Convenient preparation of N-maleoyl amino acid succinimido esters using N-trifluoroacetoxysuccinimide

Article abstract of DOI:10.1080/00397910701750151

One-pot cyclization and esterification of readily available maleamic acid derivatives using N-trifluoroacetoxysuccinimide provide a convenient and cost-effective route to a variety of useful N-maleoyl amino acid N-hydroxysuccinimido esters. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910701750151

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Convenient Preparation of N‐Maleoyl
Amino Acid Succinimido Esters using
N‐Trifluoroacetoxysuccinimide
Michael J. Paterson a & Ian M. Eggleston b
a Maternal and Child Health Sciences, Ninewells Hospital and Medical
School, University of Dundee, Dundee, United Kingdom
b Department of Pharmacy and Pharmacology, University of Bath, Claverton
Down, Bath, United Kingdom
Published online: 18 Jan 2008.
To cite this article: Michael J. Paterson & Ian M. Eggleston (2008) Convenient Preparation of N‐Maleoyl
Amino Acid Succinimido Esters using N‐Trifluoroacetoxysuccinimide, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 38:2, 303-308, DOI:
10.1080/00397910701750151
To link to this article: http://dx.doi.org/10.1080/00397910701750151
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Synthetic Communicationsw, 38: 303–308, 2008
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701750151
Convenient Preparation of N-Maleoyl
Amino Acid Succinimido Esters using
N-Trifluoroacetoxysuccinimide
Michael J. Paterson1 and Ian M. Eggleston2
1Maternal and Child Health Sciences, Ninewells Hospital and Medical
School, University of Dundee, Dundee, United Kingdom
2Department of Pharmacy and Pharmacology, University of Bath,
Claverton Down, Bath, United Kingdom
Abstract: One-pot cyclization and esterification of readily available maleamic acid
derivatives using N-trifluoroacetoxysuccinimide provide a convenient and cost-
effective route to a variety of useful N-maleoyl amino acid N-hydroxysuccinimido
esters.
Keywords: heterobifunctional
N-trifluoroacetoxysuccinimide
linker,
maleimide,
succinimido
ester,
Heterobifunctional cross-linking reagents have a wide range of important
applications in bioorganic and materials chemistry. Among the various
derivatives currently available, the succinimido esters (OSu esters) of
N-maleoyl-amino acids 1 have proved particularly popular and versatile,
providing an amine-reactive and thiol-reactive grouping, disposed within
an aliphatic or aromatic skeleton. The presence of two such complementary
reactive moieties within the same molecule has often been exploited for the
preparation of chimeric protein derivatives via side chain-selective, and
Received in the U.K. January 31, 2007
Address correspondence to Ian M. Eggleston, Department of Pharmacy and
Pharmacology, University of Bath, Claverton Down, Bath BA2 7AY, United
Kingdom. E-mail: ie203@bath.ac.uk
303

304
M. J. Paterson and I. M. Eggleston
sometimes site-selective, conjugation of synthetic peptides,[1] antibodies,[2]
or fluorescent labels[3] to various proteins, as well as in numerous
other novel applications in biological chemistry[4–7] and nanoelectronic
research.[8] Existing methods for the synthesis of N-maleoyl active ester
derivatives include conversion of amino acids to N-maleoyl amino acids
by treatment with N-methoxy carbonylmaleimide,[9] or cyclization of
preformed maleamic acids [N-(3-carboxyacroyl) amino acids], followed by
diimide-mediated esterification.[10,11] Such approaches are typically low
yielding and tend to involve cumbersome workup procedures and chromato-
graphic purification of intermediates. Indeed, although several N-maleoyl
OSu esters are commercially available, they are both costly and may
contain small but significant quantities of impurities such as dicyclohexy-
lurea (DCU).
Nielsen and Buchardt[12] have reported a one-pot preparation of these
derivatives via in situ DCC/HOSu-mediated cyclization and esterification
of an intermediate maleamic acid. In our hands, and as noted elsewhere,[13]
this proved problematic, thus prompting us to look for alternative means to
effect this overall transformation. Because pure maleamic acids can be
obtained in almost quantitative yield without recourse to chromatography,[10]
we reasoned that splitting the approach of Nielsen and Buchardt into two
discrete steps might prove advantageous [i.e., amic acid formation, then cycli-
zation and esterification). However, aside from largely ineffective diimide
reagents, there are comparatively few alternative reagent combinations that
may be employed for the concurrent cyclization and esterification steps.
Adamcyk[13] has reported the one-pot synthesis of 4-(N-maleimido methyl)cy-
clohexane-1-carboxylic acid pentafluorophenyl ester using the commercially
available reagent pentafluorophenyl trifluoroacetate 2, which presumably
first cyclizes the precursor maleamic acid via an in situ mixed trifluoroacetic
anhydride, then mediates esterification of the amino acid carboxyl via a
second such mixed anhydride species. The analogous reagent, N-trifluoroace-
toxy succinimide 3, which fitted our requirements, was originally developed as
a coupling agent for peptide chemistry[14] and has since proved effective for
other one-pot acylation or esterification reactions,[15,16] but no application to
the preparation of N-maleoyl amino acid succinimido esters had been pre-
viously reported. Compound 3 is conveniently prepared by the reaction of tri-
fluoroacetic acid anhydride and HOSu,[14] and is stable in storage for several
months at 48C. We found that treatment of the isolated maleamic acids corre-
sponding to five previously reported heterobifunctional linkers[8,11,17] with 3
(5 eq) and sym-collidine (2 eq) in DMF solution for 10 h led to a highly
efficient, gram-scale conversion to the desired N-maleoyl succinimidyl
esters 1a–e (Scheme 1). Concentration of the reaction mixture in vacuo,
followed by suspension of the residue in chloroform and successive
washing with 5% NaHCO3 and water, gave rise to almost pure products
upon drying and evaporation of the organic solution. Recrystallized
products were obtained in yields of 65–89%, representing a substantial

N-Maleoyl Amino Acid Succinimido Esters
305
Scheme 1. Reagents and conditions: (i) 3, sym-collidine, DMF, o/n.
increase in both yield and efficiency relative to previously reported methods
(Table 1). The choice of base proved critical; for example, less sterically
encumbered amines such as N,N-diisopropylethylamine gave rise to signifi-
cant amounts of colored by-products upon overnight reaction, which could
not be readily removed by crystallization.
Table 1. Preparation of N-maleoyl amino acid succinimido esters
Yield of
N-maleoyl ester
derivative
Amino acid
Maleamic acid
Mp
Lit. mp
68% (1a)
158–161 160–163[8]
170.5–173.5 165–167[11]
205–207 194–195[11]
89% (1b)
80% (1c)
75% (1d)
65% (1e)
175–178 175–177[17]
179–182 164–166.5[17]

306
M. J. Paterson and I. M. Eggleston
In conclusion, we have devised a convenient and cost-effective new
procedure for the large-scale preparation of several important N-maleoyl
active esters using N-trifluoroacetoxysuccinimide. This should increase the
scope for using these heterobifunctional linkers in many of the potential
applications cited previously. We are currently exploring the effectiveness
of N-trifluoroacetoxysuccinimide in mediating the formation of various
other maleimide derivatives from the corresponding maleamic acids.
EXPERIMENTAL
Melting points were determined on an Electrothermal IA9000 series digital
melting-point apparatus using open capillaries and are quoted uncorrected.
NMR spectra were recorded in CDCl3 on a Bruker Avance DPX 300 FT-spec-
trometer operating at 300 MHz (1H) or 75 MHz (13C).
Preparation of Maleamic Acids
Maleamic acids were prepared as described by Rich et al.,[10] by treatment of a
suspension of the amino acid (1 eq) in glacial acetic acid with an acetic acid
solution of maleic anhydride (1 eq) at room temperature. After stirring at room
temperature overnight, the suspension was filtered and the precipitate was
washed well with ether and dried in vacuo. The crude maleamic acids were
obtained in essentially quantitative yield and were used without further
purification.
Typical Procedure for Cyclization and Esterification using
N-Trifluoroacetoxysuccinimide
A cooled (08C) and stirred solution of N-(carboxyacroyl)-4-(aminomethyl)-
cyclohexane-1-carboxylic acid (1.36 g, 5.33 mmol) in anhydrous DMF
(20 mL) was treated with sym-collidine (1.48 mL, 11.22 mmol). After
30 min, a solution of N-trifluoroacetoxysuccinimide (5.64 g, 26.7 mmol) in
DMF (5 mL) was added. Cooling of the stirred solution continued for 10 h,
whereafter it was allowed to attain room temperature overnight. The
solution was concentrated under reduced pressure, and the residue was resus-
pended in chloroform. The organic solution was washed successively with
1 M hydrochloric acid, 5% sodium hydrogen carbonate, and water and then
dried (Na2SO4). Concentration of the solution under reduced pressure gave
an off-white powder, which was recrystallized from CH2Cl2–hexane to give
1b as a white powder (1.59 g, 89%).

N-Maleoyl Amino Acid Succinimido Esters
307
Data
1a: 1H NMR (CDCl3, d, ppm) 2.81 (4H, s, 2 ꢀ CH2), 3.01 (2H, t, J ¼ 6.9 Hz,
CH2CO2), 3.93 (2H, t, J ¼ 6.9 Hz, CH2N), 6.73 (2H, s, 2 ꢀ 55CH). 13C NMR
(CDCl3, d, ppm) 25.93, 30.12, 33.38, 134.69, 167.37, 169.09, 170.46.
1b: 1H NMR (CDCl3, d, ppm) 1.00–2.61 (10H, m, cyclohexyl CH2, CH), 2.81
(4H, s, 2 ꢀ CH2), 3.38 (2H, d, J ¼ 6.9 Hz, NCH2), 6.70 (2H, s, 2 ꢀ 55CH).
13C NMR (CDCl3, d, ppm) 25.98, 28.40, 29.70, 36.47, 40.80, 43.79, 134.32,
169.54, 170.99, 171.37.
1c: 1H NMR (CDCl3, d, ppm) 2.91 (4H, s, 2 ꢀ CH2), 6.90 (2H, s, 2 ꢀ 55CH),
7.63 (2H, d, J ¼ 8.7 Hz, Ar), 8.23 (2H, d, J ¼ 8.7 Hz, Ar). 13C NMR (CDCl3,
d, ppm) 26.08, 124.16, 125.61, 131.87, 134.90, 137.70, 161.55, 169.05,
169.53.
1d: 1H NMR (CDCl3, d, ppm) 2.91 (4H, s, 2 ꢀ CH2), 6.89 (2H, s, 2 ꢀ 55CH),
7.60–7.82 (2H, m, Ar), 8.12–8.18 (2H, m, Ar). 13C NMR (CDCl3, d, ppm)
26.07, 126.69, 128.10, 130.04, 130.17, 132.41, 134.80, 161.49, 169.25, 169.44.
1e: 1H NMR (CDCl3, d, ppm) 2.89 (4H, s, 2 ꢀ CH2), 3.90 (3H, s, OCH3), 6.87
(2H, s, 2 ꢀ 55CH), 7.10 (1H, d, J ¼ 8.9 Hz, Ar), 7.90 (1H, d, J ¼ 2.2 Hz, Ar),
8.22 (1H, dd, J ¼ 8.8, 2.2, Ar). 13C NMR (CDCl3, d, ppm) 26.06, 56.82,
112.43, 118.06, 120.79, 133.34, 134.31, 135.03, 161.05, 169.38, 169.59.
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