Synthesis of isotopically labelled amino acids

Article abstract of DOI:10.1002/jlcr.1301

An efficient approach to the enantioselective synthesis of a series of amino acids from either bromoacetyl bromide or glycine is described using a [2,3]-sigmatropic rearrangement to establish the stereogenic centre at C-2 under mild conditions. Protected allylglycine 5 is a valuable building block to several amino acids e.g. hydrolytic cleavage of the auxiliary in 5 followed by deprotection gave L-allylglycine in 92% yield whilst oxidative cleavage of the terminal alkene followed by deprotection gave L-aspartic acid in 67% yield over the 2 steps. Furthermore alkene 5 may be converted to hydroxy ester 8 which is an intermediate for the synthesis of various amino acids including L-lysine and L-proline. Since the enantiomer of sultam 1 is commercially available, the analogous D-amino acids may be synthesised. This chemistry is readily adapted for the incorporation of isotopic labels for example for the synthesis of [1,2-¹³C₂,¹⁵N]-L-homoserine 14. Copyright

Full text of DOI:10.1002/jlcr.1301

Journal of Labelled Compounds and Radiopharmaceuticals
J Label Compd Radiopharm 2007; 50: 399–401.
Published online in Wiley InterScience
JLCR
(www.interscience.wiley.com). DOI: 10.1002/jlcr.1301
Short Research Article
y
Synthesis of isotopically labelled amino acids
1
2
2
1
1,
DAVID O. REES , NICK BUSHBY , JOHN R. HARDING , CHUANJUN SONG and CHRISTINE L. WILLIS *
1
School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK
2
AstraZeneca, Mereside, Alderley Park, Macclesfield, Cheshire SK10 4TG, UK
Received 10 September 2006; Revised 31 January 2007; Accepted 2 February 2007
Abstract: An efficient approach to the enantioselective synthesis of a series of amino acids from either bromoacetyl
bromide or glycine is described using a [2,3]-sigmatropic rearrangement to establish the stereogenic centre at C-2
under mild conditions. Protected allylglycine 5 is a valuable building block to several amino acids e.g. hydrolytic
cleavage of the auxiliary in 5 followed by deprotection gave l-allylglycine in 92% yield whilst oxidative cleavage of the
terminal alkene followed by deprotection gave l-aspartic acid in 67% yield over the 2 steps. Furthermore alkene 5
may be converted to hydroxy ester 8 which is an intermediate for the synthesis of various amino acids including
l-lysine and l-proline. Since the enantiomer of sultam 1 is commercially available, the analogous d-amino acids may
be synthesised. This chemistry is readily adapted for the incorporation of isotopic labels for example for the
1
3
15
synthesis of [1,2-
2
C , N]-l-homoserine 14. Copyright # 2007 John Wiley & Sons, Ltd.
Keywords: amino acids; isotope labelling; [2,3]-sigmatropic rearrangement
Introduction
and the reactions are commonly carried out in two steps:
preparation of the quaternary ammonium salt followed
by treatment of the isolated salt with a strong base to
generate an ylid which undergoes the rearrangement with
Isotopically labelled amino acids are valuable for a
range of studies in bioorganic chemistry including
biosynthetic and metabolic studies. Recently the in-
corporation of stable labelled amino acids into proteins,
either uniformly labelled or selectively labelled, has
proved essential in the elucidation of the three-dimen-
3
creation of a new carbon–carbon bond. Indeed using
such a two step procedure, Oppolzer’s camphor sultam
has been employed as an auxiliary to induce intramole-
cular chirality transfer with excellent stereocontrol in the
1
4
sional structure of proteins by NMR spectroscopy.
synthesis of d-allylglycine. We have investigated an
Many methods have been developed for the selective
labelling of a-amino acids but there is a continuing
need to refine approaches and improve efficiency to
enable multigram quantities of specifically labelled
approach which would enable the stereogenic centre at
C-2 of the a-amino acid to be created under mild
conditions in a single step from a tertiary amine based
upon the in situ generation of an N-allyl ammonium ylid
as pioneered by Coldham and co-workers in their
2
amino acids to be prepared. Herein the enantioselec-
5
tive synthesis of a series of amino acids is described
using an approach which is readily adapted for the
incorporation of isotopic labels.
synthesis of racemic allylglycine methyl ester.
Results and discussion
[2,3]-Sigmatropic rearrangements of N-allyl ammo-
The substrate 3 required for the key [2,3]-Stevens
nium ylids have been widely used in organic synthesis
rearrangement was prepared in two steps and 91%
overall yield from (1S,2R)-bornane-10,2-sultam
1
4
*
Correspondence to: Christine L. Willis, School of Chemistry, Uni-
(Scheme 1). Initially the conditions reported by Cold-
ham for the [2,3]-sigmatropic rearrangement of diallyl-
versity of Bristol, Cantock’s Close, Bristol BS8 1TS, UK.
E-mail: chris.willis@bristol.ac.uk
5
glycine methyl ester were examined. Treatment of 3
Contract/grant sponsor: BBSRC
Contract/grant sponsor: AZ
2 3
with allyl bromide, K CO , DBU and tetrabutylammo-
Contract/grant sponsor: EPSRC
nium iodide at 408C in DMF gave the C-allylated
product in 85% yield as a 12:1 mixture of diaster-
eomers. The major isomer readily crystallized from
y
Proceedings of the Ninth International Symposium on the Synthesis
and Applications of Isotopically Labelled Compounds, Edinburgh,
1
6–20 July 2006.
Copyright # 2007 John Wiley & Sons, Ltd.

4
00 D. O. REES ET AL.
O
O
a
b
Br
NH
N
N
N
S
96%
95%
S
O2
O2
S
O2
1
2
3
c
7
5%
O
O
O
NH2
e,f
2%
d
NHBoc
HO
N
N
N
8
2%
9
S
O2
S
O2
5
4
L-allylglycine
g
9
6%
O
O
NHBoc
NH2
N
e,f
0%
HO
S
O2
7
CO2H
CO H
2
6
L-aspartic acid
Scheme 1
0
ethanol giving exclusively the (2 S) stereoisomer 4 in
5% isolated yield. Following further investigations the
optimum conditions were found to be reaction of 3 with
allyl iodide, K CO in DMF at 408C giving the 12:1
alcohol 8. Two approaches were examined for the
conversion of alcohol 8 to protected proline. The first
involved treatment with Ph P/Br which led to sponta-
7
3
2
2
3
neous cyclization to the target compound 10 but in our
hands in a disappointing 40% yield. A better approach
was to use a 2 step procedure in which the alcohol was
converted to tosylate 9 and then on treatment with
sodium hydride, protected l-proline 10 was isolated in
84% yield over the 2 steps. Finally, removal of the
protecting groups using TFA gave l-proline.
Since the enantiomer, (1R, 2S)-bornane-10,2-sultam,
is commercially available this chemistry may be readily
adapted for the enantioselective synthesis of d-amino
acids.
6
mixture of diastereomers in 90% yield.
The palladium(0) mediated removal of the N-allyl
7
groups gave the primary amine which was protected in
situ as the t-butyloxycarbonyl (Boc) derivative 5 in 82%
yield. Protected allylglycine 5 is a valuable intermediate
for the synthesis of a number of a-amino acids. For
example, hydrolytic cleavage of the auxiliary using lithium
hydroxide and hydrogen peroxide followed by removal of
the Boc group with acid gave l-allylglycine in 92% yield
from 5. l-aspartic acid was prepared in 67% overall yield
from 5 using ruthenium dioxide and sodium periodate to
oxidatively cleave the double bond to acid 6 followed by
removal of the auxiliary and Boc protecting group.
The approaches to the enantioselective synthesis of
a-amino acids outlined in Schemes 1 and 2 are readily
adapted for the incorporation of isotopic labels e.g.
1
3
A further demonstration of the versatility of this
chemistry is in the conversion of alkene 5 to primary
alcohol 8 which is a valuable intermediate in the
synthesis of further amino acids e.g. l-proline and
l-lysine (Scheme 2). Hydrolytic cleavage of the auxiliary
and treatment of the resultant acid with tBuOH, DCC
using [1,2- C ]-bromoacetyl bromide to acylate
2
(1S,2R)-bornane-10,2-sultam 1 in the initial step leads
1
3
to [1,2-
2
C ]-amino acids. Furthermore, amino acids
incorporating both carbon-13 and nitrogen-15 labels
may be prepared starting with commercially available
8
1
3
15
[1,2-
2
C , N]-glycine 8 as illustrated in the synthesis
0
13
15
9
(
N,N -dicyclohexylcarbodiimide) and DMAP (4-dimethy-
of [1,2- C , N]-l-homoserine 14 (Scheme 3).
2
laminopyridine) gave ester 7. Hydroboration of the olefin
This chemistry may be used for the synthesis of
under standard conditions then gave the required
further amino acids e.g. Boc homoserine t-butyl ester
Copyright # 2007 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2007; 50: 399–401
DOI: 10.1002.jlcr

SYNTHESIS OF ISOTOPICALLY LABELLED AMINO ACIDS 401
O
O
O
NHBoc
NHBoc
tBuO
NHBoc
a
tBuO
b
N
S
O2
70%
95%
5
7
OH
8
c
88%
O
NHBoc
OTs
tBuO
e
d
t
CO Bu
CO H
2
2
N
N
H
82%
95%
BOC
11
10
9
Scheme 2
O
O
tBuO
NHBoc
NH2
OH
HO C
NH2
HO
2
1
2
= carbon-13
nitrogen-15
13 OH
14
=
Scheme 3
(
unlabelled 13) has been used as an intermediate in the
References
1
0
synthesis of methionine and selenomethinoine.
1
. See for example: Watts A. Nat Rev Drug Discov
005; 4: 555–568.
2
Conclusion
2
. Reviews: (a) Kelly NM, Sutherland A, Willis CL. Nat
Prod Reports 1997; 205–219; (b) Voges R. J Label
Compd Radiopharm 2002; 45: 867–897.
In conclusion we have described a simple and efficient
method for the synthesis of protected allylglycine 4
using a base mediated [2,3]-sigmatropic rearrange-
ment of an N-allyl ammonium ylide generated in situ
from tertiary amine 3 using the camphor sultam to
induce stereocontrol at C-2. Boc protected allyl glycine
3
4
. Magewaran S, Ollis WD, Sutherland IO. J Chem
Soc, Perkin Trans 1 1981; 1953–1962.
. Workman JA, Garrido NP, Sancon J, Roberts E,
Wessel HD, Sweeney JB. J Am Chem Soc 2005;
1
27: 1066–1067.
5
is a valuable intermediate in the enantioselective
5
. Arbor e´ APA, Cane-Honeysett DJ, Coldham I, Mid-
dleton ML. Synlett 2000; 2: 236–238.
synthesis of a number of amino acids including l-
allylglycine, l-aspartic acid, l-proline, l-lysine and l-
methionine. Since the enantiomer, (1R, 2S)-bornane-
6
7
. Rees DO. Ph.D. Thesis, University of Bristol, 2007.
. Garo-Helion F, Merzouk A, Guib e´ F. J Org Chem
1
0,2-sultam, is commercially available this chemistry
1
993; 58: 6109–6113.
may be readily adapted for the enantioselective synth-
esis of a series of d-amino acids. The approach is
flexible and has been used for the incorporation of
isotopic labels for example in the synthesis of
8
. See for example: (a) Sutherland A, Willis CL. J Label
Compd Radiopharm 1996; 38: 95–102; (b) Siebum
AHG, Tsang RFT, van der Steen R, Raap J,
Lugtenburg J. Eur J Org Chem 2004; 4391–4396.
. Rees DO, Bushby N, Cox RJ, Harding JR,
Simpson TJ, Willis CL. Chem Bio Chem 2007; 8:
46–50.
1
3
15
2
[1,2- C , N]-l-homoserine 14.
9
Acknowledgements
We are grateful to the BBSRC and AZ for funding to
David O. Ress and the EPSRC for funding to Chuanjun
Song.
10. Siebum AHG, Woo WS, Raap J, Lugtenburg J. Eur
J Org Chem 2004; 2905–2913.
Copyright # 2007 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2007; 50: 399–401
DOI: 10.1002.jlcr