A new polymer-supported reagent for the Fmoc-protection of amino acids

Article abstract of DOI:10.1016/S0040-4039(01)01615-X

A new polymer-supported Fmoc-OSu (Fmoc-P-OSu) has been prepared from polymer-bound N-hydroxysuccinimide (P-HOSu), and used as a solid-supported reagent for the Fmoc-protection of amino groups. The residual P-HOSu generated after the protection reaction can be separated by simple filtration and reused.

Full text of DOI:10.1016/S0040-4039(01)01615-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 7579–7581
A new polymer-supported reagent for the Fmoc-protection of
amino acids
Rafael Chinchilla, David J. Dodsworth, Carmen N a´ jera* and Jos e´ M. Soriano
Departamento de Qu ´ı mica Org a´ nica, Universidad de Alicante, Apartado 99, 03080 Alicante, Spain
Received 21 June 2001; accepted 30 August 2001
Abstract—A new polymer-supported Fmoc-OSu (Fmoc-P-OSu) has been prepared from polymer-bound N-hydroxysuccinimide
(
P-HOSu), and used as a solid-supported reagent for the Fmoc-protection of amino groups. The residual P-HOSu generated after
the protection reaction can be separated by simple filtration and reused. © 2001 Elsevier Science Ltd. All rights reserved.
The 9-fluorenylmethoxycarbonyl (Fmoc) group enjoys
a tremendous popularity as a protecting group for
primary and secondary amines and especially for amino
acids in the synthesis of peptides due to its stability to
At the same time, the development of polymer-sup-
ported reagents for organic synthesis is also receiving
growing attention nowadays because of easy recycling
and the simplification of conventional work-up proce-
dures. A recent example on the use of a polymeric
reagent for the N-Fmoc-protection of amino groups
1
,2
9
acid and lability to base. In addition, it is readily
cleaved nonhydrolytically and the protected amine is
liberated as its free base. For the incorporation of the
Fmoc group various different reagents with the general
structure Fmoc-X have been prepared. The widely used
chloroformate ester Fmoc-Cl (1) has a series of draw-
backs such as its instability and the tendency to pro-
mote the formation of undesirable ‘Fmoc-dipeptides’,
uses an in situ generated reagent from a polystyrene-
10
bound HOBt and Fmoc-Cl (1). More recently, a
N-hydroxysuccinimide ring-opening metathesis polymer
obtained from exo-N-hydroxy-7-oxabicyclo[2.2.1]hept-
5-ene-2,3-dicarboximide using the Grubbs catalyst has
been used for the preparation of different supported
acyl-transfer reagents such as 7, although its direct
polymerization only was achieved through a HO-
involved silylation–polymerization–desilylation sequ-
3
presumably via the mixed anhydride intermediate. The
use of the azide derivative Fmoc-N (2) is claimed to
3
4
reduce dipeptide formation, although it has obvious
11
problems related to the storage and handling of poten-
tially explosive oxycarbonyl azides. Many workers
prefer the use of more shelf-stable carbonates such as
ence. In all this context, and in connection with our
project on the development of new solid-supported
reagents applicable to peptide chemistry such as P-
5
6
12
Fmoc-pfp (3, pfp=perfluorophenoxy), Fmoc-OBt (4,
Bt=benzotriazol-1-yl), 5-norbonene-2,3-dicarboximido-
TBTU, we have recently developed a co-polymer con-
taining the N-hydroxysuccinimide moiety (P-HOSu,
7
3,6,8
13
Fmoc-derivative 5, and particularly Fmoc-OSu
(6,
8) which can be easily obtained from an inexpensive
Su=succinimidyl), which also provides N-protected
amino acids in comparable yields without the formation
of the dipeptide impurities.
commercially available polymer and used as a recover-
able racemization-reducing additive for the dicyclo-
hexylcarbodiimide (DCC)-mediated coupling of amino
O
1
, X = Cl
5
, X =
O
N O
Ph
7, X =
2
3
, X = N3
n
, X = C F O-
6
5
O
X
O
N
N
O
O
N
O
4
, X =
N
O
6, X = O
O
N
O
O
Keywords: amino acids; protecting groups; polymers.
*
Corresponding author. Fax: +34 96 5903549; www.ua.es/dept.quimorg. E-mail: cnajera@ua.es
0
040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01615-X

7
580
R. Chinchilla et al. / Tetrahedron Letters 42 (2001) 7579–7581
acids. We report here the use of P-HOSu (8) for the
preparation of a polymer-bound Fmoc-OSu (Fmoc-P-
OSu, 9), which can be used as a suitable reagent for the
Fmoc-protection of amino acids. This solid-supported
reagent would allow the easy separation of the Fmoc-
protected amino acid, which could be particularly valu-
able when working on a small scale.
latter yield is considerably higher than the 43%
reported when polystyrene-supported in situ generated
10
Fmoc-OBt was employed. The isolated yield of Fmoc-
Phe and Fmoc-Pro using 9 was moderate (Table 1,
entries 9 and 11) and low in the case of Fmoc-Asn
(Table 1, entry 13). When the N-protection reaction
was performed with an alkylamine such as isopropyl-
amine, the obtained final yield was 82%, which is
slightly higher than to that reported when using in situ
14
Starting P-HOSu (8) was obtained with a loading of 1.5
10
−
1
generated Fmoc-OBt (78%). In addition, the reaction
of 9 with 2-morpholinoethylamine afforded the corre-
sponding Fmoc-protected amine in 94% yield, which is
similar to the 93% yield reported when the reagent from
mmol g by reaction of commercially available styrene/
1
5
maleic anhydride co-polymer with an 50% aqueous
13
solution of hydroxylamine. Subsequent reaction of a
mixture of 8 and K CO in water with a solution of
2
3
11
polymer 7 was employed.
9
-fluorenylmethoxycarbonyl chloride (1), afforded
Fmoc-P-OSu (9) as a white solid (Scheme 1).
We conclude that Fmoc-P-OSu can be a promising new
solid-supported amino N-Fmoc-protecting reagent
which can be prepared from inexpensive P-HOSu (8)
allowing its easy separation from the final Fmoc-amino
acid, and its recovery once the protection reaction is
finished. Further studies on the application of this
styrene/maleic anhydride co-polymer (P-HOSu, 8) for
the synthesis of other solid-supported reagents useful in
peptide synthesis are now underway.
The obtained Fmoc-P-OSu polymer (9) showed identi-
cal CꢀO and CꢁO stretching bands in the IR spectrum
3
to those of an authentic sample of Fmoc-OSu (6). The
Fmoc-P-OSu thus prepared was used as a solid-sup-
ported Fmoc-protecting reagent for the amino group of
various a-amino acids. Thus, free amino acids reacted
with 9 in the presence of a base to give the correspond-
ing protected amino acids (Table 1). After precipitation
and filtration, the obtained Fmoc-amino acids were
obtained from the aqueous solutions upon acidification.
In a typical Fmoc-protection reaction of amino acids
using Fmoc-P-OSu, to a suspension of 9 (270 mg, 0.4
mmol) in acetone (20 mL) was added a solution of the
1
The Fmoc-amino acids were pure ( H NMR) and the
filtered solid consisted of P-HOSu (8), which could be
re-used for the preparation of new 9.
Table 1. N-Fmoc-amino acids prepared using Fmoc-P-
OSu (9)
The Fmoc-protection reaction was performed in a mix-
ture of acetone/water as solvent and in the presence of
K CO as base. Changes in solvent to dichloromethane/
Entry
Fmoc-amino Solvent
acid
Base
Yield (%)a,b
2
3
water mixtures did not reported appreciable differences
Table 1, compare entries 1 and 4). In addition, chang-
ing the base to NaHCO afforded similar results (Table
1
2
3
4
5
6
7
8
9
Fmoc-Ala
Fmoc-Ala
Fmoc-Ala
Fmoc-Ala
Fmoc-Gly
Fmoc-Val
Fmoc-Leu
Fmoc-Ile
Fmoc-Phe
Fmoc-Pro
Fmoc-Ser
Fmoc-Thr
Fmoc-Asn
Acetone/water
K CO
NaHCO3 95
95
(
2
3
Acetone/water
Acetone/water
CH Cl /water
3
NaOAc
K CO
89
94
96
75
70
68
46
43
77
65
34
1, compare entries 1 and 2), whereas the use of NaOAc
2
2
2
3
3
3
3
3
3
3
3
3
3
lowered slightly the final yield (Table 1, entry 3). The
isolated yields of the obtained Fmoc-amino acids were
generally good and in some cases not very different
compare to those reported when Fmoc-OSu has been
used. Thus, Fmoc-Ala was isolated in 95% yield when 9
was used (Table 1, entry 1), whereas a 97% yield o3f
Fmoc-Ala has been reported when using Fmoc-OSu.
In addition, a 87% reported yield of the corresponding
Fmoc-protected amino acid has been obtained using3
Fmoc-OSu with the more sterically hindered Val,
whereas Fmoc-P-OSu afforded 75% yield of Fmoc-Val
Acetone/water
Acetone/water
Acetone/water
Acetone/water
Acetone/water
Acetone/water
Acetone/water
Acetone/water
Acetone/water
K CO
2
K CO
2
K CO
2
K CO
2
K CO
2
1
11
0
K CO
2
K CO
2
1
1
2
3
K CO
2
K CO
2
a
1
Isolated pure compounds ( H NMR).
b
All compounds gave satisfactory NMR ( H and 13C), IR and MS
1
(
Table 1, entry 6). It is interesting to notice that this
data.
Ph
Ph
n
n
Fmoc-Cl (1)
K CO
O
O
O
N
O
2
3
O
O
N
OH
O
8
9
Scheme 1.

R. Chinchilla et al. / Tetrahedron Letters 42 (2001) 7579–7581
7581
corresponding amino acid (0.4 mmol) and K CO (39
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2
3
mg, 0.4 mmol) in water (15 mL). The suspension was
stirred at rt for 1 day and the solvents were evaporated
in vacuo (15 Torr). Toluene was added (20 mL) and the
resulting solid was filtered. The solid was suspended in
water (20 mL) and the P-HOSu was filtered off. The
filtrate was acidified with HCl(c) (2 mL) and extracted
with AcOEt (3×20 mL). The combined organic layers
were dried (Na SO ) and evaporated in vacuo (15 Torr)
2
4
affording the pure Fmoc-amino acids.
9. For recent reviews on polymer-supported chemistry, see:
(
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Acknowledgements
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We thank the Direcci o´ n General de Ense n˜ anza Supe-
rior e Investigaci o´ n Cient ´ı fica (project no. 1FD97-0721)
of the Ministerio de Educaci o´ n y Cultura (MEC) and
the Conselleria de Cultura Educaci o´ i Ci e` ncia of the
Generalitat Valenciana (project no. GV99-33-1-02) for
financial support.
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