The synthesis and structural characterization of novel N-meta-ferrocenyl benzoyl dipeptide esters

Article abstract of DOI:10.1016/j.inoche.2005.10.027

A series of novel N-meta-ferrocenyl benzoyl dipeptide esters (2-5) have been prepared by coupling meta-ferrocenyl benzoic acid (1) to the dipeptide ethyl esters using the conventional 1,3-dicyclohexylcarbodiimide (DCC), 1-hydroxybenzotriazole (HOBt) proto

Full text of DOI:10.1016/j.inoche.2005.10.027

Inorganic Chemistry Communications 9 (2006) 152–155
www.elsevier.com/locate/inoche
The synthesis and structural characterization of novel
N-meta-ferrocenyl benzoyl dipeptide esters
a
b
b
b
David Savage , Steven R. Alley , John F. Gallagher , Alok Goel ,
b
b,
*
Paula N. Kelly , Peter T.M. Kenny
a
School of Chemical Sciences, Dublin City University, Dublin 9, Ireland
National Institute for Cellular Biotechnology, Dublin City University, Dublin 9, Ireland
b
Received 19 July 2005; accepted 22 October 2005
Available online 1 December 2005
Abstract
A series of novel N-meta-ferrocenyl benzoyl dipeptide esters (2–5) have been prepared by coupling meta-ferrocenyl benzoic acid (1) to
the dipeptide ethyl esters using the conventional 1,3-dicyclohexylcarbodiimide (DCC), 1-hydroxybenzotriazole (HOBt) protocol. The
dipeptides employed were GlyGly(OEt) (2), GlyAla(OEt) (3), GlyLeu(OEt) (4) and GlyPhe(OEt) (5). The compounds were fully char-
acterized by a range of NMR spectroscopic techniques and by mass spectrometry (MALDI-MS, ESI-MS).
Ó 2005 Elsevier Ltd. All rights reserved.
Keywords: Ferrocene; Bioorganometallic chemistry; Dipeptides; MALDI
Research in the area of bioorganometallic chemistry has
developed as a rapidly growing area, connecting organo-
metallic chemistry with the preparation of novel sensor
compounds, peptide mimetic models and unnatural drugs
[1–7]. The stability of the ferrocenyl group and its deriva-
tives, in addition to its spectroscopic, electrochemical prop-
erties and ease of use make it suitable for biological
applications and conjugation with biologically important
compounds. The synthesis and structural characterization
of novel N-ferrocenoyl and N-ferrocenyl amino acid and
peptide derivatives has been reported [8–21]. In addition
ferrocene has been incorporated in drugs such as antibiot-
ics, aspirin, anti-malarial drugs and anti-cancer drugs such
as tamoxifen [22–25]. A review on the bioorganometallic
chemistry of ferrocene has recently been published [26].
In this communication, we report the synthesis and
structural characterization of a series of novel N-meta-ferr-
ocenyl dipeptide ethyl ester derivatives (2–5). The ferrocenyl
moiety is linked to the dipeptide esters via a meta-benzoyl
group. We recently, reported the synthesis and structural
characterization of a series of N-para- and N-meta-ferroce-
nyl benzoyl amino acid ester derivatives [27–29]. The N-
meta-ferrocenyl dipeptide ethyl ester derivatives are
composed of three key moieties: (i) an electroactive core,
(ii) a conjugated linker that can act as a chromophore and
(iii) a dipeptide ethyl ester derivative that can interact with
other molecules via hydrogen bonding.
meta-Ferrocenyl benzoic acid (1) was prepared as previ-
ously reported [29]. Conventional peptide chemistry was
employed in the preparation of the dipeptide ethyl esters.
Equimolar quantities of the N-Boc protected glycine was
reacted with the amino acid ethyl ester hydrochloride salts
of glycine, L-alanine, L-leucine and L-phenylalanine under
basic conditions in the presence of dicyclohexylcarbodii-
mide (DCC), catalytic amounts of 1-hydroxybenzotriazole
(HOBt) in dichloromethane (DCM) at 0 °C yielding the
protected dipeptide ethyl esters. Deprotection of the amino
terminal was achieved using TFA. The deprotected dipep-
tide ethyl esters were then coupled to meta-ferrocenyl ben-
zoic acid using equimolar amounts of DCC and catalytic
*
Corresponding author. Tel.: +353 1 7005 689; fax: +353 1 7005 503.
E-mail address: peter.kenny@dcu.ie (P.T.M. Kenny).
1387-7003/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.inoche.2005.10.027

D. Savage et al. / Inorganic Chemistry Communications 9 (2006) 152–155
153
i
H N
2
Fe
Fe
OR'
OR'
O
O
ii
O
R'
iii
Fe
H
N
OEt
Fe
N
OH
H
O
O
R
O
2-5
1
Scheme 1. Synthesis of the N-ferrocenyl benzoyl dipeptide esters (2–5): GlyGly(OEt) (2), GlyAla(OEt) (3), GlyLeu(OEt) (4) and GlyPhe(OEt) (5). (i)
NaNO2, HCl, 5 °C, (ii) NaOH/MeOH, H2O, (iii) DCC, HOBt, Et3N, dipeptide ethyl ester.
amounts of HOBt (Scheme 1). Purification by silica gel col-
umn chromatography furnished the pure products in yields
of 50–57% and all gave analytical and spectroscopic data
in accordance with the proposed structures [30]. The N-
meta-ferrocenyl benzoyl dipeptide ethyl esters (2–5) were
70.26. The carbon atoms of the aromatic ring are non-
equivalent and therefore six signals are visible in the region
d 124.7–140.7. The methylene carbon atoms of the deriva-
tives 2–5 were identified by DEPT-135. A complete assign-
1
ment of the H and 13C NMR spectra of compound 3 is
1
characterized by a combination of H NMR, 13C NMR,
presented in Table 1.
DEPT-135 and 1H–13C COSY (HMQC) spectroscopy,
matrix assisted laser desorption ionization (MALDI) 2, 4,
5 and electrospray ionization mass spectrometry 3 (ESI).
All the proton and carbon chemical shifts for com-
pounds 2–5 were unambiguously assigned by a combination
Since the introduction of soft ionization techniques such
as fast-atom bombardment mass spectrometry (FABMS),
matrix assisted laser desorption ionization (MADLI) and
electrospray ionization mass spectrometry (ESI-MS), a
wide range of thermolabile and non-volatile compounds
of DEPT-135 and 1H–13C COSY (HMQC). The 1H and 13
C
NMR spectra for compounds 2–5 showed peaks in the fer-
rocene region characteristic of a mono substituted ferrocene
benzoyl moiety [27–29]. The protons in the ortho position of
the (g5-C5H4) ring appear in the region d 4.62–4.71,
whereas the protons in the meta position occur in the range
d 4.27–4.35. The (g5-C5H5) ring appears in the region d
3.96–4.05. The protons of the meta-disubstituted benzoyl
group appear in the region d 7.2–7.96. For example, in
the case of the L-alanine derivative 3, the aromatic protons
are present as a triplet, two doublets and a singlet at d 7.36,
d 7.64, and d 7.96, respectively. The (g5-C5H5) ring appears
as a singlet in the 1H NMR spectrum at d 4.05, whereas the
meta and ortho protons on the (g5-C5H4) ring are present at
d 4.35 and d 4.71, respectively. The glycine NH proton
appears as a triplet at d 7.31, whereas the L-alanine NH
appears as a doublet at d 7.09. The L-alanine methyl group
is present as a doublet at d 1.47 and the ethyl ester methyl
group appears as a triplet at d 1.3.
Table 1
1H and 13C spectroscopic data for 3
12
13
4
4
2
1
11
14
17
3
5
O
Fe
16 15
H
18
19
20
21
23
N
O
22
N
6-10
H
24
O
O
Site
1H NMR
13C NMR
84.4
HMQC
1
2,3
4,5
4.71
4.35
4.05
67
69.7
70.1
6–10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
134.1
7.64
7.36
7.64
129.8
124.7
129
The 13C NMR spectra of compounds 2–5 show signals
in the region d 67.02–84.7 indicative of a monosubstituted
ferrocene benzoyl subunit [27–29]. The ipso carbon of the
(g5-C5H4) ring appears in a very narrow range of d 84.4–
84.7. This signal is absent in the DEPT 135 spectra. The
carbon atoms in the ortho position of the (g5-C5H4) ring
are present at d 67.02–67.14, whereas the meta carbon
atoms appear in the range d 69.71–69.87. The carbon
atoms of the (g5-C5H5) ring appear in the range d 70.08–
140.7
168.3
169.1
7.96
125.1
44
4.19–27
4.62
1.47
48.8
18.6
173.1
4.19–27
1.3
62
14.5

154
D. Savage et al. / Inorganic Chemistry Communications 9 (2006) 152–155
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can be subjected to mass spectrometric analysis [31–34].
The N-meta-ferrocenyl benzoyl dipeptide esters were not
amenable to electron ionization (EI) or chemical ionization
(CI) studies, therefore MADLI was employed in the anal-
ysis of compounds 2, 4 and 5, whereas compound 3 was
analyzed by ESI-MS. MALDI confirmed the correct rela-
tive molecular mass for the compounds and examination
of the mass spectra revealed the presence of intense radi-
cal-cations. The formation of the radical-cation molecular
ion species was further confirmed by the detection of the
sodium adduct [M + 23]+ and potassium adduct
[M + 39]+ for each of the compounds analyzed. This is
not common for the analysis of peptides and proteins by
MADLI-MS, as the formation of [M + H]+ is favored.
The vast majority of analytes subjected to analysis by soft
ionization techniques such as FAB and MALDI furnish
protonated molecular ion species as a result of proton
transfer reactions between the analyte and matrix, and/or
cation adduction. It has been reported that the molecular
radical cation of ferrocene and not the protonated molecu-
lar ion is generated during MALDI analysis [35]. Fragment
ions were not observed or were of very low intensity in the
MALDI spectra. This is in contrast to the analysis of the
related para-ferrocenyl benzoyl amino acid derivatives by
FABMS, where important fragment ions were observed
[27,28]. Compound 3 was analyzed by electrospray ioniza-
tion mass spectrometry (ESI-MS). The ESI mass spectrum
displayed an intense [M + Na]+ species at m/z 485 and a
potassium adduct was also present at m/z 501. Fragment
ions were not observed or were of very low intensity.
In conclusion, the novel N-meta-ferrocenyl benzoyl
dipeptide ethyl esters 2–5 were prepared in good yields
using standard organic peptide synthetic protocols. The
compounds were characterized by a range of NMR spec-
troscopic techniques and by MALDI and ESI mass
spectrometry.
[30] Synthesis of compound 3. Glycine-L-alanine ethyl ester hydrochloride
(0.2 g, 1.0 mmol) was added to a solution of meta-ferrocenoyl benzoic
acid (0.3 g, 1.0 mmol), 1-hydroxybenzotriazole (0.2 g, 1.5 mmol),
triethylamine (0.5 ml), and dicyclohexylcarbodiimide (0.45 g,
2.1 mmol) in 50 ml of dichloromethane at 0 °C. After 30 min, the
temperature was raised to room temperature and the reaction was
allowed to proceed for 48 h. The precipitated N,N0-dicyclohexylurea
was removed by filtration and the filtrate was washed with water, 10%
potassium hydrogen carbonate, 5% citric acid, dried over MgSO4 and
the solvent was removed in vacuo. The product was purified by
column chromatography {eluant 2:3 petroleum ether (40–60 °C):
ethyl acetate}. Recrystallization from petroleum ether (40–60 °C):
ethyl acetate furnished the title compound as orange needles. (0.229 g,
Acknowledgments
D.S. thank the Irish American Partnership and Dublin
City University for the funding of a studentship award
1999–2002. This research was partly supported by the Na-
tional Institute for Cellular Biotechnology under the Pro-
gramme for Research in Third Level Institutions (PRTLI,
round 3, 2001–2006). We also thank Dr. Matt Openshaw
of Shimadzu Biotech, UK and John Kelly of Bruker Dal-
tonics, UK for measurement of the MALDI mass spectra.
20
50%). M.p. 87–89 °C, E00 = 131 mV, ½aꢀD ¼ ꢁ18ꢂ (c 2, EtOH). Mass
spectrum: found: [M + Na]+ 485.3, C24H26N2O4FeNa requires:
485.12. IR mmax(KBr): 3358, 3257, 3095, 2932, 1735, 1687, 1639,
1561, 1528, 1509 cmꢁ1. UV–Vis kmax EtOH; 325 (e 1290), 442 (e
370) nm. 1H NMR (400 MHz) d (CDCl3): 7.96 (1 H, s, ArH), 7.64
(2H, d, J = 8.4 Hz, ArH), 7.36 (1H, t, J = 8.4 Hz, ArH), 7.31 (1H, t,
J = 7.2 Hz, –CONH–), 7.09 (1H, d, J = 7.2 Hz, –CONH–), 4.71 {2H,
t, J = 2 Hz, ortho on (g5-C5H4)}, 4.62 {1H, quint, J = 7.6 Hz, –
CH(CH3)}, 4.35 {2H, t, J = 2 Hz, meta on (g5-C5H4)}, 4.19–4.27
(4H, m, –NHCH2CO–, –OCH2CH3), 4.05 {5H, s,(g5-C5H5)},
1.47{3H, d, J = 7.6 Hz, –CH(CH3)}, 1.30 (3H, t, J = 7.6 Hz, –
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