Synthesis and fluorescent properties of the tricyclic analogues of acyclovir linked with nitrogen hetbrocyclic units

Article abstract of DOI:10.1081/NCN-200061817

Tricyclic (T, 3,9-dihydro-9-oxo-5H-imidazo[1,2-α]purine) analogues of acyclovir (ACV, 1), substituted in the 6 position with pyrid-4-yl, 4-(pyrid-4′-yl)Ph, 4-(pyrimidin-5′-yl)Ph and 4-(thiazol-2′-yl) Ph units were synthesized. For the synthesis of the het

Full text of DOI:10.1081/NCN-200061817

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SYNTHESIS AND FLUORESCENT PROPERTIES OF THE
TRICYCLIC ANALOGUES OF ACYCLOVIR LINKED WITH
NITROGEN HETBROCYCLIC UNITS
Tomasz Goslinski a , Piotr Januszczyk a , Grazyna Wenska b , Bozenna Golankiewicz a , Erik
De Clercq c & Jan Balzarini c
a Institute of Bioorganic Chemistry, Polish Academy of Sciences , Poznan, Poland
b Adam Mickiewicz University , Poznan, Poland
c Rega Jnstitute for Medical Research, Katholieke Universiteit Leuven , Leuven, Belgium
Published online: 15 Nov 2011.
To cite this article: Tomasz Goslinski , Piotr Januszczyk , Grazyna Wenska , Bozenna Golankiewicz , Erik De Clercq & Jan
Balzarini (2005) SYNTHESIS AND FLUORESCENT PROPERTIES OF THE TRICYCLIC ANALOGUES OF ACYCLOVIR LINKED WITH
NITROGEN HETBROCYCLIC UNITS, Nucleosides, Nucleotides and Nucleic Acids, 24:5-7, 571-575, DOI: 10.1081/NCN-200061817
To link to this article: http://dx.doi.org/10.1081/NCN-200061817
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Nucleosides, Nucleotides, and Nucleic Acids, 24 (5–7):571–575, (2005)
Copyright D Taylor & Francis, Inc.
ISSN: 1525-7770 print/ 1532-2335 online
DOI: 10.1081/NCN-200061817
SYNTHESIS AND FLUORESCENT PROPERTIES OF THE TRICYCLIC
ANALOGUES OF ACYCLOVIR LINKED WITH NITROGEN
HETBROCYCLIC UNITS
5
Tomasz Goslinski and Piotr Januszczyk
Institute of Bioorganic Chemistry, Polish
Academy of Sciences, Poznan, Poland
5
Grazyna Wenska
Adam Mickiewicz University, Poznan, Poland
5
Bozenna Golankiewicz
Institute of Bioorganic Chemistry, Polish Academy of Sciences,
Poznan, Poland
5
Erik De Clercq and Jan Balzarini
Rega Jnstitute for Medical Research, Katholieke
Universiteit Leuven, Leuven, Belgium
5
Tricyclic (T, 3,9-dihydro-9-oxo-5H-imidazo[l,2-a]purine) analogues of acyclovir (ACV, 1),
substituted in the 6 position with pyrid-4-yl, 4-(pyrid-4’-yl)Ph, 4-(pyrimidin-5’-yl)Ph and 4-(thiazol-2’-
yl)Ph units were synthesized. For the synthesis ofthe heteroarylphenyl derivatives, a convenient general
route was developed, i.e., Suzuki cross-coupling between protected 6-(4-dihydroxyborylphenyl) TACV
and easily available bromoheterocycles. Fluorescent properties of newly synthesized TACV aoalogues
strongly depend on the nature of a solvent. This sensitivity of fluorescence makes the compounds
promising probes of H-bonding in the environment.
Keywords Acyclovir, Tricyclic Analogues, Synthesis, Fluorescence Probe
INTRODUCTION
Our previous studies on the fluorescent, tricyclic (T, 3,9-dihydro-9-oxo-SH-
imidazo[1,2-a] purine) analogues of two potent antivirals, acyclovir, ACV, 9-[(2-
hydroxyethoxy)methyl] guanine, 1 and its (1,3-dihydroxy-2-propoxy) congener,
ganciclovir, GCV, 2 have shown that the introduction of the 4-R-phenyl unit into 6
This work was supported by the Polish Committee for Scientific Research (KBN) Grant PBZ-KBN-059/TO9/
18, an ID0 Grant no 02/012 (to JB) from the Katholieke Universiteit Leuven and a Grant from the European
Commission (QLRT-2001-01004).
Address correspondence to Tomasz Goslinski, Institute of Bioorganic Chemistry, Polish Academy of
Sciences, Poznan, Poland.
571
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T. Goslinski et al.
position of the tricycle is the most promising modification in terms of fluorescent
and antiviral properties.[1–4]
Recently we have found that the replacement of 6-phenyl with 6-[(4’-hydroxy-
methyl)-4-biphenylyl] substituent results in the compounds 6-[(4’-HOCH2)PhPh]-
TACV 3 and -TGCV 4 with advantageous flourescense characteristics (emission in
visible region, little overlap with absorption) together with selective activity against
herpes simplex virus type 1 (HSV-1).[5] In continuation of the above approach to
improve the fluorescent properties of the tricyclic analogues, we synthesized a series
of nitrogen containing 6-heteroaryl and 6-heteroarylphenyl TACV derivatives and
characterized their fluorescence as well as biological activity.
SYNTHESIS
The synthetic route to the nitrogenheteroaryl substituted TACV analogues is
depicted in Scheme 1. The target compounds were prepared by reacting-the
l-sodium derivative of acyclovir with an appropiate bromoketone according to an
alkylation-condensation reaction using simpler reagents.[1–3,5] When using known
bromoketone 5,6-(pyrid-4-yl)TACV 6 was smoothly obtained in 75% yield. In order
to have a broad access to a variety of the heteroarylphenyl TACV derivatives, a
SCHEME 1 A: Reagents and conditions: (a) Br2, HBr, 0 ! 20ꢀC;[6] (b) NaH/DMF, r.t., 30 min. B: Reagents and
conditions: (a) DMF, pinacol, 20ꢀC, 1–3 h; (b) Br2, AlCl3, THF, 0–20ꢀC, 1.5 h; (c) NaH, DMF, 20ꢀC, h; (d) H2O,
100ꢀC, 10 min; (e) DMF-H2O (5:3 or 1:1), Pd(Ph3P)4, K2CO3 or Na2O3, 75–85ꢀC.

T. Goslinski et al.
573
general route was developed. Commercially available 4-acetylphenyldihydroxybor-
ane was transformed into a pinacol ester 7 and then into a bromoketone 8, phenacyl
bromide carrying the protected dihydroxyborane function. Acyclovir subjected to
the alkylation-condensation reaction with 8 provided a TACV substituted reagent 9
for the Suzuki cross-coupling with diverse, inexpensive, commercially available
bromoheterocycles. Hydrolytic deprotection of 9 resulted in 6-(4-dihydroxybor-
ylphenyl) TACV 10. The reconnaissance reactions, presently performed with 9,
provided TACV analogues linked in 6 position with 4-(pyrid-4’-yl)Ph 11,4-
(pyrimidin-5’-yl)Ph 12 and 4-(thiazol-2’-yl)Ph 13 units in 84, 27, and 50% yield,
respectively. The target derivatives 6,9-13 were fully characterized by elemental
1
analyses, H and 13NMR spectra, as well as absorption and fluorescence spectra.
FLUORESCENCE
The Effect of Heterocyclic Substituent on the Absorption and
Fluorescence Characteristics of 6-(4-RPh)TACV
The location of absorption maxima and the shape of the absorption bands of
synthesized heteroaryl or heteroarylphenyl substituted compounds do not
significantly differ from those of their appropriate hydrocarbon analogs: 6-(4’-R-
Ph-Ph)TACV and 6-PhTACV.[5,7] Out of 6-heteroaryl and Gheteroarylphenyl
synthesized compounds, only 6-[4-(thiazol-2’-yl)Ph] TACV showed some fluores-
cence in aqueous solution (Table 1). Unlike their hydrocarbon analogues, the
remaining heteroaryl or heteroarylphenyl substituted derivatives of TACV do not
show any measurable fluorescence in this medium (fF< 0.0001). In order to reveal
TABLE 1 Absorption (lAmax) and Fluorescence Maxima (lFmax), Fluorescence Quantum Yields (fF) and
Lifetimes (tF) of 6-[4-(Pyrid-4’-yl)Ph]TACV and 6-[4-(Thiazol-2’-yl)Ph]TACV in Selected Solvents
6-[4-(pyrid-4’-yl)Ph]TACV
6-[4-(thiazol-2’-yl)Ph]TACV
lA
[nm]
lF
[nm]
lA
[nm]
lF
[nm]
max
max
max
max
Solvent
aH2O
erb
cfF
dtF [ns]
cfF
dtF [ns]
e
e
e
78.3
270; 318
269; 327
271; 325
273; 332
250; 280;
322
505
0.005
nd
MeOH
MeCN
EtOAc
32.6
35.9
6.0
480
482
445
0.001
0.097
0.396
<0.5
3.2
250; 282;
308; 332
250; 281;
310; 336
f; 282; 309;
344
490
480
445
0.020
0.106
0.335
0.7
3.9
nd
nd
aContains ca 15% MeOH.
bRelative permittivity (dielectric constant).
clexc = 320 nm.
dlexc = 320 nm; emission at lF
eNon-fluorescent (fF< 0.0001).
was monitored.
max
fOverlapped with solvent absorption; nd—not determined.

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T. Goslinski et al.
the factors that may be responsible for the very inefficient emissive process, we
measured the fluorescence.quantum yields and lifetimes of selected compounds: 6-
[4-(pyrid-4’-yl)Ph]TACV 11 and 6-[4-(thiazol-2’-yl)-Ph]TACV 13 in protic and
aprotic organic solvents of various polarity. The results are presented in Table 1.
Their analysis indicates that the molecules are sensitive to both polarity and
hydrogen-bonding ability of a solvent. In a less polar, aprotic solvent, ethylacetate,
fluorescence is strong. In a solvent with higher polarity, acetonitrile, fluorescence
becomes weaker. In a hydroxylic solvent, MeOH, despite its polarity similar to
MeCN (expressed by c, Table 1), fluorescence quantum yield is significantly
lowered compared to aprotic, polar MeCN. The solvent induced fluorescence
quenching is even more pronounced in aqueous medium. The reduction of fF
value is accompanied by the shortening of fluorescence lifetime. It may be
suggested that the sharp decrease of fluorescence quantum yield in hyroxylic
solvents is due to the formation of hydrogen bonded pairs: excited fluorophore (H
bonding acceptor)–solvent (H bonding donor). It was demonstrated that the
intermolecular interaction of a fluorophore with hydroxylic solvents is an efficient
channel of the excited state deactivation in several systems, e.g., azaaromatics,[8]
quinones,[9] and fluorenone.[10]
The formation of H-bonded solute-solvent complexes in the ground state is not
indicated by UV spectroscopy. Except for small (<4 nm) solvatochromic shift, the
shape of the absorption bands of 6-[4-(pyrid-4’-yl)Ph]TACV and 6-[4-(thiazol-2’-yl)-
Ph]TACV in MeCN and MeOH is very similar.
BIOLOGICAL ACTIVITY AND PROSPECTS OF APPLICATION IN
BIOLOGICAL SYSTEMS
New heteroaryl TACV derivatives 6, 11, 12, and 13 were evaluated against
herpes simplex virus type 1 (HSV-1) and type 2 (HSV-2), vaccinia virus, vesicular
stomatitis virus, and herpes simplex-l TRKOS. All compounds showed SI below
10. Also the compounds demonstrated very weak inhibitory effect on the pro-
liferation of osteosarcoma cells (OST TK+, OST TK/HSV-1 TK+). However,
sensitivity of fluorescence of TACV analogues linked with nitrogen heteroaryl units
makes them promising probes of H-bonding in the environment. Such probes upon
incorporation or conjugation to macromolecules provide information about the
presence of hydrogen bonding groups, local concentration, and dynamics of water
in the close vicinity of the fluorophore. In biological systems fluorescein dianion
was reported to be a useful probe of hydrogen bonding in environment. In this
case, however, fluorescence maximum is an indicator.[11]
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