NG-hydroxyguanidines from primary amines

Article abstract of DOI:10.1021/ol061454p

A concise and general method for the preparation of N^G- hydroxyguanidines from primary amines is reported. Using available and readily prepared materials, primary amines are converted to protected N ^G-hydroxyguanidines in a one-pot p

Full text of DOI:10.1021/ol061454p

ORGANIC
LETTERS
NG-Hydroxyguanidines from Primary
Amines
2006
Vol. 8, No. 18
4035-4038
Nathaniel I. Martin,* Joshua J. Woodward, and Michael A. Marletta
Department of Chemistry, UniVersity of California Berkeley, 204 Lewis Hall,
Berkeley, California 94720
nimartin@berkeley.edu
Received June 13, 2006
ABSTRACT
A concise and general method for the preparation of NG-hydroxyguanidines from primary amines is reported. Using available and readily
prepared materials, primary amines are converted to protected NG-hydroxyguanidines in a one-pot procedure followed by deprotection under
nonreducing conditions. The method has been successfully applied to a number of examples including a high-yielding preparation of NG-
hydroxy-L-arginine, the intermediate in the enzymatic conversion of L-arginine to nitric oxide and L-citrulline by nitric oxide synthase.
NG-Hydroxyguanidines1 have been of medicinal interest for
some time having been shown to exhibit cytotoxicity in
human leukemic cells and antitumor activity in vivo.2,3
Recently, these compounds have also received attention as
protective agents in ischemia-reperfusion,4 as potential
vasodilators, and for use in treatment of diseases such as
pulmonary and cardiovascular disorders, hypoxia, sexual
dysfunction, and even memory loss.5 Our interest in this class
of molecules lies in the intermediacy of NG-hydroxy-L-
arginine in the conversion of L-arginine to nitric oxide and
L-citrulline via the action of nitric oxide synthase (Figure
1).6
fluorinated analogues of L-arginine and NG-hydroxy-L-
arginine to serve as mechanistic probes (Figure 2). The
Figure 2. NOS substrate analogues 4,4-difluoro-L-arginine (1) and
4,4-difluoro-NG-hydroxy-L-arginine (2).
synthesis of the difluorinated L-arginine analogue (1) was
straightforward and based largely upon the work of Kim and
(1) For comprehensive reviews on the chemistry and biology of
guanidines, NG-hydroxyguanidines, and other NO donors, see: (a) Katritzky,
A. R.; Rogovoy, B. V. ARKIVOC 2005, 4, 49. (b) Wang, P. G.; Xian, M.;
Tang, X.; Wu, X.; Wen, Z.; Cai, T.; Janczuk, A. J. Chem. ReV. 2002, 102,
1091.
(2) Everett, S. A.; Smith, K. A.; Patel, K. B.; Dennis, M. F.; Stratford,
M. R. L.; Wardman, P. Br. J. Cancer 1996, 74, S172.
Figure 1. Two-step NOS reaction.
(3) Chern, J. W.; Leu, Y. L.; Wang, S. S.; Lou, R.; Lee, C. F.; Tsou, P.
C.; Hsu, S. C.; Liaw, Y. C.; Lin, H. W. J. Med. Chem. 1997, 40, 2276.
(4) Dambrova, M.; Baumane, L.; Kiuru, A.; Kalvinsh, I.; Wikberg, J. E.
S. Arch. Biochem. Biophys. 2000, 377, 101.
(5) Nitromed Inc, WO Patent 99/62509, 1999.
(6) Tayeh, M. A.; Marletta, M. A. J. Biol. Chem. 1989, 264, 19654.
As part of our efforts to better characterize the mechanistic
details of this process (more specifically, the enzymatic
“second step”) we recently undertook the synthesis of
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© 2006 American Chemical Society
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co-workers.7 The preparation of the difluorinated NG-
hydroxy-L-arginine analogue (2), however, was more chal-
lenging.
Scheme 3
As a first attempt, the protected 4,4-difluoro-NG-hydroxy-
L-arginine species (5) was prepared in modest yield starting
from Kim’s precursor difluoroamine 37 and employing the
thiourea reagent (4) based method of Jirgensons and co-
workers.8 Disappointingly, the final deprotection step (under
a variety of conditions) following this route led to complete
loss of the NG-hydroxyguanidine functionality by over-
reduction (Scheme 1).9
ingly, this process yielded the exchanged thiourea product
10 in 75% yield with only trace amounts of the expected
hydroxyguanidine product. Similar results were also obtained
when using EDCI as an activating agent.14 These observa-
tions suggest that O-protected NG-hydroxythioureas are prone
to a competitive exchange of the hydroxylamine moiety in
the presence of another amine to yield a more stable thiourea.
This may also explain the modest yields (34-67%) reported
by Jirgensons and co-workers in their preparation of protected
NG-hydroxguanidines.8
Scheme 1
It was then reasoned that under forcing conditions (i.e.,
excess amounts of activating agent and O-protected hydroxyl-
amine) it might be possible to generate the NG-hydroxy-
guanidine moiety from a protected thiourea. Compound 10
was thus treated with EDCI, NEt3, and THP-ONH2. After
30 min, a new, more polar species was detected by TLC.
Additional 0.5 equiv of EDCI, THP-ONH2, and NEt3,
administered at 30-minute intervals, led to complete con-
sumption of the thiourea after 1.5 h (2.5 total equivalents of
activator, protected hydroxylamine, and amine base). Fol-
lowing workup and chromatography, the protected NG-
hydroxyguanidine 11 was obtained in 98% yield (Scheme
4).
The next approach toward 2 involved treatment of
compound 3 with cyanogen bromide. The reaction of amines
with cyanogen bromide can afford cyanamides in varying
yields.10 Cyanamides, while often unstable,11 can be treated
with amines to yield guanidines or with hydroxylamine to
provide NG-hydroxyguanidines.12 Indeed this process has
been used most often in the synthesis of NG-hydroxy-L-
arginine.10 Treatment of 3 with cyanogen bromide, however,
failed to yield the desired difluorocyanamide resulting instead
in a complex mixture.
Scheme 4
The failure of established methods to provide a route to
the desired NG-hydroxyguanidine prompted us to investigate
alternate approaches. We reasoned that by using an acid-
labile protecting group for the N-hydroxy moiety it might
still be possible to make use of a thiourea-based reagent
approach. To this end, the N-Cbz/OTHP protected NG-
hydroxythiourea 9 was prepared by reaction of Cbz-NCS
(7) (formed in situ) and O-THP-protected hydroxylamine13
(8) following the procedure of Jirgensons and co-workers
(Scheme 2).8
We next set out to optimize the preparation of the
carbamoyl isothiocyanate required for production of the
(7) Kim, K. S.; Qian, L. Tetrahedron Lett. 1993, 34, 7195.
(8) Jirgensons, A.; Kums, I.; Kauss, V.; Kalvins, I. Synth. Commun. 1997,
27, 315.
(9) The published work8 upon which this approach was based does not
describe any conditions by which the Cbz/OBn-protected species might be
deprotected to yield the desired product.
(10) Pufahl, R. A.; Nanjappan, P. G.; Woodard, R. W.; Marletta, M. A.
Biochemistry 1992, 31, 6822.
(11) Wagenaar, F. L.; Kerwin, J. F. J. Org. Chem. 1993, 58, 4331.
(12) Renodon-Corniere, A.; Dijols, S.; Perollier, C.; Lefevre-Groboillot,
D.; Boucher, J.-L.; Attias, R.; Sari, M.-A.; Stuehr, D.; Mansuy, D. J. Med.
Scheme 2
Chem. 2002, 45, 944.
(13) Other O-protected hydroxylamines (MOM-ONH2
13a
and tBu-
ONH213b were also prepared and could be used; however, THP-ONH2 was
deemed most convenient due to its ease of preparation13c and commercial
availabilty. It should be noted that the use of O-THP-protected hydroxyl-
amine does result in diastereomeric mixtures when a stereogenic centre is
present in the starting material. (a) Ullrich, T.; Sulek, P.; Binder, D.; Pyerin,
M. Tetrahedron 2000, 56, 3697. (b) Palandonken, H.; Bocian, C. M.;
McCombs, M. R.; Nantz, M. H. Tetrahedron Lett. 2005, 46, 6667. (c)
Haslanger, M. F.; Karanewsky, D. S. U.S. Patent 4 604 407, 1986.
Next, as a model, benzylamine was treated with thiourea
9 along with HgCl2 and triethylamine (Scheme 3). Surpris-
4036
Org. Lett., Vol. 8, No. 18, 2006

intermediate thioureas.15 The ideal reagent in this case would
be BocNCS allowing for a straightforward deprotection of
the product NG-hydroxyguanidine under mild acidic condi-
tions. BocNCS,16 however, is not readily prepared via
standard procedures (numerous attempts at acylation of
KNCS with Boc2O or BocF17 failed to yield BocNCS) and
so we turned our attention to improving the preparation of
CbzNCS. The reaction of benzyl chloroformate with KNCS
has been previously described along with the problematic,
competitive decarboxylation reaction to yield benzyl thio-
cyanate.18 The thiocyanate anion is a bidentate nucleophile
and the outcome of the acylation reaction is dependent upon
solvent polarity. Analysis of the product distribution obtained
using the benzene/crown ether conditions described by
Jirgensons and co-workers for their in situ preparation of
CbzNCS8 revealed formation of the undesired benzyl thio-
cyanate in major proportion (ca. 2:1 relative to CbzNCS)
also explaining our modest yield of compound 9 (Scheme
2). Other reports using ethyl acetate18a as solvent failed to
give CbzNCS. A third preparation of CbzNCS has been
described18b in which a solvent mixture of 20% acetonitrile/
80% toluene was reported to yield the desired product. While
this approach did show some selectivity for the formation
of CbzNCS it was also difficult to reproduce from one
attempt to the next and due to the lengthy reaction times
(>5 days) was deemed undesirable. We therefore screened
a number of solvents and solvent mixtures, with and without
crown ether or phase transfer catalysts. This investigation
revealed that the use of 5 mol % 18-crown-6 (relative to
CbzCl) in tetrachloroethylene at 85 °C yields the desired
CbzNCS product with only trace amounts (<5%) of benzyl
thiocyanate (Scheme 5). Contrary to the reports of others18a,b
Scheme 6
The reaction of amines with carbamoyl isothiocyanates
to yield thioureas is known to proceed rapidly and in high
yield.20 The direct activation of thioureas formed in situ,
followed by treatment with an amine, has been recently
applied to the one-pot synthesis of various substituted
guanidines.18b We therefore employed a similar approach in
Table 1. Protected NG-Hydroxyguanidines from Primary
Aminesa
Scheme 5
we found that CbzNCS is not stable to silica chromatography
or purification by distillation resulting in both instances in
decomposition to benzyl thiocyanate. Instead, the crude
product mixture can be diluted with hexanes, filtered to
remove salts, and after solvent removal, redissolved in
dichloromethane (to a concentration of 0.5 M) to provide a
stock solution of CbzNCS which can be stored at 4 °C with
no appreciable decomposition.19
a In a typical experiment, the amine (0.50 mmol) in CH2Cl2 (10.0 mL)
was treated with an equimolar quantity of CbzNCS (administered as a 0.50
M solution in CH2Cl2). Once consumption of the amine was verified by
TLC, the mixture was treated with EDCI (0.50 mmol), NEt3 (0.50 mmol),
and THP-ONH2 (0.50 mmol). Additional half-equivalents of each reagent
were then added at 15 min intervals until the intermediate thiourea was
fully consumed (generally required 2.0-3.0 total equivalents). b Isolated
yield based on amine starting material. c An equivalent of NEt3 added to
the amine hydrochloride prior to treatment with CbzNCS. d Protected
ornithine species prepared by a published procedure.10 e Prepared by method
of Kim and co-workers.7 f Yield for three-step process beginning with
hydrogenolytic removal of side-chain amine Cbz-protecting group.
(14) Poss, M. A.; Iwanowicz, E.; Reid, J. A.; Lin, J.; Gu, Z. Tetrahedron
Lett. 1992, 33, 5933.
(15) Commercially available carbamoyl isothyiocyanates (FmocNCS and
EtOCONCS) were deemed unsuitable for our application.
(16) A preparation of BocNCS has been reported via the decomposition
of BocCONCS in liquid SO2: Bunnenberg, R.; Jochims, J. C. Chem. Ber.
1981, 114, 1746.
(17) Dang, V. A.; Olofson, R. A. J. Org. Chem. 1990, 55, 1847.
(18) (a) Groziak, M. P.; Townsend, L. B. J. Org. Chem. 1986, 51, 1277.
(b) Linton, B. R.; Carr, A. J.; Orner, B. P.; Hamilton, A. D. J. Org. Chem.
2000, 65, 1566.
Org. Lett., Vol. 8, No. 18, 2006
4037

the preparation of protected NG-hydroxyguanidines (Scheme
6).
overreduced guanidine. As an alternative, the method of Kiso
and co-workers,21 using a mixture of TFA and thioanisole,
provided the fully deprotected NG-hydroxy-L-arginine product
in quantitative yield. The process was equally successful in
the global deprotection 4,4-difluoro-NG-hydroxy-L-arginine
(Scheme 8).
A variety of amines were converted to their corresponding
thioureas (formation of thiourea and concomitant consump-
tion of amine monitored by TLC) which were then directly
treated with EDCI, NEt3, and THP-ONH2 (typically 2-3
equiv) to give protected NG-hydroxyguanindine products in
high yield (Table 1).
Scheme 8
It should be noted that secondary amines are not amenable
to this procedure. While both morpholine and diallyamine
were rapidly converted to their thioureas after treatment with
CbzNCS (based on TLC observation), none of the NG-
hydroxyguanidine product was formed upon treatment with
EDCI, NEt3, and THP-ONH2. This result supports a mech-
anism whereby EDCI-mediated desulfurization is accompa-
nied by double deprotonation of the thiourea to yield a
transient carbodiimide1a which is subsequently attacked by
the amine nucleophile (Scheme 7).
In summary, we have developed a method allowing for
the rapid preparation of NG-hydroxyguanidines from primary
amines. The method has been applied to a variety of
examples including a much improved synthesis10,22 of NG-
hydroxy-L-arginine and its 4,4-difluorinated analogue. Cur-
rent work is underway evaluating the use of such analogues
as mechanistic probes for the NOS reaction cycle.
Scheme 7
Acknowledgment. This work was supported by the
Natural Sciences and Engineering Council of Canada and
the Alberta Heritage Foundation for Medical Research
(postdoctoral fellowships to N.I.M.).
Using the protected NG-hydroxy-L-arginine species (entry
7, Table 1) as a model, we next examined potential
deprotection conditions. As expected, hydrogenation was
once again problematic giving, in the best case, a 1:1 mixture
of the Cbz deprotected/O-THP protected product and the
Supporting Information Available: Experimental pro-
cedures for transformations that produced previously un-
known compounds as well as their full spectral character-
ization. This material is available free of charge via the
Internet at http://pubs.acs.org.
(19) (a) While this process does not remove the trace amount of benzyl
thiocyanate, it can be considered a “silent” impurity in the subsequent
reactions and is readily separated in the final chromatography-based
purification of the protected NG-hydroxyguanidine product. (b) This
improved preparation of CbzNCS should also be of general use in the
preparation of substitued guanidines.18b
OL061454P
(21) Kiso, Y.; Ukawa, K.; Akita, T. J. Chem. Soc., Chem. Commun. 1980,
3, 101.
(20) Esmail, R.; Kurzer, F. Synthesis 1975, 301.
(22) Feldman, P. L. Tetrahedron Lett. 1991, 32, 875.
4038
Org. Lett., Vol. 8, No. 18, 2006