Solid-Phase Assisted N-1 Functionalization of Azamacrocycles

Article abstract of DOI:10.1055/s-2004-815404

A simple solid-phase assisted strategy for the N-1 functionalization of azamacrocycles is described. Compounds such as cyclen, cyclam and piperazine can be selectively modified by temporary attachment to solid-phase resins providing an efficient and clean method to prepare biomedically interesting moieteies.

Full text of DOI:10.1055/s-2004-815404

LETTER
453
Solid-Phase Assisted N-1 Functionalization of Azamacrocycles
Solid-phase
Assisted
N
o
-1 Functiona
r
lization
a
of Azamac
g
r
ocycles Oliver,a Michael R. Jorgensen,*b Andrew D. Miller*a
a
Imperial College Genetic Therapies Centre, Department of Chemistry, Flowers Building, Armstrong Road, Imperial College London,
London, SW7 2AZ, UK
E-mail: a.miller@imperial.ac.uk
b
IC-Vec Ltd., Flowers Building, Armstrong Road, London, SW7 2AZ, UK
Fax +44(20)75945803; E-mail: m.jorgensen@icvec.com
Received 20 November 2003
Abstract: A simple solid-phase assisted strategy for the N-1 func-
tionalization of azamacrocycles is described. Compounds such as
cyclen, cyclam and piperazine can be selectively modified by
temporary attachment to solid-phase resins providing an efficient
and clean method to prepare biomedically interesting moieteies.
NH HN
HN
NH
N
N
NH HN
Key words: solid-phase synthesis, azo-macrocycles, protecting
groups, contrast agents
3
Figure 2 AMD-3100 (3).
Our interest in the synthesis and development of contrast
agents for biomedical imaging has resulted in the develop-
ment of a simple solid phase-based methodology for the
selective functionalization of azacrowns and their poly-
acetate derivatives, such as, cyclen (1,4,7,10-tetra-
typically characterized by: (i) introducing the alkyl group
prior to the cyclization step,7 a tedious method in which
the choice of alkyl group is limited, (ii) use of large ex-
cesses of the expensive polyamines in the presence of
alkylating agents8 or (iii) employing metals to afford tem-
porary protection of the appropriate amine groups.9 These
methods tend to suffer from variable yields, significant
amounts of di-and tri-finctionalized by-products and thus
difficult purification steps. More recently improved
boron10 and phosphorous11 protection approaches have
been developed, for the functionalization of cyclen (1)
and cyclam (1,4,8,11-tetraazacyclotetradecane). Howev-
er, these methods still suffer from production of by-prod-
ucts and are limited to the aforementioned tetraazacrowns
azacyclododecane,
1)
and
DOTA
(1,4,7,10-
tetraazacyclododecane-1,4,7,10-tetraacetic acid, 2; see
Figure 1).
COOH
COOH
HOOC
HOOC
NH HN
NH HN
N
N
N
N
2
1
It is known that a range of functional groups (carboxylic
acids, alcohols, amines) can be immobilized onto 2-chlo-
rotrityl resin (4), affording a temporary protection of that
functional group and thus permitting modification of the
remainder of the molecule.12 The bulky resin provides ex-
cellent protection against highly nucleophilic and basic
conditions. This concept has been used successfully for
the functionalization of linear polyamines where the ter-
minal primary amine functional group is immobilized on
the resin. For instance, in the synthesis of Philanthotoxin-
34313 and DTPA (diethylenetriaminepentaacetic acid) de-
rivatives.14 We have extended this methodology for the
functionalization of cyclic polyamines, immobilizing via
a secondary amine functionalization, to allow production
of two classes of compounds namely selectively protected
azacrowns (see Scheme 1 and Table 1) and their polyace-
tate derivatives (see Scheme 2 and Table 2). Our simple
methodology allows production of these compounds in
good yields (70–98%) and excellent purity (≥96%). These
synthons provide invaluable precursors for a range of
compounds1–5 and could also be also useful in labelling of
proteins15 and peptides16 for imaging purposes in addition
to other uses mentioned above.
Figure 1 Cyclen (1) and DOTA (2).
Azacrowns, and their polyacetic acid derivatives are a
family of unnatural compounds that display excellent
metal chelating properties, especially for heavy metal cat-
ions and lanthanides. This makes this class of compounds
useful in a host of biological and biomedical applications,
as part of MRI contrast agents,1 luminescent probes,2
DNA/RNA cleavers,3 and in radioimmunotherapy
medicines4 for treatment of diseases such as cancer and
even anti-HIV therapies. For instance, AMD-3100 (3),
was recently in a Phase II clinical trials for treatment of
HIV5 and is also being investigated for stem cell immobi-
lization (Figure 2).6
Selective N-functionalization (alkylation) is an important
step in preparing interesting variants of both azacrowns
and their polyacetate derivatives. These approaches are
SYNLETT 2004, No. 3, pp 0453–0456
Advanced online publication: 12.01.2004
DOI: 10.1055/s-2004-815404; Art ID: D28003ST
© Georg Thieme Verlag Stuttgart · New York
1
8.
0
2.
2
0
0
4

454
M. Oliver et al.
LETTER
Table 1 Selective Protection of Cyclic Polyamines and Azacrowns
Cyclic polyamine Protecting Product
Yield
N
H
(purity, %)a
a
group (R)
Cl
Cl
N
HN
H
N
Boc
98 (≥99)
96 (≥98)
96 (≥98)
88 (≥98)
4
HN
HN
NH
Boc
Cbz
HN
HN
N
5
NH HN
Cbz
NH
NH
N
N
b
NH HN
Fmoc
Boc
1
HN
HN
H
Fmoc
Boc
R
N
R
N
c
H
Boc
N
N R
NH
N R
N
N
N
N
N
N
R
7
N
H
N
H
R
6
Boc
Boc
80 (≥97)b
Boc
Boc
N
N
NH HN
NH HN
Scheme 1 a) Cyclic polyamines 1 (10 equiv), i-Pr2Et3N (20 equiv),
CH2Cl2, r.t., 12 h, MeOH (capping reagent), 10 min; b) protecting
group (Boc2O, CbzCl, FmocCl*) (5 equiv per free amine), Et3N (10
equiv), CH2Cl2, r.t., 4 h; c) 1% TFA in CH2Cl2, 3 × 90 s. Pyridine (20
equiv) was used as a base for addition of FmocCl. The scheme shown
depicts functionalization of 1, however 1 can easily be replaced with
8, 9 and 10.
N
HN
Boc
Boc
77 (≥98)b,c
Boc
NH HN
NH HN
N
N
N
HN
Boc
Selectively protected cyclic polyamines were synthesized
as follows (see Scheme 1):
a Purity determined by HPLC and 1H NMR.
First resin [2-chlorotrityl resin (4), 0.8 mmolg–1] was
loaded by treatment with excess of a given cyclic
polyamine in the presence of base. The sterics of the resin
ensured immobilization via one secondary amine func-
tional group only. Excess cyclic polyamine was isolated
by filtration and recycled. Free secondary amine function-
al groups of the resin bound cyclic polyamine were then
functionalized with a range of possible protecting groups
(Boc, Cbz and Fmoc). In each case the chloranil test17 was
used to check for incomplete reaction products. Final
products were cleaved easily from the resin under very
mild acidic conditions, affording selectively protected cy-
clic polyamines in good yields and high purity (>97%), as
determined by HPLC and 1H NMR and 13C NMR.18 This
methodology is compatible with a range of cyclic amines
(see Table 1), including the frequently used cyclen (1) and
1,4,7-triazacylonane (9).
b Yields based on amount of recovered azacrown.
c A small silica gel chromatographic was performed to improve purity.
(9) and cyclam 10 failed to yield sufficiently pure prod-
ucts under a number of conditions. We attribute this to a
combination of poor solubility of starting material and
steric factors. Final products were cleaved from the solid
support using mild acidic conditions [CH2Cl2 (2):tri-
fluoroethanol (1)], rendering the desired products, in good
yields and excellent purity.19
NH
HN
O
O
a
b
Br
N
Cl
O
O
HN
18
17
4
NH HN
Polyacetate derivatives for selective mono-functionaliza-
tion were constructed using a similar solid phase method-
ology by means of a bromoacetate 2-chlorotrityl resin (17;
see Scheme 2). This resin is commercially available,12 but
was easily made by treatment of 2-chlorotrityl resin (0.8
mmolg–1) with excess bromoacetic acid in the presence of
base. Treatment of the solid supported bromide with a giv-
en cyclic polyamine generated monosubstituted cyclic
polyamine (see Scheme 2). Excess unreacted cyclic
polyamine was easily recycled as described previously.
The remaining free secondary amine functional groups of
the resin bound cyclic polyamine were then functional-
c
NH HN
1
ROOC
O
ROOC
HOOC
COOR
COOR
N
N
N
N
N
N
COOR
d
N
O
N
COOR
20
19
Scheme 2 a) Bromoacetic acid (10 equiv), i-Pr2Et3N (20 equiv),
CH2Cl2, r.t. 12 h, MeOH (capping reagent), 10 min; b) azacrown 1 (10
ized with appropriately protected bromoacetate analogues equiv), i-Pr2Et3N (20 equiv), CH2Cl2, 4 h, r.t; c) t-butylbromoacetate/
2-benzylbromoacetate (5 equiv per free amine), Et3N, DMF, 12 h, r.t;
d) CH2Cl2 (2):trifluoroethanol (1), 2 h, r.t. The scheme shown depicts
functionalization of 1, however 1 can easily be replaced with 8.
(t-butylbromoacetate, 2-benzylbromoacetate). Attempts
to obtain polyacetate analogues of 1,4,7-triazacyclonane
Synlett 2004, No. 3, 453–456 © Thieme Stuttgart · New York

LETTER
Solid-Phase Assisted N-1 Functionalization of Azamacrocycles
455
Table 2 Selective Protection of Polyacetate Analogues
Azacrown
R
Product
Yield (purity, %)a
t-Bu
COOt-Bu
82 (≥98)
HN
NH
N
N
HOOC
t-Bu
66 (≥98)
O
O
NH HN
t-BuO
t-BuO
Ot-Bu
N
N
N
N
NH HN
OH
O
O
Bn
69 (≥95)
O
O
NH HN
NH HN
BnO
BnO
OBn
OH
N
N
N
N
O
O
a Purity determined by HPLC and 1H NMR.
(8) Helps, I. M.; Parker, D.; Morphy, J. R.; Chapman, J.
In conclusion, we have demonstrated a novel solid phase
methodology for selective functionalization of cyclic
polyamines, azacrowns and their polyacetate derivatives.
This methodology is widely applicable to a broad range of
cyclic polyamines, yielding the products in excellent yield
and purity in the absence of any purification steps.
Tetrahedron 1989, 45, 219.
(9) (a) Patinec, V.; Yaouvanc, J. J.; Clement, J. C.; Handel, H.
Tetrahedron Lett. 1995, 36, 79. (b) Patinec, V.; Yaouanc, J.
J.; Handel, H.; Clement, J. C.; Abbayes, H. D. Inorg. Chim.
Acta 1994, 347.
(10) Chuburu, F.; Baccon, M. L.; Handel, H. Tetrahedron 2001,
2385.
(11) (a) Guillaume, D.; Marshall, G. R. Synth. Commun. 1998,
28, 2903. (b) Bender, J. A.; Meanwell, N. A.; Wang, T.
Tetrahedron 2002, 3111.
(12) Novabiochem catalogue 2002/3, Merck Biosciences.
(13) Nash, I. A.; Bycroft, B. W.; Chan, W. C. Tetrahedron Lett.
1996, 37, 2625.
(14) Leclercq, F.; Cohen-Ohana, M.; Sbsrbati, A.; Herscovici, J.;
Scherman, D.; Byk, G. Bioconjugate Chem. 2003, 14, 112.
(15) Sabatino, G.; Chinol, M.; Paganelli, G.; Papi, S.; Chelli, M.;
Leone, G.; Papini, A. M.; Luca, A. D.; Ginanneschi, M. J.
Med. Chem. 2003, 46, 3170.
Acknowledgment
We thank IC-Vec Ltd. for funding.
References
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(17) Chloranil test: To 1–5 mg of resin add one drop of
acetaldehyde in DMF followed by one drop of 2% p-
chloranil in DMF. Allow to stand at r.t. for 5 min blue beads
indicate the prescence of secondary amines.
(18) Sample data for production of protected cyclic amines.
Product: Tri-Boc cyclen (15). FT-IR (film): nmax = 3417
(amine), 2960 (alkyl), 1712 (carbonyl), 1681 (amide), 1470
(alkyl) cm–1. 1H NMR (400 MHz, CDCl3): d = 1.45 [C(CH3)3
× 2], 1.46 [C(CH3)3], 3.25–3.34 (4 H, m, CH2 × 2), 3.39–3.46
(8 H, m, CH2 × 4), 3.54–3.59 (4 H, m, CH2 × 2). 13C NMR
(100 MHz, CDCl3): d = 28.1 [C(CH3)3], 28.2 [C(CH3)3 × 2],
46.8 (CH2 × 2), 47.6 (CH2 × 2), 50.7 (CH2 × 2), 52.7 (CH2 ×
2), 82.0 [C(CH3)3], 82.2 [C(CH3)3 × 2], 156.8 (COOt-Bu ×
2), 157.8 (COOt-Bu). MS (ESI+ve): m/z = 473 (M + H).
FAB-MS: m/e calcd for C23H45N4O6 (M + H): 473.3316;
found: 473.3339. HPLC analysis: tR = 24.0 min, column
Vydac C-4 peptide, mobile phases MeCN (0.1% TFA) and
H2O (0.1% TFA), gradient H2O/MeCN, 0–20 min [100/0] to
[0/100], 20–25 min [0/100], 25.1 min [100/0], 40.0 min
[100/0], flow rate 1 mL/min.
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Chem. Soc., Dalton Trans. 1984, 1937.
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M. Oliver et al.
LETTER
2), 53.3 (CH2), 53.4 (CH2), 55.6 (CH2COOt-Bu), 55.9
(19) Sample data for production of selectively protected
polyacetic acid derivatives. Product: Tri-t-Bu-DOTA (21).
FT-IR (film): nmax = 3540 (amine), 2957 (alkyl), 2933(alkyl),
2850 (alkyl), 1744(carbonyl), 1632 (amide), 1454(alkyl)
cm–1. 1H NMR (400 MHz, CDCl3): d = 1.42 [27 H, s,
C(CH3)3 × 3], 2.71–2.82 (4 H, m, CH2 × 2), 2.97–3.12 (8 H,
m, CH2 × 4), 3.26–3.31 (4 H, m, CH2 × 2), 3.37 (2 H, s,
CH2COOt-Bu), 3.36 (2 H, s, br, CH2COOH), 3.70 (4 H, s,
CH2COOt-Bu × 2). 13C NMR (100 MHz, CDCl3): d = 28.0
[C(CH3)3 × 3], 48.3 (CH2 × 2), 50.1 (CH2 × 2), 53.2 (CH2 ×
(CH2COOt-Bu × 2), 56.7 (CH2COOH), 81.6 [C(CH3)3], 81.7
[C(CH3)3 × 2], 167.4 (COOt-Bu), 169.8 (COOt-Bu × 2),
170.5 (COOH). MS (FAB+ve): m/z = 573 (M + H). FAB-
MS: m/e calcd for C28H53N4O8 (M + H): 573.3863; found:
573.3885. HPLC analysis tR = 22.0 min, column Vydac C-4
peptide, mobile phases MeCN (0.1% TFA) and H2O (0.1%
TFA), gradient H2O/MeCN, 0–20 min [100/0] to [0/100],
20–25 min [0/100], 25.1 min [100/0], 40.0 min [100/0], flow
rate 1 mL/min.
Synlett 2004, No. 3, 453–456 © Thieme Stuttgart · New York