na]+: 254.0999; found: 254.0988.
1
Valine dihydroxy acid (11b). Yield (92%). H-NMR (500
MHz, DMSO-d6): δ = 8.27 (br s, 1H), 3.68 (s, 1H), 2.01 (m,
1H), 1.13 (s, 3H), 0.93 (d, J = 7 Hz, 3H), 0.87 (d, J = 7 Hz,
3H); 13C-NMR (125 MHz, DMSO-d6): δ = 175.4, 175.1, 77.1,
73.1, 68.0, 33.1, 21.8, 18.5, 17.7; HRMS (ESI) m/z: Calcd for
C9H15NO5 [M+Na]+: 240.0842; found: 240.0830.
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127.4, 76.6, 72.8, 67.0, 40.5, 22.7; HRMS (ESI) m/z: Calcd for
C13H15NO5 [M+Na]+: 288.0842; found: 288.0817.
Leucine [2.2.1] pyroglutamate (12a). To a suspension of diol
acid 11a (1 mmol) in dichloromethane (20 mL), pyridine (3 mmol)
and BOP chloride (1.5 mmol) were added successively. The
reaction mixture was stirred at room temperature for 3 h. Water was
added and extracted with ethyl acetate, washed with brine, dried
over MgSO4, concentrated in vacuo, and purified by silica gel
column chromatography to afford pyroglutamate 12a (45% yield).
1H-NMR (500 MHz, DMSO-d6): δ = 8.51 (br s, 1H), 6.26 (d, J =
4.5 Hz, 1H), 3.95 (d, J = 4.5 Hz, 1H), 1.80 (m, 1H), 1.70 (m, 2H),
1.42 (s, 3H), 0.93 (d, J = 3.5 Hz, 3H), 0.92 (d, J = 3.5 Hz, 3H);
13C-NMR (125 MHz, DMSO-d6): δ = 175.6, 172.5, 89.6, 83.2,
68.5, 31.3, 24.2, 23.8, 10.4 (2C); HRMS (ESI) m/z: Calcd for
C10H15NO4 [M+Na]+: 236.0893; found: 236.0883.
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Valine [2.2.1] pyroglutamate (12b). Yield (51%). 1H-NMR
(500 MHz, DMSO-d6): δ = 8.94 (s, 1H), 6.5 (d, J = 4.5 Hz, 1H),
3.94 (d, J = 4.5 Hz, 1H), 2.18 (m, 1H), 1.33 (s, 3H), 1.10 (d, J = 7
Hz, 3H), 0.98 (d, J = 7 Hz, 3H); 13C-NMR (125 MHz, DMSO-
d6): δ = 174.8, 172.5, 89.4, 82.9, 71.8, 23.8, 17.6, 17.0, 10.6;
HRMS (ESI) m/z: Calcd for C9H13NO4 [M+H]+: 200.0878;
found: 200.0885.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet