A facile synthesis of fluorophores based on 5-phenylethynyluracils

Article abstract of DOI:10.1055/s-2006-948176

Compact, fluorescent uracil aglycones and derivatives suitable for incorporation into the oligonucleotide mimic peptide nucleic acid (PNA) have been prepared by Sonogashira/Castro-Stephens coupling to monosubstituted phenylacetylenes. Cyclic 6-(phenyl)furo[2,3-d]pyrimidin-2(3H)-ones were accessed by the Ag⁺-catalyzed cyclization of the 5-alkynyluracil precursors. Although this reaction was sluggish, it gave quantitative chemical yields. Electron-rich alkynes, such as p-methoxyphenylethyne, cyclize much more rapidly than electron-deficient alkynes. Adjustment of the reaction conditions permitted the synthesis of p-nitrophenylfuranouracil in excellent yield. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-948176

LETTER
2997
A Facile Synthesis of Fluorophores Based on 5-Phenylethynyluracils
S
ynthesis of Fluo
o
rescent 5-P
h
b
enylethynylu
e
racils rt H. E. Hudson,* Joanne M. Moszynski
Department of Chemistry, The University of Western Ontario, London, ON, N6A 5B7, Canada
Fax +1(519)6613022; E-mail: robert.hudson@uwo.ca
Received 23 March 2006
R'
H
H
R'
Abstract: Compact, fluorescent uracil aglycones and derivatives
N
H
N
O
N
O
N
O
H
N
suitable for incorporation into the oligonucleotide mimic peptide
nucleic acid (PNA) have been prepared by Sonogashira/Castro–
Stephens coupling to monosubstituted phenylacetylenes. Cyclic 6-
(phenyl)furo[2,3-d]pyrimidin-2(3H)-ones were accessed by the
Ag+-catalyzed cyclization of the 5-alkynyluracil precursors. Al-
though this reaction was sluggish, it gave quantitative chemical
yields. Electron-rich alkynes, such as p-methoxyphenylethyne,
cyclize much more rapidly than electron-deficient alkynes. Adjust-
ment of the reaction conditions permitted the synthesis of p-nitro-
phenylfuranouracil in excellent yield.
N
N
X
N
H
R
R
N
N
N
N
N
R
N
O
R
Figure 1 Base-pairing ability of 5-yne U (left) but not fU (right).
pairing capacity for adenine, unlike furanouracil as illus-
trated in Figure 1.6
Furthermore, the 5-phenylethynyluracils are intrinsically
luminescent – neither the phenylacetylene nor the hetero-
cycle possess any appreciable fluorescence, but once cou-
pled together the molecule is fluorescent.3,7,8 N1-
Unsubstituted uracils have potential use as selective small
molecule probes for adenine9 or may be substrates for gly-
cosylation reactions to prepare unusual nucleosides,2,10
whereas the N1-alkylated uracils are suitable for use in
PNA synthesis.11
Key words: annulations, silver(I)-catalyzed cyclization, furano-
uracils, 5-phenylethynyluracil, nucleobases
There exists a great variety of pyrimidine heterocycles
and pyrimidine nucleosides bearing substitution at the 5-
position, many of which have found some practical appli-
cation. Our particular interest has been in 5-alkynyluracils
(5-yne U) and their use as an intermediate towards the flu-
orescent furo[2,3-d]pyrimidin-2(3H)-ones (furanouracil, The 5-phenylethynyluracils and furanouracils were pre-
fU; Scheme 1), which may also serve as a precursor to pared according to Scheme 2. Uracil was iodinated to
pyrrolocytosine (pC).1
yield 5-iodouracil (1) as previously reported.12,13 Com-
pound 1 was then cross-coupled to various para-substitut-
ed phenylacetylenes using standard Sonogashira-based
conditions5 to yield the 5-phenylethynyluracils 3a–c that
are unsubstituted at the N1-position.14 Alternatively, 1 can
be alkylated using ethyl bromoacetate yielding 2,15 which
underwent analogous cross-coupling with the same
phenylacetylenes to yield the N1-substituted 5-phenyl-
ethynyluracils 4a–c.
R'
R'
R'
O
O
NH
NH
N
N
N
R
O
N
O
N
R
O
R = H, alkyl, sugar
R
5-yne U
fU
pC
Scheme 1
(p-X)-Ph
O
O
(p-X)-Ph
O
N1-Unsubstituted 5-ethynyluracils (R = H, Scheme 1)
have been a synthetic target for some time and have been
approached by the Corey–Fuchs reaction with 5-formyl-
uracil,2 or by metal-catalyzed coupling reactions with 5-
iodouracil.3 More recent work, especially with the
nucleoside, has shown the utility and reliability of the
Sonogashira/Castro–Stephens reaction.4,5 However, this
reaction has given more variable results with 5-iodouracil
as a substrate. In this letter, we report the synthesis of N1-
unsubstituted and N1-alkylated 5-phenylethynyluracils
and their efficient Ag(I)-catalyzed cyclization to furanou-
racils. We have found that simple 5-phenylethynyluracils
are fluorescent and are expected to maintain their base-
I
N
NH
NH
iii
ii
N
R
O
N
R
O
N
R
O
5-yne U
3a–c: R = H
4a–c: R = CH2CO2Et
fU
5a–c: R = H
1: R = H
2: R = CH2CO2Et
i
6a–c: R = CH2CO2Et
Scheme 2 Reagents and conditons: i) BrCH2CO2Et, DMF, K2CO3;
ii) alkyne, Pd(PPh3)4, CuI, Et3N, DMF; iii) AgNO3, acetone or
Ag2SO4, 1:1 dioxane–H2O (5c).
The cross-coupled products were achieved in very good
yields after purification as reported in Table 1.
The few existing examples of the synthesis of 5-alkynyl
N1-unsubstituted uracils in the literature indicate the
reaction may be unreliable.3 In this work, it is clear that
SYNLETT 2006, No. 18, pp 2997–3000
Advanced online publication: 04.08.2006
DOI: 10.1055/s-2006-948176; Art ID: S03306ST
© Georg Thieme Verlag Stuttgart · New York
0
3
.1
1
.2
0
0
6

2998
R. H. E. Hudson, J. M. Moszynski
LETTER
Table 1 Cross-Coupling of p-Substituted Phenylacetylenes
compounds is reflected in the recovered yield (Table 2).
The primary challenge to their purification is the water-
solubility of these compounds, which complicates catalyst
removal. Although various procedures have been attempt-
ed in order to remedy this problem, we have not yet dis-
covered an optimal method of isolation/purification for
the relatively water-soluble compounds. The small
amount of silver catalyst only needs to be rigorously re-
moved if it will interfere with subsequent chemistry,
which was found to be the case for the saponification of
compounds 6a,b.
Compound R
p-X
H
Yield (%)
3a
3b
3c
4a
4b
4c
H
88
91
86
83
87
78
H
OCH3
NO2
H
H
CH2CO2Et
CH2CO2Et
CH2CO2Et
OCH3
NO2
Under the conditions employed, for all compounds, the
cyclization is much slower than the cross-coupling reac-
tion. The N1-alkylated compounds (4a–c) tend to react
more quickly than their unsubstituted analogues (3a–c),
while electron-rich compounds such as 6b (48 h, r.t.) react
more quickly than electron-poor compounds (5c, >14 d,
reflux), which is probably related to the donicity of the
alkyne towards the Lewis acid. However, in all cases the
cyclization is very clean, and with patience, will proceed
to complete consumption of the starting material. The p-
nitrophenylethynyl-substituted uracils reacted the most
slowly and were unsatisfactory substrates under the
standard reaction conditions as reported by Agrofoglio.18
Under refluxing acetone conditions the cyclization was
80% complete after 14 days. Clearly this is not a very
practical reaction, and it was found that quantitative
cyclization could be achieved in 1:1 water–1,4-dioxane
using a stoichiometric amount of Ag2SO4 at 50 °C in six
days. The pure product was conveniently isolated by
dilution of the reaction mixture with water, which caused
precipitation of the product that led to an excellent re-
covered yield of the final product.
phenylacetylenes react cleanly to give the cross-coupled
products free from contamination of the cyclized product,
under gentle thermal conditions.3b The N1-alkylated
uracils behave well in the cross-coupling reaction as ex-
pected by analogy to the wealth of examples done on the
nucleoside. For both substrate classes, the cross-coupling
is completed within 16 hours.
With the 5-phenylethynyluracils in hand, we wished to
synthesize the corresponding furanouracils by promoting
a 5-endo-dig cyclization reaction (Scheme 2, iii). While
previous authors have reported the one-step cyclization of
halouracils to furanouracils,4a,16 and while we have
observed this in our own research with derivatives of cy-
tosine,17 we have experienced much more difficulty with
uracil-based substrates. The higher temperatures required
for this reaction appeared to facilitate undesirable side re-
actions, which often led to the formation of an intractable
solid that resisted characterization. Therefore, we chose to
use a two-step process: first cross-coupling; followed by
the use of the cyclization conditions recently reported by
Agrofoglio.18 This procedure was found to be very
successful in the case of the N1-substituted 5-phenyl-
ethynyluracils (4a–c) giving the furanouracils (6a–c) in
very good yield (Table 2).19
Both the 5-phenylethynyluracils 3a–c, 4a–c and cyclized
furanouracils 5a–c, 6a–c exhibited substitution-depen-
dent fluorescent properties as reported in Table 3.22 In
general, the trends are similar to the spectral properties
observed for pyrrolocytosines,6,17 yet with some subtle
differences. N1-Substitution benefits the intensity of fluo-
rescence as does annulation to furanouracil from the 5-
alkynyluracil such that N1-substituted > N1-unsubstituted
and cross-coupled vs. cyclized (fU > 5-yne U). The nature
of the para substituent also affects the fluorescence such
that a greater red shift through the series NO2 >> OCH3 >
H was observed. However, there was no consistent trend
for the effect of these substituents on the fluorescence
intensity. In addition, although the nitro-substituted com-
pounds were visibly fluorescent in solution or when
deposited on silica, emitting orange light when illuminat-
ed at 360 nm, their emission spectra showed a very broad
low-intensity band. The weakness of the fluorescence for
these particular compounds may be limiting in their po-
tential as a reporter group.
For cyclization of the N1-unsubstituted compounds (3a–c
→ 5a–c), while the reaction itself is quantitative as judged
by examining the 1H NMR spectrum or by TLC analysis
of the crude sample,20,21 the difficulty in purifying these
Table 2 AgNO3-Catalyzed Cyclization of 5-Alkynyluracils
Compound
R
p-X
H
Yield (%)
5a
5b
5c
6a
6b
6c
H
37a
42a
88
80
84
85
H
OCH3
NO2
H
H
CH2CO2Et
CH2CO2Et
CH2CO2Et
OCH3
NO2
In this report, we have described the synthesis of simple,
5-phenylethynyluracils that exhibit substituent-dependent
fluorescence and their conversion to phenyl-substituted
bicyclic furanouracils by treatment with silver(I) ion.18
Notably, the latter reaction may be done in an aqueous–
a These compounds have better solubility in water compared to the
N1-substituted heterocycles (6a–c) and the p-nitrophenylethynyl-
uracil (5c) and losses during aqueous–organic extractive work-up
contributed, in part, to the diminished yields.
Synlett 2006, No. 18, 2997–3000 © Thieme Stuttgart · New York

LETTER
Synthesis of Fluorescent 5-Phenylethynyluracils
2999
(5) (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron
Lett. 1975, 4467. (b) Stephens, R. D.; Castro, C. E. J. Org.
Chem. 1963, 28, 3313.
(6) A preliminary report has appeared in conference
proceedings: Hudson, R. H. E.; Dambenieks, A. K.;
Moszynski, J. M. Proc. SPIE – Int. Soc. Optical Eng. 2005,
103.
Table 3 Spectral Properties of 5-Yne Us and fUs
Compound ea (260 nm)
l
excitation (nm) lemission (nm) Relative
intensityb
3a
3b
3c
4a
4b
4c
5a
5b
5c
6a
6b
6c
1650
2840
3050
6040
4310
4920
1610
1960
3320
6310
4890
1790
330
332
435
349
325
400
342
350
420
352
360
452
410
432
520
425
436
525
420
460
530
427
449
520
1.5
2.7
NAc
1.0
4.6
NAc
23
(7) Although 5-phenylethynyluracils (ref. 3), 5-phenylethynyl-
uridine and 5-phenylethynyl-2¢-dexoyuridine have been
previously synthesized no description of their luminescent
properties has appeared: (a) Rai, D.; Johar, M.; Manning, T.;
Agrawal, B.; Kunimoto, D. Y.; Kumar, R. J. Med. Chem.
2005, 48, 7012. (b) Angell, A.; McGuigan, C.; Garcia
Sevillano, L.; Snoeck, R.; Andrei, G.; De Clercq, E.;
Balzarini, J. Bioorg. Med. Chem. Lett. 2004, 14, 2397.
(c) Aucagne, V.; Berteina-Raboin, S.; Guenot, P.;
Agrofoglio, L. A. J. Comb. Chem. 2004, 6, 717. (d) The
fluorescence of 5-tolylethynyl-2¢-dexoyuridine has been
recognized: Esho, N.; Davies, B.; Lee, J.; Dembinski, R.
Chem. Commun. 2002, 332.
(8) Intrinsically fluorescent uracils have been noted for
substantially larger ethynyl-linked aromatic chromophores:
(a) Hurley, D. J.; Seaman, S. E.; Mazura, J. C.; Tor, Y. Org.
Lett. 2002, 4, 2305. (b) Thoresen, L. H.; Jiao, G.-S.;
Haaland, W. C.; Metzker, M. L.; Burgess, K. Chem. Eur. J.
2003, 9, 4603.
15
NAc
67
118
NAc
a Molar absorptivity values (l mol–1 cm–1) measured at the standard
wavelength for nucleobases.
(9) Yoshimoto, K.; Xu, C.-Y.; Nishizawa, S.; Haga, T.; Satake,
H.; Teramae, N. Chem. Commun. 2003, 2960.
b Determined by the ratio of arbitrary fluorescence intensity (counts
per second) in degassed MeOH measured at the emission maxima.
c NA = not available. A relative comparison cannot be made reliably
because of the large difference in excitation wavelength and concen-
tration of compound required.
(10) (a) Barr, P. J.; Jones, S.; Serafinowski, P.; Walker, R. T. J.
Chem. Soc., Perkin Trans. 1 1978, 1263. (b) Valdivia, V.;
Hernandez, A.; Rivera, A.; Sartillo, F.; Loukaci, A.; Fourrey,
J.-L.; Quintero, L. Tetrahedron Lett. 2005, 46, 6511.
(11) Dueholm, K. L.; Egholm, M.; Behrens, C.; Christensen, L.;
Hansen, H. F.; Vulpius, T.; Petersen, K. H.; Berg, R. H.;
Nielsen, P. E.; Buchardt, O. J. Org. Chem. 1994, 59, 5767.
(12) Asakura, J.-I.; Robins, M. J. J. Org. Chem. 1990, 55, 4928.
(13) Compound 1: uracil (134 mmol, 15.0 g) was suspended in
800 mL of MeOH. Then, I2 (81 mmol, 20.4 g) and CAN (67
mmol, 36.7 g) were added. The reaction was refluxed for 16
h, cooled, and a white precipitate was collected by filtration.
The filtrate was evaporated, and then co-evaporated with 2:1
EtOH–H2O to remove iodine (2 × 90 mL). Both product
fractions were then combined and recrystallized from 1:1
EtOH–H2O to obtain 30.6 g (89% yield) of 2. Translucent
needles; mp >280 °C (dec.). HRMS (EI): m/z calcd for
C4H3IN2O2: 237.9239; found: 237.9246. 1H NMR (DMSO-
d6): 11.41 (s, 1 H), 11.17 (s, 1 H), 7.88 (s, 1 H).
organic solvent mixture which permits greater loading of
the Lewis acid promoter and shorter reaction times for
water-soluble substrates.
Acknowledgment
The authors wish to thank the Natural Sciences and Engineering Re-
search Council of Canada (RHEH) and the Faculty of Graduate Stu-
dies and the Department of Chemistry at Western (JMM) for
funding this work. We are also indebted to Professor M. Workentin
for helpful discussion.
(14) General Procedure for Cross-Coupling.
5-Iodouracil compound 1 or 2 (1 mmol) was stirred in
anhydrous DMF (3 mL). Then, CuI (0.2 mmol) was added
and the solvent was deoxygenated. Pd(PPh3)4 (0.1 mmol),
Et3N (2 mmol) and alkyne (1.5–3 mmol) were added
sequentially. The reaction mixture was then stirred overnight
at r.t. The completed reaction was extracted with a sat.
EDTA solution against CH2Cl2, evaporated, and dried under
vacuum. The residue was then suspended in a minimum
amount of CH2Cl2 and precipitated with Et2O. Silica gel
chromatography using gradient elution with MeOH–CH2Cl2
mixtures was used to purify the 5-alkynyluracils 3a–c and
4a–c. All 1H NMR spectra were recorded at 400.09 MHz in
DMSO-d6.
References and Notes
(1) (a) Woo, J.; Meyer, R. B. Jr.; Gamper, H. B. Nucleic Acids
Res. 1996, 24, 2470. (b) Berry, D. A.; Jung, K.-Y.; Wise, D.
S.; Sercel, A. D.; Pearson, W. H.; Mackie, H.; Randolph, J.
B.; Somers, R. L. Tetrahedron Lett. 2004, 45, 2457.
(2) Synthesis of 5-ethynyluracil was achieved in 36% yield:
Perman, J.; Sharma, R. A.; Bobek, M. Tetrahedron Lett.
1976, 28, 2427.
(3) (a) One example: 5-(o-aminophenylethynyl)uracil, 90%.
See: Arcadi, A.; Cacchi, S.; Marinelli, F. Tetrahedron Lett.
1989, 30, 2581. (b) One example: 5-(phenylethynyl)uracil,
61%. See: Farina, V.; Hauck, S. I. Synlett 1991, 157. (c)
Two examples, failed thermal reaction but 65% of 5-
(phenylethynyl)uracil under microwave irradiation. See:
Petricci, E.; Radi, M.; Corelli, F.; Botta, M. Tetrahedron
Lett. 2003, 44, 9181.
Compound 3a: white solid; elution at 8% MeOH–CH2Cl2;
mp >280 °C (dec.). HRMS (EI): m/z calcd for C12H8N2O2:
212.0586; found: 212.0594. 1H NMR: d = 11.41 (br s, 1 H)
overlapping with 11.36 (br s, 1 H), 7.88 (s, 1 H), 7.43–7.47
(m, 2 H), 7.37–7.41 (m, 3 H).
(4) (a) Robins, M. J.; Barr, P. J. Tetrahedron Lett. 1981, 22,
421. (b) Hudson, R. H. E.; Li, G.; Tse, J. Tetrahedron Lett.
2002, 43, 1381.
Synlett 2006, No. 18, 2997–3000 © Thieme Stuttgart · New York

3000
R. H. E. Hudson, J. M. Moszynski
LETTER
Compound 3b: white solid; elution at 15% MeOH; mp >270
°C (dec.). HRMS (EI): m/z calcd for C13H10N2O3: 242.0691;
found: 242.0695. 1H NMR: d = 11.40 (s, 1 H), 11.31 (br s, 1
H), 7.84 (s, 1 H), 7.37 (d, 2 H, J = 8.8 Hz), 6.96 (d, 2 H,
J = 8.9 Hz), 3.77 (s, 3 H).
Compound 3c: light orange solid; elution from 15–30%
MeOH; mp 282–286 °C (dec.). HRMS (EI): m/z calcd for
C12H7N3O4: 257.0437; found: 257.0440. 1H NMR: d = 11.54
(br s, 1 H), 11.51 (s, 1 H), 8.24 (d, 2 H, J = 9.1 Hz), 8.03 (s,
1 H), 7.68 (d, 2 H, J = 8.9 Hz).
Compound 4a: white crystals; elution from silica at 2%
MeOH; mp 193–196 °C. HRMS (EI): m/z calcd for
C16H14N2O4: 298.0954; found: 298.0498. 1H NMR:
d = 11.85 (s, 1 H), 8.55 (s, 1 H), 7.45–7.48 (m, 2 H), 7.40–
7.43 (m, 3 H), 4.55 (s, 2 H), 4.17 (q, 2 H, J = 7.0 Hz), 1.21
(t, 3 H, J = 7.0 Hz).
Compound 4b: white solid; elution at 2.5% MeOH–CH2Cl2;
mp 196–197 °C. HRMS (EI): m/z calcd for C17H16N2O5:
328.1059; found: 328.1068. 1H NMR: d = 11.82 (s, 1 H),
8.14 (s, 1 H), 7.40 (d, 2 H, J = 8.8 Hz), 6.96 (d, 2 H, J = 8.6
Hz), 4.54 (s, 2 H), 4.15 (q, 2 H, J = 7.1 Hz), 3.77 (s, 3 H),
1.20 (t, 3 H, J = 7.2 Hz).
losses were encountered during the work-up and
purification. Chromatography on an elemental sulfur
column or through a plug of EDTA·2Na+ was performed, but
neither adsorbed the catalyst. In situ generation of Cl– by
addition of chlorotrimethylsilane, or addition of brine was
unsuccessful for removal of silver ion. The reaction mixture
was subsequently purified by silica gel chromatography
(elution at 30–40% MeOH), with some of the very polar
compound remaining on the column, thereby reducing the
yield. Pale yellow solid; mp >285 °C (dec.). HRMS (EI):
m/z calcd for C13H10N2O3: 242.0691; found: 242.0693. 1H
NMR: d = 8.28 (s, 1 H), 7.75 (d, 2 H, J = 8.9 Hz), 7.05 (d, 2
H, J = 9.1 Hz), 7.04 (s, 1 H), 3.81 (s, 3 H).
Compound 5c: AgNO3 (15 mol%), reflux(acetone), 14 d,
80% yield. Alternatively, Ag2SO4 (1 equiv), 1:1 dioxane–
water, 50 °C, 6 d, isolated by precipitation caused by the
addition of one volume of H2O: 88% yield. Yellow-orange
solid; mp >320 °C (dec.). 1H NMR: d = 8.52 (s, 1 H), 8.32
(d, 2 H, J = 9.1 Hz), 8.07 (d, 2 H, J = 9.4 Hz), 7.57 (s, 1 H).
Note, the exchangeable proton was not observed. HRMS
(EI): m/z calcd for C17H16N2O5: 257.0437; found: 257.0445.
Compound 6a: AgNO3 (5 mol%), 60 h. 1H NMR: d = 8.65
(s, 1 H), 7.84 (d, 2 H, J = 7.1 Hz), 7.51 (t, 2 H, J = 7.1 Hz),
7.44 (t, 1 H, J = 7.3 Hz), 7.37 (s, 1 H), 4.79 (s, 2 H), 4.16 (q,
2 H, J = 7.1 Hz), 1.21 (t, 3 H, J = 7.1 Hz). White solid; mp
>310 °C (dec.). HRMS (EI): m/z calcd for C16H14N2O4:
298.0954; found: 298.1023.
Compound 6b: AgNO3 (5 mol%), 48 h. Off-white powder;
mp >300 °C (dec.). HRMS (EI): m/z calcd for C17H16N2O5:
328.1059; found: 328.1069. 1H NMR: d = 8.56 (s, 1 H), 7.76
(d, 2 H, J = 8.6 Hz), 7.17 (s, 1 H), 7.05 (d, 2 H, J = 8.6 Hz),
4.77 (s, 2 H), 4.17 (q, 2 H, J = 7.0 Hz), 3.81 (s, 3 H), 1.21 (t,
3 H, J = 7.0 Hz).
Compound 6c: AgNO3 (10 mol%), 8 d. Orange powder; mp
310–314 °C (dec.). HRMS (EI): m/z calcd for C16H13N3O6:
343.0804; found: 343.0799. 1H NMR: d = 8.79 (s, 1 H), 8.34
(d, 2 H, J = 9.1 Hz), 8.08 (d, 2 H, J = 8.9 Hz), 7.69 (s, 1 H),
4.81 (s, 2 H), 4.17 (q, 2 H, J = 7.1 Hz), 1.22 (t, 3 H, J = 7.1
Hz).
Compound 4c: pale yellow solid; elution at 3% MeOH–
CH2Cl2; mp >245 °C (dec.). HRMS (EI): m/z calcd for
C16H13N3O6: 343.0804; found: 343.0810. 1H NMR:
d = 11.95 (s, 1 H), 8.32 (s, 1 H), 8.25 (d, 2 H, J = 8.9 Hz),
7.71 (d, 2 H, J = 8.8 Hz), 4.57 (s, 2 H), 4.17 (q, 2 H, J = 7.0
Hz), 1.21 (t, 3 H, J = 7.1 Hz).
(15) Compound 2: 1 (63.3 mmol, 15 g) was suspended in 100 mL
DMF, to which anhyd K2CO3 (63.0 mmol, 8.7 g) was added.
Then, BrCH2CO2Et (63.1 mmol, 7.0 mL) was added
dropwise under N2. The reaction was stirred for 16 h, filtered
to remove salts and then the solvent was evaporated. The
residue was then cooled in an ice bath and acidified with
4 M HCl (65 mL). The resulting precipitate was then filtered,
washed with Et2O and dried. The crude product was
recrystallized from 1:1 EtOH–H2O to produce 10.6 g (85%
yield) of pure product. White crystals; mp 170–172 °C.
HRMS (EI): m/z calcd for C8H9IN2O4: 323.9607; found:
323.9724. 1H NMR: d = 11.82 (s, 1 H), 8.20 (s, 1 H), 4.49 (s,
2 H), 4.13 (q, 2 H, J = 7.1 Hz), 1.19 (t, 3 H, J = 7.1 Hz).
(16) Robins, M. J.; Barr, P. J. J. Org. Chem. 1983, 48, 1854.
(17) Hudson, R. H. E.; Dambenieks, A. K.; Viirre, R. D. Synlett
2004, 1400.
(20) The conversion of 3a–c → 4a–c was conveniently
monitored in the 1H NMR spectra by observation of the
appearance of the characteristic signal for the proton on the
furan ring [d(ppm) in DMSO-d6: 5a: 7.22; 5b: 7.04; 5c: 7.57;
6a: 7.37; 6b: 7.17; 6c: 7.69], disappearance of the resonance
corresponding to the N3-imino proton (ca. 11.4–11.8 ppm)
and a uniform downfield shift for the resonances associated
with the phenyl ring and H6 of the uracil ring.
(18) Aucagne, V.; Amblard, F.; Agrofoglio, L. A. Synlett 2004,
2406.
(19) General Procedure for Annulation.
The 5-alkynyluracil was suspended in deoxygenated
acetone, and 5 mol% AgNO3 was added. While the original
procedure called for 0.047 mmol of alkyne and 0.009 mmol
catalyst; scale-up was found to be effective up to 2 mmol of
starting alkynyl-uracil. The reaction mixture was stirred in
the dark for >24 h and monitored by TLC (5–15% MeOH–
CH2Cl2) until completion reaction. The completed reaction
was washed with H2O against CH2Cl2, evaporated and dried
under vacuum to provide the phenylfuranouracils 5a–c and
6a–c. As the N1-unsubstituted furanouracils 5a–c are quite
water-soluble, silver nitrate removal caused a significant
loss of product for these cases, accounting for the lower
yields. All 1H NMR spectra were recorded at 400.09 MHz in
DMSO-d6.
(21) McGuigan, C.; Brancale, A.; Barucki, H.; Srinivasan, S.;
Jones, G.; Pathirana, R.; Carangio, A.; Blewett, S.; Luoni,
G.; Bidet, O.; Jukes, A.; Jarvis, C.; Andrei, G.; Snoeck, R.;
De Clercq, E.; Balzarini, J. Antiviral Chem. Chemother.
2001, 12, 77.
(22) Fluorescence and UV measurements for N1-substituted
compounds (4a,b, 6a,b) were done in MeOH that had been
degassed by bubbling with N2 for 5 min and at a concen-
tration of 2.5 mM. Although the N1-unsubstituted com-
pounds (3a,b, 5a,b) are soluble in water, these were also
examined in degassed MeOH at 2.5 mM for comparison
purposes and the difficulty with fully deoxygenating the
water. Fluorescence excitation and emission spectra were
determined with at least 3 replicates with a 1 min rest
between scans. For comparison of intensities, excitation was
done at l = 350 nm, and emission data from the maxima
were used. At this time, we have no evidence for the
occurrence of photochemistry during the course of these
measurements. Extinction coefficients were determined for
the wavelength of interest using at least three data points.
Compound 5a: AgNO3 (5 mol%), r.t., 96 h; white solid; mp
>350 °C (dec.). HRMS (EI): m/z calcd for C12H8N2O2:
212.0586; found: 212.0574. 1H NMR: d = 8.34 (s, 1 H), 7.82
(m, 2 H), 7.51–7.40 (m, 3 H), 7.22 (s, 1 H).
Compound 5b: AgNO3 (5 mol%), 82 h. Reaction is
quantitative as measured by 1H NMR, however, significant
Synlett 2006, No. 18, 2997–3000 © Thieme Stuttgart · New York