Nitrous oxide as a primary product in base-mediated β-elimination reactions of diazeniumdiolated benzylamine derivatives

Article abstract of DOI:10.1039/c2cc32284f

We report an unexpected β-elimination pathway by which diazeniumdiolated benzylamines of structure Bn-N(R)-N(O)N-OR′ undergo base-mediated fragmentation to generate N₂O as the only gaseous product. The reaction is especially rapid for R = 2-hydroxyethyl, in which the hydroxyl group anchimerically assists benzylic proton removal with concomitant expulsion of PhCHNR and R′OH.

Full text of DOI:10.1039/c2cc32284f

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COMMUNICATION
Nitrous oxide as a primary product in base-mediated b-elimination
reactions of diazeniumdiolated benzylamine derivativesw
Debanjan Biswas,*a Zhao Cao,b Larry K. Keefera and Joseph E. Saavedrab
Received 29th March 2012, Accepted 20th April 2012
DOI: 10.1039/c2cc32284f
We report an unexpected b-elimination pathway by which diazenium-
benzylamines as potential NO prodrugs containing aromatic
chromophores and have probed into their structure–property
relationships. In this effort to develop new N-diazeniumdiolates
as efficient sources of biomedically useful NO, we have
unexpectedly encountered a pathway by which certain members
of this family fragment directly to N2O without the
formation of HNO as an initial step. Herein, we report on the
base-mediated b-elimination reaction of certain secondary
benzylamine diazeniumdiolates.
0
Q
diolated benzylamines of structure Bn–N(R)–N(O) N–OR under-
go base-mediated fragmentation to generate N2O as the only gaseous
product. The reaction is especially rapid for R = 2-hydroxyethyl, in
which the hydroxyl group anchimerically assists benzylic proton
0
Q
removal with concomitant expulsion of PhCH NR and R OH.
O2-Derivatized secondary amine diazeniumdiolate prodrugs have
emerged as efficient and reliable sources of the bio-effector molecule
nitric oxide (NO).1 Removal of the O2-protecting groups by
hydrolysis or appropriate metabolic triggers furnishes the
corresponding diazeniumdiolate anion.2 This species (1, see Fig. 1)
under physiological conditions decomposes to yield up to two
moles of NO and the nucleophilic secondary amine by initial
protonation of the amine nitrogen (Fig. 1).3 Recently, primary
amine diazeniumdiolate IPA/NO4 was reported to release
HNO, which then dimerizes to produce N2O, at both neutral
and alkaline pH. However, N-bound diazeniumdiolates have
not been found to release N2O as a direct decomposition
product, although some C-bound diazeniumdiolates are known
to furnish only N2O at neutral pH.5 Our laboratory has been
involved in diazeniumdiolate-based drug discovery focused
towards effective site-directed delivery of NO for therapeutic
applications.6 Recently, we began a study of diazeniumdiolated
Reaction of 2-benzylaminoethanol (2) with NO in the
presence of sodium methoxide afforded the corresponding
N-diazeniumdiolated sodium salt (3). Under physiological
conditions, in the presence of 7.4 pH phosphate buffer at 37 1C,
this sodium salt releases about 1.6 mole of NO per mole of the
compound (t1/2 = 8.9 min) and can be considered as a new
addition to the arsenal of NO-releasing diazeniumdiolates.6
O2-Alkylation of diazeniumdiolate salts renders enhanced
stability in acidic and alkaline media7 and accordingly the
corresponding methylated derivative (4) was prepared by the
treatment of 3 with dimethyl sulfate (Scheme 1). This analog,
barely soluble in aqueous media, was found to be fairly stable
under acidic conditions and showed no signs of decomposition
in a solution of 0.5 M HCl in ether at room temperature for over
12 h. However, a surprisingly facile chemical transformation
of 4 was observed in the presence of base.
Treating a solution of 4 in THF with LitBuO at 50 1C was
marked with a rapid disappearance of the starting material.
Under optimized conditions, the reaction completed in 1.5 h
and isolation of the product indicated the formation of Schiff
base 5 in 84% isolated yield, whose identity was confirmed by
comparing its NMR and HR-MS data with those for an
authentic sample prepared as reported.8 An effort to determine
the fate of the diazeniumdiolate nitrogen atoms in 4 using gas
Fig. 1 Decomposition mechanism of secondary amine diazeniumdiolates.
a Drug Design Section, Chemical Biology Laboratory,
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 Basic Science Program, SAIC-Frederick Inc., National Cancer
Institute at Frederick, Frederick, MD 21702, USA
w Electronic supplementary information (ESI) available: Full experi-
mental procedures and spectroscopic data for compounds 3, 4, 7a, 7b,
and 9 are available. See DOI: 10.1039/c2cc32284f
Scheme 1 Preparation of 4.
c
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Table 1 Base-mediated fragmentation of 4
Entrya
Base
Solvent
Rxn. time/h
Yieldb (%)
1
2
3
4
LitBuO
LitBuO
LiOH
Et3N
THF
DMF
EtOH
THF
1.5
2
72
24
84
71
—
o2
Scheme 2 Preparation and fragmentation of 7a and 7b.
a
b
All reactions were carried out at 50 1C. Isolated yields.
chromatographic analysis of this reaction proved to be futile due
to the use of volatile THF as a solvent. However, a careful GC
analysis of the reaction of 4 with LitBuO in DMF for 2 h at 50 1C
revealed the release of 64 mol% of N2O as a reaction product
along with the isolation of 5 in 71% yield following an aqueous
workup. 1H NMR analysis of an aliquot of this reaction mixture
also revealed the formation of methanol as a product. It is also to
be noted that the potential fragmentation of 4 in the presence of
weak bases was examined and was found to be extremely
sluggish (Table 1, entries 3 and 4) in contrast to that of reactions
with a strong base, such as LitBuO (Table 1, entries 1 and 2).
These results strongly indicate that the treatment of 4 with a
strong base, such as LitBuO, triggers in situ b-elimination of
the protected diazeniumdiolate leading to the formation of the
corresponding imine, N2O and methanol in a near stoichiometric
ratio. We postulate that the acid–base properties of the hydroxyl
group in 4 play a pivotal role in this chemical transformation.
To confirm this, we studied the effect of base on several
other protected diazeniumdiolates with structures analogous
to that of 4. Sodium salts of N-diazeniumdiolated N-benzyl-
ethylamine (6a) and N,N-dibenzylamine (6b) were prepared
following the reported procedure9 and the corresponding
O2-alkylated derivatives were synthesized. Reaction of 7a with
LitBuO in THF was found to be extremely sluggish even after
reflux for 15 h. 1H NMR analysis of the crude reaction mixture
revealed that the majority of the starting material was
unreacted. However, formation of the corresponding imine
8a in only 6% estimated yield was determined by comparing
the 1H NMR spectrum of the crude reaction mixture with that
of the authentic imine obtained commercially. Also, gas
chromatographic examination of the reaction of 7a with
LitBuO in DMF revealed the release of only 5% N2O which
is consistent with the estimated yield of 8a as determined by
1H NMR. A similar outcome was also observed for the NMR
and GC analyses of the base-mediated reaction of 7b, indicating the
formation of imine 8b and N2O in about 2% yield each. Analogs
7a and 7b, being devoid of any free N-hydroxyethyl functionality,
proved to be much less susceptible towards base-induced
fragmentation under identical conditions (Scheme 2).
Scheme 3 Preparation and reaction of 9 in base.
the reactivities of 4 and its above-mentioned analogs 7a, 7b,
and 9 points towards a key role of the N-hydroxyethyl group
in initiating a presumably concerted elimination by abstracting
one of the benzylic protons in 4 via in situ formation of the
potentially six-membered cyclic transition state (11) and leading
to the facile formation of imine 5 (Fig. 2). Based on this tentative
mechanism for the proposed b-elimination reaction, we presume
that the base-mediated reactivity of the diazeniumdiolated
benzylamines with longer N-hydroxyalkyl chains will be
substantially sluggish due to the formation of potentially
unfavorable cyclic transition states.
In summary, we have reported for the first time the preparation
of a unique secondary benzylamine diazeniumdiolate, the
O2-protected prodrugs of which can fragment directly to N2O
via a novel base-mediated b-elimination reaction triggered by
the potential acidity of the benzylic protons. Even though these
caged diazeniumdiolates have not been developed as potential
N2O donors in biological systems, this unexpected finding
should be borne in mind when working with such prodrugs,
especially diazeniumdiolated benzylamine derivatives and other
diazeniumdiolates bearing hydroxyl groups with similarly
situated acidic protons a to the amino nitrogen.
This project has been funded with Federal funds from the
National Cancer Institute, National Institutes of Health,
under contract HHSN261200800001E, and by the Intramural
Accordingly, 9, prepared by the treatment of 3 with iodomethane
in the presence of 15-crown-5 ether, when reacted with LitBuO
in THF at 50 1C, afforded only imine 10, albeit in trace quantity
(Scheme 3). Identification of the product was achieved by
comparing the NMR and HR-MS data of the crude reaction
mixture with an authentic sample of 10.10 The anomaly between
Fig. 2 Mechanism for the formation of imine 5.
c
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Research Program of the NIH, National Cancer Institute,
Center for Cancer Research.
3 K. M. Davies, D. A. Wink, J. E. Saavedra and L. K. Keefer, J. Am.
Chem. Soc., 2001, 123, 5473–5481.
4 K. M. Miranda, T. Katori, C. L. de Holding, L. Thomas,
L. A. Ridnour, W. J. McLendon, S. M. Cologna, A. S. Dutton,
H. C. Champion, D. Mancardi, C. G. Tocchetti, J. E. Saavedra,
L. K. Keefer, K. N. Houk, J. M. Fukuto, D. A. Kass, N. Paolocci
and D. A. Wink, J. Med. Chem., 2005, 48, 8220–8228.
Notes and references
1 (a) J. E. Saavedra, T. R. Billiar, D. L. Williams, Y.-M. Kim,
S. C. Watkins and L. K. Keefer, J. Med. Chem., 1997, 40,
1947–1954; (b) J. E. Saavedra, P. J. Shami, L. Y. Wang,
K. M. Davies, M. N. Booth, M. L. Citro and L. K. Keefer,
J. Med. Chem., 2000, 43, 261–269; (c) X. Wu, X. Tang, M. Xian
and P. G. Wang, Tetrahedron Lett., 2001, 42, 3779–3782;
(d) J. E. Saavedra, A. Srinivasan, C. L. Bonifant, J. Chu,
A. P. Shanklin, J. L. Flippen-Anderson, W. G. Rice,
J. A. Turpin, K. M. Davies and L. K. Keefer, J. Org. Chem.,
2001, 66, 3090–3098.
5 J. A. Hrabie and L. K. Keefer, Chem. Rev., 2002, 102, 1135–1154.
6 L. K. Keefer, ACS Chem. Biol., 2011, 6, 1147–1155.
7 J. E. Saavedra, D. S. Bohle, K. N. Smith, C. George,
J. R. Deschamps, D. Parrish, J. Ivanic, Y. N. Wang, M. L. Citro
and L. K. Keefer, J. Am. Chem. Soc., 2004, 126, 12880–12887.
8 M. Nechab, D. N. Kumar, C. Philouze, C. Einhorn and
J. Einhorn, Angew. Chem., Int. Ed., 2007, 46, 3080–3083.
9 J. Konter, G. E.-D. A. A. Abuo-Rahma, A. El-Eman and
J. Lehmann, Eur. J. Org. Chem., 2007, 616–624.
2 Z. Huang and E. E. Knaus, Org. Lett., 2011, 13, 1178–1181 and
references therein.
10 P. Mangeney, T. Tejero, A. Alexakis, F. Grosjean and J. Normant,
Synthesis, 1988, 3, 255–257.
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