362
V. A. POLYAKOV ET AL.
bonds between various structural units. Two features of
such equilibrium seem to be of particular importance for
the formation of ‘dynamic’ combinatorial pools. First, the
equilibration rates in the studied reactions are low and
negligible under physiological conditions, i.e. at ambient
temperature in neutral aqueous media. As a result, the
components of the pools based on O-substituted oximes
could be isolated as stable compounds and used for
biological testing (O-alkyl oximes and O-alkyloxyamines
have been shown previously to possess biological activity
and low toxicity, 12and may therefore constitute viable
scaffolds for libraries of drug candidates). The evolu-
tionary selection–equilibration method reported by us
previously3 is particularly promising for the use with
these pools since it involves separation of the binding and
‘scrambling’ sites. Thus, the exchange can be turned ‘off’
for selection on fragile biological ligands under physio-
logical conditions and then back ‘on,’ by increasing the
temperature and/or acidic catalysis, to bring the system to
equilibrium.
The second important attribute of the imine exchange
in the studied system is that the competing hydrolysis of
the oximes is minimal or absent under equilibrating
conditions. In that sense, the studied reactions represent a
unique balance between stability and exchangeability and
fit the requirements formulated in the Introduction for
dynamic combinatorial pools. The different properties of
the aliphatic and aromatic aminoxy compounds can be
used for choosing the proper equilibrium reaction
depending upon the selection conditions.
1.51 (d, J = 9 Hz, 2H). The product of the previous
reaction (6 g, 15 mmol) was dispersed in a solution of
hydrazine hydrate (1.75 ml, 36 mmol) in 150 ml of
ethanol and refluxed for 15 h. The reaction mixture was
then acidified with 4 ml of 3 M HCl, refluxed for another
15 min and concentrated in vacuo. The crude product was
washed with boiling EtOAc, filtered and crystallized
from EtOH as the dihydrochloride. Yield, 1.35 g (50%).
1H NMR (500 MHz, DMSO-d6), 10.98 (s, 6H), 4.06 (t,
J = 6.2 Hz, 4H), 1.93 (p, J = 6.2, 2H); 13C NMR
(126 MHz, DMSO-d6); 71.74, 27.08.
Benzaldehyde-O-methyloxime (1), Bis(aryloximes) 3,
and 3-pyridinealdehyde-O-phenyloxime (6) were synthe-
sized in 75–85% yield by stirring the mixtures of the
equivalent amounts of corresponding alkyl- or aryloxy-
amines and aldehydes in pyridine overnight, concentrat-
ing the solution in vacuo followed by crystallization of
1
the products from EtOAc–EtOH. Analytical data. 1: H
NMR (300 MHz, CDCl3), 8.07 (s, 1H), 7.59 (m, 2H),
7.38 (m, 3H), 3.99 (s, 3H); 13C (75 MHz, CDCl3), 148.6,
132.2, 129.8, 128.7, 127.0. 3a: 1H NMR (300 MHz,
DMSO-d6), 8.91 (s, 2H), 8.75 (d, J = 5.0 Hz, 2H), 8.42 (s,
2H), 8.34 (d, J = 8.1 Hz, 2H), 7.73 (q, 2H), 4.30 (t,
J = 6.5 Hz, 4H), 2.12 (p, J = 6.4 Hz, 2H); 13C NMR
(75 MHz, D2O), 151.57, 150.88, 148.69, 146.71, 139.30,
Á 2HCl:
134.72, 79.15, 35.37; Calculate for C15H16N4O2
C, 50.43, H, 5.08, N, 15.68. Found: C, 50.44, H, 5.19, N,
15.27%. 3b: 1H NMR (500 MHz, DMSO-d6), 13.19 (br s,
2H), 8.36 (s, 2H), 8.18 (s, 2H), 7.95 (d, J = 7.6 Hz, 2H),
7.84 (d, J = 7.6 Hz, 2H), 7.53 (t, J = 7.6 Hz, 2H), 4.25 (t,
J = 6.4 Hz, 4H), 2.07 (p, J = 6.4 Hz, 2H); 13C NMR
(126 MHz, DMSO-d6), 168.0, 149.4, 133.6, 132.6, 132.0,
1
131.6, 130.3, 128.7, 71.7, 40.2. 6: H NMR (300 MHz,
EXPERIMENTAL
CD3OD), 8.78 (s, 1H), 8.55 (d, J = 4.8 Hz, 1H), 8.47 (s,
1H), 8.15 (dt, J = 1.8 Hz, 8.1 Hz, 1H), 7.45 (dd,
J = 5.1 Hz, 7.8 Hz, 1H), 7.30 (t, J = 6.9 Hz, 2H), 7.20
(d, J = 7.8 Hz, 2H), 7.02 (t, J = 6.9 Hz, 1H); 13C
(75 MHz, CD3OD), 160.6, 151.7, 150.1, 149.4, 136.1,
130.5, 129.7, 125.6, 123.8, 115.5.
General. All reagents were purchased from Aldrich and
Fluka (O-phenylhydroxylamine) and used without further
purification. NMR spectra were recorded on Varian
Gemini 300 MHz and Unity 500 MHz spectrometers. pD
values presented throughout the paper were obtained by
adding the increment of 0.4 to the reading of a glass
combination electrode in the D2O solutions. Kinetic data
were processed on a Macintosh computer with the aid of
SigmaPlot 4.0 software.
For the synthesis of O-(p-nitrophenyl)hydroxylamine,
a solution of N-hydroxy-5-norbornene-2,3-dicarboxi-
mide (3.07 g, 15 mmol) and anhydrous K2CO3 (2.61 g,
18 mmol) in anhydrous DMF (50 ml) was stirred at room
temperature for 40 min, then 1-fluoro-4-nitrobenzene
(1.6 ml, 15 mmol) was added, and the reaction mixture
was stirred at 50°C overnight. After removing the solvent
in vacuo, the residue was diluted with 100 ml of saturated
NaCl and extracted twice with CHCl3 (350 ml). The
organic layers were washed with brine, dried over
Na2SO4 and, after evaporation of the solvent, yielded
4.16 g (92%) of the intermediate N-(4'-nitrophenyloxy)-
5-norbornene-2,3-dicarboximide. 1H NMR (300 MHz,
CDCl3), 8.23 (d, J = 7.5 Hz, 2H), 7.09 (d, J = 7.5 Hz, 2H),
6.32 (m, 2H), 3.54 (brs 2H), 3.41 (m, 2H), 1.86 (d,
J = 9.0 Hz, 1H), 1.60 (d, J = 9.0 Hz, 1H); 13C NMR
(75 MHz, CDCl3), 166.2, 157.5, 139.9, 130.9, 121.5,
109.8, 47.2, 40.5, 38.8. The solution of the intermediate
Syntheses. 1,3-Diaminoxypropane (4) was synthesized
by a modification of the previously described proce-
dure.13 A solution of N-hydroxy-5-norbornene-2,3-dicar-
boximide (9.94 g, 55 mmol), TEA (20 ml, 143 mmol) and
dichloropropane (2.6 g, 23 mmol) in 50 ml of DMSO was
refluxed for 15 h. After cooling, the reaction mixture was
diluted with ethyl acetate and washed with water and
brine. After drying over Na2SO4, the organic extract was
concentrated and recrystallized from EtOH to yield
7.03 g (77%) of 1,3-diaminoxypropanebis(5-norbor-
1
nene-2,3-dicarboxy)imide. H NMR (300 MHz, CDCl3),
6.19 (s, 4H), 4.17 (t, J = 6.1 Hz, 4H), 3.43 (brs, 4H), 3.19
(m, 4H), 1.98 (p, J = 6.1 Hz, 2H), 1.77 (d, J = 9 Hz, 2H),
Copyright 1999 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 12, 357–363 (1999)