2-Ureido-4[1H]-pyrimidinone Dimers
J. Am. Chem. Soc., Vol. 122, No. 31, 2000 7489
mg, 0.24 mmol), and DMAP (10 mg, 0.02 mmol) in dry dichlo-
romethane (10 mL) at 0 °C under an argon atmosphere was slowly
added diisopropylcarbodiimide (36.7 mg, 0.29 mmol) . The solution
was allowed to slowly warm to room temperature and stirred for 25 h.
The crude reaction mixture was concentrated in vacuo. The product
was purified by column chromatography (SiO2, gradient CH2Cl2/MeOH
99.5/0.5 f 95/5, Rf (95/5) ) 0.38) and recrystallization from ethyl
acetate, yielding 80 mg (49%) of pure 1b as a fine white powder. Mp
105.5 °C 1H NMR (400 MHz, CDCl3): δ 0.89 (t, 3H, CH2CH3), 1.26-
1.67 (br m, 30H), 2.20 (m, 2H, CH2CH2Pyrene), 2.36 (t, 2H, CH2-
Pyrene), 2.47 (t, 2H, C(6)CH2), 3.26 (q, 2H, NHCH2), 3.39 (t, 2H,
CH2OC(O)), 4.14 (t, 2H, OC(O)CH2), 5.76 (s, 1H, C(5)H), 7.28-8.31
(br m, 9H, Pyrene), 10.19 (t, 1H, CH2NH), 11.73 (s, 1H, C(2)NH),
13.06 (s, 1H, N(1)H). Anal. Calcd for C44H58N4O4: C 74.75, H 8.27,
N 7.92. Found: C 75.0, H 8.5, N 7.9.
2-(2,6-Diispropylphenylureido)-5,6-dimethyl-4[1H]-pyrimidino-
ne (1e). To a solution of di-tert-butyldicarbonate (0.88 g, 4.0 mmol)
in DMSO (3 mL) was added DMAP (60 mg) and 2,6-diisopropylaniline
(0.70 g, 3.9 mmol). The solution immediately showed gas evolution
(CO2) indicating the formation of an isocyanate.41 The mixture was
slowly heated to 70 °C in 15 min under argon atmosphere. A sonicated
dispersion of 5,6-dimethylisocytosine (0.50 g, 3.6 mmol) in DMSO
was added and the solution was stirred for 21 h at 70 °C. The reaction
mixture was then added to 75 mL of CHCl3 and extracted with water
(3 × 50 mL). The organic phase was dried over MgSO4 and reduced
in vacuo. The remaining solids were then recrystallized from acetic
acid to afford 390 mg (32%) of 1e as a fine white microcrystalline
powder, mp > 340 °C (degradation). 1H NMR (400 MHz, CDCl3): δ
1.20 (d, 6H, CH(CH3)), 1.25 (d, 6H, CH(CH3)), 1.87 (s, 3H,C(5)CH3),
2.17 (s, 3H, C(6)CH3), 3.19 (m, 2H, CH(CH3)), 7.20 (d, 2H, m-Ph),
7.30 (t, 1H, p-Ph), 11.76 (s, 1H, NHPh), 12.56 (s, 1H, NHC(O)NHPh),
12.94 (s, 1H, N(1)H). 13C NMR (100 MHz, CDCl3): δ 10.6 (C(5)-
CH3), 17.1 (C(6)CH3), 23.1 (CH(CH3)2), 24.2 (CH(CH3)2), 28.6 (CH-
(CH3)2), 114.0 (C(5)), 123.3 (p-Ph), 128.0 (m-Ph), 131.1 (C(6)), 142.6
(ipso-Ph), 146.6 (o-Ph), 153.2 (C(2)), 156.3 (NHC(O)NH), 172.4 (C(4)).
Anal. Calcd for C19H26N4O2: C 66.64, H 7.65, N 16.36. Found: C
66.4, H 7.8, N 16.4.
Scheme 2
measurements were performed on a Bruker AM 400 spectrometer with
a 5 mm QNP probe at 303 K. Temperature-dependent EXSY exchange
NMR measurements and all DPFGSE measurements were performed
on a Bruker AVANCE DRX 600 spectrometer with a 5 mm TXI probe.
For the DPFGSE the Bruker pulse sequence selnogp.3 was used with
a ReBurp selective refocusing pulse, which was calibrated manually
for optimal performance. Both homodimer and heterodimer peaks were
selectively refocused and yielded the same value for kd, within the error
of the measurement. For measurements in CDCl3, the methyl peak was
used, and for measurement in octadeuteriotoluene, the alkylidene proton
signal was used because of interference of the solvent signal with the
methyl peaks. The EXSY data were evaluated using the response
function φ ) ln(r + 1/r - 1) where r ) (I11 + I22)/(I12 + I21), the ratio
of diagonal- and cross-peak intensities.30,31 These data were fitted using
a linear least-squares fit to the kinetic equation φ ) kextmix. T1 relaxation
times in the DPFGSE experiments were determined according to
Dahlquist.34 The DPFGSE intensities were fitted, with a nonlinear
algorithm, to the response functions33 A ) {(1 + R)A0/(kAX + kXA)}-
{kXA exp(-Rt) + kAX exp(-(R + kAX + kXA)t)} and X ) {(1 + R)-
A0/(kAX + kXA)}kAX{exp(-Rt) - exp(-(R + kAX + kXA)t)}, with A
being the refocused peak (O, homodimer) and X being the dispersed
peak (0, heterodimer), R the inversion efficiency, A0 the equilibrium
magnetization, and R ) 1/T1. The toluene data were fitted to the
functions A and X simultaneously using Matlab; a r2 value could not
be determined.
Results
Synthesis. The syntheses of the ureidopyrimidinone molecules
depicted in Scheme 1 are outlined below. 6-Isocyanatohexanol was
prepared according to a procedure described by Versteegen39 and
Peerlings.40 6-Tridecylisocytosine was also synthesized according to
literature procedures,14 5,6-dimethylisocytosine, n-butyl-1-ureido-4[1H]-
pyrimidinone (1c), and 2-phenylureido-4[1H]-pyrimidinone (1d) were
synthesized previously.14
Fluorescence. Fluorescence spectroscopy enables the detec-
tion of strongly fluorescent species at extremely low concentra-
tions. Model compound system 1b was devised which, aside
from the 2-ureido-4[1H]-pyrimidinone moiety, also contains a
fluorescent pyrene group and a spacer of sufficient length to
form pyrene excimers within the dimer.
To ascertain that, in the concentration range of interest (10-9
-
2(6-Hydroxyhexylureido)-6-tridecyl-4[1H]-pyrimidinone (1a). To
a solution of di-tert-butyltricarbonate (5.77 g, 22 mmol) in dry
dichloromethane (25 mL) was added a solution of 6-aminohexanol (2.34
g, 20 mmol) in dry dichloromethane (40 mL) at room temperature under
argon atmosphere. The reaction mixture showed immediate gas
evolution (CO2) indicating the formation of the isocyanate.40 Stirring
was continued for 1 h. This reaction mixture was slowly added to a
solution of 6-tridecylisocytosine in dry DMF at 90 °C under argon
atmosphere. The reaction mixture was stirred for 4 h, after which it
was cooled in an ice bath. The precipitate, which was formed, was
filtered off, washed with hexane, and recrystallized from EtOH affording
5.05 g (52%) of crude product. A sample was purified using column
chromatography (90 g SiO2, CH2Cl2/MeOH 93/7, Rf ) 0.3) and
subsequently recrystallized from MeOH to yield pure product. Mp 108.5
10-5 M), excimer fluorescence only occurs within dimers,
fluorescence was measured at a concentration of 10-5 M in the
presence of a 1000-fold excess of n-butyl-1-ureido-4[1H]-
pyrimidinone (1c) in chloroform. This reduces the concentration
of 1b‚1b dimers to a negligible amount, preventing intra-dimer
excimer formation. In this case no excimer emission was
observed, while the monomer emission increased with respect
to a pure 10-5 M solution of pure 1b.
For determination of the Kdim of 1b, fluorescence spectra were
measured in the concentration range from 10-9 to 10-6 M in
chloroform, in toluene, and in chloroform saturated with water.
The excimer band (λmax ) 478 nm) was integrated from 500 to
600 nm, to exclude as much of the monomer emission bands
(λmax ) 372-399 nm) as possible. The resulting integrated
fluorescence emission intensities, normalized for concentration,
are depicted in Figure 1.
From these data, Kdim was determined to be (5.7 ( 0.6) ×
107 M-1 (r2 ) 0.992) for chloroform, (1.0 ( 0.1) × 107 M-1
(r2 ) 0.995) for wet chloroform, and (5.9 ( 0.7) × 108 M-1
1
°C. H NMR (400 MHz, CDCl3): δ 0.90 (t, 3H, CH2CH3), 1.2-1.8
(br m, 30H), 2.28 (t, 1H, OH), 2.48 (t, 2H, C(6)CH2), 3.30 (q, 2H,
NHCH2), 3.66 (q, 2H, CH2OH), 5.88 (s, 1H, C(5)H), 10.14 (t, 1H,
NHCH2), 11.90 (s, 1H, C(2)NH), 13.23 (s, 1H, N(1)H).
2-(4-(1-Pyrenyl)-1-oxo-butyloxy)hexylureido)-6-tridecyl-4[1H]-
pyrimidinone (1b). To a solution of 2, 4-(1-pyrenyl)butyric acid (68.4
(39) Versteegen, R. M.; Sijbesma, R. P.; Meijer, E. W. Angew. Chem.,
Int. Ed. Engl. 1999, 38, 2917-2918.
(40) Peerlings, H. W. I.; Meijer, E. W. Tetrahedron Lett. 1999, 40, 1021-
1024.
(41) Knolker, H.-J.; Braxmeier, T.; Schlechtingen, G. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 2497-2499.