Solid-phase synthesis of N-aryl-N′-carboalkoxy guanidines by the mitsunobu reaction of Fmoc-guanidines

Article abstract of DOI:10.1081/SCC-200026206

A new method for the solid-phase synthesis of N-aryl-N′-carboalkoxy guanidines is described. Aromatic amines were reacted with Fmoc-isothiocyanate to provide Fmoc-thioureas, which were coupled with Rink amide resin to provide the corresponding resin-bound Fmoc-guanidines. Subsequent Mitsunobu alkylation with a variety of alcohols delivered N-aryl-N′ carboalkoxy guanidines in good to high purity after resin cleavage.

Full text of DOI:10.1081/SCC-200026206

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Solid‐Phase Synthesis of
N‐Aryl‐N′‐Carboalkoxy
Guanidines by the Mitsunobu
Reaction of Fmoc‐Guanidines
Dale E. Robinson a , Punit P. Seth a & Elizabeth A.
Jefferson a
a Ibis Therapeutics, A Division of Isis
Pharmaceuticals, Inc. , 2292 Faraday Avenue,
Carlsbad, California, 92008, USA
Published online: 30 Mar 2009.
To cite this article: Dale E. Robinson , Punit P. Seth & Elizabeth A. Jefferson
(2004) Solid‐Phase Synthesis of N‐Aryl‐N′‐Carboalkoxy Guanidines by the Mitsunobu
Reaction of Fmoc‐Guanidines, Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic Chemistry, 34:15, 2743-2749, DOI:
10.1081/SCC-200026206
To link to this article: http://dx.doi.org/10.1081/SCC-200026206
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SYNTHETIC COMMUNICATIONSw
Vol. 34, No. 15, pp. 2743–2749, 2004
Solid-Phase Synthesis of N-Aryl-N0-
Carboalkoxy Guanidines by the Mitsunobu
Reaction of Fmoc-Guanidines
*
Dale E. Robinson, Punit P. Seth, and Elizabeth A. Jefferson
Ibis Therapeutics, A Division of Isis Pharmaceuticals, Inc.,
Carlsbad, California, USA
ABSTRACT
A new method for the solid-phase synthesis of N-aryl-N0-carboalkoxy
guanidines is described. Aromatic amines were reacted with Fmoc-
isothiocyanate to provide Fmoc-thioureas, which were coupled with Rink
amide resin to provide the corresponding resin-bound Fmoc-guanidines.
Subsequent Mitsunobu alkylation with a variety of alcohols delivered
N-aryl-N0 carboalkoxy guanidines in good to high purity after resin
cleavage.
Key Words: Carbamoylguanidines; Mitsunobu reactions; Rink amide
resin; Fmoc-isothiocyanate.
*
Correspondence: Punit P. Seth, Ibis Therapeutics, A Division of Isis Pharmaceuticals,
Inc., 2292 Faraday Avenue, Carlsbad, CA 92008, USA; Fax: 760-603-4653; E-mail:
pseth@isisph.com.
2743
DOI: 10.1081/SCC-200026206
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright # 2004 by Marcel Dekker, Inc.

2744
Robinson, Seth, and Jefferson
INTRODUCTION
The importance of the guanidine functionality as a biologically relevant
pharmacophore is well documented.[1,2] For applications in our small
molecule RNA targeting anti-bacterial and anti-viral programs,[3] we needed
access to a diverse set of N-aryl-N0-carboalkoxy guanidines such as compound
5. While the preparation of carboalkoxy substituted guanidines has been
described in the literature, these methods have been limited to using the
carboalkoxy substituent as a protecting/activating group which is typically
removed at the end of the synthesis.[4–7] In our case, however, we wished
to retain the carboalkoxy substituent on the guanidine and also use it as a
site to incorporate diversity. A review of the existing methods[8–11] indicated
no suitable procedure that would allow for the solid-phase synthesis of a
diverse set of carboalkoxy guanidines. As a result we decided to investigate
alternate strategies to prepare these guanidine derivatives on solid support.
Recently, Zaragoza[12] reported the preparation of O-carbamates on
solid phase by reacting support-bound Fmoc-amines with alcohols under
Mitsunobu conditions. We rationalized that a similar strategy using support-
bound Fmoc-guanidines instead of Fmoc-amines should provide access to
carboalkoxy guanidines. To examine the feasibility of this strategy, the synthetic
plan outlined in Sch. 1 was proposed. Reaction of Rink amide resin[13]a and
Fmoc-thiourea 2 in the presence of a suitable activating agent should provide
Fmoc-guanidine 3. Subsequent alkylation with alcohols under Mitsunobu
conditions should provide the desired N-aryl-N0-carboalkoxy guanidine 5.
Such a strategy would enable us to prepare the desired carboalkoxy guanidines
in a traceless manner and also allow for concurrent introduction of diversity on
the aromatic ring and the carboalkoxy substituent (Sch. 1).
A set of aromatic amines ArNH2 (A–H), were reacted with Fmoc-isothio-
cyanate in CH2Cl2 to provide the corresponding Fmoc-thioureas 2 (Sch. 2).
Rink amide resin was deprotected with piperidine and treated with
excess Fmoc-thioureas 2 and diisopropylcarbodiimide (DIC). We were
pleased to find that in all cases, arylguanidines 6 were isolated in high yield
(89–95%) and purity (.90%) after Fmoc deprotection and resin cleavage.b
aIn this report Rink amide resin was reacted with arylisothiocyanates to provide resin-
bound arylthioureas. Further coupling with primary amines in the presence of DIC pro-
vided disubstituted guanidines after resin cleavage.
bThe resin-bound guanidines obtained after deprotection of the Fmoc group in 3 (but
prior to acid cleavage) were sufficiently nucleophilic to react with other electrophiles
such as sulfonyl and acyl chlorides. We are currently investigating these reactions to
further expand the scope of this method to prepare sulfonyl- and acyl-guanidine
derivatives.

Solid-Phase Synthesis of N-Aryl-N0-Carboalkoxy Guanidines
2745
Scheme 1. Solid-phase synthesis of N-aryl-N0-carboalkoxy guanidines.
If resin-bound Fmoc-guanidines 3 were not treated with piperidine/DMF prior
to acid cleavage, then the corresponding Fmoc-guanidines were isolated as the
major products. Best results for the couplings were obtained when DIC was
used as the activating agent, although, EDC (1-[3-(dimethylamino)propyl]-
3-ethylcarbodiimide hydrochloride) could also be used. Lower yield and
purities were observed when Mukaiyama’s reagent (2-chloro-1-methylpyridi-
nium iodide) was used as the coupling reagent.[8]
Scheme 2. Preparation of N-aryl-N0-carboalkoxy guanidines.

2746
Robinson, Seth, and Jefferson
The above outcome was significant as it demonstrated the ability of the
amine functionality on the resin to essentially scavenge the activated Fmoc-
thioureas from the reaction mixture and create a guanidine linkage directly
on the resin. These results were also gratifying as they allayed our earlier con-
cerns that the Fmoc protecting group on the thiourea may be too labile to
undergo efficient coupling with the amine resin.
We next examined the utility of support-bound guanidines 3 to prepare
the requisite N-aryl-N0-carboalkoxy guanidines. Resin-bound arylguanidines
3 were reacted with alcohols ROH (a–h) under Mitsunobu conditions
(Sch. 2). Analysis of the cleaved products indicated clean formation of
carboalkoxy guanidines 5 (Table 1). In general, primary aliphatic alcohols
provided the best results (entries 1–5) although a primary alcohol containing
a basic amine functionality provided no desired product (entry 6). Secondary
alcohols provided carboalkoxy guanidines in lower yield and purity (entries
7–8). In almost all cases, the major byproduct was identified to be the
Table 1. Representative N-aryl-N0-carboalkoxy guanidines prepared by solid-phase
synthesis.
5a
(mass)
5b
Purityc
(%)
Yieldd
(%)
Entry
ROH
ArNH2
[M þ H]þ
1
2
a
b
c
A
A
A
A
A
A
A
A
B
C
D
E
F
279.16
299.13
313.14
285.09
249.11
320.18
251.13
278.14
249.15
267.14
334.20
277.18
279.16
294.13
251.14
280.2
300.2
314.2
286.1
250.2
—
95
95
95
85
70
0
91
89
85
90
77
0
3
4
d
e
5
6
f
7
g
h
a
a
a
a
a
a
a
252.2
279.2
250.2
268.2
335.3
278.2
280.3
—
71
50e
95
95
80
70
95
0
72
69
86
78
75
87
86
0
8
9
10
11
12
13
14
15
G
H
252.2
86
91
aCalculated exact mass.
bObserved m/z.
cPurity determined using an evaporative light scattering detector (ELSD).
dPercent yield determined by crude mass recovery based on initial loading of Rink-
NH2.
eProduct was contaminated with Ph3PO.

Solid-Phase Synthesis of N-Aryl-N0-Carboalkoxy Guanidines
2747
corresponding deprotected guanidine 6 (5–20%). A number of different
Fmoc-guanidines derived from substituted anilines were found to participate
in the reaction (entries 9–13). Use of 4-nitroaniline provided no desired
carbamoylguanidine, presumably due to difficulty in forming the Fmoc-thiourea
during reaction with Fmoc-isothiocyanate (entry 14). An Fmoc-guanidine
derived from a heterocyclic amine was also found to participate in the reaction
and provide the corresponding carboalkoxy guanidine (entry 15).
In order to expand the scope of the method, the Mitsunobu reaction of a
support-bound Fmoc-guanidine derived from an aliphatic amine was also
attempted. To this end, resin-bound N-Fmoc-N0-phenethylguanidine was pre-
pared and reacted with 4-methylbutanol.c Once again, the major product after
acid cleavage was identified to be that arising from O-alkylation of the carba-
moyl group (50%), while the major contaminant appeared to be unreacted
N-Fmoc-N0-phenethylguanidine (45%). From this result, it appears that
under the conditions employed, guanidines derived from aromatic amines
are more reactive that guanidines derived from aliphatic amines.
To our knowledge, this is the first report that describes the O-alkylation of
Fmoc-guanidines with alcohols under Mitsunobu conditions. No products
arising from N-alkylation of the carbamate nitrogen were detected during
LC–MS analysis of the cleavage products. This is in direct contrast to the
results obtained during Mitsunobu alkylation of N-Boc guanidines, where
the major product arises from alkylation of the carbamate nitrogen.[14,15] It
also appears that the O-alkylation under Mitsunobu conditions is limited to
Fmoc-guanidines as guanidines prepared from aromatic amines using ethoxy-
carbonylisothiocyanate or benzoylisothiocyanate were found not to participate
in the reaction. In these cases, only the corresponding ethoxycarbonyl or
benzoyl guanidines were isolated after acid cleavage. The above results
suggest that cleavage of the fluorenyl group occurs first to generate resin-
bound guanidine-carbamates, which then undergo further reaction with acti-
vated alcohols to provide the O-alkylated products.[7]
In summary, we have described a new strategy for the preparation of
N-aryl-N0-carboalkoxy guanidines. Using this method, a diverse set of carbo-
alkoxy guanidines were prepared from aromatic, heteroaromatic and aliphatic
amines, and Fmoc-isothiocyanate. The synthesis also highlights a new O-alky-
lation reaction of resin-bound Fmoc-guanidines under Mitsunobu conditions.
cA mixture of N-Fmoc-N0-phenethylthiourea (4 eq.), EDC (4 eq.), DIPEA (8 eq.) in
CH2Cl2 was reacted with Rink-NH2 using the general procedure described to prepare
the resin bound N-Fmoc-N0-phenethylguanidine. In this case very poor results were
obtained when DIC was used as the coupling agent.

2748
Robinson, Seth, and Jefferson
EXPERIMENTAL
General Procedure for the Synthesis of N-aryl-N0-
Carboalkoxy Guanidines on the Argonaut Quest 210
Argogelw-Rink-NH-Fmoc resin (0.32 mmol/g, 150 mg, 0.048 mmol) was
deprotected with 25% piperidine/DMF (2.5 mL) at rt for 2 hr, filtered and
washed with DMF (3X), CH2Cl2 (2X), MeOH (2X), and CH2Cl2 (3X). A sol-
ution of Fmoc-arylthiourea 2 (0.2 mmol, prepared in situ by reacting 0.2 mmol
of arylamine with 0.2 mmol of Fmoc-isothiocyanate in 2.5 mL of CH2Cl2 at rt
for 14 hr) was added to the resin and the mixture was agitated at rt for 14 hr,
filtered and washed with CH2Cl2 (2X), MeOH (2X), CH2Cl2 (3X), and dry
THF (2X). A solution of the desired alcohol ROH (0.24 mmol), DIAD
(2.4 mmol, 0.048 mL) and triphenylphosphine (0.24 mmol, 0.063 g) in dry
THF (2.5 mL) was added to the resin and the mixture was agitated at rt for
14 hr, filtered and washed with CH2Cl2 (3X), MeOH (3X), and CH2Cl2
(3X). The resin was cleaved with 25% TFA/CH2Cl2 (3 mL) at rt for
90 min, filtered and washed with CH2Cl2. The combined filtrate was evapor-
ated to dryness by SpeedVac (Savant Instruments, Inc.) to provide the
desired product in yield and purity as shown in Table 1.
Analytical Data for Representative Guanidines
5Aa. 1H NMR (300 MHz, DMSO-d6) d 7.33 (2H, d, J ¼ 9.0), 7.10 (2H, d,
J ¼ 9.0), 4.38 (2H, t, J ¼ 6.7), 3.88 (3H, s), 1.79 (1H, m), 1.66 (m, 2H), 1.01
(6H, d, J ¼ 6.5).
5Ba: 1H NMR (300 MHz, DMSO-d6) d 7.55 (5H, m), 4.36 (t, 2H,
J ¼ 6.7), 1.79 (1H, m), 1.66 (m, 2H), 0.99 (6H, d, J ¼ 6.5).
5Ha: 1H NMR (300 MHz, DMSO-d6) d 8.53 (s, 1H), 8.34 (m, 2H), 4.28
(t, 2H, J ¼ 6.8), 1.70 (1H, m), 1.58 (m, 2H), 0.99 (6H, d, J ¼ 6.5).
REFERENCES
1. (a) Mori, A.; Cohen, B.D.; Lowenthal, A. Eds.; Guanidines: Historical,
Biological, Biochemical and Clinical Aspects of the Naturally Occurring
Guanidino Compounds; Plenum Press: New York, 1983; (b) Mori, A.;
Cohen, B.D.; Koide, H. Eds.; Guanidines 2: Further Explorations of
the Biological and Clinical Significance of Guanidino Compounds;
Plenum Press: New York, 1987.

Solid-Phase Synthesis of N-Aryl-N0-Carboalkoxy Guanidines
2749
2. Greenhill, J.V.; Lue, P. Amidines and guanidines in medicinal chemistry.
In Progress in Medicinal Chemistry; Ellis, G.P., Luscombe, D.K., Eds.;
Elsevier Science: New York, 1993; Vol. 30, Chap. 5.
3. Ecker, D.J.; Griffey, R.H. RNA as a small-molecule drug target: doubling
the value of genomics. DDT 1999, 4, 420–429.
4. Linkletter, B.A.; Bruice, T.C. Solid–phase synthesis of oligomeric deoxy-
nucleic guanidine(DNG): a polycationic analogue of DNA. Bioorg. Med.
Chem. Lett. 1998, 8, 1285–1290.
5. Schneider, S.E.; Bishop, P.A.; Salazar, M.A.; Bishop, O.A.; Anslyn, E.V.
Solid phase synthesis of oligomeric guanidiniums. Tetrahedron 1998, 54,
15063–15086.
6. Linton, B.R.; Carr, A.J.; Orner, B.P.; Hamilton, A.D. A versatile one-pot
synthesis of 1,3-substituted guanidines from carbamoyl isothiocyanates.
J. Org. Chem. 2000, 65, 1566–1568.
7. Manimala, J.C.; Anslyn, E.V. A highly efficient method for the synthesis
of guanidinium derivatives. Tetrahedron Lett. 2002, 43, 565–567.
8. Manimala, J.C.; Anslyn, E.V. Solid-phase synthesis of guanidinium
derivatives from thiourea and isothiourea functionalities. Eur. J. Org.
Chem. 2002, 3909–3922.
9. Lin, P.; Ganesan, A. Solid phase synthesis of N-acyl-N0-carbamoylguani-
dines. Tetrahedron Lett. 1998, 39, 9789–9792.
10. Josey, J.A.; Tarlton, C.A.; Payne, C.E. Novel linker for the solid-phase
synthesis of guanidines. Tetrahedron Lett. 1998, 39, 5899–5902.
11. Ghosh, A.K.; Hol, W.G.J.; Fan, E. Solid phase synthesis of N-acyl-N0-
alkyl/aryl disubstituted guanidines. J. Org. Chem. 2001, 66, 2161–2164.
12. Zaragoza, F.; Stephensen, H. Direct conversion of Fmoc-protected amines
into O-alkyl carbamates by the Mitsunobu reaction: a practical strategy
for the solid-phase synthesis of carbamates. Tetrahedron Lett. 2000, 41,
2015–2017.
13. Li, M.; Wilson, L.J.; Portlock, D.E. A simple solid-phase synthesis of di-
substituted guanidines using rink amide resin as an amine component.
Tetrahedron Lett. 2001, 42, 2273–2275.
14. Dodd, D.S.; Wallace, O.B. Solid-phase synthesis of N,N0 substituted
guanidines. Tetrahedron Lett. 1998, 39, 5701–5704.
15. Patek, M.; Smrcina, M.; Nakanishi, E.; Izawa, H. Solid-phase synthesis of
substituted guanidines using a novel acid labile linker. J. Comb. Chem.
2000, 2, 370–377.
Received in the USA March 31, 2004