Photoisomerization as a modulator of the DNA-cleaving efficiency of novel azo bispropargyl sulfones

Article abstract of DOI:10.1039/b606653d

Novel azobenzene based bispropargyl sulfones have been prepared in the thermally stable E-form: irradiation with a high-pressure Hg lamp converted them to the Z-isomer which showed higher DNA-cleaving efficiency. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b606653d

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COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Photoisomerization as a modulator of the DNA-cleaving efficiency of
novel azo bispropargyl sulfones
Moumita Kar and Amit Basak*
Received (in Cambridge, UK) 11th May 2006, Accepted 10th July 2006
First published as an Advance Article on the web 3rd August 2006
DOI: 10.1039/b606653d
Novel azobenzene based bispropargyl sulfones have been
prepared in the thermally stable E-form: irradiation with a
high-pressure Hg lamp converted them to the Z-isomer which
showed higher DNA-cleaving efficiency.
Bispropargyl sulfones have recently emerged as a new class of
DNA-cleaving agents.1 They exert their activity via isomerization
to the allenic sulfone, which subsequently alkylates the DNA
bases, thus allowing a Maxam–Gilbert type cleavage to take
place.2 An alternative pathway, though usually not followed, is the
formation of diradicals from the bisallenic sulfone via Garratt–
Braverman rearrangement3 and subsequent oxidative DNA
damage.4 It is important to note that diradical formation can
take place only if the sulfone gets isomerized to the bisallene, which
in many cases may not be possible for stereoelectronic reasons.5
Since monoallenic sulfones show their DNA cleavage activity via
an alkylation pathway, they may be inferior in terms of their
DNA-cleaving efficiency when compared to the bisallenic sulfones
(as the latter can have dual mechanisms for cleavage, namely
mono or bis-alkylation, as well as oxidative damage through
Scheme 1 Possible studies involving the azo sulfones.
diradicals). Modulation of the reactivity of bispropargyl sulfones
has not drawn much attention, unlike the enediyne counterpart.
photoisomerized to the Z-sulfones 3 and 4. The reactivity of these
The only triggering device that is used is the variation of pH, which
is kept in the alkaline range.6a–c Dai et al. has recently exploited
isomers in presence of a base (triethylamine) as well as their DNA-
cleavage ability under alkaline pH revealed interesting results,
monopropargyl sulfones.6d–e Recently,7 we have shown how a
which are described in this Communication.
photoelectron transfer to enhance the DNA cleaving potency of
The starting material for the synthesis of both the sulfones was
photochemical E to Z isomerization can activate azoenediynes
towards Bergman cyclization (BC).8 We were curious to know
the commercially available 2,29-bishydroxy azobenzene (5). This
was bisalkylated with butyne-1,4-diol mono tosylate (7) in the
presence of K2CO3 and DMF. The resulting diol 8, isolated in 70%
yield, was converted to the dimesylate 9 and the cyclic sulfide 10
was obtained when the dimesylate was treated with Na2S,
preabsorbed in alumina (neutral) in CH2Cl2. The sulfide was then
oxidized with mCPBA to the sulfone 1 which was isolated as a
what happens to the reactivity of azo bispropargyl sulfone systems
upon such E–Z photoisomerization. Our curiosity arises from the
fact that the kinetics of BC are dependent upon the distance
between the terminal acetylenic carbons which presumably is less
in Z-azo enediynes. In a similar argument, the reactivity of the
Z-azo bispropargyl sulfones may be higher, as the allenic carbons
(formed by isomerization) should come closer, compared to the
corresponding E-system, and are thus geared towards Garratt–
Braverman cyclization. Thus, we would like to address the follow-
ing points: does the configuration of the azo moiety (E or Z) affect
the rate of isomerization to the monoallene and subsequently to
the bisallene? Does it induce any change in DNA-cleaving
efficiency and also any change in the DNA-cleaving mechanism?
All these possibilities are summarized in Scheme 1.
In order to have an answer, we synthesized the E-azobenzene
sulfones
1 and 2 (Fig. 1). These have been successfully
Bioorganic Laboratory, Department of Chemistry, Indian Institute of
Technology, Kharagpur, India. E-mail: absk@chem.iitkgp.ernet.in;
Fax: +91 3222 282252; Tel: +91 3222 283300
Fig. 1 Azobenzene based bis propargyl sulfones.
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Wittig reaction with phosphonium ylide derived from ethyl
bromoacetate. The trans ester 14 isolated in high yield (90%)
was reduced to the alcohol 15 with DIBAL-H. The latter was then
converted to the mesylate 16 which was used as the alkylation
partner. However, in this case the monoalkylated product 17 was
the major product even after prolonged hours of stirring at 50 uC.
The expected biscoupled product 18 was isolated in very poor yield
(y5%) which could not be processed any further. The mono-
coupled product was then converted to the unsymmetrical bis-
alkylated product 19 using the butyne-1,4-diol mono tosylate
which turned out to be a more reactive system. The resulting diol
20, obtained after PPTS-deprotection of THP–ether was then
converted to the sulfide 22 via the mesylate 21. Oxidation with
mCPBA finally afforded the target sulfone 2. Gratifyingly, the
double bond remained intact during the peracid oxidation.
Having successfully prepared the target sulfones,10 we turned
our attention to study the effect of UV-irradiation on these
molecules. For this, the sulfone 1 in CH2Cl2 (0.005 M) was
irradiated with a high pressure Hg lamp for 3 h. 1H-NMR showed
the occurrence of isomerization from E to Z (a ratio of 1 : 3), as
confirmed by the appearance of new singlets at d 4.59 and 3.96.
Upon heating at 60 uC for 2 h these signals disappeared, leaving
only the signals for the starting E-isomer. 1H-NMR based kinetic
study at 37 uC showed the first order rate constant for thermal Z
to E isomerization to be 5 6 103 min21. The other E-sulfone 2
behaved similarly when its methanol solution was irradiated for
3 h; in this case the ratio of E to Z isomer was also 1 : 3. The
formation of the Z-isomer was indicated by the appearance of
three new singlets at d 4.90, 4.76 and 4.10. The Z-isomer can be
thermally reisomerized back to the E isomer; the rate of thermal
reisomerization was, however, much slower (k = 3 6 102 min21) .
The chemical reactivity of the sulfones 1 and 3 were then evaluated
(Scheme 4). For this, a CDCl3 solution of the sulfone was taken
and triethylamine (1.5 eq.) added and the progress of reaction
monitored by 1H-NMR. The E-sulfone 1 rapidly forms the
monoallene, after which there was no further reaction and we
ended up with an equilibrium mixture of 1 and its monoallenic
counterpart (ratio 1 : 1). The formation of monoallene was
confirmed by formation of only the mono methanol adduct (yield
y30%) when the experiment was run in presence of MeOH at
37 uC for 12 h. In the monoallene, the methylene attached to the
Scheme 2 Synthesis of bispropargyl sulfone 1.
Scheme 3 Synthesis of bispropargyl sulfone 2.
yellow solid after purification by chromatography (Scheme 2). The
formation of the sulfone was confirmed by the appearance of two
1
4H singlets in the H NMR spectrum. The sulfoxide 11 which
could also be isolated when the oxidation was carried at 0 uC for a
short time gave a typical AB quartet for the methylene attached to
sulfur.
For the synthesis of the vinylogous sulfone 2 (Scheme 3), the
mesylate 16 was first prepared from partially THP-protected
butyne-1,4-diol 12. The alcohol was oxidized to the aldehyde 13
with Dess Martin reagent9 and the aldehyde was subjected to
Scheme 4 Reactivity of sulfones in the presence of Et3N.
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trying to design molecules for which the thermal reisomerism is
slower as compared to those reported herein, by incorporating
some strain parameters. Our strategy may find use in the
development of anticancer molecules involving photodynamic
therapy.13
MK is grateful to CSIR, Government of India for a senior
research fellowship. AB thanks DST, Government of India for a
research grant.
Fig. 2 (a) DNA cleavage experiment of compounds 1 & 3 after 1.5 h
incubation at 37 uC; lane 1: control DNA in TAE buffer (pH 8.5,
0.4 mm per bp) (7 ml) + CH3CN (10 ml); lane 2: DNA in TAE buffer
(pH 8.5, 0.4 mm per bp) (7 ml) + Z-sulfone (0.02 mM, 2.5 h) in CH3CN
(5 ml); lane 3: DNA in TAE buffer (pH 8.5, 0.4 mm per bp) (7 ml) +
E-sulfone (0.02 mM, 2.5 h) in CH3CN (5 ml). (b) DNA cleavage
experiment of compounds 2 & 4 after 2.5 h incubation at 20 uC; lane 1:
control DNA in TAE buffer (pH 8.5, 0.4 mm per bp) (7 ml) + CH3CN
(10 ml) at; lane 2: DNA in TAE buffer (pH 8.5, 0.4 mm per bp) (7 ml) +
E-sulfone (0.02 mM, 2.5 h) in CH3CN (5 ml) at; lane 3: DNA in TAE
buffer (pH 8.5, 0.4 mm per bp) (7 ml) + Z-sulfone (0.02 mM, 2.5 h) in
CH3CN (5 ml).
Notes and references
1 K. C. Nicolaou, S. Wendeborn, P. Maligres, K. Isshiki, N. Zein and
G. Ellestad, Angew. Chem., Int. Ed. Engl., 1991, 30, 418.
2 (a) A. M. Maxam and W. Gilbert, Methods Enzymol., 1980, 65, 499; (b)
A. M. Maxam and W. Gilbert, Proc. Natl. Acad. Sci. U. S. A., 1977, 74,
560.
3 (a) P. J. Garratt and S. B. Neoh, J. Org. Chem., 1979, 44, 2667; (b)
Y. S. P. Cheng, P. J. Garratt, S. B. Neoh and V. H. Rumjanek, Isr. J.
Chem., 1985, 26, 101; (c) S. Braverman, Y. Duar and D. Segev,
Tetrahedron Lett., 1979, 3181.
4 (a) Y.-j. Xu, M. S. DeMott, J. T. Hwang, M. M. Greenberg and
B. Demple, DNA Repair, 2003, 2, 175; (b) A. Liptakova, L. Svorenova,
J. Labuda and Z. Durackova, Conf. Coord. Chem., 1995, 407; (c)
C. Richter, J. W. Park and B. N. Ames, Proc. Natl. Acad. Sci. U. S. A.,
1988, 85, 6465.
5 A. Basak and U. K. Khamrai, Tetrahedron Lett., 1995, 36, 7913.
6 (a) S. M. Kerwin, Tetrahedron Lett., 1994, 35, 1023; (b) W.-M. Dai and
K. C. Fong, Tetrahedron Lett., 1995, 36, 5613; (c) W.-M. Dai,
K. C. Fong, H. Danjo, S.-i. Nishimoto, M. Solow, W. L. Mak and
M. L. Yeung, Bioorg. Med. Chem. Lett., 1996, 6, 1093; (d) K.-i. Haruna,
H. Kanezaki, K. Tanabe, W.-M. Dai and S.-i. Nishimoto, Bioorg. Med.
Chem., 2006, 14, 4427; (e) K.-i. Haruna, K. Tanabe, A. Ishii, W.-M. Dai,
H. Hatta and S.-i. Nishimoto, Bioorg. Med. Chem. Lett., 2003, 11, 5311.
7 M. Kar, A. Basak and M. Bhattacharya, Bioorg. Med. Chem. Lett.,
2005, 15, 5392.
sulfone appeared as an AB quartet reflecting their diastereotopic
nature due to the chirality of the allene. The Z-isomer on the other
hand when similarly treated first showed the formation of the
monoallene. But within a few minutes (y5 min) the signals
corresponding to monoallene disappeared and some unassignable
peaks of very small intensity appeared in the olefinic region.
Carrying out the reaction in the presence of MeOH (12 h, 37 uC)
showed the formation of both mono and bis adduct with MeOH
(appearance of peaks in the mass spectra at m/z 413 and 445). This
indicated that the monoallene from the Z-isomer got converted
into the bis allene which then rapidly decomposed (possibly via
Garratt–Braverman rearrangement in the absence of MeOH).11
Thus, there is a distinct change in reactivity between the E-and the
Z-isomer.
8 (a) R. G. Jones and R. G. Bergman, J. Am. Chem. Soc., 1972, 94, 660;
(b) R. G. Bergman, Acc. Chem. Res., 1973, 6, 25; (c) T. P. Lockhart and
R. G. Bergman, J. Am. Chem. Soc., 1981, 103, 4091.
9 D. B. Dess and J. C. Martin, J. Org. Soc., 1983, 48, 4155.
10 Selected spectral data (all 1H and 13C NMR were recorded at 200 and
50 MHz, respectively, in CDCl3 unless mentioned otherwise). For 1: dH
7.72 (1H, dd, J = 1.5 Hz, 8.5 Hz), 7.45 (2H, dt , J = 1.8, 7.5 Hz), 7.21
(4H, m), 4.93 (4H, s), 3.75 (4H, s); dC 154, 145, 131.9, 123.4, 119.6, 119,
83, 74, 60.3, 43.3; HRMS calcd for C20H16N2O4S + H+ 381.09101
found 381.0874. For 2: dH 7.70 (1H, d, J = 7.8 Hz), 7.42 (2H, m), 7.12
(5H, m), 6.30 (1H, d, J = 16 Hz), 6.10 (1H, d, J = 8.4 Hz), 4.98 (2H, s),
4.65 (2H, d, J = 5.0 Hz), 4.03 (4H, s); HRMS calcd for C22H18N2O4S +
H+ 407.1067 found 407.1096. For 3: dH 7.72 (1H, d, J = 8.4 Hz), 7.45
(1H, t, J = 7.6 Hz), 7.25 (2H, m), 7.06 (2H, m), 6.84 (2H, d, J = 8.24 Hz),
4.59 (4H, s), 3.96 (4H, s); HRMS calcd for C20H16N2O4S + H+ 381.0910
found 381.0880. For 4: dH 7.69 (1H, d, J = 2 Hz), 7.4 (3H, m), 6.9 (4H,
m), 6.31 (1H, dt, J = 2 Hz, 16 Hz), 6.11 (1H, d, J = 16 Hz), 4.90 (2H,
bs), 4.76 (2H, bs), 4.10 (4H, bs); HRMS calcd for C22H18N2O4S + H+
407.1067 found 407.1087.
11 Braverman has recently reported that the rearrangement to the
thiophene dioxides is more facile by incorporating additional conjuga-
tion to the propargyl system. For the reference see Y. Zafrani,
H. E. Gottlieb, M. Sprecher and S. Braverman, J. Org. Chem., 2005, 70,
10166. This was the rationale for designing sulfone 2.
12 The cleavage efficiency was approximately 2.5 times for the Z-isomer
than that for the E-isomer. This was determined by checking the relative
UV-absorbance of the bands at 280 nm. It is to be noted that the control
DNA specimen is usually contaminated with some nicked form (form
II).
Since the Z-isomer can isomerize up to the bisallene in the
presence of base, it is expected to show higher DNA-cleavage
efficiency as it can proceed via both an alkylation and an oxidative
diradical pathway. DNA-cleaving experiments were thus carried
out with the pure E isomer and a 1 : 3 mixture of E and Z isomer
(obtained by photoisomerism) at 37 uC using pBR 322 supercoiled
plasmid DNA at pH 8.5. The results showed the higher cleavage
efficiency12 for the Z-isomer (Fig. 2a) as compared to the E-isomer
(lane 3). However, the cleavage efficiency finally evened out as
more time was allowed. This is because of thermal reisomerization
to the E-isomer. To slow down this thermal reiomerization
process, the experiment was repeated at the lower temperature of
20 uC where the difference in cleavage efficiency became more
significant. For the other sulfone, 2, again the E-isomer was found
to be less efficient cleaving agent as compared to the Z-isomer
(Fig. 2b). In this case also, the incubation with DNA was done
with the lower temperature of 20 uC to suppress thermal
isomerization.
In conclusion, we have successfully developed a novel photo-
triggering device involving E to Z isomerism to modulate the
reactivity of azo based bis propargyl sulfones. Currently, we are
13 J. Moan and Q. Peng, Anticancer Res., 2003, 23, 3591.
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