Highly regioselective N-trans symmetrical diprotection of cyclen

Article abstract of DOI:10.1016/j.tetlet.2006.07.135

A high yielding N-trans diprotection procedure for cyclen has been developed by using succinimide carbamate derivates of Boc and Cbz and chloroformate protecting groups.

Full text of DOI:10.1016/j.tetlet.2006.07.135

Tetrahedron Letters 47 (2006) 6937–6940
Highly regioselective N-trans symmetrical diprotection of cyclen
a,
b,
a
*
*
´
´
Luis M. De Leon-Rodrıguez, Zoltan Kovacs, Ana Cristina Esqueda-Oliva
and Alma Delia Miranda-Olveraa
aInstituto de Investigaciones Cientı´ficas, Universidad de Guanajuato, Cerro de la Venada s/n, Guanajuato, Gto, CP 36040, Mexico
bUT Southwestern Medical Center, Advanced Imaging Research Center, 2201 Inwood Road NE 4.2, Dallas, TX 75390-8568, USA
Received 13 June 2006; revised 24 July 2006; accepted 26 July 2006
Abstract—A high yielding N-trans diprotection procedure for cyclen has been developed by using succinimide carbamate derivates
of Boc and Cbz and chloroformate protecting groups.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Polyaza macrocyclic systems are widely studied due to
their metal complexation properties. Particularly, the
N-functionalization of tetraaza macrocyclics such as
1,4,7,10-tetraazacyclododecane (cyclen) has been widely
pursued since the generated derivatives have wide appli-
cation as contrast agents in MRI,1 radiodiagnostic and
radiotherapeutic agents,2 fluorescent sensors3, etc. In
general, N-derivatization has been accomplished by fol-
lowing two approaches: direct derivatization and protec-
tion–derivatization–deprotection. Literature reports on
mono-, bis- and tri-derivatization by either approach
are known. However, bis-derivatization procedures are
scarce and are of particular interest given the possibility
of obtaining either symmetrical or unsymmetrical N-
trans and N-cis polyaza macrocyclic derivatives. Thus,
bis-alkylated symmetrical and asymmetrical N1,N7
(N-trans) cyclen compounds have been prepared via
phosphoryl4 and silicon5 cyclen intermediates. Addition-
ally, N-trans symmetrically alkylated cyclen derivatives
have been reported when reacting cyclen Co or Cr com-
plexes with iodoalkanes.6 N1,N7 symmetrical diprotec-
tion of cyclen has also been reported by following a
close pH control between 2 and 3 during slow addition
of chloroformates.7 The synthesis of N-trans symmetri-
cal and asymmetrical cyclen derivatives has also been re-
ported by reacting alkyl halides with the intermediate
perhydro-3,6,9,12-tetraazacyclopenteno[1,3-f,g]acenaph-
thylene in a suitable solvent.8 On the other hand, N1,N4
(N-cis) cyclen derivatives have been synthesized follow-
ing protection9 and direct derivatization approaches.10
However, the protection–functionalization–deprotec-
tion based methods have two main disadvantages: first,
the low to moderate overall yields (from 48% to 78%)
for the final functionalized cyclen derivative and second,
the harsh conditions required for the removal of some of
the suggested protecting groups. Therefore, direct func-
tionalization methods are often preferred. While these
work reasonably well for mono and trisubstitution,
direct functionalization of cyclen to obtain disubstituted
derivatives usually shows lack of regioselectivity and a
mixture of mono-, di-, tri- and tetra-functionalized
products is obtained, making their purification difficult.
Given the need for an efficient methodology to prepare
di-functionalized cyclen derivatives in this work, we
studied the regioselectivity of several reagents com-
monly used in peptide synthesis to produce readily
cleavable tert-butoxycarbonyl and benzyloxycarbonyl
protected derivatives (Scheme 1). We have found that
R2
R3
R1
R4
O
N
N
N
N
NH NH
NH
+ R
O
X
NH
R1=R2=R3= -H, R4= -CO2R N-mono
R1=R3= -H, R2=R4= -CO2R N1,N7-di
R1= H, R2=R3= R4= -CO2R N-tri
R1=R2=R3= R4= -CO2R
N-tetra
*
Corresponding authors. Tel./fax: +52 4737326252 (L.M.D.L.R.);
tel.: +1 2146452755 (Z.K.); e-mail addresses: lmdeleon@quijote.ugto.
mx; huichoncito@yahoo.com; Zoltan.Kovacs@UTSouthwestern.edu
Scheme 1. Regioselective N-protection of cyclen. Where R = benzyl or
tert-butyl and X = chloride, –OCOBn or oxysuccinimide.
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.07.135

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L. M. De Leo´n-Rodrı´guez et al. / Tetrahedron Letters 47 (2006) 6937–6940
N1,N7 diprotected products can be obtained in nearly
quantitative yields when the corresponding (oxycarbon-
yloxy)succinimide reagents are used.
this idea, we reacted 1 equiv of cyclen with 2 equiv of
benzyl chloroformate (Cbz-Cl) in chloroform. The reac-
tion proceeded quickly with precipitate formation. Thus
after reaction completion, the diCbz-cyclen dihydro-
chloride was obtained nearly quantitatively. This result
confirms that the diprotected cyclen product is acting
as a base in the reaction, followed by its precipitation
due to its low solubility in chloroform which also pre-
vents further reaction. To study the effect of the solvent,
the reaction was carried in acetonitrile yielding 73%
N1,N7-di protected cyclen with the rest being tri- and
tetra-protected products. The decrease in the yield can
be attributed to the slightly higher solubility of the
hydrochloride product in acetonitrile. This observation
provides a very simple procedure for the synthesis
of 1,7-bis(benzyloxycarbonyl)-1,4,7,10-tetraazacyclodo-
To understand the origin of the regioselectivity of these
N protection reactions, it is necessary to know the struc-
ture-reactivity characteristics of this unique macrocyclic
system. However, there are few literature reports on this
aspect although the protonation constants (pKa values)
of the four amino groups are known to be 10.5, 9.5,
1.6 and 0.8, respectively.11 These values give a relative
measure of the basicity of the amino groups. Moreover,
it is important to consider that they indicate consecutive
protonation steps, which in turn causes conformational
changes to keep apart the positively charged amino
groups. The latter has been observed in the X-ray struc-
tures of the free base12 and the tetra-protonated
cyclen.13 Thus, based on the knowledge of pKa values,
it has been shown that cyclen can be N1,N7 diprotected
with benzyloxycarbonyl (Cbz) in aqueous media with good
yields (88%) by keeping the pH between 2 and 3 during
slow addition of chloroformates.7 While this protection
gives reasonable yields, the slow addition of the chloro-
formate reagent is inconvenient, especially when the
protection is carried out on a large-scale basis. In addi-
tion, this procedure is limited to non-acid labile protect-
ing groups as the reaction is run in fairly acid solutions.
In an attempt to develop a more practical method of
cyclen protection, we assumed that the cylic tetramine
adopts a similar structure in solution as the cyclen free
base in the solid state. This has been shown to be a
[3333] quadrangular conformation with C atoms occu-
pying corner positions by X-ray structure determina-
tion. Additionally, the H atoms bound to N1,N7 are
directed inward toward the center of the ring while H
atoms bound to N4,N10 are pointed outward, away
from the center of the ring. Considering this, cyclen
has two amino groups with freely available pair of elec-
trons and therefore the addition of 2 equiv of chlorofor-
mate reagent should give a N1,N7 diprotected product.
Furthermore, if one or both of the non protected amino
groups N4,N10 are basic enough then the diprotected
cyclen hydrochloride should be formed and precipitated
from the reaction mixture in a non-polar solvent. To test
Table 1. Yields for the reaction between cyclen and dicarbonates in
chloroform at room temperature
R
Cyclen: R-X
N-mono
N1,N7-di
N-tri
N-tetra
Cbz
1:2
1:3
1:2
1:3
—
—
—
—
35.4
42.1
—
59.6
44.4
92.4
95.3
5.0
13.5
7.6
Boc
—
4.7
X = –OR.
Figure 1. 1H NMR spectra of N1,N7 diBoc-cyclen.
Table 2. Yields for the reaction between cyclen and succinimide protecting groups at room temperature (unless otherwise indicated) in different
solvents
Solvent
CH3Cl
R Cyclen: R-X
Cbz
N1,N7-di
Boc
N1,N7-di
N-mono
N-tri
N-tetra
N-mono
N-tri
N-tetra
1:2
—
—
—
97.6
94.4
94.8
91.8
93.9
90.9
91.2
95.9
96.7
91.7
2.4
1
—
—
—
—
—
—
—
—
—
—
—
99.5
99.4
100
99.9
98.9
98.9
100
100
99.7
100
0.5
0.6
—
0.1
1.1
1.1
—
—
0.3
—
—
—
—
—
—
—
—
—
—
—
1:2a
1:3
4.6
3.1
4.7
1.3
2.6
1.8
—
2.1
3.5
4.8
6.5
7.0
4.1
3.3
6.7
1:2b
1:2
CH3CN
MeOH
—
—
—
—
—
—
1:2a
1:3
1:2
1:2a
1:3
—
1.6
X = –OSucc.
a Plus 2 equiv of DIPEA.
b Temperature 60 ꢁC.

L. M. De Leo´n-Rodrı´guez et al. / Tetrahedron Letters 47 (2006) 6937–6940
6939
COOtBu
BnOOC
CO2
t
Bu
BnOOC
HOOC
CO2tBu
iii), iv)
92%
N
N
N
N
N
NH
N
N
N
N
N
i)
ii)
NH N
NH
NH
N
quant.
93%
t
BuO2C
t
BuO2C
t
BuOOC
COOBn
COOBn
COOH
1
2
3
Scheme 2. Reagents and conditions: (i) BrCH2COOBn, K2CO3, CH3CN, (ii) CF3COOH, CH2Cl2, (iii) BrCH2COOtBu, K2CO3, CH3CN, (iv) H2,
Pd/C, EtOH.
decane. However, the introduction of other protecting
groups such as Boc might be limited due to the lack of
the proper reagent.
Unfortunately, the (oxycarbonyloxy)succinimide re-
agents did not show any regioselecivity with piperazine
and 1,4,8,11-tetraazacyclotetradecan (cyclam). With
piperazine, the diprotected product was observed in high
proportion and in the case of cyclam, a mixture of com-
pounds was obtained.
Thus continuing with our quest, we tested dibenzyl and
ditert-butyl dicarbonate for the selective protection of
cyclen using chloroform as a solvent and the results
are summarized in Table 1.
The utility of the present methodology was demon-
strated with the synthesis of DOTA-bis(tert-butyl) ester)
3 (Scheme 2), which is a useful intermediate for the syn-
thesis of new cyclen derivatives. To prepare 3, the 1,7-
bis(benzyloxycarbonyl)-cyclen intermediate was alkyl-
ated at the 4 and 10 positions with benzyl bromoacetate,
followed by the removal of Boc groups in acidic media
to yield compound 2. Intermediate 2 is then alkylated
with tert-butyl bromoacetate in the presence of potas-
sium carbonate followed by the removal of the benzyl
groups by catalytic hydrogenation. Thus, 3 was pre-
pared with an overall yield of 86%.
The reaction of cyclen with ditert-butyldicarbonate gives
the N-tri-protected product in high yield as reported
previously.14 On the other hand, reaction with dibenzyl-
dicarbonate is poorly regioselective, yielding a mixture
of N1,N7-di- and N-tri- and N-tetra-protected products.
This interesting difference in the observed regioselectiv-
ity is likely due to the intramolecular steric effect inher-
ent in the protecting group. Thus, it is expected that the
bulkier Boc induces more steric hindrance, which
explains the lower amount of N-tetra protected product
compared to Cbz protection. The basicity of the leaving
group in the protecting reagent may also influence the
regioselectivity. If the basicity of the leaving group is
comparable to or higher than that of the unprotected
amino groups of cyclen, then the equilibrium will
shift to the formation of the conjugate acid of the
leaving group. Hence, the deprotonated product can
lead to the formation of tri- and tetra-protected
derivatives.
In summary, we have discovered a simple and highly
regioselective method for the preparation of the synthet-
ically useful 1,7 bis(tert-butyl)17 and bis(benzyloxycar-
bonyl)18 cyclen derivatives.
Acknowledgements
Supported by grants from CONCYTEG GTO-2002-
C01-6029 and GTO-2003-C02-11517.
Therefore, if the desired product is diBoc-cyclen then the
reagent used must have a weakly basic leaving group.
Oxycarbonyloxysuccinimide reagents were selected due
to the relatively poor basicity of the oxysuccinimide
(OSucc) leaving group (hydroxysuccinimide, pKa =
7.8).15a Indeed, it was observed that both tert-butyl-
and benzyl-(oxycarbonyloxy)succinimide (Boc-OSucc
and Cbz-OSucc, respectively) yielded the N1,N7-di-
protected cyclen product almost quantitatively (see
Fig. 1). Particularly, the tert-butoxycarbonyl diprotection
is unaffected by the addition of extra reagent, the addition
of an organic base, the change of solvent nor the change
in temperature. The results are summarized in Table 2.
References and notes
1. De Leon-Rodriguez, L. M.; Ortiz, A.; Weiner, A. L.;
Zhang, S.; Kovacs, Z.; Kodadek, T.; Sherry, A. D. J. Am.
Chem. Soc. 2002, 124, 3514–3515.
2. Liang, X.; Sadler, P. J. Chem. Soc. Rev. 2004, 33, 246–266.
3. de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.;
Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.; Rice,
T. E. Chem. Rev. (Washington, DC) 1997, 97, 1515–1566.
4. Gardinier, I.; Bernard, H.; Chuburu, F.; Roignant, A.;
Yaouanc, J. J.; Handel, H. Chem. Commun. (Cambridge)
1996, 2157–2158.
N1,N7 diprotection with Cbz-OSucc, however, is
slightly more dependent on the reaction conditions.
The somewhat lower yield for the Cbz protection using
Cbz-OSucc may be attributed to a decrease in steric hin-
drance when compared to the Boc-OSucc reagent.
5. Roignant, A.; Gardinier, I. G.; Bernard, H.; Yaouanc,
J.-J.; Handel, H. J. Chem. Soc., Chem. Commun. 1995,
1233–1234.
6. Patinec, V.; Yaouanc, J. J.; Handel, H.; Clement, J. C.; des
Abbayes, H. Inorg. Chim. Acta 1994, 220, 347–348.
7. Kovacs, Z.; Sherry, A. D. J. Chem. Soc., Chem. Commun.
1995, 185–186.
8. Rohovec, J.; Gyepes, R.; Cisarova, I.; Rudovsky, J.;
Lukes, I. Tetrahedron Lett. 2000, 41, 1249–1253.
9. Bellouard, F.; Chuburu, F.; Kervarec, N.; Toupet, L.;
Triki, S.; Le Mest, Y.; Handel, H. J. Chem. Soc., Perkin
Trans. 1: Org. Bio-Org. Chem. 1999, 3499–3505.
In additional experiments using Cbz-benzotriazoxy and
pentaflurophenoxy16 (benzotriazol and pentafluoro-
phenol pKa = 7.415b and 5.515c) derivatives, the N1,N7
di-protected cyclen derivatives were also obtained
quantitatively.

6940
L. M. De Leo´n-Rodrı´guez et al. / Tetrahedron Letters 47 (2006) 6937–6940
10. Li, C.; Wong, W.-T. J. Org. Chem. 2003, 68, 2956–2959.
11. Izait, R. M.; Pawlak, K.; Bradshaw, J. Chem. Rev. 1991,
91, 1721–2085.
12. Reibenspies, J. H. Acta Crystallogr., Sect. C: Crys. Struct.
Commun. 1992, C48, 1717–1718.
13. Reibenspies, J. H.; Anderson, O. P. Acta Crystallogr.,
Sect. C: Cryst. Struct. Commun. 1990, C46, 163–165.
14. Brandes, S.; Gros, C.; Denat, F.; Pullumbi, P.; Guilard, R.
Bulletin de la Societe Chimique de France 1996, 133, 65–73.
15. (a) Calculated using Advanced Chemistry Development
(ACD/Labs) Software V8.14 for Solaris (1994-2006 ACD/
Labs); (b) Boyle, F. T.; Jones, R. A. Y. J. Chem. Soc.,
Perkin. Trans. 2 1973, 160; (c) Birchall, J. M.; Haszeldine,
R. N. J. Chem. Soc. 1959, 3653.
extracted with chloroform (3 · 30 mL). The extracts were
combined and dried (K2CO3). The solvent was removed
by rotary evaporation and the residue was dried in
vacuum for several hours.
1,7-Bis(tert-butyloxycarbonyl)-tetraazacyclododecane: 1H
NMR (200 MHz, CDCl3/TMS): d 3.35 (8H, br m, –CH2–
N), 2.83 (10H, br m, –CH2–N, –NH–), 1.45 (18H, s,
–CH3). 13C{1H} NMR (200 MHz, CDCl3): d 155 (–CO2–),
76.45 ((CH3)3C–O), 50.9 (–CH2–N), 48.3 (–CH2–N), 28.4
(–C(CH3)3. Anal. Calcd for C18H36N4O4: C, 58.04; H,
9.74; N, 15.04. Found: C, 58.11; H, 9.82; N, 15.12.
1,7-Bis(benzyloxyycarbonyl)-tetraazacyclododecane: 1H
NMR (200 MHz, CDCl3/TMS): d 7.34 (10H, s, arom.
Bn), 5.15 (4H, s, –CH2–, Bn) 3.41 (8H, br m, N–CH2–), 3.04
(2H, s, –NH–), 2.83 (8H, br m, N–CH2–). 13C{1H} NMR
(200 MHz, CDCl3): d 158 (–CO2), 138 (–CH–, arom), 128
(–CH– arom), 127 (–CH–, arom.), 67 (–CH2–O), 52–48
(–CH2–N). Anal. Calcd for C24H32N4O4: C, 65.43; H, 7.32;
N, 12.72. Found: C, 65.32; H, 7.35; N, 12.70.
16. Benzylpentafluorophenylcarbonate: To a solution of pen-
tafluorophenol (130 mg, 0.71 mmol) and diisopropyleth-
ylamine 136.1 lL (0.78 mmol) in dichloromethane (8 mL)
was added benzyl chloroformate (Fluka) (100 mg,
0.8 equiv). The solution was stirred a room temperature
under nitrogen atmosphere and monitored by TLC.
Product was purified by column chromatography (silica
gel, dichloromethane). After solvent removal the product
was obtained quantitatively as a clear oil.
18. Typical procedure for the synthesis of 1,7-Bis(benzyloxy-
carbonyl)-tetraazacyclododecane from benzylchlorofor-
mate: To a solution of tetraazacyclododecane free base
(671 mg, 3.39 mmol) in chloroform (35 mL) in an ice bath
was added benzyl chloroformate (Fluka) (2 equiv) drop-
wise. Reaction was stirred overnight giving abundant solid
formation. Solvent was removed by rotary evaporation
and ether (30 mL) was added. The solid was filtered,
washed with ether and dried yielding 1.735 g (100%) of the
dihydrochloride salt as a white solid. The free base was
obtained by adding NaOH (30 mL, 3 M) to the solid The
aqueous phase was extracted with chloroform (3 · 30 mL).
The extracts were combined and dried (K2CO3). The
solvent was removed by rotary evaporation and the
residue was dried under vacuum for several hours giving
1.449 g (100%) of a transparent oil. The characterization
data corresponded to those indicated in Ref. 17.
1H NMR (200 MHz, CDCl3/TMS): d 7.41 (5H, s arom.),
5.32 (2H, s, –CH2–). 13C{1H} NMR (200 MHz, CDCl3): d
151.4 (–CO2–), 144.1, 140.4, 138.7, 135.3 (C–F), 133.8,
129.2, 128.9, 128.6 (arom.).
17. Typical procedure for the synthesis of N1,N7 diprotected
cyclen derivatives from succinimide reagents: To a solu-
tion of tetraazacyclododecane free base (714 mg,
4.067 mmol) in chloroform (35 mL) was added N-(Benz-
yloxycarbonyloxy)succinimide or N-(tert-Butoxycarbonyl-
oxy)succinimide (Fluka) (2 equiv). The reaction mixture
was stirred at room temperature for 1–2 days. Solvent was
removed by rotary evaporation and 30 mL of NaOH 3 M
was added to the remaining residue. The aqueous phase is