Novel rhodamine dyes via suzuki coupling of xanthone triflates with arylboroxins

Article abstract of DOI:10.1055/s-0029-1218535

Novel rhodamine dyes were prepared from xanthone precursors in a 'one-pot' procedure via reaction of the xanthone with trifluoromethanesulfonic anhydride followed by Pd-mediated Suzuki coupling between the xanthone triflate and an arylboroxin. Rhodamines with 9-(3- or 4-carboxyphenyl) and 9-(3-nitrophenyl) substituents were prepared by this procedure. The procedure also works well with thio- and selenoxanthones, but not with telluroxanthones. Georg Thieme verlag Stuttgart.

Full text of DOI:10.1055/s-0029-1218535

LETTER
A
Novel Rhodamine Dyes via Suzuki Coupling of Xanthone Triflates with
Arylboroxins
Rhodamines via
S
r
uzuki
C
a
oupling ndon D. Calitree, Michael R. Detty*
Department of Chemistry, University at Buffalo, 627 Natural Sciences Complex, North Campus, The State University of New York,
Buffalo, NY 14260-3000, USA
Fax +1(716)6456963; E-mail: mdetty@buffalo.edu
Received 28 September 2009
The 3,6-diaminoxanthylium core of most rhodamine and
rosamine dyes have been prepared by one of the synthetic
approaches outlined in Scheme 1. The reaction of a 3-ami-
nophenol with a phthalic anhydride derivative in the pres-
Abstract: Novel rhodamine dyes were prepared from xanthone
precursors in a ‘one-pot’ procedure via reaction of the xanthone
with trifluoromethanesulfonic anhydride followed by Pd-mediated
Suzuki coupling between the xanthone triflate and an arylboroxin.
Rhodamines with 9-(3- or 4-carboxyphenyl) and 9-(3-nitrophenyl) ence of strong acid gives rhodamine derivatives
(Scheme 1, a).12 Various rosamine dyes have been pre-
substituents were prepared by this procedure. The procedure also
works well with thio- and selenoxanthones, but not with telluroxan-
thones.
pared by the reaction of an aromatic aldehyde with a
3-aminophenol in the presence of acid or an oxidizing
agent such as chloranil (Scheme 1, b).13 The addition of
Key words: Suzuki coupling, arylations, palladium, rhodamine
dyes, arylboroxins
Grignard reagents or organolithium reagents to 3,6-diamino-
xanthones or 3,6-diaminochalcogenoxanthones have giv-
en various rhodamine and rosamine dyes as well as their
heavier chalcogen analogues (Scheme 1, c).10,14
The fluorescent and chemiluminescent properties of
rhodamine and rosamine dyes have been utilized in nu-
merous applications.1–5 Rhodamine and rosamine dyes
also localize preferentially in selected biological sites in-
cluding the mitochondria of tumor cells6 and in P-glyco-
protein in multidrug-resistant tumor cells.7 Rhodamine
and rosamine derivatives containing heavy atoms have in-
creased quantum yields for the generation of singlet oxy-
gen due to heavy-atom effects.8–10 These derivatives have
shown effectiveness as photosensitizers for the photody-
namic therapy of cancer as well as the photodynamic in-
activation of viral and bacterial pathogens.11
While the synthetic approaches outlined in Scheme 1 have
successfully given numerous rhodamine and rosamine de-
rivatives, these approaches still are limited to dyes derived
from acid-stable precursors and to dyes prepared from sta-
ble anionic precursors. In particular, rhodamine and ro-
samine derivatives with a 9-aryl substituent bearing a
carboxylic acid oxidation state (Figure 1) or other anion-
stabilizing group at the 3- or 4-position are not readily pre-
pared via these routes.
O
Z
O
Z
O
a)
CO2H
H
O
+
R2N
b)
OH
R2N
O
NR2
R12N
O
NR1
R12N
Z = OH, OR2, NH2, NHR2, NR2
O
NR1
2
O
2
2
Ar
Figure 1 ‘Nonaccessible’ rhodamine isomers
O
H
or
chloranil
H
+
R2N
c)
OH
O
Ar
R2N
O
NR2
An approach to ‘nonaccessible’ rhodamine isomers was
suggested by recent work in the Burgess lab where amines
reacted with xanthone triflates to give 9-amino rosamine
derivatives.15 If conditions were found where the xantho-
ne triflates and Suzuki coupling were compatible, then
rhodamine and rosamine derivatives might be prepared
with 9-substituents derived from arylboronic acids.16
Such an approach would allow the introduction of the 9-
aryl substituents of Figure 1 with carboxylic acid oxida-
tion states or anion-stabilizing groups in the 3- and 4-
position.
R2
1) R2MgX or
R2Li
2) H
R12N
E
NR1
2
R12N
E
NR1
2
E = O, S, Se, Te
E = O, S, Se, Te
Scheme 1 Three general synthetic routes to rosamine and rhodamine
dyes
SYNLETT 2010, No. x, pp 000A–000D
Advanced online publication: xx.xx.2010
x
x
.xx.
2
0
1
0
DOI: 10.1055/s-0029-1218535; Art ID: S11209ST
© Georg Thieme Verlag Stuttgart · New York

B
B. D. Calitree, M. R. Detty
LETTER
O
O O
O
O
TfO
OTf
E
O
E
S
S
F3C
CF3
PhB(OH)2
Me2N
NMe2
Me2N
NMe2
(3a)
1-E: E = O, S, Se, Te
2-E: E = O, S, Se, Te
Scheme 2 Triflate formation and reaction with phenylboronic acid
Chalcogenoxanthones 1-E reacted with 1.1 equivalents of boroxin couples to aryl halides and to one aryl triflate in
triflic anhydride (TfOTf) as shown in Scheme 2 to give 83% yield using 10 mol% Pd(PPh3)4 as catalyst in aque-
the triflated chalcogenoxanthones 2-E in quantitative ous dioxane with 2 M Na2CO3.19
yield in both anhydrous CH2Cl2 and in anhydrous MeCN.
The formation of 2-E was monitored spectrophotometri-
Suzuki coupling reactions using phenylboroxin 4a with
triflates 2-E were examined under anhydrous conditions
cally (550–600 nm) as was the loss of chalcogenoxan-
as summarized in Scheme 4.20 The addition of 1.1 equiv-
thone 1-E (350–400 nm).
alents of TfOTf to xanthones 1-E in anhydrous MeCN
The triflated xanthones 2-E react rapidly with water or al- was monitored spectrophotometrically until reaction was
cohols to regenerate the chalcogenoxanthones 1-E. The complete. Three equivalents of Na2CO3 were added fol-
triflates 2-E also reacted with phenylboronic acid (3a) in lowed by 1 equivalent of 4a and 10 mol% of
either CH2Cl2 or MeCN to regenerate the chalcogenoxan- PdCl2(PPh3)2. Pd(PPh3)4 was not an effective catalyst in
thones 1-E as shown in Scheme 2. Phenylboronic acid these reactions. While no reaction was observed below
(3a) was dehydrated to the corresponding boroxin (ben- 40 °C, at 55 °C the chromophore associated with the tri-
zeneboronic anhydride) 4a by heating at 110 °C for 6 flates 2-E was replaced by the chromophore of rosamine
hours.17 In contrast to the reactivity observed with phenyl- dyes 5-E, which we have previously prepared.10a,13b Fol-
boronic acid (3a), solutions of 2-E in either CH2Cl2 or lowing an aqueous workup, the dyes 5-E were isolated as
MeCN were stable in the presence of boroxin 4a.
a mixture of triflate and phenylboronate salts. The addi-
tion of 10% HPF6 to the workup allowed dyes 5-O, 5-S,
Commercially available boronic acids 3 were dehydrated
to boroxins 4 by heating at 110 °C for at least 6 hours
(Scheme 3).17 Boroxins have been used as coupling part-
ners in organopalladium chemistry in relatively few reac-
tions and with mixed results. Allylic alcohols are arylated
with boroxins using Pd(PPh3)4 although yields are lower
than those observed with arylboronic acids.18 3-Pyridyl-
–
and 5-Se to be isolated as the PF6 salts in 63%, 66%, and
49% yields (Scheme 4). Some unreacted xanthone 1-E
was also isolated from these reactions. The chromophore
for the formation of 5-Te was observed transiently, but 5-
Te could not be isolated from the reaction mixture. The
poor yields for the formation of 5-Te may be due to the
oxidative addition of the catalyst across the weak Te–C
bond.
Ar
O
Ar
B
B
110 °C
> 6 h
The coupling of triflates 2-O and 2-S with boroxins 4 with
10 mol% PdCl2(PPh3)2 was a general reaction20 for aryl-
boroxins with either electron-donating (4b and 4c) or
electron-withdrawing (4d–f) substituents as summarized
ArB(OH)2
O
O
B
Ar
3a, Ar = Ph
3b, Ar = 4-MeC6H4
4a, Ar = Ph
4b, Ar = 4-MeC6H4
–
in Scheme 4. Dyes 6-S10c and 7-S10c were isolated as PF6
3c, Ar = 4-MeOC6H4
3d, Ar = 4-(HO2C)C6H4
3e, Ar = 3-(HO2C)C6H4
3f, Ar = 3-O2NC6H4
4c, Ar = 4-MeOC6H4
4d, Ar = 4-(HO2C)C6H4
4e, Ar = 3-(HO2C)C6H4
4f, Ar = 3-O2NC6H4
salts while dyes 8-E–10-E were isolated as Cl– salts.21
The ‘one-pot’ procedure described here provides a new
route to rhodamine derivatives from xanthones. The
rhodamine isomers 8-E and 9-E (E = O, S) and the 9-(3-
Scheme 3 Preparation of arylboroxins 4 from arylboronic acids 3
Ar
X
1) PdCl2(PPh3)2 (10 mol%)
4 (1 equiv)
TfOTf (1.1 equiv)
MeCN
1-E
2-E
Na2CO3 (3 equiv), 55 °C
2) aq HX
Me2N
E
NMe2
5-O, Ar = Ph, X = PF6, 63% (86%)
5-S, Ar = Ph, X = PF6, 66% (99%)
5-Se, Ar = Ph, X = PF6, 49% (83%)
5-Te, Ar = Ph, X = PF6, <10%
Z
Ar =
Ar =
Z
6-S, Z = Me, X = PF6, 76% (88%)
7-S, Z = OMe, X = PF6, 79% (96%)
8-O, Z = CO2H, X = Cl, 66% (89%)
8-S, Z = CO2H, X = Cl, 60% (93%)
9-O, Z = CO2H, X = Cl, 65% (85%)
9-S, Z = CO2H, X = Cl, 65% (85%)
10-O, Z = NO2, X = Cl, 53% (86%)
10-S, Z = NO2, X = Cl, 57% (91%)
Scheme 4 Preparation of rosamine and rhodamine dyes with isolated yields (yields based on recovered xanthone 1-E)
Synlett 2010, No. x, A–D © Thieme Stuttgart · New York

LETTER
Rhodamines via Suzuki Coupling
Photochem. Photobiol. 2006, 76, 514.
(11) Detty, M. R.; Gibson, S. L.; Wagner, S. J. Med. Chem. 2004,
47, 3897.
C
nitrophenyl) derivatives 10-E (E = O, S) are novel
rhodamine structures.
(12) (a) Woodroofe, C. C.; Lim, M. H.; Bu, W.; Lippard, S. J.
Tetrahedron 2005, 61, 3097. (b) Ioffe, I. S.; Otten, V. F.
Zh. Obsh. Khim. 1962, 32, 1477.
(13) (a) Bandichhor, R.; Petrescu, A. D.; Vespa, A.; Kier, A. B.;
Schroeder, F.; Burgess, K. Bioconjugate Chem. 2006, 17,
1219. (b) Jiao, G.-S.; Castro, J. C.; Thoresen, L. H.; Burgess,
K. Org. Lett. 2005, 5, 3675.
Acknowledgment
We thank the Office of Naval Research (Grant No. N00014-09-1-
0217) and the donors of the Petroleum Research Fund, administered
by the American Chemical Society, for partial support of this work.
(14) (a) Gannon, M. K. II.; Detty, M. R. J. Org. Chem. 2007, 72,
2647. (b) Calitree, B. D.; Donnelly, D. J.; Holt, J. J.;
Gannon, M. K. II.; Nygren, C.; Sukumaran, D. K.;
Autschbach, J.; Detty, M. R. Organometallics 2007, 26,
6248.
(15) Wu, L.; Burgess, K. Org. Lett. 2009, 10, 1779.
(16) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.
(17) (a) Washburn, R. M.; Levens El Albright, C. F.; Billig, F. A.
Org. Synth., Coll. Vol. 4 1963, 68. (b) Wu, X.; Liu, X.;
Zhao, G. Tetrahedron: Asymmetry 2005, 16, 2299.
(18) Tsukamoto, H.; Uchiyama, T.; Suzuki, T.; Kondo, Y. Org.
Biomol. Chem. 2008, 6, 3005.
References and Notes
(1) As laser dyes, fluorescent labels, and probes: (a) Peterson,
O. G.; Tuccio, S. A.; Snavely, B. B. Appl. Phys. Lett. 1970,
17, 245. (b) Drexhage, K. H. Top. Appl. Phys. 1973, 1, 144.
(c) Karstens, T.; Kobs, K. J. Phys. Chem. 1980, 84, 1871.
(d) Preininger, C.; Mohr, G. J.; Klimant, I.; Wolfbeis, O. S.
Anal. Chim. Acta 1996, 334, 113. (e) Liu, J.; Diwu, Z.;
Leung, W.-Y.; Lu, Y.; Patch, B.; Hugland, R. P. Tetrahedron
Lett. 2003, 44, 4355. (f) Beija, M.; Afonso, C. A. M.;
Martinho, J. M. G. Chem. Soc. Rev. 2009, 38, 2410.
(2) Biosensing with FRET: (a) Adams, S. R.; Harootounian,
A. T.; Buechler, Y. J.; Taylos, S. S.; Tsien, R. Y. Nature
(London) 1991, 349, 694. (b) Martinez-Manez, R.;
Sancenon, F. Chem. Rev. 2003, 103, 4419.
(19) Cioffi, C. L.; Spencer, W. T.; Justin, J.; Richards, J. J.; Herr,
R. J. J. Org. Chem. 2004, 69, 2210.
(20) Trifluoromethanesulfonic anhydride (61.7 mL, 0.367 mmol,
1.1 equiv) was added to 3,6-dimethylaminoxanth-9-one 1-E
(1.0 equiv 0.334 mmol) in MeCN (15 mL). The resulting
solution was stirred for 20 min at ambient temperature.
PdCl2(PPh3)2 (23 mg, 0.033 mmol, 0.1 equiv), Na2CO3 (106
mg, 1.00 mmol, 3.0 equiv), and arylboroxin 4 (0.334 mmol,
1.0 equiv) were added, and the temperature was increased to
55 °C. The reaction was monitored by visible spectroscopy
for the appearance of the dye chromophore and disappear-
ance of the chromophore of triflate 2-E (20 min for 7-S to
4.5 h for 8-S). The reaction mixture was cooled to ambient
temperature, and H2O (15 mL) was added. The dye was
extracted with CH2Cl2 (5 × 20 mL), and the combined
extracts were concentrated onto SiO2 and dry loaded onto
a column of SiO2. A gradient elution system (10% Et2O–
CH2Cl2, 40% Et2O–CH2Cl2, 5% MeOH–CH2Cl2, and 10%
MeOH–CH2Cl2) separated recovered starting material from
rhodamine/rosamine dye isolated as a mixture of triflate and
boronate salts. The dye was dissolved in AcOH (3 mL), and
30% HPF6 or concentrated HCl was added dropwise until the
characteristic color of the rhodamine/rosamine faded. The
reaction mixture was poured into stirring ice H2O, and the
dye was collected by filtration. The ion exchange was
repeated for a total of 3 times. The success of the ion
exchange was confirmed by elemental analysis ( 0.4% in C,
H, N).
(3) Electrochemiluminescence: Richter, M. M. Chem. Rev.
2004, 104, 3003.
(4) Proton sensors: (a) de Silva, A. P.; Gunaratne, H. Q. N.;
Gunnlaugsson, T.; Huxley, A. J. M.; McCoy, C. P.;
Rademacher, J. T.; Rice, T. E. Chem. Rev. 1997, 97, 1515.
(b) Vogel, M.; Rettig, W.; Sens, D.; Drexhage, K. H. Chem.
Phys. Lett. 1988, 147, 452.
(5) Halide sensors: Anzenbacher, P. Jr.; Jursikova, K.; Sessler,
J. L. J. Am. Chem. Soc. 2000, 122, 9350.
(6) (a) Johnson, L. V.; Walsh, M. L.; Bockus, B. J.; Chen, L. B.
J. Cell. Biol. 1981, 88, 526. (b) Davis, S.; Weiss, M. J.;
Wong, J. R.; Lampidis, T. J.; Chen, L. B. J. Biol. Chem.
1985, 260, 13844. (c) Bernal, S. D.; Lampdis, T. J.;
McIsaac, R. M.; Chen, L. B. Science 1986, 222, 169.
(7) (a) Tombline, G.; Donnelly, D. J.; Holt, J. J.; You, Y.; Ye,
M.; Gannon, M. K.; Nygren, C. L.; Detty, M. R.
Biochemistry 2006, 45, 8034. (b) Tombline, G.; Holt, J. J.;
Gannon, M. K. II.; Donnelly, D. J.; Wetzel, B.; Sawada,
G. A.; Raub, T. J.; Detty, M. R. Biochemistry 2008, 47,
3294. (c) Gannon, M. K. II.; Holt, J. J.; Bennett, S. M.;
Wetzel, B. R.; Loo, T. W.; Bartlett, M. C.; Clarke, D. M.;
Sawada, G. A.; Higgins, J. W.; Tombline, G.; Raub, T. J.;
Detty, M. R. J. Med. Chem. 2009, 52, 3328.
(8) (a) Brasseur, N.; Menard, I.; Forget, A.; El Jastimi, R.;
Hamel, R.; Molfino, N. A.; van Lier, J. E. Photochem.
Photobiol. 2000, 72, 780. (b) Pal, P.; Zeng, H.; Durocher,
G.; Girard, D.; Li, T. C.; Gupta, A. K.; Giasson, R.;
Blanchard, L.; Gaboury, L.; Balassy, A.; Turmel, C.;
Laperriere, A.; Villeneuve, L. Photochem. Photobiol. 1996,
53, 161.
(9) (a) Shi, J.; Zhang, X.; Neckers, D. C. J. Org. Chem. 1992, 57,
4418. (b) Parker, C. A. In Advances in Photochemistry, Vol.
2; Noyes, W. A. Jr.; Hammond, G. S.; Pitts, J. N. Jr., Eds.;
Wiley Interscience: New York, 1964, 305.
(10) (a) Ohulchanskyy, T.; Donnelly, D. J.; Detty, M. R.; Prasad,
P. N. J. Phys. Chem. B 2004, 108, 8668. (b) Wagner, S. J.;
Skripchenko, A.; Donnelly, D. J.; Ramaswamy, K.; Detty,
M. R. Bioorg. Med. Chem. 2005, 13, 5927. (c) Gibson, S.
L.; Holt, J. J.; Ye, M.; Donnelly, D. J.; Ohulchanskyy, T. Y.;
You, Y.; Detty, M. R. Bioorg. Med. Chem. 2005, 13, 6394.
(d) Wagner, S. J.; Skripchenko, A.; Thompson-
(21) Analytical Data for 8-O
Mp >260 °C. 1H NMR (500 MHz, CD2Cl2): d = 8.35 (d, 2 H,
J = 8.0 Hz), 7.47 (d, 2 H, J = 8.0 Hz), 7.41 (d, 2 H, J = 9.5
Hz), 6.96 (dd, 2 H, J = 2.0, 9.5 Hz), 6.82 (d, 2 H, J = 2.0 Hz),
3.31 (s, 12 H). 13C NMR (75.5 MHz, CD3OD): d = 168.7,
159.2, 159.0, 158.1, 137.9, 133.8, 132.5, 131.1, 131.0,
115.8, 114.3, 97.7, 41.0. UV/vis: lmax (CH2Cl2) = 568 nm (e
+
1.10·105 M–1 cm–1). ESI-HRMS: m/z calcd for C24H23N2O3 :
387.1703; found: 387.1720.
Analytical Data for 8-S
Mp >260 °C. 1H NMR (500 MHz, CD3OD): d = 8.17 (d, 2
H, J = 7.0 Hz), 7.36 (d, 2 H, J = 7.0 Hz), 7.28 (d, 2 H, J = 9.5
Hz), 7.24 (d, 2 H, J = 2.5 Hz), 7.01 (dd, 2 H, J = 2.5, 9.5 Hz),
3.19 (s, 12 H). 13C NMR (75.5 MHz, CD3OD): d = 160.3,
154.2, 144.9, 139.3, 136.9, 130.5 (br, 2 C), 129.7, 119.4,
115.9, 106.0, 40.8. UV/vis: lmax (MeOH) = 582 nm (e
7.91·104 M–1 cm–1). ESI-HRMS: m/z calcd for
Montgomery, D.; Awatefe, H.; Donnelly, D. J.; Detty, M. R.
Synlett 2010, No. x, A–D © Thieme Stuttgart · New York

D
B. D. Calitree, M. R. Detty
LETTER
C24H23N2O2S+: 403.1475; found: 403.1468.
Analytical Data for 10-O
Analytical Data for 9-O
1H NMR (500 MHz, CD2Cl2): d = 8.53 (dd, 1 H, J = 2.0, 8.0
Hz), 8.28 (t, 1 H, J = 2.0 Hz), 7.91 (t, 1 H, J = 8.0 Hz), 7.81
(d, 1 H, J = 8.0 Hz), 7.27 (d, 2 H, J = 9.0 Hz), 6.98 (dd, 2 H,
Mp >260 °C. 1H NMR (500 MHz, CD3OD): d = 8.24 (d, 1
H, J = 8.0 Hz), 8.00 (s, 1 H), 7.72 (t, 1 H, J = 7.5 Hz), 7.62
(d, 1 H, J = 7.5 Hz), 7.26 (d, 2 H, J = 9.0 Hz), 7.03 (dd, 2 H,
J = 2.0, 9.0 Hz), 6.88 (d, 2 H, J = 2.0 Hz), 3.32 (s, 12 H). 13
NMR (75.5 MHz, CD2Cl2): d = 158.1, 157.8, 154.8, 148.7,
135.9, 131.4, 131.0, 125.4, 124.6, 115.1, 113.7, 97.2, 41.3.
UV/vis: lmax (CH2Cl2) = 574 nm (e 6.77·104 M–1 cm–1). ESI-
HRMS: m/z calcd for C23H22N3O3 : 388.1656; found:
388.1660.
C
J = 2.0, 9.0 Hz), 6.91 (d, 2 H, J = 2.0 Hz), 3.23 (s, 12 H). 13
C
NMR (75.5 MHz, CD2Cl2/CD3OD): d = 168.4, 159.0, 158.6,
158.0, 134.5, 133.4, 132.3, 132.2, 131.4, 131.2, 130.1,
115.4, 114.4, 97.5, 41.0. UV/vis: lmax (MeOH) = 562 nm (e
+
1.03·106 M–1 cm–1). ESI-HRMS: m/z calcd for C24H23N2O3 :
+
387.1703; found: 387.1704.
Analytical Data for 9-S
Analytical Data for 10-S
Mp >260 °C. 1H NMR (500 MHz, CD2Cl2): d = 8.50 (dt, 1
H, J = 1.0, 8.0 Hz), 8.20 (s, 1 H), 7.90 (t, 1 H, J = 8.0 Hz),
7.73 (d, 1 H, J = 8.0 Hz), 7.28 (d, 2 H, J = 9.0 Hz), 7.17 (d,
2 H, J = 2.0 Hz), 6.97 (dd, 2 H, J = 2.0, 9.0 Hz), 3.29 (s, 12
H). 13C NMR (75.5 MHz, CD2Cl2): d = 156.5, 153.9, 148.6,
144.7, 137.5, 136.0, 135.8, 130.8, 124.7, 124.4, 119.0,
116.1, 106.2, 41.0. UV/vis: lmax (CH2Cl2) = 593 nm
(e 1.02·105 M–1 cm–1). ESI-HRMS: m/z calcd for
C23H22N3O2S+: 404.1427; found: 404.1420.
1H NMR (500 MHz, CD3CO2D/CD2Cl2): d = 8.31 (d, 1 H,
J = 7.5 Hz), 8.02 (s, 1 H), 7.74 (t, 1 H, J = 7.5 Hz), 7.60 (d,
1 H, J = 7.5 Hz), 7.32 (d, 2 H, J = 9.5 Hz), 7.20 (d, 2 H,
J = 2.0 Hz), 6.95 (dd, 2 H, J = 2.0, 9.5 Hz), 3.25 (s, 12 H).
13C NMR (75.5 MHz, CD3OD): d = 160.6, 154.7, 145.4,
137.2, 136.5, 132.9, 131.4, 131.2, 129.5, 119.9, 116.3,
106.5, 40.8. UV/vis: lmax (MeOH) = 579 nm (e 7.26·104
M–1 cm–1). ESI-HRMS: m/z calcd for C24H23N2O2S+:
403.1475; found: 403.1467.
Synlett 2010, No. x, A–D © Thieme Stuttgart · New York

Copyright of Synlett is the property of Georg Thieme Verlag Stuttgart and its content may not
be copied or emailed to multiple sites or posted to a listserv without the copyright holder's
express written permission. However, users may print, download, or email articles for
individual use.