Facile N-1 protection of cyclam, cyclen and 1,4,7-triazacyclononane

Article abstract of DOI:10.1016/S0040-4039(03)00338-1

An exceptionally high yielding method for the tri-protection of cyclam and cyclen using ethyl trifluoroacetate is described. The selective reaction also applies to the di-protection of 1,4,7-triazacyclononane. The application of this method in the synthesis of AMD3100, a clinical candidate for stem cell mobilization is presented.

Full text of DOI:10.1016/S0040-4039(03)00338-1

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 2481–2483
Facile N-1 protection of cyclam, cyclen and
1,4,7-triazacyclononane
Wen Yang,* Christen M. Giandomenico, Michael Sartori and Dennis A. Moore
AnorMED, Inc. 200 20353 64th Ave, Langley, British Columbia, Canada V2Y 1N5
Received 16 December 2002; revised 3 February 2003; accepted 3 February 2003
Abstract—An exceptionally high yielding method for the tri-protection of cyclam and cyclen using ethyl trifluoroacetate is
described. The selective reaction also applies to the di-protection of 1,4,7-triazacyclononane. The application of this method in the
synthesis of AMD3100, a clinical candidate for stem cell mobilization is presented. © 2003 Elsevier Science Ltd. All rights
reserved.
The advent of the Richman–Atkins procedure,1 and
later improvements,2 for the synthesis of polyazamacro-
cycles have given ready access to a wide variety of
potentially useful compounds. Among the most known
ones are cyclam, cyclen and 1,4,7-triazacyclonane
(TACN). These macrocycles and especially their N-1
protected (N being the total identical ring amino func-
tion) derivatives have found use as starting materials in
the synthesis of transition metal and lanthanide-specific
chelating ligands,3,4 MRI contrast agents,5 radio-diag-
nostic and therapeutic agents.6,7 However, selective pro-
tection of the otherwise undifferentiated amine moieties
of a polyazamacrocycle can be difficult at best. As part
of our effort in our drug development program involv-
ing cyclam derivatives, we have found ethyl trifluoroac-
etate (EtOTFA) to be an ideal reagent for the selective
tri-protection of 1,4,8,11-tetraazacyclo-tetradecane
(cyclam), as well as 1,4,7,10-tetraazacyclo-dodecane
(cyclen), and similar di-protection of 1,4,7-triazacy-
clononane (TACN) under non-stoichiometrical con-
trolled conditions.
Another strategy adopts the temporary blocking of
three of the four amino groups with certain transition
metal-tricarbonyl,13 boron14 and phosphorus15 contain-
ing moieties, leaving the free amine residue open for
further manipulation. Each of these procedures has
drawbacks, ranging from the use of air sensitive materi-
als to the use of undesirable solvents.
A third approach involves differentiation prior to the
macrocyclic ring-forming step.16,17a
During the course of our drug development program
for AMD 3100 (Fig. 1), a candidate for stem cell
mobilization,18 a scalable manufacturing process for
tri-N-protected cyclam is needed. Earlier efforts mim-
icked the original medicinal chemistry route using tri-
tosyl cyclam as the intermediate.12 On kilogram scale,
as in the research laboratory, this tri-tosyl intermediate
was obtained with a relatively low yield (<40%) and
modest purity (90–95% by HPLC) after repeat recrys-
tallization. AMD 3100 was produced in less than 10%
yield from the starting cyclam (followed by N-alkyla-
tion with a,a%-dibromo-p-xylene and subsequent
Currently,8,9 there are several general methods practiced
to obtain differentiated, or selectively protected, cyclam
derivatives. The first involves the direct reaction of a
chosen protecting agent with the macrocycle, which is
typically in significant excess, under optimized condi-
tions including stoichiometry,10 pH,11 temperature and
concentration. A compromised yield and intense purifi-
cation procedure are usual outcome of this strategy
owing to the inevitable formation of so-called under-
and over-protected side-products.12
* Corresponding author. Tel.: 01-604-530-1057; fax: 01-604-530-0976;
e-mail: wyang@anormed.com
Figure 1. Structure of AMD 3100.
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00338-1

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W. Yang et al. / Tetrahedron Letters 44 (2003) 2481–2483
conditions23 (Scheme 1). The addition of triethylamine,
as previously used by other reports, may scavenge
trifluoroacetic acid, present in EtOTFA due to hydroly-
sis by adventitious water, which would otherwise proto-
nate the amino groups in the mixture. The
enhancement of reaction rate by using a combination of
MeOH and triethylamine is most likely resulted from
polarity and base catalytic effects. Use of EtOH as
solvent produced similar results, which ruled out possi-
ble base-catalyzed transesterification prior to acylation.
removal of tosyl groups using HBr/HOAc). The long
processing/reaction time, associated with the high price
of cyclam, made the overall cost of final drug substance
prohibitively high.
Ethyl trifluoroacetate has been frequently used as a
mild acetylating agent for amine protection.19,20 More
recently, Prasad et al,17b further illustrated reaction
examples displaying selectivity with EtOTFA among
primary and secondary amines and between certain
identical secondary amino groups. Interestingly, the
same authors also showed a tri-protection of a linear
tetraamine molecule using 3 equiv. of EtOTFA during
their approach to AMD 310017a,21 (previously JM
3100). In addition, they reported two examples where
mono-protection was achieved with good yield between
two identical secondary amino groups in the same
molecule. For example, when one equivalent of piper-
azine was treated with one equivalent of EtOTFA in
THF at room temperature, the mono:di ratio in the
product mixture was reported as close to 6:1. However,
these selectivity observed in linear polyamines and
small ring multiple amino containing compounds
diminished once excess EtOTFA was used. And gener-
ally, when equal or more equivalents EtOTFA (versus
free amino units present) are used, a global protection
occurs.
Adopting the new method, we have achieved the syn-
thesis of AMD 3100 with an overall yield of >72%.
Labor cost and time are both saved greatly by means of
easy protection/deprotection sequence. This resulted in
a much-reduced cost of the final drug substance. The
use of excess low cost EtOTFA offers a better kinetic
control of the reaction (accelerated rate) without posing
a problem during isolation, as the excess reagent is
easily distilled off. Tri-TFA cyclam has a much higher
Rf value (versus various di-TFA cyclam by-products)
on column chromatography. This offers an easy isola-
tion by passing through a silica gel plug. On scale, the
reaction mixture is carried on to the next step without
purification. No effort was spent to recrystallize the
white foam like Tri-TFA cyclam. Subsequent alkylation
went uneventful in acetonitrile with K2CO3 as the base.
Finally, straightforward mild deprotection (MeOH/aq.
NaOH, rt) of the amides afforded AMD3100 free base
in several hours.
While investigating alternative cyclam protection
schemes, we observed an unprecedented reaction pat-
tern of cyclam, a so-called polyamine macrocycle, with
EtOTFA. Herein, we wish to briefly disclose our
findings.22
Similarly, we found this protection chemistry to be
effective for the selective protection of both cyclen and
1,4,7-triazacyclononane (Scheme 2). In both cases, over
90% isolation was achieved for tri-TFA cyclen and
di-TFA 1,4,7-triazacyclononane, respectively. These
intermediates may find applications in the preparation
of currently marketed MRI agents.
As expected, when 1 equiv. of cyclam was let to react
with 1 equiv. of EtOTFA (CHCl3, room temperature,
15 h), LC/MS analysis of the reaction mixture revealed
the presence of 20% mono-, 30% mixed di- and 2%
tri-protected cyclam, in addition to a small amount of
unreacted cyclam itself. We were a bit surprised but not
really concerned by the absence of any detectable tetra-
protected cyclam. However, a separate reaction using
four equivalents of EtOTFA per equivalent cyclam
(CH2Cl2/MeOH, at 45°C, 15 h) produced a mixture of
1% mono-, 60% di- and 32% tri-substituted cyclam
derivatives and no detectable tetra-TFA cyclam. This
stands in sharp contrast to reactions between cyclam
and other common protecting agents, such as tosyl
chloride, where there is a great tendency for the forma-
tion of tetra-substituted cyclam.
Further experiments demonstrated that only
a
neglectable amount of tetra-derivative was formed even
when a large excess (i.e. up to using EtOTFA as the
reaction solvent) of EtOTFA was used. We also found
that the rate of the reaction was accelerated when
MeOH was adopted as the sole solvent. In the absence
of any additives, the reaction afforded 90–95% (HPLC)
tri-TFA cyclam within 4 h using 10 equiv. of EtOTFA.
The addition of triethylamine was found to allow the
reaction to approach closer to completion, giving >99%
tri-TFA cyclam under otherwise same reaction
Scheme 1. Improved synthesis of AMD 3100.

W. Yang et al. / Tetrahedron Letters 44 (2003) 2481–2483
11. Kovacs, Z.; Sherry, A. D. Synthesis 1997, 759.
2483
12. Bridger, G. J.; Skerlj, R. T.; Padmanabhan, S.; Martel-
lucci, S. A.; Henson, G. W.; Abrams, M. J.; Joao, H. C.;
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J. Med. Chem. 1996, 39, 109.
13. Patinec, V.; Yaouanc, J. J.; Clement, J. C.; Handel, H.;
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14. Bernard, H.; Yaouanc, J. J.; Clement, J. C.; des Abbayes,
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16. Helps, I. M.; Jankowski, K. J.; Nicholson, P. E.; Parker,
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17. (a) Xu, D.; Mattner, P. G.; Prasad, K.; Repic, O.;
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Scheme 2. Preparation of tri-TFA cyclen and di-TFA TACN.
18. Presentation abstracts are viewable at http://
In summary, we have discovered
a simple, but
www.anormed.com.
extremely effective and economical method for the
preparation of tri-protected cyclam/cyclen and di-pro-
tected TACN. The efficiency is exemplified in the
improved synthesis of AMD 3100, a stem cell mobiliz-
ing agent currently in clinical trial.
19. Greene, T. W.; Wuts, P. G. M. Protective Groups in
Organic Synthesis; John-Wiley and Sons: New York,
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20. Aviron-Violet, P.; Gervlas, C. US Patent No. 4,943,679,
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23. Procedure for the preparation of Tri-TFA cyclam: ethyl
trifluoroacetate (18.0 mL, 150.3 mmol) was added drop-
wise to a mixture of cyclam (7.53 g, 37.58 mmol) and
Et3N (5.20 mL, 37.58 mmol) in methanol (30 mL) at
room temperature. The addition continued over a period
of 5 min. The homogeneous reaction mixture was cooled
with an ice-water bath to control the mild exotherm.
Stirring was continued under N2 for 5 h. Volatiles were
removed in vacuo. The residue was passed through a
small silica gel plug (25 g), eluted with 100% EtOAc. The
eluent was concentrated to give the product as a white
foam (17.05 g, 92.5%). 1H NMR (300 MHz, CDCl3): l
3.85–3.25 (m, 12H), 2.90–2.80 (m, 2H), 2.74–2.50 (m,
2H), 2.30–1.90 (m, 2H), 1.85–1.63 (m, 2H), 1.25–0.60 (m,
1H). 13C NMR (75.5 MHz, CDCl3): l 158.7–157.3 (m,
CꢀO, existence of conformers and long range CꢁF cou-
pling), 122.1 (q, CF3, JCꢁFꢀ264 Hz, further split due to
conformers), 51.2–46.2 (m, CH2-N), 29.4–27.8 (m, CH2).
19F NMR (282.4 MHz, CDCl3, DMFd-7, 120°C): 7.50 (s),
6.88 (s), 6.60 (s). Anal. calcd for C16H21F9N4O3: C, 39.35;
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13C NMR is complicated by CꢁF coupling (including
long range) and the existence of conformers at room
temperature.
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