Nitrogen-bound diazeniumdiolated amidines

Article abstract of DOI:10.1039/c0cc00849d

In contrast to amidines bearing ionizable α-CH bonds, which react with nitric oxide (NO) to add diazeniumdiolate groups at their α-carbons, benzamidine forms an N-bound diazeniumdiolate that can be further derivatized at the other amidine nitrogen and/or

Full text of DOI:10.1039/c0cc00849d

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Nitrogen-bound diazeniumdiolated amidinesw
Debanjan Biswas,*a Jeffrey R. Deschamps,b Larry K. Keefera and Joseph A. Hrabie*c
Received 9th April 2010, Accepted 14th May 2010
DOI: 10.1039/c0cc00849d
In contrast to amidines bearing ionizable a-CH bonds, which
react with nitric oxide (NO) to add diazeniumdiolate groups at
their a-carbons, benzamidine forms an N-bound diazenium-
diolate that can be further derivatized at the other amidine nitrogen
and/or the terminal oxygen to form caged NO compounds as
potential NO prodrugs.
Scheme 1a. In contrast, reaction of 2-cyclohexyl-2-imidazoline
containing only one replaceable a-hydrogen atom with NO
results in the corresponding C–diazeniumdiolated product as a
zwitterionic salt (Scheme 1b).2 As a part of our ongoing
research into the synthesis and properties of diazeniumdiolated
compounds, we desired to study the reactivity of the N-atom(s)
of the amidines as a means for investigating the preparation of
new N-diazeniumdiolated products.
Preparation of carbon-bound diazeniumdiolates1 by the treat-
ment of aliphatic amidines with nitric oxide (NO) has been
reported.2 Aliphatic amidine free bases containing a-hydrogen
atom(s) react with NO to give C-diazeniumdiolates. The
a-hydrogen atom(s) of these amidines are far more acidic than
the amine protons, resulting in the reaction of an enediamine
tautomer in solution as illustrated in Scheme 1a using lysidine
as the example. Thus, the reaction of these amidines with NO
has been found to be analogous to the reaction of enamines
reported earlier.3 Amidines, such as lysidine, containing more
than one replaceable a-hydrogen atom were found to produce
polydiazeniumdiolated intramolecular salts, such as the imid-
azoline bis(diazeniumdiolate) bis(imidazolinium) salt shown in
It was envisaged that amidines containing no a-hydrogen
atom might possess different reactivity towards NO as com-
pared to those bearing one or more a-protons because reaction
of these compounds could not proceed via enediamine tautomers.
It seemed that diazeniumdiolation would most likely occur at
the amidine N-atom(s). In fact, reaction of benzamidine (1)
with NO produced the corresponding N-diazeniumdiolated
benzamidine accompanied by benzamidinium counter ion (2)
in 46% yield as a stable white powder (Scheme 2). Treatment
of a solution of 2 in basified methanol with a stoichiometric
quantity of sodium methoxide afforded the sodium salt of
N-diazeniumdiolated benzamidine (3a) in 84% yield.
The amidine moiety in benzamidine consists of a primary
amine N-atom and also an azomethine N-atom, and accordingly
N-diazeniumdiolated benzamidine can exist in either of the
two tautomeric forms (3a and 3b, depicted in Fig. 1) depending
on their relative stability. We presume the imine-diazeniumdiolate
tautomers would predominate due to their higher resonance
stabilization as compared to their amine-diazeniumdiolate
tautomers (for example 3b). However, due to the possibility
of tautomerization occurring after the reaction, it is not
possible to determine which N-atom of benzamidine actually
reacts with NO.
In order to elucidate the structural attributes of N-diazenium-
diolated benzamidine, preparation of an O2-alkylated derivative
of this compound was attempted. Accordingly, O2-methyl
N-diazeniumdiolated benzamidine (4) was prepared in 76%
yield from 3a (Scheme 3). The product was carefully
re-crystallized using a solvent combination of ethyl acetate –
hexanes and single-crystal X-ray diffraction data indeed revealed
that the N-diazeniumdiolated benzamidine exists as the imine
tautomer 4 in the solid state.
Scheme 1 Preparation of (a) C-diazeniumdiolated lysidine, (b) C-di-
azeniumdiolated 2-cyclohexyl-2-imidazoline.
a Chemistry Section, Laboratory of Comparative Carcinogenesis,
National Cancer Institute at Frederick, Frederick, MD 21702, USA.
E-mail: biswasd@mail.nih.gov; Fax: +1 301-846-5946;
Tel: +1 301-846-5421
b Center for Biomolecular Science and Engineering,
Naval Research Laboratory, Washington, D. C. 20375, USA
c Basic Science Program, SAIC-Frederick Inc., National Cancer
Institute at Frederick, Frederick, MD 21702, USA.
E-mail: hrabiej@mail.nih.gov; Fax: +1 301-846-5946;
Tel: +1 301-846-7184
w Electronic supplementary information (ESI) available: Experimental
procedures, 1H and 13C NMR spectra for all new compounds and
X-ray crystallographic data for 4 and 9. CCDC 761561 and 761562.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/c0cc00849d
Scheme 2 Preparation of N-diazeniumdiolated benzamidine.
Chem. Commun., 2010, 46, 5799–5801 | 5799
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Scheme 4 Preparation of O2-methoxymethyl N-diazeniumdiolated
Fig. 1 Tautomeric forms of N-diazeniumdiolated benzamidine.
benzamidine.
O2-methoxymethyl N-diazeniumdiolated benzamidine (5) was
prepared in 61% yield (Scheme 4).
In the past few years, preparation of a diverse class of
O2-2,4-dinitrophenyl-protected diazeniumdiolates as glutathione-
activated NO-releasing prodrugs8 has been reported and a few
have proved to be substantial anticancer lead compounds.9
Thus, we prepared O2-2,4-dinitrophenyl N-diazeniumdiolated
benzamidine (6) from 3a as a new addition to this class of
compounds (Scheme 5). O2-Glycosylated diazeniumdiolates10
have also emerged as a significant class of glycosidase-
activated NO-releasing prodrug whose decomposition products
are normal mammalian metabolites, several compounds of
this kind having been reported.11 Reaction of acetobromo-a-
D-glucose with 3a produced O2-glucosylated N-diazenium-
diolated benzamidine peracetate (7) in 61% yield (Scheme 5).
In an effort to prepare more structurally diverse N-diazenium-
diolated amidines that can act as precursors for the prepara-
tion of other bio-active molecules, we explored the possibility
of multiple functionalization of an analog of 3a. O2-Benzylated
N-diazeniumdiolated benzamidine (8) was prepared by the
treatment of 3a with benzyl bromide as an acid- and base-
stable analog and was subsequently used for further func-
tionalization. Treatment of 8 with sodium hydride and an
excess of methyl iodide produced a crystalline white solid
in 59% yield which could potentially be either O2-benzyl
N0,N0-dimethyl N-diazeniumdiolated benzamidine (9) or the
product with a methyl group on each of the amidine N-atoms
(Scheme 6). Recrystallization of the crude product using a
Scheme
benzamidine.
3
Preparation of O2-methylated N-diazeniumdiolated
There have been reports indicating preferential E stereo-
chemistry for imines of monosubstituted amidines.4 However,
the crystal structure of 4 (Fig. 2) reveals that the presence of a
hydrogen bond between the amino proton and the O1-atom
results in an unusual Z-conformation of the amidine double
bond (C10 = N3 in Fig. 2). While we have postulated the
existence of this type of hydrogen bond,5 this is the first
confirmed observation. In this amidine, the diazeniumdiolate
moiety exists in syn-periplanar conformation which is consis-
tent with the previously reported6 alkylated N-based diazenium-
diolated compounds. Examination of the crystal structure also
reveals comparable C–N bond lengths suggesting the relatively
low double-bond character of the imine group.
Next, we investigated the preparation of several other
O2-protected N-diazeniumdiolated benzamidine prodrugs for
potential therapeutic applications as NO-donating agents under
suitable conditions. It has been reported that O2-alkoxyalkyl-
protected diazeniumdiolates are acid labile7 and thus it was
envisaged that the use of this derivative would provide the
parent N-diazeniumdiolated amidine as a potential NO
source under acidic hydrolytic conditions. Accordingly,
Scheme 5 Preparation of O2-2,4-dinitrophenyl N-diazeniumdiolated
benzamidine and O2-glucosylated N-diazeniumdiolated benzamidine
peracetate.
Fig. 2 Z-conformation molecular structure and numbering scheme
for 4.
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In conclusion, we have prepared N-diazeniumdiolated amidines
as a new class of potential NO-donors. The crystal structures
of two O2-derivatized analogs of this molecule have been
determined and 1H and 13C NMR spectra of the bulk samples
fully agree with the assigned structures. We synthesized several
O2-protected derivatives of N-diazeniumdiolated benzamidine
that can potentially act as useful NO-releasing prodrugs
under appropriate conditions. N-Alkylation of O2-benzylated
benzamidine diazeniumdiolate has been shown to produce a
change in configuration about the imine bond.
Scheme 6 Preparation and methylation of O2-benzyl N-diazenium-
This project has been funded with Federal funds from the
National Cancer Institute, National Institutes of Health,
under contract HHSN261200800001E, and by the Intramural
Research Program of the NIH, National Cancer Institute,
Center for Cancer Research. Crystallographic studies were
supported by the National Institute on Drug Abuse under
contract Y1-DA6002.
diolated benzamidine.
solvent mixture of ethyl acetate and hexanes and single-crystal
X-ray diffraction analysis revealed that the product solely
exists in the N0,N0-dimethyl form (9) in the solid state (see
Supporting Information). Unlike the O2-methylated analog
(4), lack of hydrogen bonding in 9 due to the presence of a
secondary amine moiety and the steric bulk of the the dimethyl
group results in the more common E-conformation for the
imine substituents.
Notes and references
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Diazeniumdiolated molecules containing free amine func-
tionality can undergo amide coupling and this methodology
has been utilized for the preparation of diazeniumdiolated
proteins, such as bovine serum albumin and human serum
albumin.7 A similar protocol was applied to our amidine
chemistry and resulted in the preparation of a new a-amido
N-diazeniumdiolated benzamidine. Reaction of 8 with sodium
hydride, followed by the addition of 4-nitrobenzoyl chloride,
afforded the desired a-amido product (10) in 43% yield
(Scheme 7). Application of this protocol to the preparation
of other a-amido and a-sulfonamido analogs is currently being
pursued.
Preliminary results show that certain of these N- diazenium-
diolated benzamidine derivatives regenerate NO in phosphate
buffer under physiological pH at 37 1C. The NO-release
profiles of these compounds are currently under investigation.
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Scheme
7
Preparation of an a-amido-N-diazeniumdiolated
benzamidine.
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Chem. Commun., 2010, 46, 5799–5801 | 5801