An optimised synthesis of 2-[2,3-Bis(tert -butoxycarbonyl)guanidino] ethylamine

Article abstract of DOI:10.1055/s-0031-1290698

This short report describes an improved, reliable, and high-yielding (>90%) synthesis of 2-[2,3-bis(tert-butoxycarbonyl)guanidino]ethylamine. The method is scalable (>5 g), and the product obtained directly from the reaction mixture requires no further purification. In addition, this methodology can be successfully applied to other diamine substrates (1,3-propyl and 1,4-butyl; 70% and 61% yield, respectively).

Full text of DOI:10.1055/s-0031-1290698

LETTER
▌1779
lAettner Optimised Synthesis of 2-[2,3-Bis(tert-butoxycarbonyl)guanidino]ethyl-
amine
Synthesis of Aminoalkylguanidines
Shane M. Hickey, Trent D. Ashton, Simren K. Khosa, Frederick M. Pfeffer*
Centre for Biotechnology, Chemistry and Systems Biology, School of Life and Environmental Sciences, Deakin University, Waurn Ponds,
Victoria, 3216, Australia
Fax +61(3)52271040; E-mail: fred.pfeffer@deakin.edu.au
Received: 13.04.2012; Accepted after revision: 14.05.2012
number of ‘in house’ synthetic methods have become
Abstract: This short report describes an improved, reliable, and
available – each with limitations.
high-yielding (>90%) synthesis of 2-[2,3-bis(tert-butoxycarbon-
yl)guanidino]ethylamine. The method is scalable (>5 g), and the
product obtained directly from the reaction mixture requires no fur-
ther purification. In addition, this methodology can be successfully
applied to other diamine substrates (1,3-propyl and 1,4-butyl; 70%
and 61% yield, respectively).
A guanylating agent is required to synthesise aminoethyl-
guanidine 1 and typically, one of the three reagents shown
in Figure 2 is used to perform this task: N,N′-bis(tert-
butoxycarbonyl)thiourea (8),8–10 N,N′-bis(tert-butoxycar-
bonyl)-N′′-trifluoromethanesulfonylguanidine
(9),11,12
Key words: guanidine, synthesis, optimisation, cyclisation, pro-
tecting groups, NMR spectroscopy
and N,N′-bis(tert-butoxycarbonyl)-S-methylisothiourea
(10).13
The synthesis of thiourea 8 requires low temperatures (0
°C) and the use of NaH,8 which can become troublesome
A number of research groups have used 2-[2,3-bis(tert-
butoxycarbonyl)guanidino]ethylamine (1, Figure 1) as a
simple way to install a guanidine moiety with an ethyl
spacer. Indeed, aminoethylguanidine 1 has been incorpo-
rated in compounds that block tumour growth (2 and 7),1,2
engage in peptide recognition (3 and 4),3,4 act as antibac-
terial agents (5),5 and treat hypertension (6).6
9
on a larger scale. Additional reagents such as HgCl2 and
N-iodosuccinimide (NIS)10 are often required to activate
agent 8.
O
S
O
S
N
CF3
SMe
BocHN
NBoc
BocHN
NHBoc
BocHN
NHBoc
Despite the commercial availability of aminoethylguani-
8
10
9
dine 1, its cost prohibits routine use.7 As a consequence a
Figure 2 Guanylating agents
O
H
O
H
O
O
NHBoc
NBoc
N
O
N
N
H
O
S
MeO
N
NH
O
HN
MeO
NH
NBoc
O
O
O
HN
O
O
NHBoc
2
3
O
O
HO
PhO2SHN
N
O
H
H
H
H
N
N
NBoc
NHBoc
N
NBoc
HO
NBoc
NHBoc
H2N
N
N
H
H
NO2
NHBoc
O
7
1
4
O
H
O
N
NBoc
NHBoc
O
N
H
O
H
NBoc
NHBoc
N
12
MeO
N
H
O
NH
H
MeO
NBoc
NHBoc
N
6
5
Figure 1 Examples where aminoethylguanidine 1 has been used
SYNLETT 2012, 23, 1779–1782
Advanced online publication: 04.07.2012
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0031-1290698; Art ID: ST-2012-D0321-L
© Georg Thieme Verlag Stuttgart · New York

1780
S. M. Hickey et al.
LETTER
Although guanidine 9 has been shown to give excellent Synthesis of the required guanylating agent N,N′-bis(tert-
yields in guanidinylation reactions, it is expensive to butoxycarbonyl)-S-methylisothiourea (10) was carried
purchase14 or can be synthesised using a somewhat te- out as shown in Scheme 2.17
dious procedure [again, this approach requires low tem-
peratures (–78 °C) and multiple purification steps].11,12
These factors make this agent less attractive if a straight-
forward procedure is desired.
Boc2O, NaHCO3,
CH2Cl2/H2O,
21 °C, 16 h
MeI, MeOH,
65 °C, 1.5 h
S
SMe
NH2
SMe
I
H2N
NH2
H2N
74%
BocHN
NBoc
99%
As such, isothiourea 10 appears a cheaper and simpler al-
ternative to the other guanylating agents (8 and 9).
Scheme 1 depicts an approach that has been shown to be
a robust method for the synthesis of isothiourea 10 and is
amenable to gram-scale synthesis.13 In addition, the meth-
od is performed at ambient temperature; an advantage for
larger-scale preparations.
14
15
10
Scheme 2 Synthesis of N,N′-bis(tert-butoxycarbonyl)-S-methyliso-
thiourea (10)17
With guanylating agent 10 in hand, the procedure devel-
oped by Carmignani et al. for the synthesis of target 1 was
tested.6 In brief, a THF solution of guanylating agent 10
was added dropwise over 90 minutes to a solution of 2.5
equivalents of EDA in THF at 50 °C (Table 1, entry 1)
then heating was continued for four hours. In the 1H NMR
spectrum of the crude product obtained from this reaction
a singlet at δ = 3.62 ppm was observed. This singlet was
assigned to cyclic guanidine 12 which is formed from in-
tramolecular reaction of 1 as shown in Scheme 1. Forma-
tion of this side product has been described by
Castagnolo13 and in this study its identity was confirmed
Even with an efficient guanylating agent other complica-
tions exist in the current methods for the synthesis of the
target 1. For example, current methods require the slow
addition of isothiourea 10 to 1,2-ethylenediamine (EDA)
in dilute conditions (presumably to prevent the formation
of a doubly guanidinylated product). Such a slow addition
rate inevitably increases reaction time. It is also common
to perform this reaction at elevated temperatures.4–6 How-
ever, it has been shown by Castagnolo et al. that an eleva-
tion in reaction temperature facilitates the intramolecular
cyclisation of aminoethylguanidine 1 to form 12 (Scheme
1).13 As a result a purification step (chromatography) is re-
quired.15 Alternatively, crude samples of 1 have been used
directly when the next step is high yielding and the prod-
uct is easily isolated from a complex reaction mixture.5
1
after subsequent comparison with published H NMR
spectroscopic data.18
As aminoethylguanidine 1 has been shown to cyclise (to
12) at elevated temperature,13 the reaction was repeated at
21 °C using CH2Cl2 as the solvent (Table 1, entry 2) such
that the reaction mix could be easily concentrated without
heating. Gratifyingly, the reaction proceeded smoothly,
and after aqueous workup the product contained only a
trace of the cyclised product (as evidenced in the 1H NMR
spectra), decreasing from 17% to 3% (Table 1, entries 1
and 2).
NBoc
NH
n = 1
THF, 71 °C
HN
12
H
N
NBoc
NHBoc
H2N
n
The reaction was repeated on a larger scale (Table 1, entry
3), and an increase in yield was noted (87%). In an attempt
to achieve faster reaction times the effect of concentration
was investigated (increased from 0.02 M to 0.07 M, Table
1, entries 3–5) and in less than six hours a 74% yield of
target 1 was realised (with only a trace of 12 detected by
1H NMR spectroscopy; Table 1, entry 5).
NBoc
NH
1 n = 1
11 n = 2
n = 2
HN
Et3N, CH2Cl2,
21 °C
13
Scheme 1 Reported cyclisation of aminoalkylguanidine substrates13
Using these more concentrated conditions the reaction
was again scaled up (Table 1, entry 6), however, a new
side product was noted which was subsequently isolated
using column chromatography and identified as tert-butyl
N-(2-aminoethyl)carbamate (BocEDA, 16).19 Although
the formation of this side product was minimal (<10% by
1H NMR spectroscopy), purification would be required to
remove BocEDA as it has a free amine that would com-
pete with aminoethylguanidine 1 in ensuing reactions.
Suspecting that the long reaction time was responsible for
the additional side product and given that an excess of
EDA (2.5 equiv) was present the reaction was repeated
with the guanylating agent added in a single portion (Ta-
ble 1, entry 7). Careful monitoring of the reaction mix by
TLC indicated that the reaction was complete in only 3.5
An orthogonal protecting-group strategy (such as that
used by Wakimoto in the synthesis of hordatine and
aperidine16) would avoid intramolecular cyclisation and
ultimately afford highly pure samples of aminoethylgua-
nidine 1, however, the multistep nature of such an ap-
proach is not ideal.
In order to pursue analogues of antibacterial agent 5,5 an
optimised method for the synthesis of aminoethylguani-
dine 1 was sought. Herein, a practical, reliable, scalable,
and operationally simple synthesis to access gram-scale
quantities of 1 using cheap and readily accessible starting
materials is described.
Synlett 2012, 23, 1779–1782
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of Aminoalkylguanidines
1781
Table 1 Optimisation of the Synthesis of Aminoethylguanidine 1
NH2
H2N
(2.5 equiv)
NBoc
NH
SMe
NBoc
H
N
NBoc
NHBoc
NHBoc
H2N
H2N
HN
BocHN
10
1
12
16
Entry
Scale (g)
Conc. (M)
Temp (°C)
Addition time (min) Reaction time
Ratio (1/12/16)b
Yield (%)d
1a
2
0.2
0.2
0.6
0.6
0.6
1.8
1.8
1.8
1.8
5.8
0.02
0.02
0.02
0.03
0.07
0.07
0.14
0.27
0.67
0.27
50
21
21
21
21
21
21
21
21
21
90
90
90
90
60
90
–
4 h
24 h
24 h
23 h
83:17:0
97:3:0
95:5:0
97:3:0
99:1:0
86:5:9
98:2:0
98:2:0
92:3:5c
98:2:0
69
64
87
85
74
78
76
82
55
90
3
4
5
5.5 h
20 h
3.5 h
6
7
8
–
80 min
50 min
70 min
9
–
10
–
a This reaction was performed in THF. All other reactions were performed in CH2Cl2.
b Ratio determined from integration of the NMR spectra (see Supporting Information for example).
c An additional unidentified side product was formed.
d Isolated yield after aqueous workup (see Supporting Information for full experimental).
hours and upon workup no trace of BocEDA could be de- sired aminopropylguanidine 11 was a colourless oil. The
tected in the product. Indeed, formation of both side prod- observed morphology of the products was in contrast to
1
ucts 12 and 16 could be minimised (<2% by H NMR that previously reported by Nnanabu and Burgess who
spectroscopy) if the more concentrated reaction was used claimed that the white solid formed by recrystallisation of
with the guanylating agent 10 added in one portion (Table the crude compound was the desired compound.21 Our
1
1, entry 7).
findings (including the comparison of H NMR spectra)
conclude that the compound reported by Nnanabu and
Burgess to be 11 was indeed the cyclised side product
13.21
It was subsequently confirmed through reactions of vari-
ous durations and concentration (Table 1, entries 7–9) that
BocEDA only formed when longer reaction times and/or
a concentration greater than 0.27 M was used.
H2N
NH2
n
The optimised reaction conditions were shown to be
amendable for multigram-scale reactions, which was
highlighted by a yield of 90% on a 5.8 gram scale (Table
1, entry 10).20 In addition, no evidence of a diguanidinyl-
ated product was observed throughout this study, even at
higher reagent concentrations. It is also worth noting that
we found it necessary to store this compound at less than
–20 °C as slow cyclisation was observed even at 4 °C.
(2.5 equiv)
CH2Cl2, 21 °C
SMe
NBoc
NBoc
N NHBoc
BocHN
H2N
n
H
10
11 n = 1 (70%)
17 n = 2 (61%)
Scheme 3 Synthesis of analogues 11 and 17
The synthesis of 2-[2,3-bis(tert-butoxycarbonyl)guanidi-
no]butylamine (17), a precursor to the neurotransmitter
agmatine,22 was also achieved in good yield (61%) and
high purity (>90%) using this methodology.
To investigate the scope of the methodology the synthesis
of two related aminoalkylguanidines was pursued
(Scheme 3). Castagnolo et al.13 previously reported that
the propyl analogue 2-[2,3-bis(tert-butoxycarbonyl)gua-
nidino]propylamine (11) would immediately cyclise to
form the six-membered ring (13, Scheme 1). However, by
employing the methodology developed in this study –
which does not require heating – only a minimal amount
of cyclised product was observed (<5% by 1H NMR spec-
troscopy). Using column chromatography the cyclised
side product was isolated as a white solid, whilst the de-
A cheap, scalable (90% yield on >5 g scale), and opera-
tionally simple synthesis of 2-[2,3-bis(tert-butoxycarbon-
yl)guanidino]ethylamine (1) has been developed.
Compared to existing methods, reductions in reaction du-
ration, solvent volume, and purification steps have been
achieved. The product is of a high purity (>98%) follow-
ing a simple extractive workup. The scope of the method-
© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 1779–1782

1782
S. M. Hickey et al.
LETTER
(9) Hernandez, M. I. R.; Royo, F. R.; Meana, J.; Callado, L.
WO 2009080818, 2009.
(10) Ohara, K.; Vasseur, J.-J.; Smietana, M. Tetrahedron Lett.
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(11) Feichtinger, K.; Sings, H. L.; Baker, T. J.; Matthews, K.;
Goodman, M. J. Org. Chem. 1998, 63, 8432.
(12) Feichtinger, K.; Zapf, C.; Sings, H. L.; Goodman, M.
ChemInform 1998, 29, 3804.
(13) Castagnolo, D.; Raffi, F.; Giorgi, G.; Botta, M. Eur. J. Org.
Chem. 2009, 334.
(14) Sigma-Aldrich online catalogue; Product code: 15033;
US$209.69/g.
(15) Lim, H. A.; Joy, J.; Hill, J.; San Brian Chia, C. Eur. J. Med.
Chem. 2011, 3130.
ology was successfully expanded to the synthesis of
related aminoalkylguanidines 11 and 17. Given the conve-
nience of the compound 1 as a means to install a guanidine
group with an ethyl spacer and the current cost to purchase
this reagent, the method described herein should be of
practical use to researchers in the fields of organic and
medicinal chemistry.
Acknowledgment
The authors would like to thank Deakin University and in particular
the Strategic Research Centre for Biotechnology, Chemistry and
Systems Biology for their financial support and a scholarship for
S.M.H.
(16) Wakimoto, T.; Nitta, M.; Kasahara, K.; Chiba, T.; Yiping,
Y.; Tsuji, K.; Kan, T.; Nukaya, H.; Ishiguro, M.; Koike, M.;
Yokoo, Y.; Suwa, Y. Bioorg. Med. Chem. Lett. 2009, 19,
5905.
(17) Radau, G.; Schermuly, S.; Fritsche, A. Arch. Pharm. 2003,
336, 300.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnoIufproig
m
iotSrat
ungIifoop
r
t
(18) Yuan, J. H.; Yang, X. X.; Lin, H.; Wang, D. X. Chin. Chem.
Lett. 2011, 22, 1399.
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References and Notes
(1) Novoa, A.; Pellegrini-Moïse, N.; Bechet, D.; Barberi-
Heyob, M.; Chapleur, Y. Bioorg. Med. Chem. 2010, 18,
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(2) Nicolaou, K. C.; Trujillo, J. I.; Jandeleit, B.; Chibale, K.;
Rosenfeld, M.; Diefenbach, B.; Cheresh, D. A.; Goodman,
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2950.
(7) Sigma-Aldrich online catalogue; Product code: 84190;
US$338.81/g.
(20) 2-[2,3-Bis(tert-butoxycarbonyl)guanidino]ethylamine (1)
Boc-protected methylisothiourea 10 (5.81 g, 20.0 mmol) in
CH2Cl2 (30 mL) was added in one portion to a stirred
solution of 1,2-ethylenediamine (3.3 mL, 50.0 mmol) in
CH2Cl2 (44 mL). The reaction was allowed to stir at 21 °C
for 70 min. The reaction mixture was washed with H2O (3 ×
25 mL), brine (30 mL), then dried (MgSO4) and filtered.
Solvent was removed in vacuo at ambient temperature to
afford 1 (5.37 g, 90%) as a white powder; mp 96.2–100.1 °C.
1H NMR (270 MHz, CDCl3): δ = 1.50 (9 H, br s, t-Bu), 1.51
(9 H, br s, t-Bu), 2.90 (2 H, t, J = 6.2 Hz, CH2), 3.49 (2 H,
app q, Japp = 5.5 Hz, CH2), 8.67 (1 H, br s, NH), 11.51 (1 H,
br s, NH). 13C NMR (67.5 MHz, CDCl3): δ = 28.2, 28.4,
41.1, 43.5, 79.4, 83.2, 153.3, 156.5, 163.7. ESI-HRMS: m/z
calcd for C13H26N4O4 [M + H]+: 303.2027; found: 303.2032.
(21) Nnanabu, E.; Burgess, K. Org. Lett. 2006, 8, 1259.
(22) Li, G.; Regunathan, S.; Barrow, C.; Eshraghi, J.; Cooper, R.;
Reis, D. Science 1994, 263, 966.
(8) Expósito, A.; Fernández-Suárez, M.; Iglesias, T.; Muñoz, L.;
Riguera, R. J. Org. Chem. 2001, 66, 4206.
Synlett 2012, 23, 1779–1782
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