Click reactions with nitroxides

Article abstract of DOI:10.1055/s-0028-1088018

Copper-catalyzed azide-alkyne 1,3-dipolar cycloadditions (CuAAC) with paramagnetic azide and alkyne building blocks are described. This method provides a route to the synthesis of complex spin-labeled molecules such as amino acids, carbohydrates, drug mol

Full text of DOI:10.1055/s-0028-1088018

1336
PAPER
Click Reactions with Nitroxides
C
lick Reactions
a
with
N
itro
m
xides ás Kálai,a Wayne L. Hubbell,b Kálmán Hideg*a
a
Institute of Organic and Medicinal Chemistry, University of Pécs, P.O. Box 99, 7602 Pécs, Hungary
Fax +36(72)536219; E-mail: kalman.hideg@aok.pte.hu
Jules Stein Eye Institute and Department of Chemistry & Biochemistry, University of California Los Angeles,
Los Angeles, CA 90095-1662, USA
b
Received 24 November 2008
In the technique of site-directed spin-labeling, nitroxide
spin-labels are selectively introduced into proteins by re-
placing a native amino acid with cysteine, followed by se-
lective modification of the sulfhydryl group with a thiol-
specific reagent, most often a methanethiosulfonate.13
Proteins that contain native and reactive cysteine residues
pose a problem that is generally solved by replacing the
native cysteine residues with inert amino acids. Since this
is often not practical, a method for site-selectively intro-
ducing a nitroxide side chain based on chemistry foreign
to natural proteins would be extremely valuable. Click
chemistry is a possible candidate. In this study we evalu-
ate the applicability of both paramagnetic azides and acet-
ylenes in copper-catalyzed azide-alkyne 1,3-dipolar
cycloaddition (CuAAC) reactions (Scheme 1) that can po-
tentially be used to introduce nitroxide side chains into
proteins in a manner similar to that used earlier with fluo-
rophores.14 This chemistry was used by Jawalekar and co-
workers for the modification of adenosine using the para-
magnetic azide 1.15,16 In this paper we report an extension
of the click reaction to other biomolecules (BioMol) and
nitroxide-containing click reaction partners.
Abstract: Copper-catalyzed azide-alkyne 1,3-dipolar cycloaddi-
tions (CuAAC) with paramagnetic azide and alkyne building blocks
are described. This method provides a route to the synthesis of com-
plex spin-labeled molecules such as amino acids, carbohydrates,
drug molecules and biradicals.
Key words: acetylenes, amino acids, azides, click chemistry, free
radicals
The incorporation of nitroxides into various biological
and non-biological structures, followed by EPR analysis,
has emerged as an important technology during the last
two decades. The spin-labeling method, introduced by
McConnell and co-workers,1 is an effective tool with
which to explore the structure, dynamics and interactions
of complex biomolecules such as proteins, nucleic acids,
enzymes, and polysaccharides.2–4
Spin-labels can be introduced into selected parts of mac-
romolecules by covalent chemical bond formation with
site-directed specific reagents, such as methanethiosul-
fonates,5 or by genetic site-directed incorporation.6
The concept of ‘click chemistry’ was introduced by Kolb,
Finn and Sharpless7 as a ‘near perfect’ (very selective,
modular, high-yield and wide in scope) carbon–heteroat-
om bond forming reaction. The 1,3-dipolar cycloadditions
of organic azides with alkynes in the presence of Cu(I) or
Ru-catalysts fulfill the click criteria,8,9 albeit that the basic
azide-alkyne 1,3-dipolar cycloaddition has been well
known since the works of Huisgen.10 This reaction is an
established procedure for triazole synthesis in heterocy-
clic chemistry11 and its copper-catalyzed variation has
been established as one of the most wide-spread means for
the covalent assembly of complex molecules.12
N3
N3
N
N
O
2
•
•
O
1
N
O
5
N
O
3
N
•
•
•
O
4
Figure 1 Paramagnetic azides and acetylenes used for click reac-
tions
N
•
BioMol
N
N
+
R
BioMol N3
Paramagnetic azides 115 and 2,17 and acetylenes 3,18 419
and 5,20 have been synthesized previously (Figure 1).
Azides are available by nucleophilic substitution from
mesylates. Acetylenes can be prepared by reaction of 4-
oxo-2,2,6,6-tetramethylpiperidin-1-yloxil with the lithi-
um salt of triisopropylsilylacetylene, followed by elimina-
tion and deprotection.18 Alternatively, an elimination
reaction of a 1,2-dibromo derivative17 or Grignard reac-
tion of ethynylmagnesium bromide with 2,2,5-trimethyl-
3,4-dihydo-2H-pyrrol-1-oxide can be employed.20
•
R
N
•
R
•
N
N
+
R
N3
BioMol
BioMol
Scheme 1 Possible modification of biomolecules (Biomol) with
stable free nitroxide radical (R) spin-labels
SYNTHESIS 2009, No. 8, pp 1336–1340
Advanced online publication: 16.03.2009
x
x
.
x
x
.2
0
0
9
DOI: 10.1055/s-0028-1088018; Art ID: P13008SS
© Georg Thieme Verlag Stuttgart · New York

PAPER
Click Reactions with Nitroxides
1337
In addition to these monofunctional click reaction part- ples, biomolecules with an acetylene moiety were modi-
ners, cross-linking building blocks can also be accessed fied with spin-labeled azides. This idea can be inverted;
from the dibromo compound 6.19 Thus, reaction of com- for example, azido-containing biomolecules can be modi-
pound 6 with one equivalent of sodium azide in aqueous fied by paramagnetic acetylenes such as compound 4.
acetone, yielded the mixture of monoazido21 and diazido Thus, reaction of 4 with b-azido-pentaacetyl-D-glucopyr-
derivatives 7 and 8, respectively (Scheme 2).
anozide (19)23 or with protected DL-4-azidophenylalanine
21,24 gave the spin-labeled carbohydrate derivative 20 and
the spin-labeled DL-paramagnetic amino acid 22, respec-
tively (Scheme 3).
Br
Br
Br
N3
N3
N3
a
+
In conclusion, we have demonstrated with several exam-
ples that both paramagnetic azides and both paramagnetic
acetylenes are reasonable click reaction partners. All react
with the corresponding biomolecules under mild condi-
tions and the reaction can be accomplished using relative-
ly simple procedures. The application of this [3+2]
cycloaddition reaction for spin-labeling of a native protein
at properly modified side chains is underway.
N
O
6
N
N
O
8
•
•
•
O
c
b
7
CO2Me
S
MeO2SS
N3
BocHN
N3
N
O
9
•
N
O
•
Melting points were determined with a Boetius micro melting point
apparatus and are uncorrected. Elemental analyses (C, H, N, S)
were performed on a Fisons EA 1110 CHNS elemental analyzer.
The IR (Specord 85) spectra were in each case consistent with the
assigned structure. Mass spectra were recorded on a Thermoquest
Automass Multi and VG TRIO-2 instruments in the EI mode. The
ESI MS were recorded on Perkin–Elmer Sciex API III instrument
(0.1% TFA in H2O–MeCN, 1:1; flow rate: 0.2 mL/min). ESR spec-
tra were taken on Miniscope MS 200 in 10–4 M CHCl3 solution and
all mono-radicals gave a triplet line aN = 14.7–16.5 G. Optical rota-
tions were recorded at 25 °C, with a Perkin–Elmer 343 polarimeter.
Flash column chromatography was performed using Merck Kiesel-
gel 60 (0.040–0.063 mm). Qualitative TLC was carried out on com-
mercially prepared plates (20 × 20 × 0.02 cm) coated with Merck
Kieselgel GF254. Compounds 12, 14, N-(tert-butoxycarbonyl)-L-
cysteine methyl ester and all other reagents were purchased from
Fluka or Aldrich. Compounds 1,15 2,17 3,18 4,19 5,20 6,18 17,22 19,23
and 2124 were prepared according to published procedures.
10
Scheme 2 Reagents and conditions: (a) NaN3 (1.2 equiv), aq ace-
tone, 40 °C, 30 min.; 7 (16%), 8 (40%); (b) NaSSO2Me (2.0 equiv),
aq acetone, 50 °C, 30 min (59%); (c) N-(tert-butoxycarbonyl)-L-cy-
steine methyl ester (1.0 equiv), K2CO3 (1.0 equiv), CHCl3, reflux, 1 h
(48%).
The former can be functionalized further with nucleo-
philic substitution by heating with sodium methanethio-
sulfonate (NaSSO2CH3) in aqueous acetone, to yield
compound 9 as a cross-linking spin-label with both thiol-
specific and acetylene-specific functional groups. S-
Alkylation of N-Boc-L-cysteine methyl ester with com-
pound 7 in chloroform, in the presence of potassium car-
bonate, afforded amino acid 10 as an unnatural,
paramagnetic L-amino acid with an azido side chain capa-
ble of further conjugation in a click reaction (Scheme 2).
3-Azidomethyl-4-bromomethyl-2,2,5,5-tetramethyl-2,5-dihy-
dro-1H-pyrrol-1-yloxyl Radical (7) and Bis-3,4-diazidomethyl-
2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrol-1-yloxyl Radical (8)
To a solution of compound 6 (3.26 g, 10.0 mmol) in acetone (20
mL), NaN3 (780 mg, 12.0 mmol) in H2O (5 mL) was added and the
mixture was heated at 40 °C for 30 min. The mixture was diluted
with H2O (20 mL), acetone was evaporated off in vacuo, the aque-
ous phase was extracted with EtOAc (2 × 20 mL). The organic
phase was dried (MgSO4), filtered and evaporated. The residue was
purified by flash column chromatography (hexane–Et2O, 2:1). The
first band contained the dibromo compound 6, the second band con-
sisted of compound 7 [Rf = 0.31 (hexane–Et2O, 2:1)], then com-
pound 8 [Rf = 0.25 (hexane–Et2O, 2:1)].
Reaction of equimolar amounts of compounds 1 and 3, in
the presence of copper(I) iodide (0.4 equiv) in dimethyl
sulfoxide at 40 °C, gave the biradical compound 11
(Scheme 3). This reaction was applied to introduce a para-
magnetic group into biomolecules with an acetylene func-
tion, such as 17a-ethynylestradiol (12) or mestranol (14),
to give spin-labeled steroid derivatives 13 and 15, respec-
tively. Reaction of compound 8 with two equivalents of
17a-ethynylestradiol (12) and copper(I) iodide (0.8
equiv), gave the cross-linked bisteroid compound 16. Al-
ternative conditions for this reaction were found to be
treatment of a 2:1 mixture of the acetylene and azide in the
presence of ascorbate (1.5 equiv) and aqueous copper(II)
sulfate solution in N,N-dimethylformamide (Method B).
An excess of ascorbate was required because of consump-
tion of the reducing agent by nitroxide reduction to N-hy-
droxylamine. This reaction can be applied to the synthesis
of paramagnetically modified amino acids containing an
acetylene moiety. For example, treatment of the protected
S-propargyl cysteine22 (17) with azide 1, in the presence
of copper(I) iodide in dimethyl sulfoxide, yielded the
spin-labeled L-cysteine derivative 18. In the above exam-
7
Yield: 460 mg (16%); yellow solid; mp 58–60 °C.
IR (Nujol): 2100 (N3), 1650 (C=C) cm–1.
MS (EI): m/z (%) = 287/289 (9/9) [M+], 228/230 (18/18), 193 (40),
152 (68), 41 (100).
Anal. Calcd for C10H16BrN4O: C, 41.68; H, 5.60; N, 19.44. Found:
C, 41.55; H, 5.71; N, 19.25.
8
Yield: 1.12 g (45%); yellow solid; mp 95–97 °C.
IR (Nujol): 2100 (N3), 1660 (C=C) cm–1.
Synthesis 2009, No. 8, 1336–1340 © Thieme Stuttgart · New York

1338
T. Kálai et al.
PAPER
•
O
O
N
•
N
N
N
N
OH
OH
a
N
N
a
N
1 + 3
1 +
N
RO
R
RO
•
O
12 13
14 15
11
12, 14
13, 15
H
Me
N
OH
OH
b or c
N
N
12 + 8
HO
N
N
N
HO
N
O
•
16
BocHN
S
CO2Me
BocHN
S
CO2Me
a
•
+
1
O
N
N
N
N
17
18
CO2Et
CO2Et
a
BocHN
BocHN
OAc
OAc
N
O
N
N
O
OAc
N3
a
4 +
N3
N
4
+
OAc
O
N
•
AcO
N
AcO
N
OAc
OAc
N
•
22
21
O
19
20
Scheme 3 Reagents and conditions: (a) CuI (0.4 equiv), DMSO, 40 °C, 30–60 min (32–79%); (b) CuI (0.8 equiv), 12 (2.0 equiv), DMSO,
40 °C, 1 h (48%); (c) aq 10% CuSO4 (0.2 equiv), sodium ascorbate (2.2 equiv), DMF, r.t., 2 h, under N2, then MnO2 (1.0 equiv), O2, 15 min
(73%).
MS (EI): m/z (%) = 250 (64) [M+], 235 (11), 193 (73), 77 (100).
Methyl 3-(1-Oxyl-4-azidomethyl-2,2,5,5-tetramethyl-2,5-dihy-
dro-1H-pyrrol-3-ylmethylsulfanyl)-2-tert-butoxycarbonylami-
nopropionate Radical (10)
Anal. Calcd for C10H16N7O: C, 47.99; H, 6.44; N, 39.17. Found: C,
47.89; H, 6.35; N, 39.21.
A mixture of compound 7 (286 mg, 1.0 mmol) and N-(tert-butoxy-
carbonyl)-L-cysteine methyl ester (235 mg, 1.0 mmol) and K2CO3
(138 mg, 1.0 mmol) in CHCl3 (10 mL) was stirred under reflux for
1 h. After cooling, the inorganic salts were filtered off and washed
with CHCl3 (5 mL). The filtrate was washed with brine (10 mL),
and the organic phase was dried (MgSO4), filtered and evaporated.
The residue was purified by flash column chromatography (hex-
ane–EtOAc, 2:1) to yield compound 10.
3-Azidomethyl-4-methanethiosulfonylmethyl-2,2,5,5-tetra-
methyl-2,5-dihydro-1H-pyrrol-1-yloxyl Radical (9)
To a solution of compound 7 (286 mg, 1.0 mmol) in acetone (10
mL) and H2O (3 mL), NaSSO2CH3 (270 mg, 2.0 mmol) was added
and the solution was heated at 50 °C until complete consumption of
starting material was observed (~30 min). After cooling, the acetone
was evaporated off, H2O (7 mL) was added and the aqueous phase
was extracted with CHCl3 (2 × 10 mL). The organic phase was dried
(MgSO4), filtered and evaporated, then the residue was purified by
flash column chromatography (CHCl3–Et2O, 2:1) to give the title
compound.
Yield: 212 mg (48%); yellow oil; Rf = 0.30 (hexane–EtOAc, 2:1);
[a]D25 –17.6 (c 0.49, MeOH).
IR (neat): 3330 (NH), 2105 (N3), 1730, 1700 (C=O) cm–1.
MS (EI): m/z (%) = 442 (48) [M+], 412 (1), 299 (4), 182 (37), 150
(90), 57 (100).
Yield: 188 mg (59%); orange solid; Rf = 0.27 (CHCl3–Et2O, 2:1);
mp 32–34 °C.
IR (Nujol): 2100 (N3), 1650 (C=C) cm–1.
MS (EI): m/z (%) = 319 (5) [M+], 305 (4), 166 (12), 79 (100).
Anal. Calcd for C19H32N5O5S: C, 51.57; H, 7.29; N, 15.82. Found:
C, 51.55; H, 7.38; N, 15.66.
Cu-Catalyzed 1,3-Dipolar Addition; General Procedure
A solution of azide 1, 8, 19 or 21 (2.0 mmol) and acetylene 3, 4, 12,
14 or 17 (2.0 mmol or 4. 0 mmol for reaction with 8) and CuI (152
mg, 0.8 mmol or 304 mg, 1.6 mmol for compound 8) was heated to
Anal. Calcd for C11H19N4O3S2: C, 41.36; H, 6.00; N, 17.54. Found:
C, 41.18; H, 6.02; N, 17.50.
Synthesis 2009, No. 8, 1336–1340 © Thieme Stuttgart · New York

PAPER
Click Reactions with Nitroxides
1339
40 °C in DMSO (8 mL) and kept at this temperature until complete
consumption of starting materials was observed (indicated by TLC;
30–60 min). The solution was diluted with H2O (30 mL) and the
precipitate was filtered and purified further by flash column chro-
matography. When no precipitation occurred, the mixture was ex-
tracted with EtOAc (2 × 10 mL) and the organic phase was dried
(MgSO4), filtered and evaporated. The residue was purified by flash
column chromatography (CHCl3–Et2O, 2:1 or CHCl3–MeOH, 9:1)
to yield compounds 13, 15, 16, 18, 20 or 22 as solids in 32–79%
yields.
Yield: 538 mg (32%); beige solid; mp 220–222 °C; Rf = 0.50
(CHCl3–MeOH, 4:1); [a]D25 +63 (c 0.17, MeOH).
IR (Nujol): 3350 (OH), 1655 (C=C) cm–1.
MS (ESI): m/z = 843 [M + H]+.
Anal. Calcd for C50H64N7O5: C, 71.23; H, 7.65; N, 11.63. Found: C,
71.05; H, 7.54; N, 11.68.
Method B: A mixture of diazide 8 (125 mg, 0.5 mmol), a-ethy-
nylestradiol 12 (296 mg, 1.0 mmol), sodium L-ascorbate (217 mg,
1.1 mmol) and 10% aq CuSO4 (0.16 mL, 0.1 mmol) in DMF (7 mL),
was stirred under N2 at r.t. for 2 h. The mixture was poured onto a
H2O-ice mixture (30 mL) and allowed to reach r.t. The mixture was
filtered and the collected solid was washed with H2O (10 mL). The
solid was dried at r.t. then dissolved in a mixture of CHCl3–MeOH
(2:1, 30 mL), MnO2 (43 mg, 0.5 mmol) was added and O2 was bub-
bled through for 15 min. The MnO2 was filtered off and the solvent
was removed under vacuum to afford compound 16 with the same
spectroscopic and physical data as listed above for Method A.
4-{1-(1-Oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-1H-[1,2,3]tri-
azol-4-yl}-2,2,6,6-tetramethyl-1,2,3,6-tetrahydropyridin-1-yl-
oxyl Biradical (11)
Compounds 1 (2.0 mmol, 394 mg) and 3 (356 mg, 2.0 mmol) were
used to obtain compound 11, which was purified by column chro-
matography (CHCl3–Et2O, 2:1).
Yield: 420 mg (56%); pink solid; mp 129–132 °C; Rf = 0.23
(CHCl3–Et2O, 2:1).
IR (Nujol): 1675 (C=N), 1650 (C=C) cm–1.
Yield: 307 mg (73%); beige solid.
ESR (10–4 M solution in H2O containing 1% MeOH) consisted of
11 lines of 5:12:13:1:5:12:5:1:13:12:5 intensity ratio.
MS (EI): m/z (%) = 375 (8) [M+], 346 (22), 330 (20), 140 (80), 74
Methyl (S)-2-tert-Butoxycarbonylamino-3-[{1-(1-oxyl-2,2,6,6-
tetramethylpiperidin-4-yl)-1H-[1,2,3]triazol-4-yl}methyl-
thio]propionate Radical (18)
Compounds 1 (2.0 mmol, 394 mg) and 17 (546 mg, 2.0 mmol) were
used to obtain compound 18, which was purified by column chro-
matography (CHCl3–MeOH, 9:1).
(100).
Anal. Calcd for C20H33N5O2: C, 63.97; H, 8.86; N, 18.65. Found: C,
64.05; H, 8.75; N, 18.53.
Yield: 742 mg (79%); pink solid; mp 101–103 °C; Rf = 0.28
(CHCl3–MeOH, 9:1); [a]D25 –7 (c 0.17, MeOH).
IR (Nujol): 3400 (NH), 1740, 1700 (C=O) cm–1.
17-{4-(1-Oxyl-2,2,6,6-tetramethylpiperidin-4-yl)-1H-[1,2,3]tri-
azol-4-yl}-13-methyl-7,8,9,11,12,13,14,15,16,17-decahydro-6H-
cyclopenta[a]phenanthren-3,17-diol Radical (13)
MS (ESI): m/z = 471 [M + H]+.
Compounds 1 (2.0 mmol, 394 mg) and 12 (592 mg, 2.0 mmol) were
used to obtain compound 13, which was purified by column chro-
matography (CHCl3–MeOH, 9:1).
Anal. Calcd for C21H36N5O5S: C, 53.60; H, 7.71; N, 14.88. Found:
C, 53.53; H, 7.64; N, 14.75.
Yield: 424 mg (43%); orange solid; mp 162–164 °C; Rf = 0.33
(CHCl3–MeOH, 9:1); [a]D25 +31 (c 0.16, MeOH).
IR (Nujol): 3300 (OH), 1610, 1580 (C=C) cm–1.
MS (EI): m/z (%) = 493 (2) [M+], 476 (7), 270 (34), 140 (80), 41
(100).
1-b-D-{4-(1-Oxyl-2,2,5,5-tetramethyl-2,5-dihydro-1H-pyrrol-3-
yl)-1H-[1,2,3]triazol-1-yl}-2,3,4,6-tetra-O-acetylglucopyranose
Radical (20)
Compounds 4 (328 mg, 2.0 mmol) and 19 (746 mg, 2.0 mmol) were
used to obtain compound 20.
Yield: 730 mg (68%); pale-yellow solid; mp 88–89°C; Rf = 0.34
(CHCl3–MeOH, 9:1); [a]D25 –50 (c 0.18, MeOH).
IR (Nujol): 1760, 1750 (C=O), 1630 (C=C) cm–1.
Anal. Calcd for C29H41N4O3: C, 70.56; H, 8.37; N, 11.35. Found: C,
70.48; H, 8.44; N, 11.33.
MS (ESI): m/z = 538 [M + H]+.
4-{4-(17-Hydroxy-3-methoxy-13-methyl-
7,8,9,11,12,13,14,15,16,17-decahydro-6H-cyclopenta[a]phenan-
thren-17-yl)-1H-[1,2,3]triazol-1-yl}-2,2,6,6-tetramethylpiperi-
din-1-yloxyl Radical (15)
Anal. Calcd for C24H33N4O10: C, 53.63; H, 6.19; N, 10.24. Found:
C, 53.80; H, 6.11; N, 10.41.
Compounds 1 (2.0 mmol, 394 mg) and 14 (620 mg, 2.0 mmol) were
used to obtain compound 15.
Ethyl 2-tert-Butoxycarbonylamino-3-{4-[4-(1-oxyl-2,2,5,5-tet-
ramethyl-2,5-dihydro-1H-pyrrol-3-yl)-1H-[1,2,3]triazol-1-
yl]phenyl}propionate Radical (22)
Reaction of D/L-amino acid 21 (468 mg, 2.0 mmol) and compound
4 (328 mg, 2.0 mmol) afforded a brown residue after work-up.
Chromatographic purification (CHCl3–Et2O, 2:1) gave compound
22.
Yield: 608 mg (60%); orange solid; mp 106–108 °C; Rf = 0.41
(CHCl3–MeOH, 9:1); [a]D25 +29 (c 0.16, MeOH).
IR (Nujol): 3300 (OH), 1610, 1590 (C=C) cm–1.
MS (EI): m/z (%) = 507 (1) [M+], 492 (5), 284 (5), 227 (100), 124
(55), 41 (59).
Yield: 637 mg (64%); pale-yellow solid; mp 125–126 °C; Rf = 0.35
(CHCl3–Et2O, 2:1).
IR (Nujol): 3280 (NH), 1700, 1695 (C=O), 1620 (C=C) cm–1.
Anal. Calcd for C30H43N4O3: C, 70.97; H, 8.54; N, 11.04. Found: C,
70.88; H, 8.62; N, 10.95.
MS (ESI): m/z = 499 [M + H]+.
3,4-Bis-{4-(3,17-dihydroxy-13-methyl-
7,8,9,11,12,13,14,15,16,17-decahydro-6H-cyclopenta[a]phenan-
thren-17-yl)-1H-[1,2,3]triazol-1-ylmethyl}-2,2,5,5-tetramethyl-
2,5-dihydro-1H-pyrrol-1-yloxyl Radical (16)
Anal. Calcd for C26H36N5O5: C, 62.63; H, 7.28; N, 14.05. Found: C,
62.58; H, 7.14; N, 14.10.
Method A: Compounds 8 (2.0 mmol, 500 mg) and 12 (1.18 g, 4.0
mmol) were used to obtain compound 16.
Synthesis 2009, No. 8, 1336–1340 © Thieme Stuttgart · New York

1340
T. Kálai et al.
PAPER
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Acknowledgment
This work was supported by a grant from the Hungarian National
Research Fund (OTKA-NKTH K67597, T48334 and M045190).
The authors thank to Dr. József Jekő (Alkaloida, Tiszavasvár,
Hungary) for mass spectral measurements, Maria Balog for techni-
cal assistance and Krisztina Kis for elemental analysis.
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Synthesis 2009, No. 8, 1336–1340 © Thieme Stuttgart · New York