Synthesis and spectroscopic properties of rosamines with cyclic amine substituents

Article abstract of DOI:10.1021/jo800902j

(Chemical Equation Presented) There is a close structural similarity between rosamines A and rhodamines B, yet a diversity of structures in the rosamine class and their spectral properties have yet to be explored in depth. This manuscript describes a concise, scalable, solution-phase method to obtain rosamines 1-5 and 12-15, which include some water-soluble derivatives. In one test case (for 15) an illustrative protein conjugate was also formed. Throughout these products were isolated and purified, and the syntheses were found to be scalable. Further, the rosamines with these cyclic amine substituents display solvent-dependent fluorescence intensities, and high quantum yields in chlorinated hydrocarbons. In some cases the nature of the cyclic amine substituent was shown to modulate the fluorescence of the parent molecules in pH-dependent ways. The ring size of those amine substituents also correlated with some of their spectroscopic properties. Several water-soluble rosamines were prepared from some of the addition products 1-5, and one of these, 15, was efficiently conjugated to avidin via an amide linkage. The spectroscopic properties of 15 and 15-avidin in aqueous media were very similar.

Full text of DOI:10.1021/jo800902j

Synthesis and Spectroscopic Properties of Rosamines with Cyclic
Amine Substituents
Liangxing Wu and Kevin Burgess*
Department of Chemistry, Texas A & M UniVersity, Box 30012, College Station, Texas 77841
burgess@tamu.edu
ReceiVed April 25, 2008
There is a close structural similarity between rosamines A and rhodamines B, yet a diversity of structures
in the rosamine class and their spectral properties have yet to be explored in depth. This manuscript
describes a concise, scalable, solution-phase method to obtain rosamines 1-5 and 12-15, which include
some water-soluble derivatives. In one test case (for 15) an illustrative protein conjugate was also formed.
Throughout these products were isolated and purified, and the syntheses were found to be scalable. Further,
the rosamines with these cyclic amine substituents display solvent-dependent fluorescence intensities,
and high quantum yields in chlorinated hydrocarbons. In some cases the nature of the cyclic amine
substituent was shown to modulate the fluorescence of the parent molecules in pH-dependent ways. The
ring size of those amine substituents also correlated with some of their spectroscopic properties. Several
water-soluble rosamines were prepared from some of the addition products 1-5, and one of these, 15,
was efficiently conjugated to avidin via an amide linkage. The spectroscopic properties of 15 and 15-
avidin in aqueous media were very similar.
Introduction
aromatic ring attenuates nonradiative decay via free rotation of
the aryl group, while lack of a carboxylic acid makes the system
impossible to lactonize to much less fluorescent forms at low
pH, i.e., it is less acid-sensitive than fluorescein.
There is considerable potential for modulating the chemical
and spectroscopic properties of rosamines by varying the C-meso
substituent, but the scope of such research is limited by synthetic
constraints. The first syntheses of rosamines center around
condensation reactions of the type shown in Figure 1a.1,2,8,9 Such
approaches tend to involve high temperatures. They can be
facilitated via microwave heating,10 but even in those cases the
conversion to product is not very efficient. Other work deals
with modification of the 2-carboxylate group of rhodamines (not
From a structural perspective, rosamines A1,2 relate to
rhodamines B in much the same way that Tokyo Green C3,4
and Pennsylvania Green D5,6 are similar to fluorescein E. The
recent excitement caused by these fluorescein derivatives7 can
be attributed to the fact that the methyl group on the meso
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10.1021/jo800902j CCC: $40.75
Published on Web 10/18/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 8711–8718 8711

Wu and Burgess
FIGURE 1. Syntheses of rosamine dyes via (a) the conventional
condensation approach and (b) nucleophilic additions to xanthones
(shown for a solid-supported synthesis).
shown),11,12 but the parent systems are also made via condensa-
tion reactions at elevated temperatures. An alternative approach
to syntheses of rosamine dyes is to add an organo-lithium or
-magnesium compound to the carbonyl group of a xanthone
and then protonate to cause loss of water and generate the
product.13 That approach has been applied extensively by Detty
and co-workers14 to make rosamines (and especially S-, Se-,
and Te-analogs),15-17 for their photodynamic and other me-
dicinal properties.18-22 Similarly, attack of aryl Grignards on
solid-supported xanthones F was used in a supported parallel
synthesis approach to a library of rosamine derivatives G (Figure
1b).23
For several years, work in our laboratory has focused on a
solution-phase approach to the rosamine dye cores 1-5 with a
variety of meso-substituents a-i. The attributes of this approach
are that (i) the xanthone starting materials were obtained via
SNAr reactions of a relatively accessible ditriflate; (ii) the meso-
substituent can be alkyl or aryl; and (iii) the products were
isolated as discrete compounds that could be purified to obtain
accurate spectroscopic data, whereas this is difficult via a solid-
phase approach. The spectroscopic data obtained on the products
show some unexpected dependence on solvent and amine ring
size. Further, some of the dyes could be modified to water-
soluble forms with functional groups to allow conjugation to
proteins.
unsymmetrical product was also explored briefly, and this
proved to be feasible (reaction 1), presumably because the
product of the first addition (a vinylogous amide) is less reactive
than the starting material.
Results and Discussion
Synthesis. The ditriflate 6 was easily prepared on a multigram
scale without chromatography via a slight modification of a
known procedure.24,25 This reacted with piperidine, morpholine,
Boc-piperazine, and pyrrolidine to give the symmetrical products
7-10 as indicated in Table 1. Stepwise addition to give an
Ketones 7-11 are not exceptionally reactive. They are
vinylogous ureas, but they combined efficiently with organo-
lithium and Grignard reagents to give tertiary alcohols. These
adducts were not isolated; instead they were treated with aqueous
(11) Dujols, V.; Ford, F.; Czarnik, A. W. J. Am. Chem. Soc. 1997, 119, 7386–
7387.
(12) Adamczyk, M.; Grote, J. Bioorg. Med. Chem. Lett. 2000, 10, 1539–
1541.
8712 J. Org. Chem. Vol. 73, No. 22, 2008

Rosamine DeriVatiVes
TABLE 1. Amination of the Ditriflyl Xanthone 6
The dimethoxy compound 1h was also modified to give a
water soluble analog. Scheme 1 outlines how this was achieved
via a demethylation procedure to give the phenol 14. That
derivative has no functional group for direct attachment to a
biomolecule, and in any case it is only slightly water-soluble.
Consequently, the two phenol groups were alkylated with tert-
butyl bromoacetate, and then the esters were deprotected to give
the diacid 15. Diacid 15 is a polar, water-soluble compound
that was conveniently isolated by washing away residual starting
materials using acetonitrile/ether. This derivative was conjugated
to avidin to explore how its spectroscopic properties are
modulated by coupling to this protein (see below).
Another modification to give a water-soluble probe is shown
in reaction 3. Here the two tert-butyloxycarbonyl groups of
rosamine 4h were removed with acid to give, after neutralization
and ion exchange, the bisamine 5h. Compound 5h dissolves
easily in water; its spectroscopic properties in buffers were
explored extensively (see below).
acid to give the desired products as indicated in Table 2. All
the compounds were isolated in tens to hundreds of milligram
amounts via flash chromatography. However, in three cases (1d,
1h, and 4d), several grams of product were isolated via
recrystallization of crude materials obtained on a larger scale.
The benzenoid compounds shown in Table 2 have methyl-,
iodo-, and one or two methoxy substituents on the meso-aryl
group. Steric effects associated with ortho-substitution did not
seem to have any significant adverse effects on the syntheses
of any compounds in this series.
None of the compounds shown in Table 2 are water-soluble,
but several modifications were tested to demonstrate that water-
soluble derivatives are accessible from some of the products.
Reaction 2, for instance, illustrates how the piperazine derivative
4d could be unmasked to give a bisammonium salt. That salt
was not isolated, but an aqueous solution of it was strongly
fluorescent, and it was neutralized then reacted with tert-butyl
bromoacetate to give the diester 13. Treatment of diester 13
with trifluoroacetic acid gave a product that was water-soluble
and fluorescent but difficult to isolate. The inference of these
experiments is that the intermediate ammonium salt and the
diacid derived from it could be prepared via deprotection of
their precursors and then used to couple to biomolecules.
(13) Beach, S. F.; Hepworth, J. D.; Mason, D.; Swarbrick, E. A. Dyes Pigm.
1999, 42, 71–77.
(14) Gannon, M. K., II.; Detty, M. R. J. Org. Chem. 2007, 72, 2647–2650.
(15) Brennan, N. K.; Donnelly, D. J.; Detty, M. R. J. Org. Chem. 2003, 68,
3344–3347.
(16) Del Valle, D. J.; Donnelly, D. J.; Holt, J. J.; Detty, M. R. Organome-
tallics 2005, 24, 3807–3810.
(17) Holt, J. J.; Calitree, B. D.; Vincek, J.; Gannon, M. K., II.; Detty, M. R.
J. Org. Chem. 2007, 72, 2690–2693.
(18) Holt, J. J.; Gannon, M. K.; Tombline, G.; McCarty, T. A.; Page, P. M.;
Bright, F. V.; Detty, M. R. Bioorg. Med. Chem. 2006, 14, 8635–8643.
(19) Gibson, S. L.; Holt, J. J.; Ye, M.; Donnelly, D. J.; Ohulchanskyy, T. Y.;
You, Y.; Detty, M. R. Bioorg. Med. Chem. 2005, 13, 6394–6403.
(20) Wagner, S. J.; Skripchenko, A.; Donnelly, D. J.; Ramaswamy, K.; Detty,
M. R. Bioorg. Med. Chem. 2005, 13, 5927–5935.
(21) Tombline, G.; Donnelly, D. J.; Holt, J. J.; You, Y.; Ye, M.; Gannon,
M. K.; Nygren, C. L.; Detty, M. R. Biochemistry 2006, 45, 8034–8047.
(22) Tombline, G.; Holt, J. J.; Gannon, M. K.; Donnelly, D. J.; Wetzel, B.;
Sawada, G. A.; Raub, T. J.; Detty, M. R. Biochemistry 2008, 47, 3294–3307.
Spectroscopic Properties of Rosamine Derivatives. Table
3 shows UV-vis absorption wavelength maxima, molar extinction
(23) Ahn, Y.-H.; Lee, J.-S.; Chang, Y.-T. J. Am. Chem. Soc. 2007, 129, 4534–
4535.
(24) Miller, E. W.; Albers, A. E.; Pralle, A.; Isacoff, E. Y.; Chang, C. J.
J. Am. Chem. Soc. 2005, 127, 16652–16659.
(25) Wu, L.; Burgess, K. Org. Lett. 2008, 10, 1779–1782.
J. Org. Chem. Vol. 73, No. 22, 2008 8713

Wu and Burgess
TABLE 2. Synthesis of Rosamine Derivatives
coefficients, fluorescence emission maxima, fluorescence peak
broadness (measured by full width at half-maximum height, fwhm),
and quantum yields for the target compounds. These data were
collected for all the compounds when dichloromethane was used
as solvent. However, we observed that the meso aliphatic com-
pounds 1a and 1b tend to be less stable in polar media.
Relative to the other compounds, the meso-methyl derivative
1a has blue-shifted absorption and fluorescence maxima, but
8714 J. Org. Chem. Vol. 73, No. 22, 2008

Rosamine DeriVatiVes
TABLE 3. Spectral Properties of Rosamines
SCHEME 1. Synthesis of Water-Soluble Rosamine
λmax abs
(nm)
λfluor
(nm)
fwhm
(nm)a
ꢀmax
Φfb
CH2Cl2
1a
1b
1c
1d
1e
1f
1g
1h
1i
2i
3i
12i
4i
4d
4h
13
555
566
565
570
562
566
582
565
565
554
558
559
555
561
555
564
101,900
95,900
570
582
585
590
582
585
603
586
582
575
573
579
577
585
580
586
35
34
37
38
35
37
41
36
34
37
31
36
40
42
40
44
0.91 ( 0.05
0.87 ( 0.01
0.89 ( 0.01
0.85 ( 0.01
0.91 ( 0.02
0.92 ( 0.03
0.33 ( 0.01c
0.86 ( 0.04
0.89 ( 0.01
0.80 ( 0.01
1.00 ( 0.09
0.59 ( 0.01
0.47 ( 0.01
0.41 ( 0.01
0.49 ( 0.02
0.007
106,000
115,400
116,700
118,000
107,600
87,300
123,600
106,900
127,400
111,000
120,800
77,900
109,300
62,800
EtOH
1c
1d
1e
1f
1g
1i
2i
3i
12i
4i
14
15
5h
564
568
561
564
581
564
556
558
560
556
557
557
561
86,700
70,700
100,400
115,600
56,500
119,500
98,600
120,500
97,000
108,000
71,000
84,800
81,000
588
592
582
586
606
584
582
576
582
580
580
580
582
39
40
37
37
43
36
38
33
38
39
37
38
45
0.30 ( 0.01
0.28 ( 0.01
0.31 ( 0.01
0.36 ( 0.01
0.10 ( 0.01c
0.28 ( 0.01
0.16 ( 0.01
0.95 ( 0.01
0.14 ( 0.01
0.19 ( 0.01
0.26 ( 0.01
0.49 ( 0.02
0.008
the extinction coefficient and the quantum yield are comparable
with the rosamines having aromatic meso-substituents. This
indicates that the meso-aryl groups are not significantly conju-
gated to the xanthene core, presumably because steric factors
involving these substituents make it energetically unfavorable
for the molecules to become planar. Curiously, the benzyl
compound 1b is spectroscopically more similar to the meso-
phenyl compound 1c than to the methyl derivative 1a. The
emission peak broadness values in this series range from 31 to
44 nm (CH2Cl2); these values are comparable with a typical
rhodamine, rhodamine B (36 nm in CH2Cl2).
0.1 M phosphate buffer, pH ) 7.4
15
5h
567
542
76,500
71,500
594
568
42
41
0.13 ( 0.01
0.14 ( 0.02
a Full width at half-maximum height: a measure of the sharpness of
the fluorescence peaks. b Average of three measurements and rhodamine
101 was used as a standard (Φ ) 1.0 in EtOH).32 c Cresyl Violet (Φ )
0.54 in MeOH)33 was used as a standard.
on similar effects in the context of 4-amino-1,8-napthalimide27
and 4-aminophthalimide derivatives.28 The origin of these effects
is thought to be that smaller-ring cyclic amine substituents are
less inclined to invert at nitrogen atom in the excited state, so
this pathway for nonradiative loss is disfavored and the overall
quantum yields are higher.29
Differences between the piperidine derivatives 1 and the
pyrrolidine derivative 3i are accentuated in a more polar medium
(ethanol, second section of Table 3). This is also consistent with
previous reports; 27-29 more polar solvents favor intramolecular
charge transfer from cyclic amines where the N-atom more
closely approximates a planar arrangement as a result of facile
interconversion between ring forms. The interconversion is more
rapid for piperidine derivatives than the pyrrolidine-based
compounds.27 Overall, the quantum yield data for the com-
pounds in ethanol are, in all the cases measured, less than that
in dichloromethane. This is a somewhat unexpected finding since
other rosamines are known to be very highly fluorescent even
Several trends emerge from the spectroscopic data recorded
for the compounds dissolved in dichloromethane. Compound
1g is most red-shifted in terms of the wavelengths for absorption
maxima and the fluorescence emission maxima. However, the
quantum yield for this compound was about a third of that
observed for other members in this series; this observation can
be attributed to partial fluorescence quenching via electron
transfer from the S-containing heterocycle.26 Quantum yields
for the Boc-piperazine derivatives were also low relative to other
members of the series. The very best quantum yield observed
was for the pyrrolidine derivative 3i. To the best of our
knowledge, this work is the first to uncover a relationship
between the ring size of the peripheral amines in rosamines
and their fluorescent quantum yields, but others have commented
(27) Saha, S.; Samanta, A. J. Phys. Chem. A 2002, 106, 4763–4771.
(28) Bhattacharya, B.; Samanta, A. Chem. Phys. Lett. 2007, 442, 316–321.
(29) Zachariasse, K. A.; Grobys, M.; von der Haar, T.; Hebecker, A.; Il’ichev,
Y. V.; Morawski, O.; Rueckert, I.; Kuehnle, W. J. Photochem. Photobiol., A
1997, 105, 373–383.
(26) DeSilva, A. P.; Gunaratne, H. G. N.; Gunnlaugsson, T.; Huxley, A. J. M.;
McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997, 97, 1515–1566.
J. Org. Chem. Vol. 73, No. 22, 2008 8715

Wu and Burgess
TABLE 4. Spectral Properties of Rosamines 1i in Different
Solvents
solvent
dielectric
spectral properties
dipole
λmax abs
(nm)
λfluor
constant
moment
(nm)
Φfa
CHCl3
EtOAc
THF
CH2Cl2
acetone
EtOH
MeOH
CH3CN
DMF
4.8
6.1
7.5
1.04
1.78
1.75
1.60
2.88
1.69
1.70
3.92
3.82
566
567
567
565
564
564
562
562
570
580
586
588
582
587
584
582
586
594
0.95 ( 0.02
0.19 ( 0.01
0.32 ( 0.01
0.89 ( 0.01
0.15 ( 0.01
0.28 ( 0.01
0.19 ( 0.01
0.14 ( 0.01
0.20 ( 0.01
8.9
21.0
24.3
33.0
36.6
38.3
a Rhodamine 101 (Φ ) 1.0 in EtOH) was used as a standard.
FIGURE 2. (a) UV-vis absorption (10-6 M) and (b) fluorescence
spectra (10-7 M, excited at λmax abs), for representative rosamines in
CH2Cl2.
in alcohols; for instance, tetramethyl rosamine, has a quantum
yield of 0.84 in MeOH.30,31
FIGURE 3. Fluorescence intensity (excited at 562 nm) of 1i at 1.4 ×
10-7 M in a CH2Cl2/MeOH mixed solvent system. The shoulder
observed at around 560 nm is an artifact caused by the excitation source,
which was necessarily close in wavelength.
Quantum yields of 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene
(BODIPY) dyes with meso-aryl substituents are thought to be
increased if the aryl group has an ortho-substituent, or if rotation
of the aryl is otherwise restricted.34 Similarly, fluorescein and
derivatives with 2-substituted meso-aryl groups are far more
fluorescentthanthecorrespondingmeso-phenylcompound.3,4However,
it is remarkable that in the series outlined in Table 3, unlike
the analogous BODIPY dyes, rosamines with ortho-substi-
tuted meso-aryl groups (1i {2-Me}, 1f {2-OMe}, 1h {2,6-
OMe}) have similar quantum yields to the phenyl derivative
(1c).
emission maxima, and quantum yields (Table 4). It emerged
that the fluorescence efficiency of the rosamines is quite
dependent on the solvent used. Two of those with small dipole
moments (CH2Cl2 and CHCl3, the compounds are not soluble
in aliphatic hydrocarbons) gave optimal fluorescence quantum
yields. No significant differences were observed in the UV-vis
absorption maxima for these compounds. Similarly, the Stokes
shifts observed are all within the range 14-24 nm.
The only nonsymmetrical compound (with respect to a plane
through the meso-substituent) is the piperidine/morpholine
derivative 12i. This has a lower quantum yield than similar
symmetrical compounds (i.e., 1i and 2i). The water-soluble
derivatives 15 and 5h had quantum yields of approximately 0.13
in 0.1 M phosphate buffer (pH 7.4).
Some illustrative UV-vis absorption and fluorescence emis-
sion spectra for molecules of Table 3 are shown in Figure 2.
These data show that the meso-thiophene derivative 1g is red-
shifted, both in terms of absorption and fluorescence, relative
to the other compounds. Absorption and emissions from all the
other probes are clustered in a relatively narrow wavelength
range, with the meso-methyl derivative 1a at the blue end of
the scale.
Data presented in Table 4 indicate the intensity of fluores-
cence emission from 1i can be used as a probe for solvent
polarity. This assertion was confirmed via a series of measure-
ments in a dichloromethane/methanol mixed solvent system
(Figure 3). The intensity of the fluorescence of this compound
(at 1.4 × 10-7 M) increased nearly linearly with the proportion
of dichloromethane (see Supporting Information).
The fact that the molecules were most fluorescent in two less
polar solvents could be due to aggregation in polar media. This
possibility was tested by measuring fluorescence wavelength
maxima for 1i as a function of concentration in ethanol (a
representative polar solvent; Figure 4). In fact, the wavelength
for emission maxima did not change significantly over a
concentration range of 5 × 10-9 to 5 × 10-6 M. This implies
aggregation is not significant at those concentrations. Experi-
ments with the nonionic detergent Triton-X 100 gave no
appreciable change in the fluorescence spectra of the dye in
this concentration range (see Supporting Information), further
supporting the assertion that 1i does not aggregate. At a
relatively high concentration of the dye, 2 × 10-5 M, there was
a 7 nm red shift in the emission maximum for the fluorescence
spectrum, indicating aggregation may only be a problem under
such conditions.
The piperidine derivative 1i was chosen as a typical dye to
study solvent effects on absorption maxima, fluorescence
(30) http://probes.invitrogen.com. In Molecular Probes; Invitrogen Corpora-
tion: Carlsbad, CA, 2006.
(31) Detty, M. R.; Prasad, P. N.; Donnelly, D. J.; Ohulchanskyy, T.; Gibson,
S. L.; Hilf, R. Bioorg. Med. Chem. 2004, 12, 2537–2544.
(32) Karstens, T.; Kobs, K. J. Phys. Chem. 1980, 84, 1871–1872.
(33) Magde, D.; Brannon, J. H.; Cremers, T. L.; Olmsted, J., III. J. Phys.
Chem. 1979, 83, 696–699.
(34) Loudet, A.; Burgess, K. Chem. ReV. 2007, 107, 4891–4932.
8716 J. Org. Chem. Vol. 73, No. 22, 2008

Rosamine DeriVatiVes
FIGURE 4. Aggregation study of 1i in EtOH (excited at 564 nm).
Predictably, the quantum yields of the piperazine derivatives
4, 5h, and 13 are somewhat dependent on the nitrogen atoms
at the periphery of the molecule. Derivatives for which this
nitrogen is protected with a tert-butyloxycarbonyl groups have
moderate quantum yields. However, the free secondary amine
of compound 5h, for example, almost completely quenches the
fluorescence unless it is protonated; this is because photoinduced
electron transfer (PET) mechanisms may be possible for the
amine.26 Similarly, compound 13 in the free base form is almost
completely nonfluorescent in dichloromethane, but solutions of
this dye become strongly fluorescent upon protonation. The
photophysical properties of rosamine 5h are pH-dependent. The
absorption and fluorescence maxima of dye 5h slightly red-
shifted with increasing pH from 2 to 12 (see Supporting
Information for details). Figure 5a shows how the fluorescence
of 5h drops dramatically when the pH is raised above ∼6, while
Figure 5b demonstrates that these fluorescence changes are
reversible.
FIGURE 5. Dependence of electronic spectra of 5h on pH. (a)
Fluorescence intensities (λmax emiss, excited at λmax abs, 5.7 × 10-7 M)
versus pH in phosphate solutions (50 mM, H3PO4 + NaOH); (b)
reversibility (excitation at λmax abs 538 nm and detection at λmax emiss 565
nm for pH 2.9; excitation at λmax abs 560nm and detection at λmax emiss
584 nm for pH 11.3; throughout the dye was used at 5.7 × 10-7
M
and the pH was adjusted with 2 M HCl or NaOH).
Finally, the water-soluble probe 15 was conjugated with
avidin via activation of the dye-carboxylic acids (N-hydrox-
ysuccinimide and N,N′-diisopropylcarbodiimide in DMF) and
then addition of the activated probe to the protein in 0.1 M
NaHCO3 at pH 8.3. The dye:protein ratio was calculated to be
0.8 by UV-vis when 3 equiv of the dye was used; this
corresponds to 27% labeling efficiency. When 5 equiv of dye
was used then a dye:protein ratio of 1.4 (28% labeling
efficiency) was observed. The latter sample was used to obtain
the spectral data shown in Figure 6. The UV-vis absorption
and fluorescence emission maxima of 15 on avidin were
observed to be within a few nanometers of the dye in phosphate
buffer. The quantum yield of the protein conjugate was 0.06 in
phosphate buffer compared with 0.13 for the free dye 15.
FIGURE 6. UV-vis absorption and fluorescence spectra of rosamines
15 and 15-labeled avidin in 0.1 M phosphate buffer, pH 7.4.
Conclusions
conjugated to a protein was not significantly different to that
of the free dye in aqueous media.
This paper describes efficient solution-phase syntheses of
rosamines with cyclic amine substituents. It is amenable to scale-
up and purification of the products. Further, water-soluble
derivatives can be prepared via modification of the initial
products. The rosamine products are shown to be more
fluorescent in dichloromethane and chloroform than in more
polar solvents; this is not the case for some commercially
available rosamines with acyclic amine substituents. That solvent
dependence might be useful in some applications. Further, the
data collected here illustrate that the ring size of the cyclic amine
substituents has significant effects on the quantum yields; this
trend has not been noted previously for rosamine dyes. Finally,
the spectroscopic properties of the water-soluble rosamine 15
Experimental Section
General Procedure for Amination of Ditriflyl Xanthone. 3,6-
Di-OTf-xanthone 6 (1.0 equiv) was dissolved in DMSO (0.2 M)
and the amine (10 equiv) was added. The reaction mixture was
heated to 90 °C and stirred for 12 h. After cooling to room
temperature, the reaction was quenched with water. The precipitate
was collected, washed with saturated aqueous Na2CO3 and water
to give the crude product, which was recrystallized from EtOAc/
hexanes to afford the pure product.
Spectral data for a few representative compounds are listed here.
Complete data are given in Supporting Information.
J. Org. Chem. Vol. 73, No. 22, 2008 8717

Wu and Burgess
3,6-Dipiperidin-1-yl-xanthen-9-one (7). Yellow solid (3.0 g,
89%). 1H NMR (300 MHz, DMSO-d6) δ 7.86 (d, 2H, J ) 9.0 Hz),
6.98 (dd, 2H, J ) 9.0, 2.3 Hz), 6.74 (d, 2H, J ) 2.3 Hz), 3.41 (br,
8H), 1.60 (br, 12H); 13C NMR (75 MHz, DMSO-d6) δ 173.0, 157.7,
154.8, 126.8, 111.7, 111.3, 98.8, 47.8, 24.8, 23.9. MS (ESI) m/z
calcd for (M + H)+ C23H26N2O2 363.21; found 363.21.
7.30-7.23 (m, 7H), 3.87-3.85 (m, 8H), 3.79-3.77 (m, 8H), 2.06
(s, 3H); 13C NMR (125 MHz, CD3OD) δ 160.2, 159.9, 159.1, 137.2,
133.0, 132.8, 131.9, 131.4, 130.1, 127.3, 116.3, 115.8, 98.6, 67.4,
48.5, 19.6; IR (thin film) 1646, 1590, 1481, 1415, 1383, 1235, 1190
cm-1; HRMS (ESI) m/z calcd for (M - Cl)+ C28H29N2O3 441.2178;
found 441.2184.
Rosamine 3i. Green solid (47 mg, 53%). Rf ) 0.28 (10% MeOH/
CH2Cl2). 1H NMR (500 MHz, CD3OD) δ 7.58-7.45 (m, 3H), 7.26
(d, 1H, J ) 7.5 Hz), 7.16 (d, 2H, J ) 9.5 Hz), 6.95 (dd, 2H, J )
9.5, 2.2 Hz), 6.84 (d, 2H, J ) 2.2 Hz), 3.63 (br, 8H), 2.15 (br,
8H), 2.06 (s, 3H); 13C NMR (125 MHz, CD3OD) δ 159.3, 159.1,
156.4, 137.2, 133.4, 132.4, 131.9, 131.2, 130.1, 127.3, 116.6, 114.7,
97.9, 50.1, 26.3, 19.6. IR (thin film) 1648, 1596, 1413, 1378, 1344,
1189 cm-1; HRMS (ESI) m/z calcd for (M - Cl)+ C28H29N2O
409.2280; found 409.2277.
3,6-Dimorpholin-4-yl-xanthen-9-one (8). White solid (308 mg,
1
93%). H NMR (500 MHz, CDCl3) δ 8.14 (d, 2H, J ) 9.0 Hz),
6.86 (dd, 2H, J ) 9.0, 2.4 Hz), 6.67 (d, 2H, J ) 2.4 Hz), 3.86 (t,
8H, J ) 4.9 Hz), 3.33 (t, 8H, J ) 4.9 Hz); 13C NMR (125 MHz,
CDCl3) δ 175.1, 158.0, 155.3, 127.7, 114.1, 111.2, 100.0, 66.5,
47.5. MS (ESI) m/z calcd for (M + H)+ C21H23N2O4 367.17; found
367.17.
3,6-Bis(4-Boc-piperazin-1-yl)-xanthen-9-one (9). White solid
1
(680 mg, 60%). H NMR (500 MHz, CDCl3) δ 8.13 (d, 2H, J )
9.0 Hz), 6.86 (dd, 2H, J ) 9.0, 2.3 Hz), 6.66 (d, 2H, J ) 2.3 Hz),
3.59 (t, 8H, J ) 5.2 Hz), 3.37 (t, 8H, J ) 5.2 Hz), 1.47 (s, 18H);
13C NMR (125 MHz, CDCl3) δ 175.0, 158.1, 155.0, 154.6, 127.8,
114.0, 111.7, 100.3, 80.2, 47.3(2C), 28.4. MS (ESI) m/z calcd for
(M + H)+ C31H41N4O6 565.30; found 565.29.
Rosamine 4d. Green solid (107 mg, 68%). Rf ) 0.31 (10%
1
MeOH/CH2Cl2). H NMR (500 MHz, CDCl3) δ 7.94 (d, 2H, J )
8.3 Hz), 7.37 (d, 2H, J ) 9.5 Hz), 7.19-7.15 (m, 4H), 7.11 (d,
2H, J ) 8.3 Hz), 3.80 (br, 8H), 3.67-3.65 (m, 8H), 1.45 (s, 18H);
13C NMR (125 MHz, CDCl3) δ 158.2, 156.9, 156.8, 154.4, 138.2,
131.8, 131.0, 130.8, 115.4, 114.0, 98.3, 97.3, 80.7, 47.2(2C), 28.3;
IR (thin film) 1694, 1644, 1592, 1479, 1415, 1387, 1226, 1161
cm-1; HRMS (ESI) m/z calcd for (M - Cl)+ C37H44N4O5 751.2356;
found 751.2342.
3,6-Dipyrrolidin-1-yl-xanthen-9-one (10). Yellow solid (90 mg,
1
18%). H NMR (500 MHz, CDCl3) δ 8.08 (d, 2H, J ) 8.9 Hz),
6.50 (dd, 2H, J ) 8.9, 2.2 Hz), 6.28 (d, 2H, J ) 2.2 Hz), 3.34 (t,
8H, J ) 6.6 Hz), 2.01 (t, 8H, J ) 6.6 Hz); 13C NMR (125 MHz,
CDCl3) δ 175.0, 158.1, 151.6, 127.6, 111.5, 109.1, 96.5, 47.6, 25.4.
MS (ESI) m/z calcd for (M + H)+ C21H23N2O2 335.18; found
335.18.
Rosamine 12i. Green solid (93 mg, 98%). Rf ) 0.30 (10%
1
MeOH/CH2Cl2). H NMR (500 MHz, CD3OD) δ 7.58-7.45 (m,
3H), 7.29-7.19 (m, 7H), 3.86-3.81 (m, 8H), 3.74-3.72 (m, 4H),
2.07 (s, 3H), 1.85-1.75 (m, 6H); 13C NMR (125 MHz, CD3OD) δ
160.3, 159.5, 159.3, 158.7, 158.6, 137.3, 133.1, 133.0, 132.5, 131.9,
131.3, 130.1, 127.3, 116.8, 115.7, 115.6, 115.3, 98.7, 98.1, 67.4,
50.2, 48.4, 27.3, 25.3, 19.6; IR (thin film) 1646, 1590, 1480, 1415,
1387, 1236, 1189 cm-1; HRMS (ESI) m/z calcd for (M - Cl)+
C29H31N2O2 439.2386; found 439.2384.
General Procedure for the Synthesis of Rosamine Derivatives.
A Grignard reagent or lithium reagent (1.0 mmol) was added
dropwise over 1 min to the solution of 3,6-diamino-xanthen-9-one
(0.2 mmol) in 5 mL of THF at 0 °C. After stirring for 12 h at room
temperature, the reaction mixture was quenched by addition of 2
mL of 2 M aqueous HCl, stirred for 10 min, and then diluted with
20 mL of CH2Cl2. The organic layer was washed with water and
brine, dried over Na2SO4, and concentrated under reduced pressure.
The residue was purified by flash chromatography (5% to 10%
MeOH/CH2Cl2) to give the pure product.
Acknowledgment. We thank the TAMU/LBMS-Applications
Laboratory directed by Dr. Shane Tichy for assistance with mass
spectrometry. Support for this work was provided by The
National Institutes of Health (GM72041) and by The Robert
A. Welch Foundation.
Rosamine 1a. Purple solid (71 mg, 89%). Rf ) 0.28 (10%
1
MeOH/CH2Cl2). H NMR (500 MHz, CDCl3) δ 8.01 (d, 2H, J )
9.6 Hz), 7.22 (dd, 2H, J ) 9.6, 2.5 Hz), 6.77 (d, 2H, J ) 2.5 Hz),
3.66 (br, 8H), 2.87 (s, 3H), 1.72 (br, 12H); 13C NMR (125 MHz,
CDCl3) δ 157.6, 156.4, 156.3, 130.1, 114.8, 114.0, 96.8, 48.8, 25.8,
Supporting Information Available: Complete experimental
details and characterization data for all the new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
24.1, 14.8; IR (thin film) 1643, 1597, 1486, 1401, 1235, 1200 cm-1
;
HRMS (ESI) m/z calcd for (M - Cl)+ C24H29N2O 361.2280; found
361.2287.
Rosamine 2i. Green solid (94 mg, 98%). Rf ) 0.26 (10% MeOH/
CH2Cl2). 1H NMR (500 MHz, CD3OD) δ 7.59-7.46 (m, 3H),
JO800902J
8718 J. Org. Chem. Vol. 73, No. 22, 2008