Synthesis of unsymmetric cyanine dye via merocyanine and their interaction with DNA

Article abstract of DOI:10.1135/cccc2009034

An effective method has been developed for synthesis of neutral merocyanine dye 4 from (4-nitrophenyl)malondialdehyde. Merocyanine 4 was used as a basic building block for preparation of unsymmetric cyanine dyes 9 and 10, both derived from the same aromat

Full text of DOI:10.1135/cccc2009034

Synthesis of Unsymmetric Cyanine Dye
1081
SYNTHESIS OF UNSYMMETRIC CYANINE DYE VIA MEROCYANINE
AND THEIR INTERACTION WITH DNA
c
b2
Tomáš BŘÍZAa1,b,*, Zdeněk KEJÍKb1, Jarmila KRÁLOVÁ , Pavel MARTÁSEK and
a2,d
Vladimír KRÁL
a
Department of Analytical Chemistry, Institute of Chemical Technology, Prague,
Technická 5, 166 28 Prague 6, Czech Republic; e-mail: 1 ftor@seznam.cz, 2 vladimir.kral@vscht.cz
b
First Faculty of Medicine, Charles University, Kateřinská 32, 121 08 Prague 2, Czech Republic;
1
2
e-mail: zkejik@centrum.cz, pavel.martasek@gmail.com
c
Institute of Molecular Genetics, Academy of Sciences of the Czech Republic, v.v.i.,
Flemingovo nám. 2, 166 10 Prague 6, Czech Republic; e-mail: kralova@img.cas.cz
Zentiva R & D, U Kabelovny 130, 102 37 Prague 10, Czech Republic
d
Received March 11, 2009
Accepted May 11, 2009
Publish ed on lin e July 15, 2009
An effective m eth od h as been developed for syn th esis of n eutral m erocyan in e dye 4 from
(4-n itroph en yl)m alon dialdeh yde. Merocyan in e 4 was used as a basic buildin g block for prep-
aration of un sym m etric cyan in e dyes 9 an d 10, both derived from th e sam e arom atic
m alon dialdeh yde. With th e cyan in e 10, pH-depen den t affin ity to DNA h as been studied.
Un sym m etric cyan in e dye 10 h as been com pared with sym m etric dye 8. Th e correspon din g
bin din g con stan ts (K) h ave been calculated for various pH.
Keyw ord s: Malon dialdeh yde; Merocyan in e; Sym m etric; Un sym m etric; Cyan in e; DNA.
Cyan in e dyes are very in terestin g an d in ten sively studied fam ily of ch rom o-
ph oric com poun ds1,2. Th ey are widely used in in dustrial ch em istry, in th e
ch em istry of dyes, in m edicin e, an d are used in specific bran ch es of n on -
lin ear optics1,3 (NLO), optical recordin g3, ph otodyn am ic th erapy1,3 (PDT),
can cer treatm en t1,3 an d optical sen sin g1,3 wh ile n ew structures with in ter-
estin g properties are still em ergin g every year.
Cyan in e dyes from m alon dialdeh ydes are m en tion ed in literature on ly
rarely. We h ave exten ded th is area an d prepared som e sym m etric cyan in e
dyes derived from 2-arylm alon dialdeh ydes. We studied affin ity an d selec-
tivity to an ion s an d biologically im portan t sulfated biopolym er h eparin 4.
In our furth er research , we h ave focused on a study of affin ity of th e dyes
to DNA as a m ost biologically im portan t biopolym er with several bin din g
sites for various guest m olecules5. Cyan in e dye–DNA in teraction s h ave been
exten sively studied an d a variety of n on covalen t bin din g m odes h ave been
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 7–8, pp. 1081–1090
© 2009 Institute of Organic Chemistry and Biochemistry
doi:10.1135/cccc2009034

1082
Bříza, Kejík, Králová, Martásek, Král:
iden tified5,6. It is well kn own th at DNA can be used as a tem plate for form a-
tion of supram olecular structures of cyan in e dyes7; certain dyes can also
in fluen ce ph ysiological fun ction s of DNA5. Guest m olecules h ave a h igh
poten tial as ch em oth erapeutic drugs wh ich m ay suppress th e gen e repli-
cation or tran scription in tum or cells. In addition , such a h ost–guest in ter-
action m ay be used for detection of n ucleic acids wh en th e ph ysical
properties of th e guest m olecule ch an ge upon bin din g an d m ay be easily
m on itored8. Especially useful alon g th ese lin es is DNA stain in g, wh ich is
based on th e color ch an ge of th e dye upon bin din g to th e m acrom olecule7.
Despite of m an y differen t types of cyan in e dyes wh ich h ave been studied
in in teraction with DNA, cyan in e dyes derived from arom atic m alon -
dialdeh ydes h ave been om itted. Moreover, from our previous research , dyes
of th is type in th e presen ce of an ion ic biopolym er h eparin exh ibit sign ifi-
can t spectral ch an ges in th e visible region 4. Th erefore, our goal was com -
parison of affin ity of sym m etric an d un sym m etric cyan in e dyes based on
arom atic m alon dialdeh ydes to DNA an d to try to evaluate th e role of both -
side h eteroarom atic rin gs of th e m eth in ium ch ain .
RESULTS AND DISCUSSION
Preparation of Unsymmetric Cyanine Dyes
At th e first stage, it was n ecessary to fin d a syn th etic way for preparation of
un sym m etric cyan in e dyes with two differen t h eteroarom atic rin gs on both
sides of th e ch ain . Preparation of un sym m etric dyes is well docum en ted in
literature1,3. On th e oth er h an d, th e syn th esis of un sym m etric cyan in es
from arom atic m alon dialdeh ydes h as n ot been described.
Our first syn th etic strategy for preparation of un sym m etric dyes was
based on a previous m eth od wh ich was used for preparation of sym m etric
dyes4. We used two differen t salts of correspon din g 2-m eth ylben zoth iazoles
in th e 1:1 ratio an d reacted th em with on e equivalen t of selected m alon -
dialdeh yde 1 (Sch em e 1).
Th is m eth od afforded com plex m ixtures con tain in g probably traces of
un sym m etric an d sym m etric dyes, wh ich could n ot be separated an d puri-
fied. Th erefore we focused our effort on th e secon d approach . It is described
in th e literature th at un sym m etric cyan in es can be prepared via n eutral
m erocyan in es. Th e m eth od utilizes tetraalkoxy-2-alkylpropan es as basic
buildin g blocks9,10 for con den sation with various h eteroarom atic salts. In
th e first step of th e described m eth od, a tetraalkoxypropan e is reacted with
on e equivalen t of h eteroarom atic salt to afford correspon din g n eutral
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 7–8, pp. 1081–1090

Synthesis of Unsymmetric Cyanine Dye
1083
m erocyan in e dye. Th e dye is th en con verted with a secon d, differen t
h eteroarom atic salt in to fin al un sym m etric dye10. We h ave tried to apply
th is prin ciple in th e case of arom atic m alon dialdeh yde (Sch em e 1), but th e
m eth od failed in syn th esis of n eutral m erocyan in e. We obtain ed on ly com -
plex m ixtures con tain in g traces of th e product. Moreover, th e product was
probably accom pan ied by a sym m etric cyan in e dye, wh ich was form ed by
th e reaction between n ascen t n eutral m erocyan in e an d startin g h etero-
arom atic salt.
SCHEME 1
Our furth er idea h ow to reach n eutral m erocyan in e from arom atic
m alon dialdeh yde, was based on th e kn own 11,12 con version of m alon di-
aldeh yde with an arom atic am in e in to correspon din g acrylaldeh yde
(Sch em e 2). Th e subsequen t reaction of th e acrylaldeh yde with ben zo-
th iazolium salt in alkalin e m edium sh ould lead to th e desired n eutral
m erocyan in e dye (Sch em e 3).
SCHEME 2
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 7–8, pp. 1081–1090

1084
Bříza, Kejík, Králová, Martásek, Král:
In th e first step, 2-(4-n itroph en yl)m alon dialdeh yde (1) was reacted with
several arom atic am in es (Sch em e 2). Th e con den sation with an ilin e af-
forded th e product accom pan ied by a com plex m ixture of oth er com -
poun ds wh ich m ake separation an d purification of acrylaldeh yde difficult.
In th e case of 4-n itroan ilin e, as an oth er am in e wh ich was used in con -
den sation with m alon dialdeh yde, th e correspon din g product 2, 3-(4-n itro-
an ilin o)-2-(4-n itroph en yl)acrylaldeh yde was clearly form ed. Un fortun ately,
th e solubility of th e aldeh yde was very lim ited un der th e con dition s of th e
con den sation with ben zoth iazolium salt an d th is fact com plicated its utili-
zation . Th e best results were obtain ed by con den sation of m alon dialdeh yde
1 with 2,5-dim eth oxyan ilin e, wh ich form ed th e correspon din g acryl-
aldeh yde 3 in th e yield of 76%. Com poun d 3 was easily separated by m ere
filtration ; it was sufficien tly soluble for th e n ext step.
Th e n ext step, con den sation with 2-m eth yl-3-propylben zoth iazol-3-ium
iodide (5), was accom plish ed un der reflux in eth an ol in th e presen ce of
trieth ylam in e. Th e reaction sm ooth ly afforded th e desired m erocyan in e dye
4 in 82% yield (Sch em e 3).
SCHEME 3
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 7–8, pp. 1081–1090

Synthesis of Unsymmetric Cyanine Dye
1085
Th e fin al step was syn th esis of un sym m etric ch arged cyan in e dyes
from n eutral m erocyan in e 4. First, we tried to con vert th e dye 4 in to sym -
m etric cyan in e dye to justify th e reactivity of 4 to ben zoth iazolium salt 5
(Sch em e 3). Merocyan in e 4 was reacted with th e salt 5 an d th e correspon d-
in g sym m etric cyan in e dye 8 was clearly form ed. Th en we studied con den -
sation of 4 with salt 6, wh ich sh ould lead to un sym m etric cyan in e dye 9.
In th is case, un der con dition s successfully used for preparation of cyan in e
dye 8, th e salt 6 did n ot afford th e product; startin g com poun ds were foun d
in th e reaction m ixture. Th erefore we ch an ged th e reaction con dition s. Th e
tem perature of th e m ixture was in creased to 130 °C an d acetic an h ydride,
wh ich is frequen tly used in syn th eses of cyan in e dyes10, was used as a sol-
ven t. Un der th ese con dition s, un sym m etric cyan in e dye 9 with two dif-
feren t ben zoth iazolium un its on both sides was form ed in a good yield
(Sch em e 3). In th e last step we decided to prepare an un sym m etric cyan in e
dye with ben zoth iazolium un it on on e side an d a un it with differen t
h eterocycle on th e oth er. We ch ose 2-m eth ylpyridin ium iodide (7) as a salt
for th e con den sation with m erocyan in e 4. Th e reaction was accom plish ed
in butan -1-ol with piperidin e in stead of trieth ylam in e as a base. Com poun d
10 was form ed in a good yield (Sch em e 3).
Study of the Affinity of Prepared Dyes to DNA
Th e prepared un sym m etric salt 10 h as been studied an d com pared with
sym m etric salt 8, wh ich was already studied in detail as a optical sen zor for
sulfates4. In th e case of un sym m etric dye 9, stron g aggregation was ob-
served un der con dition s of th e m easurem en t an d th erefore th e dye was
excluded from furth er experim en ts. We h ave studied th e in fluen ce of struc-
ture of cyan in e dyes 8 an d 10 on stability of th eir com plex with DNA at
various pH (Figs 1–4).
First of all, we com pared dyes 8 an d 10 in acidic m edium , pH 5.50 (Figs 1
an d 2). We h ave observed a stron ger affin ity of th e sym m etric dye 8 to
DNA, wh ereas un sym m etric dye 10 in teracted m ore weakly. In n eutral m e-
dium , pH 7.00 (Figs 3 an d 4), th e situation was sim ilar. Sym m etric dye 8,
in com parison with dye 10, sh owed a stron ger affin ity to DNA. Gen erally
we h ave observed a stron ger in fluen ce of pH on th e value of K for sym m et-
ric cyan in e dye 8 an d sm all depen den ce for un sym m etric cyan in e dye 10.
In th e case of sym m etric dye 8, th e 1:1 com plex was observed in both types
of th e m edium – at pH 5.50 an d 7.00, respectively. Con cern in g th e
un sym m etric dye 10, 1:1 an d 2:1 com plexes of DNA an d th e dye h ave been
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 7–8, pp. 1081–1090

1086
Bříza, Kejík, Králová, Martásek, Král:
FIG. 1
Addition of DNA (0–40 equiv.) to th e un sym m etric cyan in e dye 8 in ph osph ate buffer,
pH 5.50, wavelen gth 643 n m
FIG. 2
Addition of DNA (0–80 equiv.) to th e un sym m etric cyan in e dye 10 in ph osph ate buffer,
pH 5.50, wavelen gth 643 n m
FIG. 3
Addition of DNA (0–20 equiv.) to th e sym m etric cyan in e dye 8 in ph osph ate buffer, pH 7.00,
wavelen gth 643 n m
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 7–8, pp. 1081–1090

Synthesis of Unsymmetric Cyanine Dye
1087
observed. Th e calculated K an d th e observed com plexes at various pH for
dyes 8 an d 10 are sum m arized in Table I.
FIG. 4
Addition of DNA (0–100 equiv.) to th e un sym m etric cyan in e dye 10 in ph osph ate buffer,
pH 7.00, wavelen gth 643 n m
TABLE I
Calculated values of K of sym m etric an d un sym m etric cyan in e dyes 8 an d 10 with DNA
K for com plexes DNA–dye
pH 5.50
pH 7.00
Stoich iom etry of com plex
1:1
2:1
–
1:1
2:1
K for sym m etric cyan in e dye 8
K for un sym m etric cyan in e dye 10
5.0
4.8
–5.7
4.9
–
9.3
9.5
CONCLUSIONS
We h ave developed an efficien t m eth od for preparation of n eutral m ero-
cyan in e dye 4 from (4-n itroph en yl)m alon dialdeh yde (1). Merocyan in e 4
h as been used as a key buildin g block in th e syn th esis of un sym m etric
cyan in e dyes 9 an d 10, wh ereas dye 9 h as been excluded from our study
due to stron g aggregation . We studied an d com pared in teraction of sym -
m etric dye 8 an d un sym m etric dye 10 with DNA. Our pilot exploration
sh owed differen ce between un sym m etric an d sym m etric dyes in affin ity to
DNA at various pH. Sligh tly h igh er affin ity to DNA was eviden t in th e case
of sym m etric dye 8 in com parison with un sym m etric dye 10. Neverth eless,
a precise evaluation of con tribution s of both h eteroarom atic rin gs on th e
m eth in ium ch ain to th e affin ity of th e dyes to DNA is n ot possible at th is
stage of research . It is n ecessary to prepare larger series of un sym m etric an d
sym m etric dyes an d to study an d com pare th eir affin ity to DNA.
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 7–8, pp. 1081–1090

1088
Bříza, Kejík, Králová, Martásek, Král:
EXPERIMENTAL
NMR spectra were obtain ed with a Varian 300 (300.077 MHz for 1H an d 75.460 MHz for
13C) at 23 °C in DMSO-d6. Ch em ical sh ifts (δ) are presen ted in ppm , couplin g con stan ts (J)
in Hz. Mass spectra were obtain ed by electron im pact (EI+) with a VG An alytical ZAB-EQ
spectrom eter.
3-(2,5-Dim eth oxyan ilin o)-2-(4-n itroph en yl)acrylaldeh yd (3)
Th e reaction flask was ch arged with 2-(4-n itroph en yl)m alon dialdeh yde (200 m g, 1.04 m m ol),
2,5-dim eth oxyan ilin e (150 m g, 0.98 m m ol) an d dry aceton itrile (15 m l). Th e m ixture was
stirred at 75 °C for 18 h . Th en th e volum e of th e reaction m ixture was reduced to a h alf an d
precipitated acrylaldeh yde 3 was filtered off, wash ed with dieth yl eth er (3 m l) an d dried in
vacuo. Th e filtrate was allowed to crystallize an d th e product was separated as m en tion ed
above. Total yield of com poun d 3 was 221 m g (76%). 1H NMR (DMSO-d6): 12.41 d, 1 H, J =
13.8; 9.78 d, 2 H, J = 3.8; 8.22 dt, 2 H, J = 8.8, J = 2.6; 7.74 dd, 1 H, J = 13.2, J = 3.8; 7.47 dt,
2 H, J = 8.8, J = 2.6; 6.88 d, 1 H, J = 8.8; 6.81 d, 1 H, J = 2.9; 6.64 dd, 1 H, J = 8.8, J = 2.6;
3.94 s, 3 H; 3.8 s, 3 H. 13C NMR (DMSO-d6): 189.2 (CH=O), 154.3 (C), 145.6 (C), 144.8 (C),
143.4 (C), 143.2 (CH), 129.3 (C), 125.5 (CH), 124.4 (CH), 112.3 (CH), 111.1 (C), 108.7 (CH),
101.7 (CH), 56.5 (CH3), 55.9 (CH3). HRMS (EI+): for C17H16N2O5 [MH] calculated 328.1046,
foun d 328.1059.
2-(4-Nitroph en yl)-3-(3-propyl-2,3-dih ydroben zoth iazol-2-yliden e)but-1-en al (4)
Th e reaction flask was ch arged with acrylaldeh yde 3 (100 m g, 0.30 m m ol), ben zoth iazolium
salt 5 (97 m g, 0.30 m m ol), dry trieth ylam in e (0.5 m l), 1,2-dim eth oxyeth an e (3 m l) an d dry
eth an ol (7 m l). Th e stirred m ixture was refluxed for 18 h . Th e m ixture was reduced to h alf
volum e an d placed in a fridge for 5 h . Precipitated m erocyan in e 4 was filtered off, wash ed
with dieth yl eth er (1 m l) an d dried in vacuo. Th e filtrate was allowed to crystallize an d th e
solid was separated as m en tion ed above. Total yield of com poun d 4 was 91 m g (82%).
1H NMR (DMSO-d6): 9.39 s, 1 H; 8.25 d, 2 H, J = 8.8; 7.77 d, 1 H, J = 7.6; 7.63 d, 2 H, J =
8.9; 7.40 m , 3 H; 7.20 t, 1 H, J = 7.0; 5.94 d, 1 H, J = 13.0; 4.00 t, 2 H, J = 7.0; 1.64 sextet,
2 H, J = 7.3; 0.87 t, 3 H, J = 7.3. 13C NMR (DMSO-d6): 188.8 (CH=O), 160.1 (C), 145.3 (C),
142.4 (C), 141.8 (C), 130.5 (CH), 127.2 (CH), 125.4 (C), 123.6 (C), 123.2 (CH), 122.3 (CH),
111.6 (CH), 99.1 (CH), 89.0 (CH), 46.1 (CH2), 19.9 (CH2), 10.9 (CH3). MS: for C20H18N2O3S
calculated 366, foun d (M + H) 367. For C20H18N2O3S calculated: 65.56% C, 4.95% H; foun d:
65.72% C, 4.98% H.
3-(4-Nitroph en yl)-1-(3-propyl-2,3-dih ydroben zoth iazol-2-yliden e)-
5-[3-(3-sulfon atopropyl)ben zoth iazol-2-yl]pen tam eth in ium (9)
Th e reaction flask was ch arged with m erocyan in e 4 (50 m g, 0.14 m m ol), ben zoth iazolium
salt 6 (37 m g, 0.14 m m ol) an d acetic an h ydride (7 m l). Th e reaction m ixture was h eated to
130 °C for 3 h . Th e m ixture was th en cooled to laboratory tem perature an d th e precipitated
cyan in e dye 9 was filtered off, wash ed with cold eth an ol (1 m l) an d dried in vacuo. Com -
poun d 9 was obtain ed as a green crystalin e powder (47 m g, 55%). 1H NMR (DMSO-d6):
8.40 d, 2 H, J = 8.8; 7.39–8.08 m , 12 H; 6.17 d, 1 H, J = 14.0; 5.96 d, 1 H, J = 13.5; 4.41 t,
2 H, J = 7.0; 4.16 t, 2 H, J = 7.0; 2.46 t, 2 H, J = 6.7; 1.96 quin tet, 2 H, J = 5.9; 1.64 sextet,
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 7–8, pp. 1081–1090

Synthesis of Unsymmetric Cyanine Dye
1089
2 H, J = 7.0; 0.82 t, 3 H, J = 7.3. 13C NMR h as n ot been obtain ed due to very lim ited solubil-
ity of dye 9. HRMS (EI+): for C31H29N3O2S2 [M – SO3] calculated 539.1698, foun d 539.1701.
1-(1-Meth ylpyridin -1-ium -2-yl)-3-(4-n itroph en yl)-5-(3-propyl-2,3-dih ydroben zoth iazol-
2-yliden e)-3-(4-n itroph en yl)pen tam eth in ium Iodide (10)
Th e reaction flask was ch arged with m erocyan in e 4 (54 m g, 0.15 m m ol), pyridin ium salt 7
(35 m g, 0.15 m m ol), piperidin e (0.3 m l) an d dry butan -1-ol (7 m l). Th e reaction m ixture
was h eated to 115 °C for 18 h . Th e m ixture was th en cooled to laboratory tem perature an d
th e precipitated com poun d 10 was filtered off, wash ed with dieth yl eth er (2 × 20 m l) an d
dried in vacuo. Th e product 10 was obtain ed as a dark green crystallin e powder (57 m g,
66%). 1H NMR (DMSO-d6): 8.59–8.10 m , 6 H; 7.75 d, 1 H, J = 7.7; 7.64 d, 2 H, J = 8.3;
7.48 bs, 1 H; 7.38 m , 3 H; 7.17 t, 1 H, J = 7.7; 5.94 m , 1 H; 5.49 m , 1 H; 3.90 s, 5 H; 1.57 bs,
2 H; 0.81 bs, 3 H. 13C NMR (DMSO-d6): 156.8 (C), 153.0 (C), 146.7 (CH), 146.3 (C), 144.6
(CH), 144.2 (C), 142.8 (CH), 141.9 (C), 141.7 (CH), 131.3 (CH), 127.2 (CH), 126.8 (C), 124.1
(2 × CH), 123.6 (C), 122.9 (CH), 122.3 (CH), 120.8 (CH), 111.3 (CH), 106.8 (CH), 91.2 (CH),
46.0 (CH2), 44.7 (CH3), 19.9 (CH2), 11.0 (CH3). HRMS (EI+): for C27H25N3O2S [M – H] calcu-
lated 455.1668, foun d 455.1667. UV-Vis (30% MeOH): λm ax = 600 n m , εm ax = 6.4 × 104
m ol–1 l cm –1
.
Study of Bin din g Cyan in e Dyes 8 an d 10 to DNA
Th e association of th e cyan in e dyes 8 an d 10 with DNA was studied by UV-Vis spectrom etry
accordin g to th e m eth od reported previously4. Bin din g con stan ts (K) were calculated from
ch an ges in absorban ce of cyan in e dyes (∆A) by n on lin ear regression usin g th e program
Letagroup spefo 2005. As polym er DNA ch ain s h ave various len gth s, K for dyes 8 an d 10
were calculated usin g con cen tration s of each repeatin g base pairs. Dyes 8 an d 10 were used
as 2.7 µM solution s in 1 m M ph osph ate buffer in H2O–MeOH–DMSO (68:30:2, v/v/v). DNA
was used as a 1 m M solution in 1 m M ph osph ate buffer in H2O–MeOH–DMSO (68:30:2,
v/v/v), an d th e solution was added to a solution of th e cyan in e dye.
This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic
(Projects MSM0021620806, KAN200200651, LC 512) and the Czech Science Foundation (Grant
No. 203/09/1311).
REFERENCES
1. Mishra A., Behera K. R., Behera K. P., Mishra K. B., Behera G. P.: Chem. Rev. 2000, 100,
1973.
2. Shirinian V. Z., Shimkin A. A.: Top. Heterocycl. Chem. 2008, 14, 75.
3. Fabian J., Nakazumi H., Matsuoka M.: Chem. Rev. 1992, 92, 1197.
4. Bříza T., Kejík Z., Císařová I., Králová J., Martásek P., Král V.: Chem. Commun. 2008,
1901.
5. Ihmels H., Otto D.: Top. Curr. Chem. 2005, 258, 161.
6. Armitage B. A.: Top. Heterocycl. Chem. 2008, 14, 11.
7. Hannah K. C., Armitage B. A.: Acc. Chem. Res. 2004, 37, 845.
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 7–8, pp. 1081–1090

1090
Bříza, Kejík, Králová, Martásek, Král:
8. Hilal H., Taylor J. A.: J. Biochem. Biophys. Methods 2008, 70, 1104.
9. Kazymov A. V.: Dokl. Akad. Nauk SSSR 1975, 220, 159.
10. Monich N. V., Lyubich M. S., Larina S. M., Khusid A. K.: Zh. Org. Khim. 1991, 27, 1761.
11. Rupe H.: Helv. Chim. Acta 1927, 10, 848.
12. Rupe H.: Helv. Chim. Acta 1927, 10, 305.
Collect. Czech. Chem. Commun. 2009, Vol. 74, Nos. 7–8, pp. 1081–1090