A mild and inexpensive procedure for the synthesis of N, N′ -di-boc-protected guanidines

Article abstract of DOI:10.1055/s-0029-1218365

A novel and efficient synthetic procedure for converting a diverse set of amines to N,N′-di-Boc-protected guanidines is described. The methodology comprises the use of cyanuric chloride (TCT) as activating reagent for di-Boc-thiourea. The employ of inexpensive TCT instead of classical HgCl ₂ eliminates the environmental hazard of heavy-metal waste without appreciable loss of yield or reactivity. This protocol provides an alternative route for the guanylation of amines from those currently employed. Georg Thieme Verlag Stuttgart - New York.

Full text of DOI:10.1055/s-0029-1218365

3368
LETTER
A Mild and Inexpensive Procedure for the Synthesis of N,N¢-Di-Boc-Protected
Guanidines
Procedure for the
S
n
ynthesis of
N
,
N
¢-Di-Boc
r
-protected
G
uanidin
a
es Porcheddu,* Lidia De Luca, Giampaolo Giacomelli
Dipartimento di Chimica, Università degli Studi di Sassari, Via Vienna 2, 07100 Sassari, Italy
Fax +39(079)212069; E-mail: anpo@uniss.it.
Received 7 September 2009
these guanylating reagents have led to the development of
Abstract: A novel and efficient synthetic procedure for converting
a diverse set of amines to N,N¢-di-Boc-protected guanidines is de-
scribed. The methodology comprises the use of cyanuric chloride
(TCT) as activating reagent for di-Boc-thiourea. The employ of in-
milder, efficient, and environmentally benign methodolo-
gies, though not all are devoid of disadvantages. Most
procedures run into difficulties at the purification step,
expensive TCT instead of classical HgCl2 eliminates the environ- usually compounded by the high basicity of guanidines.
mental hazard of heavy-metal waste without appreciable loss of
The employ of more activated carbamate-guanylating re-
yield or reactivity. This protocol provides an alternative route for
agents allows for the direct synthesis of protected
the guanylation of amines from those currently employed.
guanidines introducing several advances13 in the guanyla-
Key words: cyanuric chloride, TCT, guanidine, N,N¢-di-Boc-pro-
tected guanidines, guanylating reagent
tion of amines. Unlike free guanidines, protected
guanidines are less polar and basic, and unreacted amines
can be easily removed by washing with an acidic aqueous
solution.
The guanidine moiety is an important structural motif,
which often occurs in many biologically and pharmaceu-
tically relevant compounds.1 Recent studies have dis-
closed that guanidine-containing molecules exhibit
antiviral, antifungal, and antitumorous activities.2 The
wide range of biological activities found for guanidines
are mainly due both to the high stability of the protonated
guanidine and to the hydrogen-bond mediated interaction
of guanidinium ions.3 In fact, under physiological condi-
tions, the guanidine framework is fully protonated due to
its strongly basic character, a detail that is essential to un-
derstand specific noncovalent ligand–receptor interac-
tions.4 Taking into account the growing importance of
guanidine derivatives in the field of medicinal chemistry,
there is significant interest in new methods to synthesize
guanidines. Typical synthetic routes to guanidines involve
the reaction of a primary or secondary amine with an elec-
trophilic guanylating reagent.5 To date, a variety of such
agents have been documented for this conversion, includ-
ing cianamide,6 pyrazole-1-carboxamidine derivatives,7
O-methylisourea hydrogen sulfate,8 diprotected triflyl-
guanidines,9 and protected thioureas as well as S-methyl-
isothioureas derivatives, which need to be activated by
toxic Hg salts10 or by Mukaiyama’s reagent.11 However,
some shortcomings and limitations12 inherent in using
In continuation of our interest in the preparation of
amidines14 and guanidines,15 we herein report an eco-
friendly, efficient, and inexpensive synthesis of N,N¢-di-
Boc-protected guanidines making use of di-Boc-thiourea
as guanylating agent, and 2,4,6-trichloro-1,3,5-triazine16
(TCT, cyanuric chloride) as initial activating17 reagent
(promoter).18
In order to find the best reaction conditions, we have per-
formed the guanylation reaction using benzylamine as
model target. Preliminary efforts were mainly focused on
the evaluation of the optimum amount of TCT with differ-
ent solvents. In the first instance, the reaction was accom-
plished by treating di-Boc-thiourea 1 with an equimolar
amounts of TCT and three equivalents of N-methylmor-
pholine (NMM) in anhydrous THF (Scheme 1). The acti-
vated thiourea was successively reacted with benzylamine
2a and NMM19 in the presence of a catalytic amount of
4-(N,N-dimethylamino)pyridine20 (DMAP) until comple-
tion. The product 3a was isolated in low yield (28%) after
aqueous workup and an exceptionally tedious column
chromatographic purification.21
After a careful analysis of reaction products, we have dis-
covered the formation of alkyamino-substituted triazines
Cl
Boc
S
N
1. NMM, THF
0 °C, 10 min; reflux, 1 h
Boc
Boc
N
N
Boc
N
N
+
Ph
N
N
2. BnNH2 (2a), DMAP (cat.)
THF, r.t., 6 h
H
H
H
H
Cl
N
Cl
1
3a (28%)
TCT
Scheme 1 Guanylation reaction by using an equimolar amount of TCT promoter
SYNLETT 2009, No. 20, pp 3368–3372
x
x
.x
x
.2
0
0
9
Advanced online publication: 11.11.2009
DOI: 10.1055/s-0029-1218365; Art ID: G29409ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Procedure for the Synthesis of N,N¢-Di-Boc-Protected Guanidines
3369
as major, undesirable side products along with many other teraction of tert-butylamine (Table 1, entry 11) and diiso-
unidentified byproducts.
propylamine (Table 1, entry 12).27 Any attempts to
increase the yield by prolonging reaction time and heating
the reaction mixture proved unsuccessful. The majority of
the guanidines isolated were pure enough for immediate
spectroscopic characterization. Chemical identity was es-
In order to improve the conversion of amine 2a into pro-
tected guanidine 3a, we have successively carried out our
guanylation reaction reducing the amount of TCT by two-
thirds. At one, this ratio di-Boc-thiourea/TCT22 (3:1) has
shown a positive effect on the reactivity, and the desired
product 3a was achieved in 95% yield without any notice-
able unpleasant side products.23 The course of the reaction
(8 h) was conveniently monitored by TLC analysis, and
isolation of the final product involved simple aqueous
workup24 procedure and purification through a short silica
gel chromatography column.
13
tablished by 1H NMR, C NMR, and elemental analysis
in order to ensure that the reaction procedure has been
successfully accomplished.28 Using these optimized con-
ditions, we have also investigated the guanylation of the
amino group of a L-a-amino acid methyl ester (Table 1,
entry 15). The results indicate that the guanylation reac-
tion proceeds cleanly and in good yield providing ready
access to protected L-guanidino methyl ester29 with reten-
tion of stereochemistry.30
Based on literature precedents,31 we suppose that TCT
promotes the removal of H2S from the thiourea 1 giving a
bis-Boc-carbodimide intermediate 4 (nonisolable under
this conditions) as reactive species (Scheme 2). The
amine then adds to the carbodiimide to yield the corre-
sponding guanidine.32
In order to test the influence of the solvent in this protocol,
we have duplicated the target experiment both in DMF
and CH2Cl2. Interestingly, a significant decrease in the
reaction rate and chemical yield was observed carrying
25
out the guanylation reaction in CH2Cl2 but it was also
substantial in DMF.26 Use of 1,2-dimethoxyethane as sol-
vent has shown a decreased conversion of benzylamine 2a
into the corresponding guanidine, adversely affecting the
isolated yield of 3a (51%). The replacement of THF with
different solvents in the second step of guanylation reac-
tion did not allow any improvement. Among the solvents
tested, THF was found the best in terms of chemical yield
and purity.
O
O
Cl
N
N
Me
N
Cl
Me
N
N
N
THF
Cl
Me
Me
Cl
N
Cl
To investigate the scope and limitations of this procedure,
a series of structurally and electronically diverse amines
were subjected to reaction with TCT and carbamate-
protected thiourea 1. The results and some experimental
details are illustrated in Table 1. Several conclusions can
be reached from the analysis of these data. In all cases in-
vestigated, the guanidinylation of unhindered primary al-
iphatic (Table 1, entry 1–3 and 8–10) and cyclic
secondary amines (Table 1, entries 13 and 14) with re-
agent 1 is extremely facile and proceed in high yield
(>90%, based on mass recovery) and purity (>95%, based
on LC-MS and 1H NMR analyses). We also observed that
the method is applicable to several aromatic amines
(Table 1, entry 4–7), and only the electron-poor 4-nitro-
aniline 3f gave somewhat lower yield. Difficulties were
only observed with protected guanidine 3k and 3l, which
were obtained in low yields, presumably due to steric in-
0 °C, 10 min
N
N
N
TCT
O
S
O
Cl
NHBoc
R1
Boc
Boc THF, reflux, 12 h
BocN
N
R2
N
N
H
H
1
3a–o
NBoc
NHBoc
NBoc
NHBoc
NHR1R2 (2a–o)
DMAP (cat.)
8–36 h
Boc
S
N
NMM
N
C
N Boc
NBoc N
S
N
THF, r.t.
4
BocHN
S
Scheme 2 Proposed mechanism for the TCT-promoted guanylation
of di-Boc-protected thiourea 1 with a set of different amines 2a–o
Table 1 Conversion of Several Amines to Di-Boc-protected Guanidines Using Cyanuric Chloride (TCT) as Activating Agent
Cl
Boc
S
N
1. NMM, THF
0 °C, 10 min; reflux, 12 h
R1
Boc
Boc
Boc
N
N
N
N
+
2. R1R2NH (2a–o), DMAP (cat.)
THF, 8–36 h, r.t.
N
R2
N
H
H
H
1
Cl
N
Cl
TCT
3a–o
R1, R2 = H, alkyl, aryl
Entry
1
Amine
Time (h)
8
Producta
Yield (%)b
95
NBoc
NHBoc
NH2
2a
3a
N
H
Synlett 2009, No. 20, 3368–3372 © Thieme Stuttgart · New York

3370
A. Porcheddu et al.
LETTER
Table 1 Conversion of Several Amines to Di-Boc-protected Guanidines Using Cyanuric Chloride (TCT) as Activating Agent (continued)
Cl
Boc
S
N
1. NMM, THF
0 °C, 10 min; reflux, 12 h
R1
Boc
Boc
Boc
N
N
N
N
+
2. R1R2NH (2a–o), DMAP (cat.)
THF, 8–36 h, r.t.
N
R2
N
H
H
H
1
Cl
N
Cl
TCT
3a–o
R1, R2 = H, alkyl, aryl
Entry
2
Amine
Time (h)
8
Producta
Yield (%)b
97
NBoc
NHBoc
NH2
2b
3b
N
H
MeO
MeO
MeO
NBoc
NHBoc
NH2
3
2c
9
3c
94
N
H
MeO
H
N
NH2
NHBoc
4
5
2d
2e
12
12
3d
3e
69
74
NBoc
H
NH2
N
NHBoc
NBoc
MeO
O2N
MeO
O2N
H
NH2
N
NHBoc
NBoc
6
7
2f
24
24
3f
32
47
Me
N
H
N
NHBoc
2g
3g
Me
NBoc
H
N
NHBoc
8
9
2h
2i
8
9
3h
3i
93
94
NH2
NBoc
H
N
NHBoc
NBoc
NH2
NBoc
10
11
2j
12
18
3j
88
27
NH2
N
NHBoc
H
H
N
NH2
NHBoc
NBoc
2k
3k
12
13
14
2l
36
8
3l
21
94
91
N
NHBoc
NH
NBoc
NHBoc
2m
2n
3m
3n
NH
N
NBoc
8
N
NHBoc
NBoc
NH
2o
12
3o
83
15
NHBoc
N NBoc
MeOOC
MeOOC
NH2
H
a All the products were characterized by 1H NMR, 13C NMR, and HRMS or elemental analysis.
b Overall isolated yield.
Synlett 2009, No. 20, 3368–3372 © Thieme Stuttgart · New York

LETTER
Procedure for the Synthesis of N,N¢-Di-Boc-Protected Guanidines
3371
(7) (a) Bernatowicz, M. S.; Wu, Y.; Matsueda, G. R. J. Org.
Chem. 1992, 57, 2497. (b) Drake, B.; Patek, M.; Lebl, M.
Synthesis 1994, 579. (c) Patek, M.; Smrcina, M.; Nakanishi,
E.; Izawa, H. J. Comb. Chem. 2000, 2, 370. (d) Drager, G.;
Solodenko, W.; Messinger, J.; Schön, U.; Kirschning, A.
Tetrahedron Lett. 2002, 43, 1401. (e) Castillo-Melendez,
J. A.; Golding, B. T. Synthesis 2004, 1655. (f) Solodenko,
W.; Bröker, P.; Messinger, J.; Schön, U.; Kirschning, A.
Synthesis 2006, 461.
In conclusion, we have developed a novel and efficient
synthetic procedure for converting a diverse set of amines
into N,N¢-di-Boc-protected guanidines. The reaction con-
sists of an attack of primary and secondary amines on di-
Boc-thiourea activated by a very cheap promoter (TCT).
This protocol provides an alternative route for the guany-
lation of amines from those currently employed. An at-
tractive feature of this methodology is that the reaction
occurs under mild conditions, and without producing any
significant side and/or environmental hazardous products.
Furthermore, at the end of the reaction, a simple aqueous
workup followed by filtration through a short plug of sili-
ca gel affords the desired products pure and in moderate
to high yields.
(8) Palmer, D. C. O-Methylisourea, In Encyclopedia of
Reagents for Organic Synthesis; Paquette, L. A., Ed.; Wiley:
Sussex, 1995, 3525–3526.
(9) Feichtinger, K.; Zapf, C.; Sings, H. L.; Goodman, M. J. Org.
Chem. 1998, 63, 3804.
(10) (a) Levallet, C.; Lerpiniere, J.; Ko, S. Y. Tetrahedron 1997,
53, 5291. (b) Guo, Z. X.; Cammidge, A. N.; Horwell, D. C.
Synth. Commun. 2000, 30, 2933. (c) Gers, T.; Kunce, D.;
Markowski, P.; Izdebski, J. Synthesis 2004, 37.
(11) Yong, Y. F.; Kowalski, J. A.; Lipton, M. A. J. Org. Chem.
1997, 62, 1540.
(12) Many of these reagents present difficulties such as toxicity,
odour, moisture-sensitivity, harsh reaction conditions, and
long reaction time.
(13) The application of reagents containing two urethane-type
protecting groups is beneficial since two electron-
withdrawing groups in positions conjugated with the
reaction center increase the electrophilicity and the
solubility of the guanylating agent.
Acknowledgment
This work was financially supported by the Università degli Studi
di Sassari and MIUR (ROME) within the project PRIN: ‘Structure
and Activity Studies of DNA Quadruplex through the Exploitation
of Synthetic Oligonucleotides and Analogues’.
References and Notes
(1) (a) Guanidines: Historical, Biological, Biochemical and
Clinical Aspects of the Naturally Occurring Guanidino
Compounds; Mori, A.; Cohen, B. D.; Lowenthal, A., Eds.;
Plenum Press: New York, 1985. (b) Guanidines 2: Further
Explorations of the Biological and Clinical Significance of
Guanidino Compounds; Mori, A.; Cohen, B. D.; Koide, H.,
Eds.; Plenum Press: New York, 1987. (c) Feichtier, K.;
Sings, H. L.; Baker, T. J.; Matthews, K.; Goodman, M.
J. Org. Chem. 1998, 63, 8432. (d) Le, V.-D.; Wong, C.-H.
J. Org. Chem. 2000, 65, 2399. (e) McAlpine, I. J.;
Armstrong, R. W. Tetrahedron Lett. 2000, 41, 1849. (f) De
Clercq, E. Nat. Rev. Drug Discovery 2006, 5, 1015.
(2) (a) Petersen, M. J.; Nielsen, C. K.; Arrigoni-Martelli, E.
J. Med. Chem. 1978, 21, 773. (b) Ganelin, C. R. Chronicles
of Drug Discovery, Vol. 1; Bindra, J. S.; Lednicer, D., Eds.;
Wiley: New York, 1982, 1–38. (c) Taniguchi, K.;
Shigenaga, S.; Ogahara, T.; Fujitsu, T.; Matsno, M. Chem.
Pharm. Bull. 1993, 41, 301. (d) Yoshiizumi, K.; Seko, N.;
Nishimura, N.; Ikeda, S.; Yoshino, K.; Kondo, H.;
Tanizawa, K. Bioorg. Med. Chem. Lett. 1998, 8, 3397.
(3) (a) Heys, L.; Moore, C. G.; Murphy, P. J. Chem. Soc. Rev.
2000, 29, 57; and references cited there. (b) Schug, K. A.;
Lindner, W. Chem. Rev. 2005, 105, 67.
(4) (a) Cotton, F. A.; Day, V. W.; Hazen, E. E. Jr.; Larsen, S.
J. Am. Chem. Soc. 1973, 95, 4834. (b) Schneider, S. E.;
Bishop, P. A.; Salazar, M. A.; Bishop, O. A.; Anslyn, E. V.
Tetrahedron 1998, 54, 15063. (c) Linton, B.; Hamilton,
A. D. Tetrahedron 1999, 55, 6027. (d) Linton, B. R.; Carr,
A. J.; Orner, B. P.; Hamilton, A. D. J. Org. Chem. 2000, 65,
1566.
(5) (a) Orner, B. P.; Hamilton, A. D. J. Inclusion Phenom.
Macrocyclic Chem. 2001, 41, 141. (b) Manimala, J. C.;
Anslyn, E. V. Eur. J. Org. Chem. 2002, 3909. (c)Katritzky,
A. R.; Rogovoy, B. V. ARKIVOC 2005, (iv), 49. (d) Ohara,
K.; Vasseur, J.-J.; Smietana, M. Tetrahedron Lett. 2009, 50,
1463.
(14) Porcheddu, A.; Giacomelli, G.; Piredda, I. J. Comb. Chem.
2009, 11, 126.
(15) Porcheddu, A.; Giacomelli, G.; Chighine, A.; Masala, S.
Org. Lett. 2004, 6, 4925.
(16) (a) Falorni, M.; Porcheddu, A.; Taddei, M. Tetrahedron Lett.
1999, 40, 4395. (b) Blotny, G. Tetrahedron 2006, 62, 9507;
and references cited therein. (c) Giacomelli, G.; Porcheddu,
A. 1,3,5-Triazines, In Comprehensive Heterocyclic
Chemistry III, Vol. 9; Katritzky, A. R.; Ramsden, C. A.;
Scriven, E. F. V.; Taylor, R. J. K., Eds.; Elsevier: Oxford,
2008, 197–290.
(17) Generally, the conversion of protected thiourea into a
guanidine requires initial activation.
(18) In particularly, we were interested in replacing the
traditional activating Mukaiyama reagent with the readily
available cyanuric chloride for the guanylation of di-Boc-
protected thiourea.
(19) The use of NMM or Et3N proved to be crucial to reaction
success. Without a base, the reaction did not take place.
(20) Adding a catalytic amount of DMAP we have detected an
increased reaction rate.
(21) The Boc protecting group was then removed by treatment
with 3 M anhyd methanolic HCl, to yield the guanidine as
the HCl salt in 96% isolated yield.
(22) The ability to use only 0.33 equiv of the TCT as guanylating
agent is advantageous since it minimizes reagent
consumption and byproduct generation compared to the
Mukaiyama reagent and S-methylisothioureas derivatives.
Moreover our method does not give off toxic gaseous side
product such as methyl mercaptan that is generated using
N,N¢¢-bis-Boc-S-methylisotiourea as guanylating reagent
(see ref. 10a–c).
(23) Representative Procedure for the Synthesis of N,N¢¢-
Di-Boc-protected Guanidines: N,N¢-Bis(tert-butoxy-
carbonyl)-N¢¢-benzylguanidine (3a)
(6) Schow, S. Cyanamide, In Encyclopedia of Reagents for
Organic Synthesis; Paquette, L. A., Ed.; Wiley: Sussex,
1996, 1408–1410.
To a solution of cyanuric chloride (185 mg, 1.0 mmol) in dry
THF (20 mL), N-methylmorpholine (303 mg, 330 mL, 3.0
mmol) was added at 0 °C under argon and with vigorous
stirring. A white suspension was formed to which a solution
Synlett 2009, No. 20, 3368–3372 © Thieme Stuttgart · New York

3372
A. Porcheddu et al.
LETTER
reagent from the reaction mixture. Reagents for High-
Throughput Solid-Phase Organic Synthesis, In Handbook of
Reagents for Organic Synthesis; Wipf, P., Ed.; Wiley:
Sussex, 1999, 72–74.
of the N,N¢-di-Boc-thiourea 1 (830 mg, 3.0 mmol) and N-
methylmorpholine (606 mg, 660 mL) in anhydrous THF (20
mL) was added, and the stirring was continued at reflux
temperature for 12 h. The slurry was cooled to r.t. and to this
mixture, benzylamine 2a (482 mg, 491 mL, 4.5 mmol) and
DMAP (10 mg) were added, and the stirring was run for
additional 8 h at r.t. The reaction was judged to be complete
by TLC analysis. After completion of the reaction, solid
material was collected by suction, followed by successive
washing with a minimal amount of THF, and the filtrates
were combined and concentrated. The residue was dissolved
in CH2Cl2, and the resulting solution was washed
(25) Performing the model reaction in CH2Cl2 we have recovered
the desired protected guanidine 3a in very low yields (<5%).
(26) TCT react DMF to give an insoluble Vilsmeier–Haack-type
specie, which precipitates: De Luca, L.; Giacomelli, G.;
Porcheddu, A. Org. Lett. 2002, 4, 553; under these
conditions, we observed a partial removal of Boc-protective
group, and the formation of several side products in the
crude reaction mixture..
successively with H2O, HCl (1 N), NaHCO3 (sat. solution),
and then with brine. The organic layer was dried over anhyd
Na2SO4, passed through short a silica gel column (hexane–
EtOAc = 8:2), and the solvent removed under reduced
pressure to give 3a (1.0 g, 95%) pure as an off-white solid;
mp 125–126 °C [lit.:7b mp 126–127 °C]. TLC: Rf = 0.46
(hexane–EtOAc = 8:2). 1H NMR (300 MHz, CDCl3): d =
1.47 (s, 9 H), 1.51 (s, 9 H), 4.59 (d, J = 5.2 Hz, 2 H), 7.20–
7.39 (m, 5 H), 8.43 (br s, 1 H), 11.41 (br s, 1 H). 13C NMR
(75 MHz, CDCl3): d = 27.9, 28.3, 44.9, 80.5, 82.9, 127.8,
128.2, 129.0, 137.2, 152.9, 156.4, 163.4. ESI-HRMS: m/z
[M + H]+ calcd for C18H28N3O4: 350.2080; found: 350.2092.
Anal. Calcd for C18H27N3O4: C, 61,87; H, 7,79; N, 12,03.
Found: C, 61.71; H, 7.95, N, 11.92.
(27) Unchanged reagents were present still after 36 h at r.t.
(28) All the analytical data are consistent with those described in
the literature.
(29) Identified by matching all the analytical data with those
described in literature, see: Balakrishnan, S.; Zhao, C.;
Zondlo, N. J. J. Org. Chem. 2007, 72, 9834.
(30) In the reaction of TCT/di-Boc-thiourea 1 with phenylalanine
methyl esther 2o, we have not observed significant
racemization of the stereogenic center as revealed by the
optical rotation value of the products 3o if compared with
that reported in the literature (ref. 29).
(31) (a) Ho, K.-C.; Sun, C. Bioorg. Med. Chem. Lett. 1999, 9,
1517. (b) See also ref. 5c.
(32) The exact intermediates for this TCT-promoted guanylating
reaction formation are unknown.
(24) The triazine ring is weakly basic therefore a dilute acid wash
is able to remove any byproduct as well as any excess
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