Efficient routes for the synthesis of 1,4,7,10,13-pentaazacyclohexadecane- 14,16-dione

Article abstract of DOI:10.1055/s-2007-965924

The synthetic access to 1,4,7,10,13-pentaazacyclohexadecane-14,16-dione was significantly improved from an overall yield of 1% to 33%. The procedure involved the use of COCF₃ or DDE as highly selective protecting groups for primary amines and o

Full text of DOI:10.1055/s-2007-965924

PAPER
679
Efficient Routes for the Synthesis of
1,4,7,10,13-Pentaazacyclohexadecane-14,16-dione
S
ynthesis of 1,4,7
h
,10,13-Pent
i
a
lohexadec
a
r
ne-14,16
a
-dione Da Pieve,1 Alfredo Medina-Molner, Bernhard Spingler*
University of Zürich, Winterthurerstr. 190, 8057 Zürich, Switzerland
Fax +41(44)6356803; E-mail: spingler@aci.unizh.ch
Received 11 October 2006; revised 24 November 2006
ber of possible side products. In order to selectively pro-
tect the secondary amines, one first has to protect the
primary amines. Recently there have been several reports
about the synthesis of oligoamines with all their second-
Abstract: The synthetic access to 1,4,7,10,13-pentaazacyclohexa-
decane-14,16-dione was significantly improved from an overall
yield of 1% to 33%. The procedure involved the use of COCF3 or
DDE as highly selective protecting groups for primary amines and
of Boc to protect the secondary amine functions. The reaction of a ary amines Boc-protected. One group chose to use tri-
fluoroacetate,7 and two others 1-(4,4-dimethyl-2,6-dioxo-
tris-Boc protected tetraethylenepentamine with malonic acid gave
cyclohexylidene)ethyl (DDE8) to yield the linear oligo-
the macrocycle in a 44% yield, the same reaction with malonyl chlo-
ride, however, yielded only 3.6% of the desired product. Boc depro-
amines protected at their a- and w-positions.9
tection and removal of the trifluoroacetate salts gave access to the
Based on these reports, we chose to compare the efficien-
cy of COCF3 and DDE as protecting groups for the syn-
thesis of tetraethylenepentamine having its primary amine
functions protected. The obtained products were then re-
acted with (Boc)2O for the protection of the secondary
amines. The COCF3 and DDE protecting groups were se-
lectively removed under mild conditions (Scheme 1),
leaving the secondary amines protected by the Boc
groups. In effect, the trifluoroacetamide path gave a high-
er overall yield and in addition did not require any purifi-
cation by chromatography.
final product, 1,4,7,10,13-pentaazacyclohexadecane-14,16-dione,
in an overall yield of 33%.
Key words: azamacrocycles, 1,4,7,10,13-pentaazacyclohexadec-
ane-14,16-dione, protecting groups, cyclization, ligands
In recent years there has been a continuing interest in the
field of macrocyclic complexes because of their close re-
semblance with naturally occurring biological systems,
their stability (macrocyclic effect), and well-behaved co-
ordination chemistry. A variety of macrocyclic crown
ethers, cryptands, and azamacrocycles have been synthe-
sized. The latter have a strong tendency to form stable
complexes with transition metals,2 such as Ni2+, Cu2+,
Co3+ and Zn2+. These macrocyclic systems can have a
varying ring size, number and kind of donor atoms, as
well as substituents. Kimura et al. introduced the macro-
cyclic bisamide copper(II) complexes of a,w-oligoamines
when working on superoxide dismutase model systems.3
In order to gain access to the macrocycle 7, we performed
various cyclization reactions of malonic acid derivatives
H2N
DDE
N
H
N
H
N
H
NH2
CF3COOEt
It is known that Ni(II) and Cu(II) complexes of
1,4,7,10,13-pentaazacyclohexadecane-14,16-dione (7)
are able to promote the conversion of poly d(GC) from B
to Z form.4 During our studies aimed at a better under-
standing of the transition metal induced B- to Z-DNA
transformation,4b,5 we initially synthesized the ligand
1,4,7,10,13-pentaazacyclohexadecane-14,16-dione (7)
according to the published procedure.6 However, we were
not able to reproduce the published yield of 15%. The iso-
lation of the product, in fact, required purification by
chromatography on silica gel and preparative HPLC, and
yielded only 1% of the desired product. Alternative reac-
tion routes were thus considered. The protection of the
three secondary amine groups of the tetraethylenepent-
amine would leave only its terminal primary amine groups
free to react with the diethyl malonate reducing the num-
HN
DDE
N
H
N
H
N
H
NH
HN
N
N
H
N
H
NH
H
DDE
COCF3
COCF3
3
1
(Boc)2O
(Boc)2O
HN
N
N
N
NH
HN
N
N
N
NH
DDE Boc Boc Boc DDE
COCF3Boc Boc Boc COCF3
2
4
25% NH2NH2
25% NH4OH
H2N
N
N
N
NH2
Boc Boc Boc
5
SYNTHESIS 2007, No. 5, pp 0679–0682
Advanced online publication: 08.02.2007
DOI: 10.1055/s-2007-965924; Art ID: P11706SS
x
x
.
x
x
.2
0
0
7
Scheme 1 Two synthetic paths leading to 4,7,10-tris(Boc)tetra-
ethylenepentamine (5).
© Georg Thieme Verlag Stuttgart · New York

680
C. Da Pieve et al.
PAPER
were recorded on a Varian Mercury 200 MHz, Gemini 300 MHz, or
Bruker DRX 500 MHz spectrometer. The chemical shifts are rela-
tive to residual solvent protons as reference. Elemental analyses
were performed on a Leco CHNS-932 elemental analyzer.
with 4,7,10-tris(Boc)tetraethylenepentamine (5). As al-
ready mentioned in the introduction, direct condensation
of tetraethylenepentamine with diethyl malonate gave
completely unsatisfactory results. The reaction of 5 with
malonic acid under standard peptide coupling conditions
gave the highest yield (44%) of the desired product 6. On
the other hand, when the protected pentamine 5 was react-
ed with malonyl dichloride, only 3.6% of 6 could be iso-
lated (Scheme 2). This result is in line with a publication
from the group of Picard, who cyclized Boc-protected
amines only with the help of anhydride activated diacids,
but not with the corresponding diacid dichlorides.10 Brad-
shaw et al., however, reported a relatively high yield for
the azamacrocycle formation with the help of a diacid
chloride.11 After deprotection of the Boc groups of 6 and
removal of the trifluoroacetate salts, we obtained the mac-
rocycle 1,4,7,10,13-pentaazacyclohexadecane-14,16-di-
one (7) in an overall yield of 33% starting from
tetraethylenepentamine.
1,13-Di(DDE)tetraethylenepentamine (1)
Tetraethylenepentamine (1 g, 5.3 mmol) was dissolved in anhyd
EtOH (30 mL). 2-Acetyl-5,5-dimethyl-1,3-cyclohexandione (1.93
g, 10.6 mmol) and activated molecular sieves (5 g) were added to
the solution and the mixture was stirred at r.t. for 24 h. The solvent
was then removed under vacuum to yield a yellow oil; yield: 1.84 g
(98%); HPLC: tR = 18.3 min.
1H NMR (500 MHz, DMSO-d6): d = 0.92 (s, 12 H), 2.23 (s, 8 H),
2.46 (s, 6 H), 2.56 (t, J = 5.2 Hz, 4 H), 2.59 (t, J = 5.4 Hz, 4 H), 2.73
(t, J = 5.9 Hz, 4 H), 3.43 (t, J = 5.9 Hz, 4 H).
13C NMR (125 MHz, DMSO-d6): d = 17.69, 27.90, 29.75, 42.90,
47.37, 48.52, 48.98, 52.40, 106.92, 172.31, 196.20.
MS (ESI): m/z (%) = 518 (100, [M + H]+), 1057 (5, [2 M + Na]+).
Anal. Calcd for C28H47N5O4·2CH3OH: C, 61.93; H, 9.53; N, 12.04.
Found: C, 61.55; H, 9.17; N, 11.87.
1,13-Di(DDE)-4,7,10-tris(Boc)tetraethylenepentamine (2)
A solution of (Boc)2O (3.23 g, 18.5 mmol) in MeOH (15 mL) was
added dropwise over 10 min at 0 °C to a solution of 1 (1.90 g, 3.68
mmol) in MeOH (30 mL). The mixture was stirred at r.t. for 24 h.
H2O (15 mL) was added to the solution and the stirring continued at
40 °C for 2 h. The solvent was removed in vacuo and the residue
was purified by chromatography on silica gel. Elution was started
with 1:1 EtOAc–hexane. The polarity of the eluent was gradually
increased to 100% EtOAc. The product was obtained as a white sol-
id; mp 77–80 °C; yield: 2.37 g (79% based on 1); Rf = 0.15 (100%
EtOAc); HPLC: tR = 24.2 min.
O
O
NH HN
a or b
H2N
N
N
N
NH2
N
N
Boc
Boc
N
Boc Boc Boc
Boc
5
c, d
6
O
O
NH HN
1H NMR (500 MHz, MeOH-d4): d = 1.01 (s, 12 H), 1.42–1.45 (m,
27 H, CH3), 2.34 (s, 8 H), 2.55 (s, 6 H), 3.35 (br t, 8 H), 3.48 (br t,
4 H), 3.68 (br t, 4 H).
H
N
NH
HN
13C NMR (125 MHz, MeOH-d4): d = 18.45, 28.56, 28.88, 31.18,
7
42.36, 46.36, 47.55, 53.64, 81.73, 109.09, 156.89, 176.03, 199.97.
MS (ESI): m/z = 817 (100%, [M]+).
Scheme 2 Reagents and conditions: (a) malonyl dichloride, Et3N,
CH2Cl2, 1 mM soln of 5, r.t., 12 h, 3.6% yield; (b) malonic acid,
HOBt, DCC, CH2Cl2, 4 mM soln of 5, r.t., 12 h, 44% yield; (c) TFA,
CH2Cl2, r.t., 24 h, quantitative yield, (d) anion-exchange column,
83% yield.
Anal. Calcd for C43H71N5O10: C, 63.11; H, 8.75; N, 8.56. Found: C,
63.33; H, 8.92; N, 8.62.
1,13-Di(trifluoroacetyl)tetraethylenepentamine (3)7
Ethyl trifluoroacetate (1.38 mL, 22.6 mmol) was added dropwise to
a solution of tetraethylenepentamine (1.0 g, 5.3 mmol) in MeOH
(25 mL) at –78 °C. The mixture was stirred for 1 h at –78 °C and
then at r.t. for 24 h. The solvent was removed under vacuum to yield
a yellowish oil; yield: 2.02 g (quant).
IR (KBr): 1709 cm–1 (C=O).
1H NMR (300 MHz, MeOH-d4): d = 2.79–2.99 (m, 16 H).
13C NMR (75 MHz, D2O): d = 34.01, 41.15, 41.32, 45.05, 113.61
(q, J = 284.1 Hz), 174.21.
MS (ESI): m/z (%) = 381 (100, [M]+), 363 (16, [M – F + H]+).
All chemicals were purchased from Aldrich or Fluka (Buchs, Swit-
zerland) and used without further purification. Tetraethylenepent-
amine was obtained as a free base according to the literature.6 1,13-
Di(trifluoroacetyl)tetraethylenepentamine (3) and 1,13-di(trifluoro-
acetyl)-4,7,10-tris(Boc)tetraethylenepentamine (4) were synthe-
sized by a modification of the procedure described in literature.7 All
reactions were performed under N2. The reactions were monitored
by HPLC or TLC. TLC were carried out on 0.25 mm Merck silica
gel aluminum plates (60 F254) or aluminum oxide pre-coated plastic
sheets (alox N/UV254) using UV light or Schlittler staining solutions
as visualizing agent. Column chromatography was performed on
silica gel (particle size 0.040–0.063 mm). HPLC analyses were per-
formed on Merck-Hitachi D-7000 system using a Macherey-Nagel
Nucleosil C18 EC RP column (5 mm particle size, 100 Å pore size,
250/3 mm) at a wavelength of 215 nm. The solvents for analytical
HPLC were: eluent A = 0.1% trifluoroacetic acid in H2O (pH 2);
eluent B = MeOH. The gradient used for all analyses was: 0–3.0
min, 100% A; 3.1–9.0 min, 75% A; 9.1–20.0 min, 66% to 0% A,
20.0–25.0 min 0% A; 25.1–30.0 min, 100% A. The flow rate was
0.5 mL/min. Electrospray ionization (ESI) mass spectra were re-
corded on a Merck Hitachi M-8000 spectrometer. NMR spectra
1,13-Di(trifluoroacetyl)-4,7,10-tris(Boc)tetraethylenepent-
amine (4)7
A solution of (Boc)2O (4.29 g, 19.68 mmol) in MeOH (15 mL) was
added dropwise, over a period of 10 min, to a solution of 3 (1.5 g,
3.93 mmol) in MeOH (25 mL) at 0 °C. The solution was stirred at
r.t. for 2 d. The excess of (Boc)2O was quenched with H2O (2 mL).
The solvent was then removed under vacuum and the residue was
purified by chromatography on silica gel. The elution was started
with 100% CH2Cl2 and then changed to 30:1 CH2Cl2–MeOH; yield:
2.4 g (90%); Rf = 0.5 (20:1 CH2Cl2–MeOH); HPLC: tR = 22.4 min.
Synthesis 2007, No. 5, 679–682 © Thieme Stuttgart · New York

PAPER
Synthesis of 1,4,7,10,13-Pentaazacyclohexadecane-14,16-dione
681
1H NMR (300 MHz, MeOH-d4): d = 1.45 (s, 27 H), 3.34–3.44 (m,
16 H).
Tris(trifluoroacetate) Salt of 1,4,7,10,13-Pentaazacyclohexa-
decan-14,16-dione
Compound 6 (69.2 mg, 0.124 mmol) was dissolved in CH2Cl2 (4
mL). TFA (0.2 mL, 2.5 mmol) was added and the solution was
stirred at r.t. for 24 h. The solvent was then removed under vacuum
to afford the tris(trifluoroacetate) salt of 7 as a yellowish oil; yield:
74.2 mg (quant).
13C NMR (75 MHz, MeOH-d4): d = 28.71, 39.26, 46.24, 47.47,
48.16, 81.63, 117.62 (q, J = 285.5 Hz), 154.68, 156.34 (q, J = 24.5
Hz).
4,7,10-Tris(Boc)tetraethylenepentamine (5) by DDE Deprotec-
tion
IR (KBr): 1679 cm–1 (C=O).
A solution of 2 (505 mg, 0.618 mmol) in EtOH (9 mL) and 25% aq
NH2NH2 (10 mL) were stirred together at r.t. for one day. The com-
pletion of the reaction was checked by TLC (eluent = 9:1:0.1
CH2Cl2–MeOH–concd soln of NH4OH). The solvent was removed
under vacuum to afford a colorless oil that was further purified by
chromatography on silica gel (eluent: 9:1 CH2Cl2–MeOH); yield:
0.30 g (quant); Rf = 0.05 (9:1:0.1 CH2Cl2–MeOH–concd NH4OH);
HPLC: tR = 17.6 min.
1H NMR (300 MHz, MeOH-d4): d = 3.08–3.11 (m, 4 H), 3.28–3.31
(m, 8 H), 3.57–3.61 (m, 4 H).
13C NMR (75 MHz, MeOH-d4): d = 37.08, 43.56, 45.74, 47.20,
49.39, 172.62.
MS (ESI): m/z (%) = 258 (100) [M + H]+.
1,4,7,10,13-Pentaazacyclohexadecan-14,16-dione (7)6
1H NMR (500 MHz, MeOH-d4): d = 1.45 (s, 27 H, CH3), 2.76 (br s,
4 H), 3.26–3.35 (m, 12 H).
13C NMR (125 MHz, MeOH-d4): d = 28.90, 40.75, 45.96, 46.42,
51.50, 81.56, 157.24.
To isolate the free base 7, the tris(trifluoroacetate) salt of 7 was dis-
solved in a minimum amount of a 1:1 mixture of H2O and MeOH.
This solution was run through an activated Amberlyst A26-OH,
macromolecular ion-exchange resin in H2O. Fractions containing
product 7 were combined together and concentrated under reduced
pressure to give a yellow oil;12 yield: 26.5 mg (83%).
MS (ESI): m/z (%) = 490 (33, [M]+), 390 (100, [M – Boc]+), 289
(27, [M – 2 Boc – H]+).
1H NMR (500 MHz, D2O): d = 2.74 (br s, 8 H), 2.77–2.81 (m, 4 H),
3.82–3.46 (m, 4 H).
13C NMR (125 MHz, D2O): d = 38.69, 46.94, 47.58, 47.68, 170.47.
4,7,10-Tris(Boc)tetraethylenepentamine(5)by Trifluoroacetate
Deprotection7
Compound 4 (2.24 g, 3.28 mmol) was dissolved in MeOH (50 mL)
followed by the addition of 25% NH4OH (10 mL). The mixture was
stirred at r.t. for 3 days. The reaction was followed by HPLC. The
solvent was evaporated to obtain a white very hygroscopic product;
yield: 1.6 g (quant).
Acknowledgment
This research was supported by the Swiss National Science Found-
ation and the Forschungskredit of the University of Zürich. We
thank Prof. Dr R. Alberto, Dres J. K. Pak and Ph. Kurz for valuable
discussions. We further thank B. and H. Spring for performing the
elemental analyses and Dr T. Fox and P. Ruiz-Sánchez for NMR
measurements.
The analytical characteristics of the product were identical to those
given above, obtained by deprotection of 2.
4,7,10-Tris(Boc)-14,16-dioxo-1,4,7,10,13-pentaazacyclohexa-
decane (6) by Cyclization of 5 with Malonyl Dichloride
A solution of malonyl dichloride (29.5 mL, 0.30 mmol) in CH2Cl2 (5
mL) was added dropwise during a period of 90 min to a solution of
5 (0.149 g, 0.30 mmol) in CH2Cl2 (30 mL). The mixture was stirred
at r.t. overnight. After removing the solvent, the residue was dis-
solved in 0.1 M HCl (30 mL) and extracted with CH2Cl2 (3 × 10
mL) The organic phase was then dried over MgSO4 and evaporated.
The residue was purified by chromatography on silica gel. The elu-
tion was started with 2:1 EtOAc–hexane and then was switched to
9:1 EtOAc–MeOH; yield: 6.1 mg (3.6%).
References
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4,7,10-Tris(Boc)-14,16-dioxo-1,4,7,10,13-pentaazacyclohexa-
decane (6) by Cyclization of 5 with Malonic Acid
Compound 5 (0.138 g, 0.282 mmol), malonic acid (0.029 g, 0.280
mmol), and HOBt (0.086 g, 0.56 mmol) were dissolved in CH2Cl2
(70 mL). Dicyclohexylcarbodiimide (DCC, 0.174 g, 0.846 mmol)
was added and the mixture was stirred overnight at r.t. under N2.
The solvent was removed under vacuum and the residue was puri-
fied by chromatography on silica gel. The elution was started with
1:1 EtOAc–hexane and then was changed to 100% EtOAc; yield:
69.2 mg (44%); Rf = 0.35 (100% EtOAc).
1H NMR (500 MHz, acetonitrile-d3): d = 1.21–1.45 (m, 27 H, CH3),
2.16 (s, 4 H, CH2), 3.03 (s, 2 H, CH2), 3.1–3.4 (m, 12 H, CH2).
13C NMR (125 MHz, acetonitrile-d3): d = 20.98, 28.96, 39.43,
43.51, 45.47, 48.65, 49.45, 80.59, 156.35, 166.75.
MS (ESI): m/z (%) = 580 (53, [M + Na]+), 557 (11, [M]+), 458 (100,
[M – Boc + H]+), 401 (41, [M – Boc – (CH3)2C=CH2]+), 357 (16,
[M – 2 Boc]+).
Anal. Calcd for C26H47N5O8: C, 55.99; H, 8.49; N, 12.55. Found: C,
55.71; H, 8.21; N, 12.60.
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682
C. Da Pieve et al.
PAPER
have previously found that it is extremely difficult to
completely remove all the hydrochlorides from
tetraethylenepentamine. Therefore derivatives of
1,4,7,10,13-pentaazacyclohexadecane-14,16-dione are
frequently isolated as their hydrochloride salts and
purified.4b
(10) Arnaud, N.; Picard, C.; Cazaux, L.; Tisnes, P. Tetrahedron
Lett. 1995, 36, 5531.
(11) Bradshaw, J. S.; Krakowiak, K. E.; Izatt, R. M.
J. Heterocycl. Chem. 1989, 26, 1431.
(12) In ref. 6, compound 7 has been described as a solid.
However, following the synthetic pathways in ref. 6, we
Synthesis 2007, No. 5, 679–682 © Thieme Stuttgart · New York