Concise syntheses of enantiomerically pure protected 4-hydroxypyroglutamic acid and 4-hydroxyproline from a nitroso-cyclopentadiene cycloadduct

Article abstract of DOI:10.1016/j.tetasy.2008.12.013

O-TBS-protected methyl trans-4-hydroxypyroglutamate and methyl trans-4-hydroxyproline ester were synthesized from nitroso-cyclopentadiene Diels-Alder cycloadducts. Enzymatic resolution of the key intermediate, 4-amino-cyclopent-2-enol, provides access to both l- and d-amino acids.

Full text of DOI:10.1016/j.tetasy.2008.12.013

Tetrahedron: Asymmetry 19 (2008) 2835–2838
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Concise syntheses of enantiomerically pure protected 4-hydroxypyroglutamic
acid and 4-hydroxyproline from a nitroso-cyclopentadiene cycloadduct
Weiqiang Huang, Marvin J. Miller *
Department of Chemistry and Biochemistry, University of Notre Dame, 251 Nieuwland Science Hall, Notre Dame, IN 46556, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
O-TBS-protected methyl trans-4-hydroxypyroglutamate and methyl trans-4-hydroxyproline ester were
synthesized from nitroso-cyclopentadiene Diels–Alder cycloadducts. Enzymatic resolution of the key
Received 13 October 2008
Accepted 10 December 2008
Available online 29 Januaray 2009
intermediate, 4-amino-cyclopent-2-enol, provides access to both
L
- and
D-amino acids.
Ó 2008 Elsevier Ltd. All rights reserved.
1
. Introduction
2. Results and discussion
Syntheses and studies of both non-proteinogenic natural and
Nitroso Diels–Alder reactions with 1,3-cycloalkadienes afford
cycloadducts 2. These versatile compounds have been used to
synthesize many natural products and other biologically interest-
ing compounds. The same scaffolds are readily converted to no-
vel amino acids. Oxidative cleavage of the double bond of 2 gives
cyclic oxyamino acids 3. Subsequent N–O bond reduction pro-
unnatural amino acids continue to be of considerable interest,
often because of their enzyme inhibitory and antimetabolite
properties. Among these amino acids, substituted glutamic acids
attract particular attention because of their interaction with glu-
tamate receptors in the central nervous system (CNS) and their
involvement in many other biological processes.1 Proline and
its derivatives are of considerable interest because their cyclic
structures play a critical role in forming protein and peptide sec-
7
vides the corresponding acyclic hydroxy
a-amino acids 5 (Route
A). Alternatively, N–O reduction of 2 gives amino hydroxy cyclo-
alkenes 4 that can also be oxidatively converted to amino acids 5
(Route B). We anticipated that these intermediates could also
serve as precursors of substituted pyroglutamates 6 and prolines
7. Thus, we became interested in developing asymmetric synthe-
ses of these useful amino acids.
ondary structures such as b-turns and
a
-helices.2 4-Substituted
prolines are particularly interesting because the C-4 substituents
can influence not only the conformation of the pyrrolidine ring,
but also the rate of cis–trans isomerization about the amide
bond. In the investigation of collagenous peptides that have
repeating units of Gly-X–Y (X is often occupied by proline resi-
dues, and Y is frequently occupied by hydroxyproline residues),
hydroxylation of proline at the 4-position is found to affect
Using generalized route A (Scheme 1), we previously reported
specific examples of the syntheses of dipeptides such as 12 that
contain carboxy proline analogs and conformationally restricted
glutamate analogs 3. The method involved oxidation of amino
acid hydroxamates 9 in the presence of dienes, to trap the tran-
sient nitroso agent 10 giving adduct 11 and subsequent alkene
3
greatly the conformational stability of the collagen triple helix.
In addition, many biologically active natural products including
4
7a
kainic acid have a 4-substituted proline framework. Due to
oxidation to generate 12. Diastereoselectivity of the cycloaddi-
the biological importance of substituted glutamic acids and
tion (10?11) depended on the size of the starting amino acid
side chain (R of 8) and, using route B, enantiomerically enriched
forms of 4 could be obtained by removal of the starting amino
acid either hydrolytically or under Edman degradation condi-
4
-substituted prolines, extensive efforts have focused on devel-
opment of their syntheses. Among these studies, 4-hydroxypyro-
glutamates have received particular interest5 because they are
able to be transformed to both 4-substituted glutamic acids
and prolines. In addition, they also have been employed as key
intermediates in the syntheses of many biologically interesting
7
f
tions. Addition of appropriate side chains, such as a phenylace-
tyl group (penicillin G side chain), imparted antibiotic activity,
similar to that of b-lactam antibiotics, to amino acid 3-contain-
6
8
heterocycles. Herein, we report asymmetric syntheses of substi-
ing peptides such as 13. Heinz also used nitroso cycloaddition
tuted pyroglutamate and proline derivatives from oxazines that
are readily derived from nitroso cycloaddition reactions.
chemistry and a variant of route A to synthesize racemic 4-
hydroxypyroglutamate 14 in a sequence that also included high
5
a
pressure N–O bond reduction (Scheme 2).
While route A does provide access to novel amino acids and the
use of amino acid hydroxamates 9 gives diastereoselectively
*
Corresponding author.
E-mail address: mmiller1@nd.edu (M.J. Miller).
0
957-4166/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2008.12.013

2
836
W. Huang, M. J. Miller / Tetrahedron: Asymmetry 19 (2008) 2835–2838
enriched cycloadducts and eventual peptides, we sought alterna-
tive methods that would provide access to substituted glutamates,
prolines and analogs in enantiomerically pure form. No catalytic
asymmetric intermolecular acylnitroso cycloaddition reaction
Reaction of (+)-17 with TBSCl gave silyl ether 19, which was
oxidized to the diacid with RuCl /NaIO which upon treatment
with excess diazomethane provided protected methyl trans-N-
Boc-4-hydroxypyroglutamate 20 in moderate yield. The R-config-
3
4
9
has yet been developed despite considerable effort although a
uration of the
a-amino acid carbon (C-2) in 20 was confirmed to
moderately successful intramolecular example has been de-
correspond with that of
specific rotation data (Scheme 4).
D
-glutamate by comparison to reported
1
0
5b
scribed. Yamamoto has reported very effective catalytic asym-
metric cycloadditions using pyridyl nitroso agents, but removal
of the pyridyl moiety requires several steps and is often low
Reduction of pyroglutamates to proline derivatives has been ef-
1
2
13
fected with BH
3
2
ꢁMe S
or NaBH(OMe)
3
.
While subjection of 20
7
c,7d
yielding.
Consequently, with a renewed focus on the poten-
to these conditions resulted in decomposition or no desired prod-
uct, we did find that a two-step process that proceeded through a
tial of route B, we sought to capitalize on our previously re-
1
1
14
ported enzymatic resolution of amino cyclopentenols 4
allow access to either enantiomer of the targeted glutamate
and proline analogs. Thus, NaIO -mediated oxidation of Boc-
to
hemiaminal intermediate was effective. Thus, treatment of 20
with LiEt
3
BH at ꢀ78 °C gave isolable hemiaminal 21, which upon
ꢁOEt and Et SiH provided protected 4-hydroxy-
proline 22. The NMR spectrum of 22 was consistent with that re-
4
reaction with BF
3
2
3
1
5
hydroxylamine 15 in the presence of cyclopentadiene afforded
cycloadduct 16. Then, N–O reduction with 20 mol % of molybde-
1
6
ported by Gellman (Scheme 5).
numhexacarbonyl in the presence of excess NaBH
4
gave
The methodology described here provides access to 4-substi-
tuted pyroglutamates and prolines from readily prepared 4-amino-
cyclopent-2-enols in two to three steps. The availability of both
enantiomers of 4-aminocyclopent-2-enol via enzymatic resolution
(
±)-aminocyclopentenol 17 in 60% overall yield from N-hydrox-
ycarbamate 15. Multigram kinetic resolution of 17 with immobi-
lized Candida antartica lipase B (CALB) provided acetate (ꢀ)-18 in
9
2–98% ee after 43% conversion (98% after one recrystallization)
makes it feasible to synthesize both the L- and D-amino acids. Use
as well as (ꢀ)-17 in 46% (98% ee after one recrystallization).
of homologous or other dienes in the initial cycloaddition will allow
syntheses of numerous analogs of potential interest.
Methanolysis of acetate 18 gave alcohol (+)-17 (Scheme 3).
R
R
HO HN
C=C oxdn
route A
O
N
N-O Rdn
HO C
CO H
2
n
2
HO C
CO H
2
3
n
2
n
R
5
N
O
R
HO
O
HO
n
+
N
n
n
O
route B
N-O Rdn
C=C oxdn
n = 1, 2, 3
HO
NHR
1
2
CO2H
CO2H
n
N
H
N
H
4
6
7
Scheme 1. Generalized nitroso cycloaddition followed by elaboration to novel amino acids.
For R' = Me:
R
R
R
O
N
HCl•H NOH, KOH, MeOH
R
2
R
OR'
NHOH
O
O
N
+
PNH
PNH
PNH
N
or: 1. HCl•H NOBn/AlMe
PNH
2
3
PNH
O
O
O
O
2
. H2 Pd-C
O
8
, P = Boc, Cbz
ArSO2
O
9
1
0
For R' = H: EDC/H NOBn/H O
11a, Major
11b, Minor
2
2
O
O
CO H
CO2H
HO
O
2
H
N
PHN
13a R1 = R = H
2
O
N
O
Ph
N
O
13b R = H, R2 = CH
3c R = CH , R = H
antibiotic activity
1
11
R1 R2
3
CO2Me
R
O
1
2
N
1
3
12
H
CO H
CO H
14, racemic
2
2
Scheme 2. Cycloadditions of amino acid-based acylnitroso agents gives precursors to novel peptides and antibiotics.
NHBoc
CpH, NaIO4
HO
O
Mo(CO)6, NaBH4
CH3CN/H2O
BocNHOH
BocN
MeOH/H O
2
15
16
(±)-17
(4, R=Boc, n=1)
60% (two steps)
(
2, R=Boc, n=1)
vinyl acetate
BocHN
NHBoc
NHBoc
OH
AcO
HO
Candida antartica lipase B
+
MeOH
heptane(wet)/CH2Cl2
(-)-18
43% conversion
8%ee
K CO
2
3
(+)-
17
(
-)-17
6%, 98% ee
4
99%
9
Scheme 3. Enzymatic resolution of racemic aminoalcohols obtained from nitroso cycloadducts provides enantiomerically pure amino cyclopentols.

W. Huang, M. J. Miller / Tetrahedron: Asymmetry 19 (2008) 2835–2838
2837
TBSO
O
1
. RuCl , NaIO4
3
NHBoc
TBSO
NHBoc
EtOAc/CH CN/H O
3
2
HO
TBSCl
CO Me
2
o
N
2
. CH N , Et O, -78 C
imidazole,
DMF, 99%
2
2
2
Boc
(+)-17
1
9
44%
20
Scheme 4. Conversion of aminocyclopentenol, 17, to pyroglutamate 20.
TBSO
HO
TBSO
TBSO
O
3 3 2
Et SiH, BF -OEt
LiEt BH
3
CO Me
o
2
CO Me
CO
2
Me
o
2
CH Cl , -78 C
2 2
N
THF, -78 C
N
N
Boc
Boc
6
3% (two steps)
Boc
2
1
22
2
0
Scheme 5. Conversion of pyroglutamate, 20, to proline derivative, 22.
3
3
. Experimental
.1. General
4
then dried (MgSO ), filtered, and concentrated to give a brown oil
that was chromatographed on silica gel eluting with EtOAc/hex-
anes (1:6–1:3) to give compound 20 (82 mg, 44%) as a clear oil.
2
D
0
1
½
aꢂ
¼ ꢀ34:7 (c 1.0, CHCl
3
), H NMR (300 MHz, CDCl
= 1.5 Hz, 1H), 4.42 (dd, J = 10.2 Hz, J
1H), 3.79 (s, 3H), 2.37 (ddd, J = 13.2 Hz, J = 8.4 Hz, J
1H), 2.21 (dt, J = 13.2 Hz, J
(s, 9H), 0.18 (s, 3H), 0.13 (s, 3H); C NMR (75 MHz, CDCl
d: 172.2, 171.9, 149.7, 84.0, 69.8, 55.1, 52.9, 32.0, 28.0, 25.8, 18.4,
3
), d: 4.59
Tetrahydrofuran (THF) was distilled from sodium metal/ben-
(dd, J
1
= 9.6 Hz, J
2
1
2
= 8.1 Hz,
3
= 1.5 Hz,
zophenone ketyl, and methylene chloride was distilled from cal-
cium hydride. All other solvents and chemicals were purchased
from Acros or Aldrich and used as is. Silica gel flash column
chromatography was performed using Silica Gel 60 (30–70 lm
irregular particles). All specific rotations were measured using
1
2
1
2
= 10.2 Hz, 1H), 1.50 (s, 9H), 0.89
1
3
3
)
+
-4.3, -5.1. HRMS Calcd for C17
374.1972.
6
H32NO Si (M+H) : 374.1999, found:
a Perkin Elmer model 343 polarimeter at 589 nm and 20 °C. Pro-
ton ( H NMR) and carbon ( C NMR) nuclear magnetic resonance
1
13
spectra were recorded on a Varian UnityPlus 300 NMR at
3.1.3. 4-(tert-Butyl-dimethyl-silanyloxy)-pyrrolidine-1,2-
dicarboxylic acid 1-tert-butyl ester 2-methyl ester 22
3
00 MHz and 75 MHz, respectively. High resolution mass spectra
were recorded on a JEOL JMS-AX505 HA Double Sector Mass
Spectrometer.
A
1.0 M solution of lithium triethylborohydride in THF
(0.225 mL, 0.225 mmol) was added to a solution of 20 (70 mg,
0
.187 mmol) in THF (4 mL) at ꢀ78 °C under a nitrogen atmosphere.
3
.1.1. 4-(tert-Butyl-dimethyl-silanyloxy)-cyclopent-2-enyl]-
After 30 min, the reaction mixture was quenched with saturated
carbamic acid tert-butyl ester 19
aqueous NaHCO3 (0.35 mL) and warmed to 0 °C. H O2 (30%) (1
drop) was added, and the mixture was stirred at 0 °C for 20 min.
The organic solvent was removed in vacuo, and the aqueous layer
2
To a solution of (+)-1711 (243 mg, 1.22 mmol) in DMF (3 mL)
were added TBDMSCl (276 mg, 1.83 mmol) and imidazole
(
249 mg, 3.66 mmol), and the mixture was stirred overnight at rt
under N . DMF was removed in vacuo, and 20 mL of EtOAc was
added to dissolve the residue. The EtOAc solution was washed with
% Na CO solution and brine, and then dried over Na SO . Re-
moval of the Na SO and solvent gave the title product as a light
yellow oil (380 mg, 99%). ½
300 MHz, CDCl ) d: 5.80 (m, 2H), 4.69 (m, 2H), 4.57 (br, 1H),
.70 (dt, J = 13.5 Hz, J = 8.4 Hz, 1H), 1.43 (s, 9H), 1.34 (dt,
= 13.5 Hz, J = 5.1 Hz, 1H), 0.89 (s, 9H), 0.07 (s, 6H); C NMR
75 MHz, CDCl ) d: 155.4, 136.7, 133.9, 79.5, 75.6, 54.5, 42.9,
was extracted with CH Cl (3 ꢃ 5 mL). The combined organic layers
2
2
2
2 4
were dried over Na SO , filtered, and concentrated. The crude
product 21 (78 mg) was used without further purification.
A solution of 21 and triethylsilane (0.030 mL, 0.187 mmol) in
CH Cl (3 mL) was cooled to ꢀ78 °C, and boron trifluoride etherate
5
2
3
2
4
2
4
2
2
2
D
0
1
a
ꢂ
ꢀ 0:3 (c 1.0, CHCl
3
), H NMR
(0.026 mL, 0.206 mmol) was then added dropwise under a nitrogen
atmosphere. After 30 min, 0.030 mL of triethylsilane and 0.026 mL
of boron trifluoride etherate were added. The resulting mixture
was stirred for 2 h at ꢀ78 °C, and was then quenched with satu-
(
2
3
1
2
1
3
J
1
2
(
2
3
3
rated aqueous NaHCO (0.3 mL). The mixture was extracted with
3
+
8.6, 26.1, 18.4, ꢀ3.4, ꢀ4.5. HRMS Calcd for C16
14.2151, found: 314.2144.
3
H32NO Si (M+H) :
CH C1 (3 ꢃ 5 mL), dried over Na SO , and filtered. Evaporation
2
2
2
4
of the solvent and purification by flash chromatography (EtOAc/
hexanes: 10–30%) gave 42 mg (63%) of product 22 as a clear oil.
2
D
0
1
3
1
.1.2. 4-(tert-Butyl-dimethyl-silanyloxy)-5-oxo-pyrrolidine-
,2-dicarboxylic acid 1-tert-butyl ester 2-methyl ester 20
½aꢂ
¼ þ39:2 (c 1.0, CHCl ), H NMR (300 MHz, CDCl ), d: 4.41
3
3
(m, 1H), 4.33 (t, J = 7.8 Hz, 1H), 3.73 (s, 3H), 3.59 (td, J = 10.5 Hz,
1
To a solution of 19 (157 mg, 0.5 mmol) in EtOAc/CH
.8 mL) at 0 °C was added NaIO (481 mg, 2.25 mmol) in H
(18.4 mg, 0.0885 mmol). The mixture
3
CN (1:1,
J = 4.8 Hz, 1H), 3.34 (m, 1H), 2.17 (m, 1H), 2.02 (m, 1H), 1.46–
2
1
3
2
4
2
O
1.41 (d, 9H), 0.87 (s, 9H), 0.06 (s, 6H). C NMR (75 MHz, CDCl3)
mixture of rotamers d: 174.0, 173.8, 154.8, 154.1, 80.3, 70.6, 69.9,
58.3, 57.8, 55.1, 54.8, 52.4, 52.2, 40.0, 39.1, 28.6, 28.5, 25.9, 18.2,
(
5.5 mL) followed by RuCl
3
was stirred at 0 °C open to the air for 15 min and then at room tem-
+
perature for 2 h. The reaction mixture was suction filtered through
1.22, ꢀ4.6, ꢀ4.7. HRMS Calcd for C17H34NO Si (M+H) : 360.2206,
5
Ò
a short pad of Celite . The layers were separated, and the aqueous
found: 360.2203.
layer was saturated with NaCl and was extracted with EtOAc
(
3 ꢃ 3 mL). The combined organic extracts were dried over MgSO
filtered, and concentrated to give the crude diacid as a brown oil.
The brown oil was dissolved in Et O (8.0 mL) and treated with
excess CH in Et
O at ꢀ78 °C. After 30 min, the excess CH
was quenched with AcOH in Et O. The reaction mixture was
washed with saturated NaHCO solution. The aqueous layer was
extracted with Et
O (1 ꢃ 8 mL). The combined organic layers were
4
,
Acknowledgments
2
This work was supported by grants from the National Institutes
of Health (GM 68012 and GM 075855). We gratefully acknowledge
the Lizzadro Magnetic Resonance Research Center at Notre Dame
for NMR facilities and Nonka Sevova for mass spectrometry data.
Special thanks are given to Baiyuan Yang.
2
N
2
2
2 2
N
2
3
2

2
838
W. Huang, M. J. Miller / Tetrahedron: Asymmetry 19 (2008) 2835–2838
Baldwin, J. E.; Aldous, D. J.; Chan, C.; Harwood, L. M.; O’Niel, I. A.; Peach, J. M.
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