Altering Fundamental Trends in the Emission of Xanthene Dyes

Article abstract of DOI:10.1021/acs.joc.8b03030

Fluorescent small molecules enable researchers and clinicians to visualize biological events in living cells, tissues, and organs in real time. Herein, the focus is on the structure and properties of the relatively rare benzo[a]xanthenes that exhibit enhanced steric and electronic interactions due to their annulated structures. Three types of fluorophores were synthesized: (i) pH- and solvent-dependent seminaphthorhodafluors, (ii) pH- and solvent-independent seminaphthorhodafluors, and (iii) pH-independent but solvent-sensitive seminaphthorhodamines. The probes exhibited promising far-red to near-infrared (NIR) emission, large Stoke shifts, broad full width at half-maximum (fwhm), relatively high quantum yields, and utility in immunofluorescence staining. Deviation of the π-system from planarity due to changes in the fluorophore ionization state resulted in fluorescence properties that are atypical of common xanthene dyes.

Full text of DOI:10.1021/acs.joc.8b03030

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Article
Altering Fundamental Trends in the Emission of Xanthene Dyes
Lei G Wang, Ian Munhenzva, Martha Sibrian-Vázquez, Jorge O.
Escobedo, Catherine H. Kitts, Frank R Fronczek, and Robert M. Strongin
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b03030 • Publication Date (Web): 05 Feb 2019
Downloaded from http://pubs.acs.org on February 5, 2019
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Altering Fundamental Trends in the Emission of Xanthene Dyes
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Lei G. Wang,† Ian Munhenzva,‡ Martha Sibrian-Vazquez,‡ Jorge O. Escobedo,‡ Catherine H. Kitts,‡
Frank R. Fronczek,§ and Robert M. Strongin*‡
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†Department of Biomedical Engineering, Oregon Health & Science University, 3181 SW Sam Jackson Park Road, Portland,
Oregon 97239, USA
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‡Department of Chemistry, Portland State University, 1719 SW 10th Avenue, Portland, Oregon 97201, USA
§Department of Chemistry, Louisiana State University, 232 Choppin Hall, Baton Rouge, Louisiana 70803, USA
Keywords: xanthenes, red-to-NIR dye, mega Stokes dye, rhodol, annulation
ABSTRACT: Fluorescent small molecules enable researchers and clinicians to visualize biological events in living cells, tissues,
and organs in real time. Herein, the focus is on the structure and properties of the relatively rare benzo[a]xanthenes that exhibit
enhanced steric and electronic interactions due to their annulated structures. Three types of fluorophores were synthesized: (i) pH
and solvent dependent seminaphthorhodafluors, (ii) pH and solvent independent seminaphthorhodafluors, and (iii) pH-independent
but solvent sensitive seminaphthorhodamines. The probes exhibited promising far-red to near-infrared (NIR) emission, large Stoke
shifts, broad full width at half maximum (FWHM), relatively high quantum yields and utility in immunofluorescence staining.
Deviation of the π-system from planarity due to changes in the fluorophore ionization state resulted in fluorescence properties that
are atypical of common xanthene dyes.
INTRODUCTION
Xanthenes, including benzoxanthene derivatives (Figure 1), are of considerable interest in chemical biology. They are an important
class of heterocyclic compounds in both photophysical and medicinal chemistry.1-6 Benzoxanthenes have been employed as
fluorescent materials for the recognition of biologically important molecules.7,8 Their light conversion ability enables applications
ranging from optical fibers to filters.9 Much attention has been paid to the structural design and modification of benzoxanthenes due
to their attractive biological and antiproliferative properties.10-12 We have previously reported a family of water-soluble, low molecular
weight, NIR-emitting seminaphthofluorones wherein modifications to the regiochemistry of the ionizable groups and annulation
directions have led to significant modulation of spectral properties.13 Transposition of the ionizable moieties, as a design variable of
benzo[c]xanthene analogs, resulted in unusually large Stokes shifts (~200 nm) and emissions in the NIR (> 750 nm).14,15 In our
previous work, a seminaphthorhodafluor dye featured an inherent affinity for pancreatic ductal adenocarcinoma (PDAC) cells without
the need for conjugation to any targeting agents.16 This was accomplished through a systematic modification of three classes of
benzo[c]xanthene fluorophores with an emphasis on pharmacochemical properties. The study described herein focuses on a series of
red-to-NIR emitting fluorescent benzo[a]xanthene annulated probes containing various polar groups (Y, on C-5, Figure 1), including
their synthesis, spectral properties, computational modeling and in vitro imaging.
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c
1
10
1
O
X
O
2
b
3
4
7
R
a
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5
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7
Y
X
=
+NH2, +NMe2
1
Y
10
= OH, OMe, NH2
N
O
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9
R
=
7
R
O
O
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5
Y
Figure 1. Annulation of the xanthene chromophore can lead to three types of benzoxanthenes. The benzo[a]xanthene framework is annulated on the side
opposite to the X-group.
The design of a library of nine fluorophores was based on systematic functional group variation (Figure 2). At C-5, the
hydroxyl, methoxy or amine functionality, as well as different levels of rigidity at the C-10 iminium group were varied. All structures
possess a methyl benzoate at C-7, allowing for eventual conjugation to biological molecules. Compounds 1, 4 and 7 are the only pH-
and solvent-dependent fluorophores in the series. Substitution of the hydroxyl at C-5 with a methoxy group results in analogs 2, 5,
and 8. The corresponding naphthylamine are compounds 3, 6, and 9.
H2N
O
N
O
N
O
O
O
O
O
O
O
R
R
R
1
2
3
, R = OH
, R = OMe
, R = NH2
4
5
6
7
8
9
, R = OH
, R = OMe
, R = NH2
, R = OH
, R = OMe
, R = NH2
Figure 2. Seminaphthorhodafluors and seminaphthorhodamines.
RESULTS AND DISCUSSION
Fluorophore synthesis and characterization. Seminaphthorhodafluor analogs (Figure 2) were synthesized in three steps (Scheme
1). Briefly, condensation of the corresponding hydroxybenzophenone with the appropriate 2-naphthol derivative in a mixture of
CH3SO3H:TFA (1:1) at 80 °C for 16-24 h produced a single isomer (benzoxanthene type a) as the major product. Single-crystal X-
ray structure analysis and 2D NMR (COSY), confirmed the structure assignment. Subsequent Fischer esterification to produce the
methyl ester derivatives was carried out under reflux conditions in anhydrous MeOH and catalyzed by HCl. The required starting
materials 2-(2,4-dihydroxybenzoyl)benzoic acid, 2-(4-amino-2-hydroxybenzoyl)benzoic acid, 2-(8-hydroxy-1,2,3,5,6,7-
hexahydropyrido[3,2,1-ij]quinoline-9-carbonyl)benzoic acid, and 2,7-naphthalene derivatives were synthesized according to
published literature protocols.10,16,17 In the case of the seminaphthorhodamines (Figure 2), we followed a protocol developed recently
by our group.18 All compounds were isolated by flash column chromatography on silica gel, preparative TLC or medium-pressure
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reversed phase chromatography, and characterized by 1D and 2D NMR as well as HR-ESI-MS. Detailed characterization data and
procedures are described in the experimental section.
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Scheme 1. Synthesis of seminaphthorhodafluors from the benzo[a]xanthenes series.
5
6
7
8
HO
R1
HO
R1
O
(i)
O
R =
OH
, R1 = OH
, R1 = OMe
H2N
12
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9
O
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H2N
O
NH2
H2N
O
(iv)
H2N
OH
R
(ii)
O
(vi)
O
(v)
O
O
R1
OH
R1
11
O
, R1 = OH
, R1 = OMe
, R1 = OH
1
, R1 = OMe
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10
2
N
O
N
O
(iv)
(v)
N
OH
N
OH
(iii)
(vi)
+
O
O
O
R
O
R1
19
20
R1
, R1 = OH
, R1 = OMe
, R1 = OH
, R1 = OMe
21
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5
22
N
O
N
O
(iv)
(v)
(iii)
N
OH
R
N
OH
(vi)
+
O
O
O
R1
O
R1
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, R1 = OH
, R1 = OMe
, R1 = OH
, R1 = OMe
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7
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12
(i) NaH, MeI, DMF 12 h; (ii) NaOH, 160 ºC 3h; (iii) phthalic anhydride, toluene, reflux 12 h; (iv) MeSO3H:TFA 1:1, , 80 ºC 24h;
13
(v) MeSO3H:TFA 1:1, , 80 ºC 24h; (vi) MeOH, CH3COCl, 50 ºC 72h
Spectroscopic studies of the pH-dependent seminaphthorhodafluors. The products were screened for pH and solvent
dependence, relative quantum yields are reported as the average of multiple measurements using multiple references (SI).19 Their
optical properties in MeOH are summarized in Table 1. The absorption and emission spectra of all fluorophores included in this series
display broad, structured peaks, which enables efficient use over a wide range of wavelengths. All exhibit broad FWHM ranges from
101 to 182 nm, covering a significant window of the NIR region (Figure 3). Stokes shifts range from 40 to 100 nm.
Table 1. Spectral properties of compounds 1-9 in MeOH.
Compound
λmax abs
λmax em
Stokes
Shift
Extinction
Coefficient
(M -1·cm -1)
Quantum
Yield
Brightness
FWHM
(nm)
527
520
568
556
520
584
(nm)
630
590
610
610
610
640
(nm)
103
70
(%)
4.6
(nm)
151
132
182
110
114
114
1
2
3
4
5
6
29,276
1347
11027
195
26,261
41.99
1.08
3.97
11.65
0.68
42
18,013
54
28,896
1147
3210
178
90
27,549
56
26,110
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7
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9
538
535
592
630
630
670
92
95
78
25,562
26,275
21,690
20.52
18.36
1.6
5245
4824
347
114
116
148
1
2
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5
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7
8
9
The absorption and fluorescence spectra (Figure 3a) of seminaphthorhodafluor naphthols 1, 4 and 7 exhibit pH dependence
over the physiological range, as well as sensitivity to other solvent systems (MeOH, DMSO). In aqueous solution, ratiometric
absorption spectra with well-defined isosbestic points are observed (Figure 4a). At low pH, 1, 4 and 7 are red-to-NIR emitting. Their
absorption spectra display structured peaks with maxima ranging from 520 to 540 nm, large Stokes shifts (60 - 92 nm), and emission
maxima between 580 and 630 nm. At high pH, the neutral forms (deprotonated naphthols) display red-shifted absorption peaks at ca.
600 nm. Surprisingly, as the pH of the aqueous solutions of 1, 4 and 7 is increased further, the fluorescence intensity decreases to a
non-fluorescent state (pH =12.1, Figure 4g). Overall, the pKa values range from 8.07 for compound 1 to 8.39 for compound 7,
embodying enhanced acidity as compared to 2-naphthol (pKa = 9.51). Although small differences in pKa for rhodols 1, 4 and 7 are
observed, they follow the expected trend associated with N-substitution at C-10 (7>4>1). Solvent dependence can be at least partially
attributed to the differences in the equilibria of the tautomers of the compounds. In organic solvents, absorptions from each tautomer
are observed, with the shorter wavelength species predominating, similar to what is observed in the neutral buffer (Figure 4d).
Quantum yields are generally about 3-27 times higher in organic solvents as compared to aqueous buffer.
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Figure 3. Spectral properties of compounds 1−9 in aqueous media. (a) Absorption spectra (solid lines) and normalized fluorescence emission spectra (dash
lines) of pH-dependent seminaphthorhodafluors (1, 4, and 7) in acidic solution 1:9 DMSO:HCl (12.5 mM in H2O, pH 1.9). (b) pH-independent
seminaphthorhodafluors (2, 5, and 8) in 1:9 DMSO:buffer (12.5 mM phosphate, pH 7.4). (c) pH-independent seminaphthorhodamines (3, 6, and 9) in 1:9
DMSO:buffer (12.5 mM phosphate, pH 7.4). Emission spectra are normalized to their corresponding absorption peaks. Excitation Emission Matrices (EEMs)
are provided in the Supporting Information.
1
2
3
4
5
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7
8
9
Spectroscopic studies of pH-independent seminaphthorhodafluor methyl ethers. Replacing the hydroxyl moiety of 1,
4 and 7 with a methyl ether (compounds 2, 5, and 8) eliminates both pH and solvent dependence (Figure 4b, and 4e). At pH 7.4,
compounds 2, 5, and 8 exist as single species, similar to what is observed for 1, 4 and 7 in acidic solution. Absorption spectra are
nearly identical to the corresponding hydroxyl analogs. The emission wavelengths of the methyl ethers are also in the red-to-NIR
region, (Figure 4g), including large Stokes shifts (90-105 nm). Quantum yields, ranging from about 12-42%, are higher compared to
1, 4 and 7 (~ 64-fold higher for compound 2). No spectral evidence for tautomeric equilibria is observed for the methyl ethers, with
the absorption spectrum of compound 8 being the most stable over the pH range from 1.9 to 12.1 (Figure 4b), followed by compound
5, then 2 (SI). There is negligible change in the absorption spectrum upon changing solvents from DMSO to MeOH to neutral pH
buffer (Figure 4e).
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Spectroscopic studies of pH-independent seminaphthorhodamines. Seminaphthorhodamines 3, 6 and 9 are pH-
insensitive, but exhibit greater solvent-dependent spectral changes compared to seminaphthorhodafluors 1, 4, and 7. Compound 9
shows the highest stability over the pH range of 1.9 to 12.1 (Figure 4c), followed by 6, then 3 (SI). Their emission maxima range is
from 610 to 670 nm, with broad FWHM of up to 182 nm (in MeOH), covering a significant portion of the NIR region. Stokes shifts
range from 50 to 105 nm in aqueous media. Quantum yields are generally two times higher in MeOH compared to neutral buffer.
Solvatochromic behavior (up to 60 nm) is observed for the longest wavelengths in DMSO and for the shortest in aqueous media
(Figure 4f).
Molecular simulations. Density functional theory (DFT) calculations were performed to further investigate the relationship
between the structural and electronic properties of the benzo[a]xanthenes. The highest occupied molecular orbitals (HOMOs) and the
lowest unoccupied molecular orbitals (LUMOs) were obtained from the optimized molecular geometries in the ground state at the
B3LYP/6-31+G(d,p) level of theory using Gaussian 09 (see Table 2).20 Notably, electrons are preferentially localized on the naphthyl
annulation region of the molecule within the HOMO, and more delocalized over the xanthene dye core of the molecule in the LUMO
(Figure S9, SI).
Table 2. Dihedral angles (C(7-10)) and dipole moment for compounds 7, 8 and 9 in their cationic and neutral forms from DFT-optimized geometries in the
ground state.
Compound
dihedral angle (ϕ, º)
dipole moment
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C(7-10)
cation
(μ, Debye)
1
2
3
4
5
6
7
8
neutral
-11.2
N/Aa
-12.4
cation
3.1
neutral
11.6
7
8
9
-1.4
-3.1
-0.2
7.5
N/Aa
7.8
2.7
9
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aNot applicable. The neutral form cannot be produced.
To understand the effects of pH on the emission (as shown in Figure 4g) of compounds 7, 8, and 9 at the molecular level,
we further examined geometries and electronic properties. DFT calculations revealed a distortion of the conjugated π-systems from
planarity for the neutral forms of 7 and 9 (Figure 5). The dihedral angles (C7-10) are > -11° (Table 2). In contrast, the dihedral angles
in the cationic forms are closer to 0°, consistent with the more typical planar conformation. Similarly, compound 8, which exists only
as a cation, has a relatively small dihedral angle of -3.1° (Table 2). Analogous relationships are observed for the other two pH-
dependent fluorophores, 1 and 4 (Table S11, SI), wherein the neutral molecules exhibit greater dihedral angles compared to the
corresponding cations.
It is not surprising that pH-insensitive methoxy-functionalized 8 affords relatively consistent emission over the entire pH
range studied. However, the emission from naphthol 7 is dim at pH ca. 9 and above as shown in the EEMs (Figure 4g). This is not
typical for xanthenes such as fluorescein or even the type[c] benzoxanthenes, wherein higher pH enhances and red-shifts fluorescence
emission.21 This unique effect is ascribed to the aforementioned increased dihedral angle distortion in the neutral form of 7 (pKa =
8.39 ± 0.06).
Since the neutral and cationic forms of amino analog 9 have very similar dihedral angles compared to neutral and cationic
7, compound 9 should exhibit enhanced fluorescence at low pH levels, for instance at levels below the pKa of an ammonium cation.
However, rhodamine and congeners are well-known to be relatively pH-insensitive, and to display relatively bright emission over
broad pH ranges. In the case of 9, however, the EEMs in Figure 4g show relatively bright fluorescence at only the lowest solution pH
studied (1.9). The emission diminishes sharply as the equilibrium shifts towards the neutral form at higher pH values. This is in
keeping with the emissive properties of neutral versus cationic 7, and serves as further evidence that fluorescence modulation in
benzo[a]xanthenes is related to increases in the dihedral angle formed by carbons 7-10 (Figure 5 and Table 2).
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Figure 4. Comparison of the optical properties of compounds 7-9. Absorption spectra as a function of pH for compound 7 (a), 8 (b), and 9 (c). Inset figures
are ambient light photographs of the compounds in solution. Absorption spectra as a function of solvents including DMSO, MeOH, and 1:9 DMSO:buffer
(12.5 mM phosphate, pH 7.4) for compounds 7 (d), 8 (e), and 9 (f). (g) Normalized excitation emission matrices (EEMs) as a function of pH for compounds
7, 8, and 9.
Our calculations also show an increased charge separation in the neutral 7 and 9, with dipole moment values 3-4 times
higher compared to the cationic forms (Table 2). Molecular electrostatic potential surfaces calculations using the natural bound order
(NBO) display a significant charge repulsion between the centroid of the benzoate ring and the heteroatom at the edge of the [a]
annulated ring system (Figure 6). The result of the charge repulsion of the naphthyl moiety is an increase in the dihedral angle formed
by C7-C10.
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O
 C(7-10) = -1.4º
 C(7-10) = 0.2º
O
N
1
2
N
8
9
6
7
1.47
1.43
1.47
1.41
1.41
1.41
10 11
1.41
1.43
NH2
3
4
OH
O
O
5
6
7
O
O
7
9
H+
-H+
H+
-H+
8
9
O
N
O
N
 C(7-10) = -12.4º
 C(7-10) = -11.2º
1.49
1.49
1.40
1.45
1.44
1.38
1.38
1.40
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1.45
NH
1.46
O
O
O
O
O
Figure 5. DFT-optimized geometries of the cationic (top) and the neutral forms (bottom) of compounds 7 (left) and 9 (right). The torsion in the naphthyl
annulation region increases upon deprotonation of the hydroxyl and amino groups, respectively. The bond lengths in angstroms and dihedral angles (ϕ) for
C7-C10 of the extended benzoxanthene region are shown for clarity.
Figure 6. Calculated molecular electrostatic potential surfaces of DFT optimized geometries of the cationic and the neutral forms of compounds 7 and 9 in
the ground-state.
In vitro studies. To investigate their biocompatibility and utility in live cell imaging, compounds 1 through 9 were screened
for cytotoxicity in Capan-1 cells (a human PDAC cell line) (Figure 7). Cell viability is reported as the half maximal inhibitory
concentration (IC50) for each compound. Interestingly, methoxy analogs (2, 5, and 8) exhibited higher cytotoxicity as compared to
their corresponding hydroxyl analogs (1, 4, and 7) and amine analogs (3, 6, and 9). Moreover, alkylation at the C-10 nitrogen seemed
to result in increased cytotoxicity, with compound 1 the least toxic (IC50 30.96 μM), followed by compound 4 (IC50 26.30 μM), and
then 7 (IC50 13.45 μM) among three pH-dependent seminaphthorhodafluors. The same relationship was also observed for the other
two sets of fluorophores, with IC50 value comparisons of 2>5>8 for pH-independent seminaphthorhodafluors, and 3>6>9 for the pH-
independent seminaphthorhodamines. The evidence indicates that lipophilicity may play a role in the cytotoxicity of these
compounds.
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Concentration (
M)
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Concentration (M)
Figure 7. Cell viability of compounds 1-9. Cytotoxicity was determined by Cell Titer-Blue assay. Capan-1 cells were incubated with decreasing concentrations
from 100 μM of each compound in growth media. The half maximal inhibitory concentrations (IC50) determined by interpolating values in the graph (% cell
viability vs fluorophore concentration).
The subcellular localization (Figure 8) of compound 8 was investigated in Capan-1 cells. Organelle-specific fluorescent
probes for the nucleus (DAPI), endoplasmic reticulum (ER), lysosomes (Lyso), and mitochondria (Mito) were used to assess the site-
specific uptake. Compound 8 displays homogeneous distribution in the cytoplasm with limited accumulation in the nucleus,
endoplasmic reticulum, and mitochondria, showing utility as a cell membrane permeable fluorescent probe for live cell imaging.
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Tracker
Compound 8
Merge
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Figure 8. Subcellular localization of compound 8. Images of subcellular organelles stained with commercial fluorescent trackers are labeled as DAPI (blue),
Lyso (green), Mito (green), ER (green). Mid-column shows the images of cells stained with compound 8 (red). The merge column shows images of (top)
colocalization of DAPI + 8, followed successively below by, colocalization of Lyso + 8, colocalization of Mito + 8, and colocalization of ER + 8.
Since compound 8 exhibits a large Stokes shift, broad FWHM, appreciable brightness, and high stability across organic and
aqueous buffer solutions, it was modified to produce the NHS activated probe 8-NHS (Scheme 2). Compound 8-NHS was conjugated
to bovine alpha-tubulin mouse antibody as the primary and donkey anti-mouse antibody as the secondary antibody. The fluorophore
to antibody-labeling conjugation ratio was 2.8:1, as determined by UV-Vis analysis. Immunoconjugates were used for indirect
immunofluorescence staining in fixed U2OS cells at a 200 nM concentration. Figure 9 shows high contrast and detailed microtubular
network visualization, demonstrating benzo[a]xanthene’s utility for specific staining applications.
Scheme 2. Synthesis of 8-NHS.
O
N
OH
O
O
N
O
O
13
toluene
O
N
OH
+
O
Reflux, 24 h
MSA/TFA
80 oC, 12h
OH
O
O
O
O
OH
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O
29
HO
24
OH
N+
30
N+
O
PBS buffer
O
TEA
DMAP
DSC
H2N
pH, 8.3
OH
OH
O
O
O
DCM, 2 h
O
O
O
O
O
HN
N
O
8-NHS
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100 µm
Figure 9. Immunofluorescence imaging of Microtubule. Tubulin was labeled using indirect immunofluorescence with compound 8-NHS in fixed U2OS cell
at 200 nM concentration (left); negative control fluorescence image of fixed U2OS cells incubated with donkey, anti-mouse secondary antibody (pre-
conjugated with compound 8-NHS) only (right). Scale bar 100 m.
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CONCLUSION
In summary, an efficient synthesis of red-to-NIR benzo[a]xanthene analogs has been developed. Spectral and physicochemical
properties were tuned by varying the substituent at C-5 (-OH, -OMe or –NH2). These changes afforded three types of fluorophores,
based on their absorption spectra: (i) pH and solvent dependent seminaphthorhodafluors, (ii) pH and solvent independent
seminaphthorhodafluors and (iii) pH independent but solvent dependent seminaphthorhodamines. Fluorescence properties observed
included far red-to-NIR emission, large Stoke shifts (103 nm at neutral pH), broad FWHM (182 nm in MeOH) and relatively high
quantum yields (42% in MeOH). Minimal cytotoxicity and relatively bright fluorescence rendered the dyes suitable for live cell
imaging, as well as site-specific immunofluorescence imaging applications. Changes to the C-5 substituent led to significant
modulation of fluorescence emission as a function of ionization state. This was ascribed to deviations from planarity in the
chromophore.
This effect was associated with brighter fluorescence observed at lower solution pH values, which is opposite the trend
typically exhibited by other xanthene and benzoxanthene dyes. The type [a] annulated fluorophores thus add architectural diversity
and complementary fluorescent properties to the common xanthene dyes. Since they are analogs of the type [c] benzoxanthenes that
have previously exhibited inherent cancer tissue targeting,16 the type [a] fluorophores should enable future insight into how structure
changes modulate not only the optical but also the targeting and biological properties of the benzoxanthenes. The investigation of the
benzoxanthenes in targeted imaging and sensing applications is ongoing in our labs.
EXPERIMENTAL SECTION
General. Unless otherwise indicated, all commercially available starting materials were used directly without further purification.
1
Silica gel Sorbent Technologies 32-63 m was used for flash column chromatography. H-NMR was obtained on an ARX-400
Advance Bruker spectrometer. Chemical shifts () are given in ppm relative to DMSO-d6 (2.50 ppm, 1H, 39.52 13C) unless otherwise
indicated. MS (HRMS, ESI) spectra were obtained at the PSU Bioanalytical Mass Spectrometry Facility on a ThermoElectron LTQ-
Orbitrap high resolution mass spectrometer with a dedicated Accela HPLC system, or an Agilent 6244 time-of-flight LCMS with
diode array detector VL+.
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Synthesis of 2-(4-amino-2-hydroxybenzoyl)benzoic acid (11). Rhodamine 110 hydrochloride (1 g, 2.725 mmol) is mixed with
NaOH (1.875 g, 4.635 mol) and 1 mL of water. The mixture is stirred and heated in an oil bath at 160 oC for two hours, 3 mL of 50%
NaOH are added in one portion and the mixture heated and stirred at 160 oC for an additional one hour. The mixture is allowed to
cool down to room temperature and diluted with 50 mL of water. The mixture is acidified to pH 1 with concentrated HCl. The
resulting mixture is extracted with ethyl ether (2 × 50 mL), the organic extracts combined, dried over anhydrous Na2SO4, filtered and
the solvent evaporated under reduced pressure to leave a pale yellow solid. Yield 0.61 g, 87 %. 1H NMR (400 MHz, DMSO-d6) δ
13.01 (s, 1H), 12.60 (s, 1H), 7.95 (dd, J = 7.8, 1.2 Hz, 1H), 7.67 (td, J = 7.5, 1.4 Hz, 1H), 7.60 (td, J = 7.6, 1.4 Hz, 1H), 7.37 (dd, J =
7.6, 1.2 Hz, 1H), 6.71 (d, J = 8.5 Hz, 1H), 6.44 (s, 2H), 6.04 – 5.97 (m, 2H). 13C{1H} NMR (101 MHz, DMSO-d6) δ 198.3, 166.9,
165.0, 156.8, 140.1, 134.5, 132.0, 129.9, 129.7, 129.3, 127.7, 109.8, 106.5, 106.2, 98.2, 48.6. HRMS (ESI-Orbitrap) m/z: [M - H] -
Calcd for C14H10NO4 256.0615; Found 256.0627.
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Synthesis of 2-(4-(dimethylamino)-2-hydroxybenzoyl)benzoic acid (20). Compound 18, 3-dimethyl amino phenol (5 g, 36.44
mmol) and 19, phthalic anhydride (5.39 g, 36.44 mmol) are dissolved in 30 mL of toluene and refluxed with an oil bath for 12 h. The
solvent is evaporated under vacuum to leave a purple residue. The residue is dissolved in ethyl acetate and the mixture passed through
a plug of silica gel using EtOAc:Hexanes 1:1, EtOAc:Hexanes 3:1, and EtOAc for elution. Yield 4.32 g, 42%. 1H NMR (400 MHz,
DMSO-d6) δ 13.07 (s, 1H), 12.54 (s, 1H), 7.97 (dd, J = 7.8, 1.2 Hz, 1H), 7.69 (td, J = 7.5, 1.4 Hz, 1H), 7.61 (td, J = 7.6, 1.4 Hz, 1H),
7.38 (dd, J = 7.6, 1.1 Hz, 1H), 6.81 (d, J = 9.1 Hz, 1H), 6.21 (dd, J = 9.2, 2.5 Hz, 1H), 6.10 (d, J = 2.5 Hz, 1H), 3.00 (s, 6H). 13C{1H}
NMR (101 MHz, DMSO-d6) δ 198.7, 166.9, 164.3, 155.7, 140.0, 133.9, 132.1, 129.9, 129.7, 129.5, 127.6, 109.7, 104.3, 97.0, 39.6.
HRMS (ESI-Orbitrap) m/z: [M - H] - Calcd for C16H14NO4 284.0928; Found 284.0938.
Synthesis of 2-(8-hydroxy-1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinoline-9-carbonyl)benzoic acid (25). Phthalic anhydride
(0.392 g 2.64 mmol) and 8-hydroxyjulolidine (0.5 g, 2.64 mmol) are dissolved in 12 mL of toluene. The mixture is refluxed in an oil
bath for 24 h, then the solvent evaporated under vacuum to leave a deep red residue. The target compound is isolated as a pale yellow
solid by flash column chromatography on silica gel using CH2Cl2:MeOH 95:5 for elution. Yield: 622 mg, 70%. 1H NMR (400 MHz,
DMSO-d6) δ 12.94 (s, 2H), 7.94 (dd, J = 7.8, 1.3 Hz, 1H), 7.66 (td, J = 7.5, 1.4 Hz, 1H), 7.58 (td, J = 7.6, 1.4 Hz, 1H), 7.33 (dd, J =
7.5, 1.3 Hz, 1H), 6.39 (s, 1H), 3.27 – 3.19 (m, 4H), 2.59 (t, J = 6.4 Hz, 2H), 2.40 (t, J = 6.2 Hz, 2H), 1.90 – 1.80 (m, 2H), 1.80 – 1.70
(m, 2H). 13C{1H} NMR (101 MHz, DMSO-d6) δ 197.9, 167.0, 159.7, 148.8, 140.1, 131.7, 129.9, 129.8, 129.6, 129.2, 127.7, 112.4,
108.3, 104.6, 49.4, 48.9, 26.6, 21.1, 20.1, 19.5. HRMS (ESI-Orbitrap) m/z: [M + H]+ Calcd for C20H20NO4 338.1387; found 338.1391;
[M + Na]+ Calcd for C20H19NO4Na 360.1206; Found 360.1210.
Synthesis of 7-methoxynaphthalen-2-ol (13). 2,7-naphthalenediol (1 g, 6.24 mmol, 1.0 equiv) was dissolved in dry THF (14 mL)
and cooled to 0 °C. NaH (60% dispersion in mineral oil, 250 mg , 6.24 mmol, 1.0 equiv) was added portion-wise and the reaction
contents were stirred at 0 °C for 10 min; Me2SO4 (886 µL, 6.24 mmol, 1.0 equiv) was then added dropwise at 0 °C. The reaction
contents were stirred for 14 h with slow warming from 0 °C to 25 °C. Upon completion, the contents were quenched by the addition
of saturated NH4Cl (50 mL). They were then poured into water (50 mL) and extracted with EtOAc (3 × 50 mL). The combined
organic extracts were washed with water (50 mL) and brine (50 mL), dried (MgSO4), and concentrated. The resultant crude grey solid
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was purified by flash column chromatography (silica gel. hexanes/Et2O, 2/1). monomethylated product (334 mg. 30%) as a purple
solid. Rf = 0.32 (silica gel, hexanes/EtOAc, 2/1); 1H NMR (400 MHz, DMSO-d6) δ 9.63 (s, 1H), 7.64 (dd, J = 8.7, 2.7 Hz, 2H), 7.07
(d, J = 2.6 Hz, 1H), 7.03 (d, J = 2.4 Hz, 1H), 6.94 – 6.85 (m, 2H), 3.83 (s, 3H). 13C{1H} NMR (101 MHz, DMSO-d6) δ 157.6, 155.8,
136.0, 129.0, 129.0, 123.1, 115.9, 115.1, 108.1, 104.5, 55.0. HRMS (ESI-TOF) m/z: [M + H] + Calcd for C11H11O2 175.0754; Found
175.0731.
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Synthesis of 7-aminonaphthalen-2-ol (14).17 2,7-dihydroxy naphthalene (5 g, 31.22 mmol), formamide (1.37 mL, 34.34 mmol), and
sodium sulfite (8.26 g, 65.56 mmol) are suspended in 50 mL of DI water. The reaction mixture is refluxed in an oil bath overnight.
The solid formed is collected by vacuum filtration and washed with 100 mL DI water. The collected solid is dissolved in 100 mL of
diethyl ether, and washed with 100 mL of 6 N HCl. The aqueous layer is basified to pH 13 with 50% NaOH solution, and washed
with 100 mL of diethyl ether. The aqueous layer is then neutralized to pH 7 with 2N HCl solution, and extracted with ethyl acetate
(3 x 100 mL). The organic extract is combined, dried over anhydrous Na2SO4, and the solvent evaporated under vacuum. Yield: 3.02
g, 61%. 1H NMR (400 MHz, DMSO-d6) δ 9.30 (s, 1H), 7.44 (d, J = 8.8 Hz, 1H), 7.42 (d, J = 8.8 Hz, 1H), 6.72 (d, J = 2.3 Hz, 1H),
6.67 (dd, J = 8.9, 2.1 Hz, 1H), 6.65 (dd, J = 9.0, 2.2 Hz, 1H), 6.59 (d, J = 2.1 Hz, 1H), 5.20 (s, 2H). 13C{1H} NMR (101 MHz, DMSO-
d6) δ 155.4, 146.8, 136.6, 128.9, 128.3, 121.2, 115.2, 113.4, 106.3, 104.6. HRMS (ESI-Orbitrap) m/z: [M + H] + Calcd for C10H10NO
160.0757; Found 160.0755.
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General condensation procedure. Hydroxybenzophenone (0.918 mol) and 1,8-naphthalene derivative (1.380 mol) are dissolved in
1.5 mL of methanesulfonic acid, then 1.5 mL of TFA are added. The mixture is heated and stirred in an oil bath at 80 oC for 16-24 h.
The reaction mixture is allowed to warm to room temperature, then poured into 50 mL of DI water. If no precipitate is formed, the
mixture is neutralized to pH 6-7 by portion wise addition of solid NaHCO3. The resulting precipitate is filtered, washed with DI water
and air dried. The target compound isolated by flash column chromatography on silica gel, preparative TLC or medium pressure
reversed chromatography as required.
General esterification procedure. Under Ar atm, compound (0.131 mmol) is dissolved in 25 mL of anhydrous MeOH. The solution
is cooled to 0 oC in an ice bath. Acetyl chloride (750 L) is added dropwise. The mixture is stirred and kept at 50 oC for 48 h. Then
acetyl chloride (300 L) is added dropwise and the mixture kept at 50 oC for additional 24 h. The mixture is allowed to cool down
to room temperature and the solvent is evaporated under vacuum.
Synthesis of 9-amino-2-hydroxy-3'H-spiro[benzo[a]xanthene-12,1'-isobenzofuran]-3'-one (15). Following general condensation
method, compound 11(100 mg, 0.389 mmol) and 12 (62.3 mg, 0.389mmol) were condensed to provide compound 15, yield 93 mg,
63%. 1H NMR (400 MHz, DMSO-d6) δ 9.75 (s, 1H), 8.13 – 8.06 (m, 1H), 7.94 (d, J = 8.9 Hz, 1H), 7.75 (d, J = 8.7 Hz, 1H), 7.72 –
7.63 (m, 2H), 7.26 (d, J = 8.9 Hz, 1H), 7.16 – 7.10 (m, 1H), 6.90 (dd, J = 8.7, 2.3 Hz, 1H), 6.42 (d, J = 2.2 Hz, 1H), 6.34 (dd, J = 8.7,
2.3 Hz, 1H), 6.25 (d, J = 8.7 Hz, 1H), 6.22 (d, J = 2.2 Hz, 1H), 5.62 (s, 2H). 13C{1H} NMR (101 MHz, DMSO-d6) δ 169.4, 156.4,
154.6, 150.9, 150.7, 149.8, 135.7, 132.8, 132.5, 130.9, 129.7, 127.7, 126.4, 125.3, 125.0, 123.2, 116.1, 114.6, 111.7, 106.6, 106.5,
106.4, 98.4, 83.9. HRMS (ESI-Orbitrap) m/z: [M + H] + Calcd for C24H16NO4 382.1074; Found 382.1068.
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Synthesis of 9-amino-2-methoxy-3'H-spiro[benzo[a]xanthene-12,1'-isobenzofuran]-3'-one (16). Following general condensation
method, compound 11(100 mg, 0.389 mmol) and 13 (67.7 mg, 0.389 mmol) were condensed to provide compound 16, yield 93 mg,
61%. 1H NMR (400 MHz, DMSO-d6) δ 8.20 – 8.10 (m, 1H), 8.03 (d, J = 8.9 Hz, 1H), 7.86 (d, J = 8.9 Hz, 1H), 7.77 – 7.70 (m, 2H),
7.37 (d, J = 8.9 Hz, 1H), 7.27 – 7.20 (m, 1H), 7.00 (dd, J = 8.9, 2.4 Hz, 1H), 6.45 (d, J = 2.1 Hz, 1H), 6.37 (dd, J = 8.7, 2.1 Hz, 1H),
6.33 (d, J = 8.7 Hz, 1H), 6.27 (d, J = 2.4 Hz, 1H), 5.67 (s, 2H), 3.26 (s, 3H). 13C{1H} NMR (101 MHz, DMSO-d6) δ 169.4, 157.8,
154.8, 151.1, 150.9, 149.9, 136.1, 132.5, 132.2, 131.0, 130.0, 127.9, 126.6, 125.9, 124.9, 123.7, 115.9, 115.8, 111.8, 107.2, 105.9,
103.1, 98.5, 84.2, 54.5. HRMS (ESI-Orbitrap) m/z: [M + H] + Calcd for C25H18NO4 396.1230; Found 396.1228.
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Synthesis of 9-(dimethylamino)-2-hydroxy-3'H-spiro[benzo[a]xanthene-12,1'-isobenzofuran]-3'-one (21). Following general
condensation method, compound 20 (100 mg. 0.350 mmol) and 12 (56.1 mg, 0.350 mmol) were condensed to provide compound 21,
yield 67 mg, 45%. 1H NMR (400 MHz, DMSO-d6) δ 9.76 (s, 1H), 8.14 – 8.09 (m, 1H), 7.96 (d, J = 8.7 Hz, 1H), 7.76 (d, J = 8.8 Hz,
1H), 7.73 – 7.64 (m, 2H), 7.27 (d, J = 8.9 Hz, 1H), 7.16 – 7.10 (m, 1H), 6.91 (dd, J = 8.8, 2.2 Hz, 1H), 6.56 – 6.50 (m, 2H), 6.39 (d,
J = 8.8 Hz, 1H), 6.25 (d, J = 2.2 Hz, 1H), 2.93 (s, 6H). 13C{1H} NMR (101 MHz, DMSO-d6) δ 169.4, 156.39, 154.6, 151.4, 150.9,
149.7, 135.7, 132.7, 132.6, 130.9, 129.7, 127.5, 126.3, 125.3, 125.1, 123.2, 116.1, 114.5, 109.8, 106.71, 106.6, 106.4, 97.4, 83.6,
79.2. HRMS (ESI-Orbitrap) m/z: [M + H] + Calcd for C26H20NO4 410.1387; Found 410.1386.
Synthesis of 9-(dimethylamino)-2-methoxy-3'H-spiro[benzo[a]xanthene-12,1'-isobenzofuran]-3'-one (22). Following general
condensation method, compound 20 (100 mg, 0.350 mmol) and 13 (61.0 mg, 0.350 mmol) were condensed to provide compound 22,
yield 104 mg, 70%. 1H NMR (400 MHz, DMSO-d6) δ 8.21 – 8.13 (m, 1H), 8.05 (d, J = 8.7 Hz, 1H), 7.87 (d, J = 8.9 Hz, 1H), 7.76 –
7.71 (m, 2H), 7.38 (d, J = 8.9 Hz, 1H), 7.26 – 7.20 (m, 1H), 7.01 (dd, J = 8.9, 2.4 Hz, 1H), 6.58 – 6.53 (m, 2H), 6.47 (d, J = 8.2 Hz,
1H), 6.29 (d, J = 2.4 Hz, 1H), 3.28 (s, 3H), 2.94 (s, 6H). 13C{1H} NMR (101 MHz, DMSO-d6) δ 169.4, 157.9, 154.7, 151.6, 151.2,
149.8, 136.2, 132.6, 132.2, 131.0, 130.1, 127.8, 126.5, 125.9, 124.9, 123.7, 115.9, 115.7, 109.9, 107.2, 106.2, 103.1, 97.4, 83.9, 54.49,
39.8. HRMS (ESI-Orbitrap) m/z: [M + H] + Calcd for C27H22NO4 424.1543; Found 424.1542.
Synthesis
of
11-hydroxy-1,2,3,5,6,7-hexahydro-3'H-spiro[benzo[5,6]chromeno[2,3-f]pyrido[3,2,1-ij]quinoline-9,1'-
isobenzofuran]-3'-one (26). Following general condensation method, compound 25 (100 mg, 0.296 mmol) and 12 (47.5 mg, 0.296
mmol) were condensed to provide compound 26, yield 39 mg, 29%. 1H NMR (400 MHz, DMSO-d6) δ 9.71 (s, 1H), 8.11 – 8.06 (m,
1H), 7.93 (d, J = 8.6 Hz, 1H), 7.74 (d, J = 8.8 Hz, 1H), 7.71 – 7.61 (m, 2H), 7.28 (d, J = 8.9 Hz, 1H), 7.14 – 7.08 (m, 1H), 6.90 (dd,
J = 8.7, 2.2 Hz, 1H), 6.20 (d, J = 2.2 Hz, 1H), 5.91 (s, 1H), 3.16 (t, J = 5.6 Hz, 2H), 3.10 (ddd, J = 7.2, 4.7, 3.0 Hz, 2H), 2.89 (t, J =
6.6 Hz, 2H), 2.58 – 2.34 (m, 2H), 1.99 – 1.88 (m, 2H), 1.82 – 1.71 (m, 2H). 13C{1H} NMR (101 MHz, DMSO-d6) δ 169.4, 156.3,
154.6, 150.9, 145.1, 143.7, 135.6, 132.7, 132.4, 130.9, 129.7, 126.4, 125.3, 125.0, 123.3, 123.2, 117.9, 116.0, 114.8, 106.7, 106.3,
105.8, 105.8, 84.3, 49.0, 48.6, 26.9, 21.2, 20.6, 20.5. HRMS (ESI-Orbitrap) m/z: [M + H] + Calcd for C30H24NO4 462.1700; Found
462.1702.
Synthesis
of
11-methoxy-1,2,3,5,6,7-hexahydro-3'H-spiro[benzo[5,6]chromeno[2,3-f]pyrido[3,2,1-ij]quinoline-9,1'-
isobenzofuran]-3'-one (27). Following general condensation method, compound 25 (100 mg, 0.296 mmol) and 13 (51.6 mg, 0.296
mmol) were condensed to provide compound 27, yield 45 mg, 32%. 1H NMR (400 MHz, DMSO-d6) δ 8.19 – 8.11 (m, 1H), 8.02 (d,
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J = 8.9 Hz, 1H), 7.85 (d, J = 8.9 Hz, 1H), 7.76 – 7.69 (m, 2H), 7.39 (d, J = 8.9 Hz, 1H), 7.25 – 7.18 (m, 1H), 6.99 (dd, J = 8.8, 2.4
Hz, 1H), 6.26 (d, J = 2.4 Hz, 1H), 5.99 (s, 1H), 3.27 (s, 3H), 3.17 (t, J = 5.6 Hz, 2H), 3.12 (ddd, 2H), 2.90 (t, J = 6.5 Hz, 2H), 2.59 –
2.36 (m, 2H), 2.01 – 1.90 (m, 2H), 1.83 – 1.70 (m, 2H). 13C{1H} NMR (101 MHz, DMSO-d6) δ 169.4, 157.8, 154.7, 151.1, 145.2,
143.8, 136.1, 132.4, 132.2, 131.0, 130.0, 126.6, 125.8, 124.9, 123.7, 123.5, 118.1, 115.9, 115.8, 106.9, 105.9, 105.3, 103.1, 84.6,
54.5, 49.0, 48.6, 26.9, 21.2, 20.5, 20.5. HRMS (ESI-Orbitrap) m/z: [M + H] + Calcd for C31H26NO4 476.1856; Found 476.1845.
Synthesis of 2-hydroxy-12-(2-(methoxycarbonyl)phenyl)-9H-benzo[a]xanthen-9-iminium (1). Compound 1 was prepared
following general esterification method, yield 92%. 1H NMR (400 MHz, DMSO-d6) δ 10.38 (s, 1H), 9.11 (d, J = 73.7 Hz, 2H), 8.55
– 8.47 (m, 1H), 8.40 (dd, J = 7.2, 2.0 Hz, 1H), 8.02 (d, J = 8.8 Hz, 1H), 8.00 – 7.92 (m, 2H), 7.81 (d, J = 8.9 Hz, 1H), 7.48 (dd, J =
6.9, 2.0 Hz, 1H), 7.23 (dd, J = 8.7, 2.0 Hz, 1H), 7.14 (dd, J = 9.5, 2.0 Hz, 1H), 7.07 (d, J = 2.0 Hz, 1H), 7.04 (d, J = 9.4 Hz, 1H), 6.31
(d, J = 2.2 Hz, 1H), 3.56 (s, 3H). 13C{1H} NMR (101 MHz, DMSO-d6) δ 165.1, 161.5, 160.7, 158.9, 157.6, 156.7, 141.0, 137.1,
134.6, 132.4, 131.8, 131.7, 131.6, 130.8, 128.9, 128.4, 125.2, 121.2, 117.9, 117.9, 114.8, 113.8, 109.9, 96.0, 52.5. HRMS (ESI-
Orbitrap) m/z: [M] + Calcd for C25H18NO4 396.1230; Found 396.1229.
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Synthesis of 2-methoxy-12-(2-(methoxycarbonyl)phenyl)-9H-benzo[a]xanthen-9-iminium (2). Compound 2 was prepared
following general esterification method, yield 90%. 1H NMR (400 MHz, DMSO-d6) δ 9.39 (s, 1H), 9.18 (s, 1H), 8.59 (d, J = 9.0 Hz,
1H), 8.42 (dd, J = 7.9, 1.3 Hz, 1H), 8.13 (d, J = 8.9 Hz, 1H), 8.05 (td, J = 7.6, 1.3 Hz, 1H), 7.94 (dd, J = 8.3, 6.7 Hz, 2H), 7.58 (dd, J
= 7.6, 1.2 Hz, 1H), 7.28 (dd, J = 8.8, 2.4 Hz, 1H), 7.17 (dd, J = 9.5, 2.1 Hz, 1H), 7.13 – 7.06 (m, 2H), 6.61 (d, J = 2.4 Hz, 1H), 3.56
(s, 3H), 3.26 (s, 3H). 13C{1H} NMR (101 MHz, DMSO-d6) δ 165.5, 162.3, 160.5, 160.2, 158.2, 156.8, 140.8, 137.7, 135.3, 132.8,
132.4, 132.1, 131.6, 131.4, 129.7, 129.0, 126.6, 122.0, 118.9, 118.3, 115.5, 115.4, 106.7, 96.6, 55.2, 53.1. HRMS (ESI-Orbitrap) m/z:
[M] + Calcd for C26H20NO4 410.1387; Found 410.1395.
Synthesis of N-(2-hydroxy-12-(2-(methoxycarbonyl)phenyl)-9H-benzo[a]xanthen-9-ylidene)-N-methylmethanaminium (4).
Compound 4 was prepared following general esterification method, yield 85%. 1H NMR (400 MHz, DMSO-d6) δ 10.45 (s, 1H), 8.54
(d, J = 8.9 Hz, 1H), 8.45 – 8.35 (m, 1H), 8.04 (d, J = 8.8 Hz, 1H), 7.98 (td, J = 7.2, 1.6 Hz, 2H), 7.78 (d, J = 8.9 Hz, 1H), 7.54 – 7.47
(m, 1H), 7.39 (dd, J = 9.8, 2.5 Hz, 1H), 7.27 (d, J = 2.5 Hz, 1H), 7.26 (dd, J = 9.2, 2.4 Hz, 1H), 7.01 (d, J = 9.8 Hz, 1H), 6.36 (d, J =
2.1 Hz, 1H), 3.56 (s, 3H), 3.39 (s, 6H). 13C{1H} NMR (101 MHz, DMSO-d6) δ 165.2, 160.6, 159.0, 158.1, 157.0, 156.8, 141.3, 137.1,
134.6, 132.5, 131.7, 131.6, 130.9, 130.4, 128.9, 128.4, 125.1, 118.9, 118.0, 117.6, 115.1, 113.6, 110.0, 95.6, 52.6, 41.4, 40.9. HRMS
(ESI-Orbitrap) m/z: [M] + Calcd for C27H22NO4 424.1543; Found 424.1547.
Synthesis of N-(2-methoxy-12-(2-(methoxycarbonyl)phenyl)-9H-benzo[a]xanthen-9-ylidene)-N-methylmethanaminium (5).
Compound 5 was prepared following general esterification method, yield 95%. 1H NMR (400 MHz, DMSO-d6) δ 8.61 (d, J = 9.0 Hz,
1H), 8.44 (dd, J = 8.0, 1.2 Hz, 1H), 8.15 (d, J = 8.9 Hz, 1H), 8.06 (td, J = 7.6, 1.3 Hz, 1H), 7.95 (td, J = 7.7, 1.3 Hz, 1H), 7.90 (d, J
= 9.0 Hz, 1H), 7.61 (dd, J = 7.8, 1.2 Hz, 1H), 7.43 (dd, J = 9.8, 2.5 Hz, 1H), 7.31 (d, 2.4 Hz, 1H), 7.30 (dd, J = 8.8, 2.4 Hz, 1H), 7.05
(d, J = 9.9 Hz, 1H), 6.64 (d, J = 2.4 Hz, 1H), 3.56 (s, 3H), 3.41 (d, J = 11.3 Hz, 6H), 3.27 (s, 3H). 13C{1H} NMR (101 MHz, DMSO-d6)
δ 165.1, 160.0, 159.8, 158.4, 156.9, 156.6, 140.6, 137.2, 134.9, 132.4, 131.6, 131.1, 131.0, 130.6, 129.2, 128.6, 126.1, 119.3, 118.2,
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117.9, 115.2, 114.9, 106.3, 95.7, 54.7, 52.7, 41.5, 41.0. HRMS (ESI-Orbitrap) m/z: [M] + Calcd for C28H24NO4 438.1700; Found
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11-hydroxy-9-(2-(methoxycarbonyl)phenyl)-1,2,3,5,6,7-hexahydrobenzo[5,6]chromeno[2,3-f]pyrido[3,2,1-
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ij]quinolin-4-ium (7). Compound 7 was prepared following a general esterification method, yield 71 %. 1H NMR (400 MHz, DMSO-
d6) δ 10.24 (s, 1H), 8.42 (d, J = 8.7 Hz, 1H), 8.40 – 8.34 (m, 1H), 7.98 (d, J = 5.9 Hz, 1H), 7.97 – 7.91 (m, 2H), 7.80 (d, J = 8.9 Hz,
1H), 7.49 – 7.44 (m, 1H), 7.19 (dd, J = 8.7, 2.3 Hz, 1H), 6.62 (s, 1H), 6.23 (d, J = 2.2 Hz, 1H), 3.67 (t, J = 5.7 Hz, 2H), 3.62 (t, J =
5.9 Hz, 2H), 3.55 (s, 3H), 3.09 (t, J = 6.0 Hz, 2H), 2.70 (t, J = 6.5 Hz, 2H), 2.14 – 2.00 (m, 2H), 1.93 – 1.81 (m, 2H). 13C{1H} NMR
(101 MHz, DMSO-d6) δ 165.1, 158.4, 157.0, 155.7, 153.1, 151.4, 139.4, 137.3, 134.5, 132.2, 131.5, 130.7, 129.3, 128.7, 128.4, 125.3,
125.2, 117.8, 117.7, 114.4, 113.8, 109.6, 104.1, 52.4, 51.1, 50.7, 27.0, 19.7, 18.9, 18.7. HRMS (ESI-Orbitrap) m/z: [M] + Calcd for
C31H26NO4 476.1856; Found 476.1846.
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Synthesis
of
11-methoxy-9-(2-(methoxycarbonyl)phenyl)-1,2,3,5,6,7-hexahydrobenzo[5,6]chromeno[2,3-f]pyrido[3,2,1-
ij]quinolin-4-ium (8). Compound 8 was prepared following general esterification method, yield 81%. 1H NMR (400 MHz, DMSO-
d6) δ 8.50 (d, J = 8.9 Hz, 1H), 8.41 (dd, J = 7.9, 1.0 Hz, 1H), 8.09 (d, J = 8.9 Hz, 1H), 8.06 – 8.00 (m, 1H), 7.96 – 7.88 (m, 1H), 7.92
(d, J = 8.9 Hz, 1H), 7.57 (dd, J = 7.6, 0.9 Hz, 1H), 7.24 (dd, J = 8.9, 2.4 Hz, 1H), 6.66 (s, 1H), 6.55 (d, J = 2.3 Hz, 1H), 3.69 (s, 2H),
3.64 (s, 2H), 3.55 (s, 3H), 3.24 (s, 3H), 3.10 (d, J = 5.9 Hz, 2H), 2.71 (s, 2H), 2.14 – 2.01 (m, 2H), 1.96 – 1.82 (m, 2H). 13C{1H}
NMR (101 MHz, DMSO-d6) δ 165.0, 159.4, 156.3, 155.4, 153.4, 151.4, 138.9, 137.5, 134.7, 132.1, 131.6, 131.0, 130.8, 129.6, 129.06,
128.7, 126.1, 125.4, 118.3, 117.6, 115.1, 114.6, 105.9, 104.3, 54.7, 52.6, 51.3, 50.8, 27.0, 19.7, 18.9, 18.7. HRMS (ESI-Orbitrap)
m/z: [M] + Calcd for C32H28NO4 490.2013; Found 490.2005.
Compounds 3, 6, and 9 were synthesized according to a previous published protocol.18
Synthesis of isomers 4-(8-hydroxy-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinoline-9-carbonyl)isophthalic acid and 2-(8-
hydroxy-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinoline-9-carbonyl)terephthalic acid (29). 8-hydroxyjulolidine (3.00 g,
15.85 mmol) was dissolved in toluene 45 mL, heated to 60 oC with an oil bath, and pounded trimellitic anhydride (3.65 g, 19.02
mmol) was added with magnetic stirring. The mixture was refluxed for 24 h and cooled. The residue was filtered off, washed with
toluene (3×7.5 mL), dissolved in MeOH (45 mL) refluxed for 10 minutes. Then acetic acid (15 mL) was added and the mixture was
evaporated to dryness. Crystalline solid was refluxed with MeOH (30 mL) for 2 h and kept at 4 oC overnight. The crystals formed
were filtered off and washed with MeOH (7.5 mL) to give the product as a yellow crystalline solid (1.90 g, 20 % yield). 1H NMR
(400 MHz, DMSO-d6) δ 13.40 (s, 4H), 12.83 (d, J = 13.0, 2.8 Hz, 2H), 8.47 (s, 1H), 8.19 (d, J = 8.1 Hz, 1H), 8.15 – 8.09 (dd,
1H), 8.04 (dd, J = 8.7, 2.8 Hz, 1H), 7.79 (s, 1H), 7.48 (dd, J = 8.2, 2.9 Hz, 1H), 6.40 (dd, J = 12.8, 2.7 Hz, 2H), 3.34 (s, 4H),
3.18 (s, 2H), 2.59 (m, J = 6.8 Hz, 5H), 2.46 – 2.38 (m, 5H), 1.86 (s, 4H), 1.77 (s, 4H). 13C{1H} NMR (101 MHz, DMSO-d6) δ
197.3, 197.1, 167.0, 166.7, 166.6, 166.6, 160.3, 160.2, 149.5, 149.5, 144.3, 140.6, 134.2, 133.9, 133.0, 131.9, 131.2, 130.8, 130.6,
130.4, 130.0, 130.0, 128.8, 128.7, 113.2, 108.6, 108.6, 105.1, 105.1, 49.9, 49.4, 49.1, 27.1, 21.5, 20.5, 20.0. HRMS (ESI-TOF) m/z:
[M + H] + Calcd for C21H20NO6 382.1285; Found 382.1286 and 382.1289.
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Synthesis of isomers 11-methoxy-3'-oxo-2,3,6,7-tetrahydro-1H,3'H,5H-spiro[benzo[5,6]chromeno[2,3-f]pyrido[3,2,1-
ij]quinoline-9,1'-isobenzofuran]-5'-carboxylic acid and 11-methoxy-3'-oxo-2,3,6,7-tetrahydro-1H,3'H,5H-
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spiro[benzo[5,6]chromeno[2,3-f]pyrido[3,2,1-ij]quinoline-9,1'-isobenzofuran]-6'-carboxylic acid (30). The direct condensation
with 1:1 MSA/TFA gave 60 % yield of 1:1 5’ and 6’ carboxy isomers. Started with 100 mg of the 8-hydroxybenzophenone resulted
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in 60 mg of the dye. Rf 0.35 (9:1:0.5 MeOH/DCM/HCOOH). H NMR (400 MHz, Methanol-d4) δ 8.77 (s, 1H), 8.19 (d, J = 6.9
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Hz, 5H), 8.04 (dd, J = 9.0, 5.0 Hz, 2H), 7.72 (d, J = 8.9 Hz, 2H), 7.65 (s, 1H), 7.46 (dd, J = 8.9, 7.4 Hz, 2H), 7.11 (d, J = 7.9
Hz, 1H), 6.94 (ddd, J = 8.9, 2.4, 0.9 Hz, 2H), 6.65 (d, J = 2.4 Hz, 1H), 6.57 (d, J = 2.3 Hz, 1H), 6.49 (s, 1H), 6.43 (s, 1H),
3.36 (dq, J = 18.3, 6.6 Hz, 8H), 3.16 (d, J = 6.9 Hz, 6H), 2.93 (m, J = 6.0 Hz, 4H), 2.53 (ddt, J = 16.0, 10.2, 6.0 Hz, 4H),
1.98 (q, J = 6.0 Hz, 4H), 1.82 (q, J = 6.2 Hz, 4H). 13C{1H} NMR (101 MHz, MeOD) δ 166.4, 159.4, 159.4, 133.8, 132.1, 130.9,
125.6, 125.4, 117.3, 114.7, 114.6, 105.6, 105.3, 104.9, 78.2, 77.9, 77.6, 54.2, 50.6, 50.5, 50.2, 50.1, 27.2, 20.5, 20.5, 19.6, 19.5.
HRMS (ESI-TOF) m/z: [M + H] + Calcd for C32H26NO6 520.1755; Found 520.1747 and 520.1771.
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Synthesis of isomers 9-(2-carboxy-4-(((2,5-dioxopyrrolidin-1-yl)oxy)carbonyl)phenyl)-11-methoxy-1,2,3,5,6,7-
hexahydrobenzo[5,6]chromeno[2,3-f]pyrido[3,2,1-ij]quinolin-4-ium and 9-(2-carboxy-5-(((2,5-dioxopyrrolidin-1-
yl)oxy)carbonyl)phenyl)-11-methoxy-1,2,3,5,6,7-hexahydrobenzo[5,6]chromeno[2,3-f]pyrido[3,2,1-ij]quinolin-4-ium (8-
NHS). Compound 30 (68 mg, 0.13 mmol), DMAP (79.95 mg, 0.65 mmol) and TEA (0.091 mL, 0.65 mmol) were dissolved in dry
DCM (5 mL). DSC (67.06 mg, 0.26 mmol) was added and the reaction stirred for 2 h. The reaction was quenched with AcOH
(0.075 mL, 1.31 mmol) and the solution was directly loaded onto an equilibrated flash chromatography column and eluted with
0.5% HCOOH in CHCl3/MeOH 98/2 to afford the pure compound (54 mg, 80%). Rf 0.22 (98:2:0.5 MeOH/DCM/HCOOH). 1H
NMR (400 MHz, DMSO-d6) δ 8.58 (dd, J = 1.5, 0.7 Hz, 1H), 8.30 – 8.25 (m, 2H), 8.22 (s, 2H), 8.11 – 8.04 (m, 2H), 7.89
(dd, J = 9.0, 1.1 Hz, 2H), 7.59 (t, J = 1.0 Hz, 1H), 7.43 (d, J = 8.9 Hz, 2H), 7.35 (dd, J = 8.0, 0.7 Hz, 2H), 7.04 (dt, J = 8.9,
2.5 Hz, 2H), 6.27 (dd, J = 6.0, 2.4 Hz, 2H), 6.07 (d, J = 8.3 Hz, 2H), 3.32 (d, J = 5.9 Hz, 4H), 3.20 (t, J = 5.6 Hz, 4H), 3.17 –
3.09 (m, 4H), 2.97 – 2.89 (m, 4H), 2.63 (s, 8H), 2.03 – 1.94 (m, 4H), 1.81 (dd, J = 11.3, 5.7 Hz, 4H), 1.18 (s, 6H). 13C{1H}
NMR (101 MHz, DMSO-d6) δ 208.8, 174.4, 173.5, 173.3, 169.3, 166.7, 166.6, 163.7, 158.4 158.3, 155.4, 155.3, 151.6, 151.5,
145.6, 145.6, 144.4, 144.4, 137.1, 134.2, 133.0, 133.0, 132.5, 132.5, 131.5, 131.3, 127.5, 126.3, 125.9, 125.7, 124.2, 124.0, 123.9,
118.8, 118.7, 116.4, 116.4, 116.2, 106.9, 106.9, 106.4, 106.4, 106.3, 105.1, 105.0, 103.5, 103.5, 85.4, 69.0, 56.3, 55.0, 54.9, 49.5,
49.0, 40.6, 40.4, 40.2, 40.0, 39.8, 39.5, 39.3, 32.6, 30.2, 30.1, 29.5, 27.3, 25.7, 21.6, 21.0. HRMS (ESI-TOF) m/z: [M + H] + Calcd
for C36H29N2O8 617.1918; Found 617.1922 and 617.1914.
UV−Vis Absorption and Fluorescence Spectroscopy. UV−vis spectra were collected using a Cary 50 UV−vis spectrophotometer
at room temperature (rt) using a reduced- volume 1 cm quartz cuvette. Fluorescence spectra were collected on a Cary Eclipse
fluorescence spectrophotometer (Agilent Technologies). All absorption spectra were reference corrected. Fluorescence spectra were
corrected for the wavelength-dependent response of the R928 photomultiplier tube using a manufacturer-generated correction file.
Quantum yields were reported as the average of multiple measurements using multiple references. EEMs were collected over
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various spectral regions, using 5 or 10 nm step sizes for emission and excitation. The band pass for excitation and emission was
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Cell Culture. The human PDAC cell line Capan-1 was obtained from Dr. Rosalie Sears’s laboratory at Oregon Health and Science
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University (OHSU) and was maintained in RPMI medium (Gibco) in a humid atmosphere at 37 °C with 5% CO . All media were
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supplemented with 10% (v/v) fetal bovine serum (FBS) and 1% (v/v) penicillin/streptomycin. Cell viability was determined by Cell
Titer-Blue assay. Monolayers of 104 Capan-1 cells were seeded in triplicate in 96-well plates and incubated with decreasing
concentration from 100 μM of each compound in growth media containing 10% FBS. After 24 h incubation time, 20 μL of Cell
Titer-Blue reagent was added into each well and culture cells were incubated for additional 2 h. Fluorescence intensity in each well
was recorded at 560/590 nm using a SpectraMax M5 Microplate Reader. The half-maximal inhibitory concentrations (IC50
determined by interpolating values in the graph (% cell viability vs fluorophore concentration).
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In Vitro Live Cell Imaging. Monolayers of 104 Capan-1 cells were seeded in triplicate in eight-well plates and incubated for 24 h
in growth medium containing 10% FBS and were allowed to attach. For subcellular colocalization experiments, the medium was
extracted and cells were washed with phosphate-buffered saline (PBS), and phenol red-free growth medium was added to each
well. Organelle trackers were added to each well, and incubated with the fluorophore of interest (final concentration 0.5 μM) for 1
h. Cells were washed with PBS and fixed with 2% paraformaldehyde (PFA). Fluorescence microscopy was carried out using a
Zeiss inverted microscope with an Axioscan fluorescence camera for imaging. The final concentration of organelle trackers used
were as follows: DAPI 0.5 μM, Mito-tracker green 0.5 μM, and ER-tracker green 1 μM.
Fluorophore-labeled Antibodies. Compound 8 was conjugated to donkey anti-mouse antibody (Jackson ImmunoResearch) for
immunofluorescence microtubule imaging. The antibody was buffer exchanged into 1× phosphate buffered saline (PBS) and pH
adjusted to 8.3 with 35 mM disodium phosphate. The conjugation reactions were set up with fluorophore and antibody mixed at a
10:1 molar ratio for immunofluorescence microtubule imaging measurements. The fluorophore-antibody conjugation reactions
were rocked gently at room temperature protected from light for 3 hr and were washed and concentrated in 10 kDa MWCO spin
filters. The fluorophore to antibody-labeling conjugation ratio was 2.8:1, which was determined using an absorbance spectrometer
(SpectraMax M5 Microplate Reader).
Immunofluorescence Labeling. U2OS Cells were pre-extracted with 0.5% Triton X-100 in PEM (100 mM PIPES buffer pH 7.0,
1 mM EGTA and 1 mM MgCl2) for 20 s, fixed with 0.4% glutaraldehyde (Electron Microscopy Science) and 0.25% Triton X-100
in PEM for 90 s, washed with PBS before fixing with 3% glutaraldehyde in PEM 15 min. Cells were washed with PBS (3 × 5 min),
then reduced with 10 mM sodium borohydride for 10 min, washed again with PBS (3 × 5 min), and blocked with 5% bovine serum
albumin (BSA) in PBS. Cells were incubated with bovine alpha-tubulin mouse primary antibody (Thermo Fisher Scientific) at 2 μ
gml−1 in BSA for 4 h at room temperature and washed with PBS (3 × 5 min). Lastly, cells were incubated with fluorophore-labeled
donkey anti-mouse secondary antibody at 0.2 μM fluorophore concentration in PBS for 30 min at room temperature protected from
light and washed with PBS (3 × 5 mins).22 Images were acquired on an Axio Observer inverted fluorescence microscope (Zeiss,
Thornwood, NY) at 20× magnification. A PhotoFluor II was used and filtered using a 545 ± 12.5 bandpass excitation filter.
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Fluorescence images were collected using an Axiocam 506 camera (Zeiss), where a 605 ± 35 nm bandpass emission filter was used
for image collection.
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ASSOCIATED CONTENT
Supporting Information
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The Supporting Information is available free of charge on the ACS Publications website. Optical properties of new compounds,
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computer-assisted molecular calculations, NMR and HRMS spectra, and X-ray crystallographic information file (CIF) for compound
21.
Author Information
Corresponding Author
*E-mail: strongin@pdx.edu.
ORCID
Lei Wang: 0000-0002-0664-2704
Robert M. Strongin: 0000-0003-3777-8492
Author Contributions
All authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the National Institutes of Health (via grant R15EB016870). The authors acknowledge Dr. Summer L. Gibbs
and Dr. Xiaolin Nan of the Oregon Health & Sciences University for shared imaging facilities and insightful discussions.
REFERENCES
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(4) Pouli, N.; Marakos, P. Fused xanthone derivatives as antiproliferative agents. Anticancer Agents Med. Chem. 2009, 9, 77-
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(6) Chen, W.; Xu, S.; Day, J. J.; Wang, D.; Xian, M. A general strategy for development of near-infrared fluorescent probes for
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