A trifluoroacetic acid-labile sulfonate protecting group and its use in the synthesis of a near-IR fluorophore

Article abstract of DOI:10.1021/jo302065u

Sulfonated molecules exhibit high water solubility, a property that is valuable for many biological applications but often complicates their synthesis and purification. Here we report a sulfonate protecting group that is resistant to nucleophilic attack but readily removed with trifluoroacetic acid (TFA). The use of this protecting group improved the synthesis of a sulfonated near-IR fluorophore and the mild deprotection conditions allowed isolation of the product without requiring chromatography.

Full text of DOI:10.1021/jo302065u

Note
pubs.acs.org/joc
A Trifluoroacetic Acid-labile Sulfonate Protecting Group and Its Use
in the Synthesis of a Near-IR Fluorophore
Steven M. Pauff and Stephen C. Miller*
Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, 364 Plantation Street,
Worcester, Massachusetts 01605, United States
S
* Supporting Information
ABSTRACT: Sulfonated molecules exhibit high water solubil-
ity, a property that is valuable for many biological applications
but often complicates their synthesis and purification. Here we
report a sulfonate protecting group that is resistant to
nucleophilic attack but readily removed with trifluoroacetic
acid (TFA). The use of this protecting group improved the
synthesis of a sulfonated near-IR fluorophore and the mild
deprotection conditions allowed isolation of the product
without requiring chromatography.
ulfonation of hydrophobic fluorescent dyes greatly
improves both their solubility and fluorescence properties
highly reactive electrophiles that alkylate a wide range of
nucleophilic molecules. Protecting groups currently available
for sulfonates all have caveats that limit their general
applicability.8 Isopropyl (iPr) and isobutyl (iBu) sulfonates
retain lability to nucleophiles,9−11 while trichloroethyl (TCE)
sulfonates are unstable to mildly basic conditions.12 Neopentyl
(Neo) groups are highly stable to nucleophiles but removal
requires either strongly acidic conditions8,12 or heating in DMF
with strong nucleophiles.11,13 Trifluoroethyl (TFE) sulfonates
are stable to both nucleophilic and acidic conditions, but need
strongly basic conditions for removal.8 For our purposes,
existing sulfonate protecting groups are either insufficiently
stable, require cleavage conditions that are too harsh, and/or
deprotection would require subsequent HPLC purification. For
example, attempted cleavage of the sulfonate ester dye 2 with
NaOH led to dye decomposition.
S
in aqueous media.1−5 However, sulfonated molecules can be
tedious to prepare and purify because their high polarity often
necessitates the use of aqueous conditions during synthesis and
purification. For example, we found that HPLC was required
for purification of sulfonated oxazine near-IR fluorophores such
as 1 (Figure 1).6 On the other hand, purification of stable
A convenient cleavage reagent for many protecting groups is
trifluoroacetic acid (TFA).7 While TFA-labile protecting groups
for carboxylates, alcohols, amines, and other functionalities are
common,7 most sulfonate esters that are stable to nucleophiles
are also stable to TFA.8 An exception is the “triggered” TFA-
labile sulfonate ester protecting group reported by Roberts et
al., Neo N−B, which is stable to nucleophiles by virtue of its
neopentyl structure.13 However, cleavage of Neo N−B is a two-
step process that requires initial cleavage of a Boc-protected
amine followed by neutralization to effect release of the
sulfonate. Isolation of the liberated sulfonate requires the
chromatographic removal of nonvolatile side-products, and the
protecting group itself is not readily available, requiring a four-
Figure 1. Bis-sulfonate oxazine 1 and bis-sulfonate ester 2.
sulfonate ester dyes such as 2 was possible via conventional
silica gel flash chromatography.6 Thus, the synthesis of
sulfonated molecules could be aided by a protecting group
which allows solubility in standard organic solvents and
purification by conventional chromatography, but is readily
removed in a final cleavage step that does not itself require
HPLC purification.
Compared to carboxylates, there are relatively few choices for
Received: September 19, 2012
Published: November 20, 2012
sulfonate protecting groups.7 Sulfonate esters are generally
© 2012 American Chemical Society
711
dx.doi.org/10.1021/jo302065u | J. Org. Chem. 2013, 78, 711−716

The Journal of Organic Chemistry
Note
Scheme 1. Ruppert-Prakash Synthesis of Potential Reagents for the Development of Sulfonate Protecting Groups
Scheme 2. Synthesis of TFMT Sulfonate Esters
step synthesis from commercially available materials. Other
related neopentyl-based protection strategies for sulfonates
have similar limitations.5,14 We therefore sought to develop a
sulfonate protecting group which could satisfy three criteria: (i)
intrinsic TFA-lability not requiring any additional treatment for
cleavage to occur; (ii) easy separation of the cleaved sulfonate
from any byproducts; and (iii) ready access from commercially
available materials.
In a recent survey of commercially available alcohols that
could potentially serve as protecting groups for sulfonates, we
found that α-trifluoromethylbenzyl (TFMB) sulfonates (as in 2,
Figure 1) are stable to nucleophiles and could function as a
scaffold for the design of sulfonate ester protecting groups with
engineered lability.8 For example, we have prepared an esterase-
labile sulfonate protecting group by incorporating an acetoxy
group into the para position.15 Thus, while TFMB sulfonates
themselves are stable to TFA,8 we hypothesized that electron-
rich variants could be created that would be labile to TFA yet
retain stability to nucleophiles.
Solvolysis studies of 1-aryl-2,2,2-trifluoromethyl tosylates
have shown that the rate of carbocation formation in ionizing
solvents is dependent on the nature of the aryl group.16 We
therefore introduced the strongly electron-donating para-
methoxy group onto the TFMB scaffold by treating p-
anisaldehyde with TMSCF3 under Ruppert-Prakash condi-
tions17 to afford 3 (Scheme 1). More weakly electron-donating
p-methyl and p-isopropyl substituents were similarly introduced
to yield alcohols 4 and 5, respectively (Scheme 1). Typically,
the Ruppert-Prakash reaction is quenched with HCl to
hydrolyze the initially formed TMS ether,17 followed by
chromatographic purification. However, we found that the
starting benzaldehydes and trifluoromethylated alcohol prod-
ucts closely elute during flash column purification. Thus, if the
reaction is incomplete, purification becomes tedious. We
therefore first purified the TMS ether, which was very well
resolved from the starting aldehyde by flash chromatography.
Subsequent treatment with aqueous acid cleanly cleaved the
TMS ether to yield the desired product after extractive workup.
We have adopted this improved synthesis protocol for all
Ruppert-Prakash reactions with benzaldehydes.
the dansylate and 3-chloropropanesulfonate decomposed
during purification on a silica gel column). On the other
hand, sulfonate esters 6 and 7 derived from alcohol 4 (Scheme
2) were stable to isolation and to treatment with nucleophiles.
Like the corresponding TFMB sulfonate esters,8 6 and 7 were
quantitatively recovered after 2 h of treatment with 20%
piperidine in DMF at room temperature or 4 h of treatment
with 1.1 equiv of sodium iodide in refluxing acetone (Table S2,
Figure S1, Supporting Information). While stable to
nucleophilic attack, the p-methyl group is sufficiently
electron-donating that sulfonate esters of 4, dubbed “TFMT”
sulfonates, can be cleaved by solvolysis in TFA (Table 1 and
Table 1. TFA Cleavage of Sulfonate Esters 6 and 7
entry
substrate
% water
time (h)
yield (%)
a
1
6
7
6
7
6
6
6
0
0
1
1
1
2
3
5
2
2
39
84
41
99
88
84
82
89
2
b
3
2
4
2
b
5
16
2
c
6
b
7
2
8
6
2
a
b
c
Average of n = 3. n = 2. n = 4.
Table S1, Supporting Information). Inclusion of water as a
cation scavenging agent at 2% v/v or greater improves the
cleavage yield in the case of the tosyl ester 6. This may reflect
the formation of a tight ion pair that can recombine unless
quenched by water.18 Under these conditions, cleavage is
readily achieved in 2 h at room temperature (Table 1), and the
pure sulfonate could be conveniently isolated by extraction and
evaporation, without requiring chromatography. Tosylate 8
derived from p-isopropyl alcohol 5 was also found to be TFA-
labile and nucleophile stable, but provided no obvious
advantages over TFMT. Finally, the TFMT sulfonate ester is
stable to K2CO3 in hot DMF and to hot dilute aqueous acidic
conditions that cleave acetals and carboxylate esters (2 M HCl/
We next tested the suitability of 3-5 as sulfonate protecting
groups. Reaction of alcohol 3 with sulfonyl chlorides yielded
sulfonate esters that were too labile to be easily isolated (e.g.,
712
dx.doi.org/10.1021/jo302065u | J. Org. Chem. 2013, 78, 711−716

The Journal of Organic Chemistry
Note
Scheme 3. Synthesis of Bis-sulfonated Oxazine Dye 1 using the TFA-labile TFMT Sulfonate Protecting Group
Scheme 4. Synthesis of Bis-sulfonate Oxazine Dye 1 using 1,3-Propanesultone
acetone, 70 °C),19 suggesting that TFMT sulfonates could
survive similar conditions necessary for oxazine dye synthesis
(Table S2, Supporting Information).
In conclusion, TFMT is a new sulfonate protecting group
that is readily accessible in a single high-yield step from
commercially available materials. TFMT-protected sulfonates
are resistant to nucleophilic attack but can be unmasked under
relatively mild conditions by treatment with TFA, and the
cleavage product separated by extraction and evaporation. The
TFMT group thus complements and expands the repertoire of
practical and useful sulfonate protecting groups. We have found
it to be advantageous for the synthesis and purification of
sulfonated oxazine dyes and their intermediates, and anticipate
that it will be generally useful for the synthesis of a wide variety
of sulfonated molecules. For example, the stability of the
TFMT group to 20% piperidine in DMF and facile cleavage by
TFA suggests that it may also find application for the synthesis
of sulfonated peptides under Fmoc solid-phase peptide
synthesis (SPPS) conditions.
Having established that TFMT met our three criteria for a
new sulfonate protecting group, we next sought to test its utility
in the context of sulfonated near-IR fluorophore synthesis. To
access the TFMT-protected sulfonated dyes, we first
synthesized the TFMT-protected sulfopropyl iodide 10
(Scheme 2). As with TFMB,6 the remarkable stability of
TFMT sulfonates to nucleophilic attack by iodide is manifestly
demonstrated by the selective substitution of the chloride in 9
to cleanly afford 10. Iodide 10 was then used to alkylate the
tetrahydroquinolines 116 and 126 to afford 13 and 14 (Scheme
3). Like TFMB,6 the TFMT group was stable to these
nucleophilic substitution reactions. Similarly, acidic diazonium
coupling conditions (10% aqueous H2SO4) and dye coupling
conditions (hot acetic acid or HCl in refluxing aqueous
ethanol) were tolerated, allowing access to the bis-TFMT
protected oxazine dye 16 (Scheme 3), which could be purified
by silica gel chromatography.
Treatment of the purified bis-TFMT dye 16 with TFA and
2% H2O at rt for 2h liberated the bis-sulfonate dye 1. After
reaction, the volatiles were removed and the dye was extracted
into water and lyophilized. Notably, complete cleavage was
observed, and no impurities were introduced (Figure S2,
Supporting Information). Compared to the synthesis of the bis-
sulfonated oxazine dye 1 using the conventional sulfopropyla-
tion reagent 1,3-propanesultone (Scheme 4), the TFMT
approach was higher-yielding, easier to perform, and more
rapid as all products could be purified by standard silica gel
flash chromatography methods rather than HPLC.
EXPERIMENTAL SECTION
■
General Procedure for Ruppert-Prakash Reactions.
Trimethyl(trifluoromethyl)silane (1.4 equiv) was added to a solution
of the benzaldehyde (1.0 equiv) in THF (0.5 M). The solution was
cooled to 0 °C followed by the dropwise addition of a catalytic amount
(∼1 drop/mmol) of 1 M tetra-n-butylammonium fluoride (TBAF) in
THF. The reaction was brought to room temperature and stirred for 1
h. The solution was then concentrated via rotary evaporation and
purified by flash column chromatography (0−1% ethyl acetate/
hexanes) giving the TMS-protected alcohol as a clear, pale-yellow
liquid. This intermediate was dried under vacuum and then dissolved
in 1:1 THF−1 M HCl (0.25 M) and stirred for 2 h at room
temperature to cleave the TMS ether. The solution was then poured
into water and the product was extracted with ethyl acetate. The
combined organic phases were washed with 0.1 M HCl, water, brine,
and then dried over Na2SO4. Removal of solvent in vacuo yielded the
product.
713
dx.doi.org/10.1021/jo302065u | J. Org. Chem. 2013, 78, 711−716

The Journal of Organic Chemistry
Note
2,2,2-Trifluoro-1-(4-methoxy-phenyl)-ethanol 3:20 yellow oil (1.4
g, 94%). H NMR (400 MHz, CDCl3): δ 7.40 (d, 2H, J = 8.9 Hz),
General Procedure for Determining Stability to 20%
Piperdine/DMF. Protected sulfonate (0.1−0.4 mmol) was dissolved
in 1 mL of a freshly prepared 20% piperdine/DMF solution and stirred
at room temperature for 2 h. The reaction was then poured into 1 M
HCl and extracted with ethyl acetate. The combined organic phases
were washed with water and brine and then dried over sodium sulfate.
The recovered compound was then isolated by removal of the solvent
via rotary evaporation.
General Procedure for Determining Stability to NaI in
Refluxing Acetone. The protected sulfonate (0.1 mmol) was
dissolved in a solution of NaI (16 mg, 0.11 mmol) in 0.2 mL acetone
and refluxed for 4 h. The reaction was diluted into 1:1 EtOAc/H2O
and the organic phase was collected. The aqueous phase was further
extracted with ethyl acetate and the combined organic phases were
washed with brine and dried over sodium sulfate. The compound was
then isolated by removal of the solvent via rotary evaporation.
General Procedure for Determining Stability to Potassium
Carbonate. K2CO3 (0.24 mmol) was added to a solution of the
protected sulfonate (0.2 mmol) in DMF (0.4 mL) and the mixture was
heated to 65 °C for 24 h. The reaction was then poured into water and
acidified with 1 M HCl. The aqueous phase was extracted with ethyl
acetate and the combined organic phases were washed with water and
brine and then dried over sodium sulfate. The solvent was removed by
rotary evaporation and the recovered material was purified by flash
column chromatography (0−10% ethyl acetate/hexanes). Both 6 and
7 were recovered in 92% yield.
1
6.93 (d, 2H, J = 8.9 Hz), 4.97 (q, 1H, JHF = 6.7 Hz), 3.83 (s, 3H), 2.46
(br s, 1H). 19F-NMR (376 MHz, CDCl3): δ −79.0 (d, J = 6.7 Hz). 13
C
NMR (100 MHz, CDCl3): δ 160.5, 129.1, 126.5, 124.6 (q, 1JCF = 282
Hz), 114.2, 72.5 (q, 2JCF = 32 Hz), 55.5. HRMS (EI) m/z: [M + H]+
Calcd for C9H10F3O2: 207.0633, found: 207.0625.
2,2,2-Trifluoro-1-p-tolyl-ethanol 4:18 pale yellow oil (1.5 g, 95%).
1
The product becomes a soft, off-white solid when stored at 4 °C. H
NMR (400 MHz, CDCl3): δ 7.36 (d, 2H, J = 8.2 Hz), 7.22 (d, 2H, J =
7.9 Hz), 5.02−4.96 (dq, 1H, J = 4.5 Hz, JHF = 6.7 Hz), 2.48 (d, 1H, J =
4.6 Hz), 2.37 (s, 3H). 19F-NMR (376 MHz, CDCl3): δ −78.9 (d, J =
6.8 Hz). 13C NMR (100 MHz, CDCl3): δ 139.8, 131.3, 129.6, 127.6,
124.6 (q, 1JCF = 282 Hz), 72.9 (q, 2JCF = 32 Hz), 21.4. HRMS (EI) m/
z: [M + Na]+ Calcd for C9H9F3ONa: 213.0503, found: 213.0497.
2,2,2-Trifluoro-1-(4-isopropyl-phenyl)-ethanol 5: pale yellow oil
(1.3 g, 94%). 1H NMR (400 MHz, CDCl3): δ 7.46 (d, 2H, J = 8.2 Hz),
7.38 (d, 2H, J = 8.2 Hz), 4.92 (q, 1H, JHF = 6.5 Hz), 4.05 (br s, 1H),
3.11−3.00 (m, 1H), 1.40 (s, 3H), 1.38 (s, 3H). 19F-NMR (376 MHz,
CDCl3): δ −78.5 (d, J = 6.8 Hz). 13C NMR (100 MHz, CDCl3): δ
150.8, 131.7, 127.9, 127.0, 124.7 (q, 1JCF = 282 Hz), 72.9 (q, 2JCF = 32
Hz), 34.2, 24.0. HRMS (EI) m/z: [M + H]+ Calcd for C11H14F3O:
219.0997, found: 219.0999.
General Procedure for Sulfonate Ester Formation. Sulfonyl
chloride (1.1 equiv) and a trifluoromethyl benzyl alcohol (1 equiv)
were dissolved in CH2Cl2 (0.7 M). To this solution was added
dropwise a solution of DABCO (1.2 equiv) in CH2Cl2 (2 M), resulting
in precipitate formation. The mixture was then stirred for 4 h at room
temperature. The reaction was quenched by the addition of 1 M
NaOH (1 mL) resulting in elimination of the precipitate. The solution
was diluted into ethyl acetate and extracted with saturated NaHCO3,
0.1 M HCl, water, and brine. The organic phase was then dried over
Na2SO4 and the solvent was removed via rotary evaporation.
Stability of 6 to Hot Dilute Aqueous Acid. Compound 6 (75
mg, 0.22 mmol) was dissolved in a 1 mL solution of 1:1 2 M HCl/
acetone and heated to 70 °C for 2 h. The reaction was then poured
into ethyl acetate and the organic phase was washed with water and
brine and dried over sodium sulfate. The solvent was removed by
rotary evaporation and the recovered material was purified by flash
column chromatography (0−10% ethyl acetate/hexanes) to return the
starting material as a white solid (70 mg, 93%).
Toluene-4-sulfonic acid 2,2,2-trifluoro-1-p-tolyl-ethyl Ester 6:18
white solid (1.17 g, 85%). mp 83−85 °C; 1H NMR (400 MHz,
CDCl3): δ 7.65 (d, 2H, J = 8.3 Hz), 7.23−7.20 (m, 4H), 7.11 (d, 2H,
7.7 Hz), 5.62 (q, 1H, JHF = 6.4 Hz), 2.40 (s, 3H), 2.33 (s, 3H). 19F-
NMR (376 MHz, CDCl3): δ −76.5 (d, J = 6.4 Hz). 13C NMR (100
MHz, CDCl3): δ 145.5, 140.7, 133.3, 129.9, 129.5, 128.3, 128.1, 126.9,
3-Chloro-propane-1-sulfonic acid 2,2,2-trifluoro-1-p-tolyl-ethyl
Ester 9. Compound 4 (1 g, 5.3 mmol) was dissolved in anhydrous
dichloromethane (48 mL) and cooled to 0 °C in an ice bath.
Triethylamine (1.46 mL, 10.5 mmol) was then added to the solution.
3-Chloropropanesulfonyl chloride (0.83 mL, 6.8 mmol) dissolved in
anhydrous dichloromethane (5 mL) was added dropwise via syringe to
the reaction flask at a rate of 20 drops/min. Upon completion of
compound addition, the reaction was stirred at 4 °C overnight. The
reaction was poured into 1 M HCl and the product was extracted with
dichloromethane. The combined organic phases were washed with
water, saturated sodium bicarbonate and brine, then dried over sodium
sulfate. Removal of the solvent under vacuum resulted in a pale yellow
oil. The crude product was purified by flash column chromatography
(0−15% ethyl acetate/hexanes) to yield the product as a white solid
(1.64 g, 97%). mp 61−63 °C; 1H NMR (400 MHz, CDCl3): δ 7.38 (d,
2H, J = 8.0 Hz), 7.26 (d, 2H, J = 7.8 Hz), 5.74 (q, 1H, JHF = 6.4 Hz),
3.63−3.54 (m, 2H), 3.27−3.14 (m, 2H), 2.39 (s, 3H), 2.31−2.21 (m,
2H). 19F-NMR (376 MHz, CDCl3): δ −76.3 (d, J = 6.4 Hz). 13C
NMR (100 MHz, CDCl3): δ 141.4, 130.0, 128.3, 126.8, 122.5 (q, 1JCF
1
2
122.5 (q, JCF = 281 Hz), 78.3 (q, JCF = 34 Hz), 21.8, 21.5. HRMS
(EI) m/z: [M + Na]+ Calcd for C16H15F3O3SNa: 367.0592, found:
367.0583.
5-Dimethylamino-naphthalene-1-sulfonic acid 2,2,2-trifluoro-1-
1
p-tolyl-ethyl Ester 7: yellow solid (436 mg, 98%). mp 97−99 °C; H
NMR (400 MHz, CDCl3): δ 8.46 (d, 1H, J = 8.5 Hz), 8.23 (d, 1H, J =
8.7 Hz), 8.08 (dd, 1H, J = 1.3 Hz, 7.4 Hz), 7.56 (dd, 1H, J = 7.6 Hz,
8.6 Hz), 7.37 (dd, 1H, J = 7.4 Hz, 8.5 Hz), 7.16 (d, 1H, J = 7.1 Hz),
6.98 (d, 2H, J = 8.0 Hz), 6.81 (d, 2H, J = 7.9 Hz), 5.6 (q, 1H, JHF = 6.4
Hz), 2.83 (s, 6H), 2.17 (s, 3H). 19F-NMR (376 MHz, CDCl3): δ
−76.3 (d, J = 6.4 Hz). 13C NMR (100 MHz, CDCl3): δ 151.8, 140.3,
132.1, 131.8, 130.4, 130, 129.8, 128.9, 128.2, 126.1, 122.9, 122.5 (q,
1JCF = 281 Hz), 119.7, 115.7, 78.8 (q, 2JCF = 34 Hz), 45.6, 21.3. HRMS
(EI) m/z: [M + H]+ Calcd for C21H21F3NO3S: 424.1194, found:
424.1202.
2
= 281 Hz), 78.3 (q, JCF = 35 Hz), 49.8, 42.3, 26.7, 21.6. HRMS (EI)
m/z: [M + Na]+ Calcd for C12H14ClF3O3SNa: 353.0202, found:
353.0190.
Toluene-4-sulfonic acid 2,2,2-trifluoro-1-(4-isopropyl-phenyl)-
ethyl Ester 8: off-white solid (349 mg, 94%). mp 81−83 °C; 1H
NMR (400 MHz, CDCl3): δ 7.61 (d, 2H, J = 8.4 Hz), 7.20 (d, 2H, J =
8.2 Hz), 7.17 (d, 2H, J = 8 Hz), 7.12 (d, 2H, 8.2 Hz), 5.63 (q, 1H, JHF
= 6.4 Hz), 2.92−2.82 (m, 1H), 2.37 (s, 3H), 1.22 (s, 3H), 1.20 (s,
3H). 19F-NMR (376 MHz, CDCl3): δ −76.5 (d, J = 6.4 Hz). 13C
NMR (100 MHz, CDCl3): δ 151.4, 145.3, 133.4, 129.8, 128.4, 128.1,
3-Iodo-propane-1-sulfonic acid 2,2,2-trifluoro-1-p-tolyl-ethyl
Ester 10. Compound 9 (500 mg, 1.51 mmol) was added to a solution
of sodium iodide (906 mg, 6.05 mmol) in acetone (5 mL). The
reaction was heated to reflux and stirred for 16 h. After cooling to
room temperature, the mixture was filtered to remove sodium chloride
and the filtrand was washed with acetone (25 mL). Acetone was
removed via rotary evaporation and the resulting residue was extracted
between ethyl acetate and water. The organic phase was collected and
the aqueous phase was washed with ethyl acetate. The combined
organic phases were washed with brine, dried over sodium sulfate, and
the solvent removed via rotary evaporation giving a bronze oil. The
crude material was purified with flash column chromatography (0−
10% ethyl acetate/hexanes) to yield the product as a viscous, pale
1
2
127.0, 126.9, 122.5 (q, JCF = 281 Hz), 78.5 (q, JCF = 34 Hz), 34.1,
24.04, 24, 21.8. HRMS (EI) m/z: [M + Na]+ Calcd for
C18H19F3O3SNa: 395.0905, found: 395.0899.
General Procedure for TFA Cleavage of Sulfonate Esters.
Sulfonate ester (∼0.2 mmol) and water (0−30 μL) were dissolved in
TFA (1 mL) and stirred at room temperature for 2−16 h. After rotary
evaporation of TFA, the residue was taken up into 80 mL water. The
aqueous phase was extracted with ethyl acetate and lyophilized to yield
the cleaved tosylate or dansylate.
1
yellow oil (571 mg, 90%). H NMR (400 MHz, CDCl3): δ 7.39 (d,
714
dx.doi.org/10.1021/jo302065u | J. Org. Chem. 2013, 78, 711−716

The Journal of Organic Chemistry
Note
2H, J = 8.1 Hz), 7.27 (d, 2H, J = 7.9 Hz), 5.73 (q, 1H, JHF = 6.4 Hz),
3.22−3.07 (m, 4H), 2.39 (s, 3H), 2.33−2.21 (m, 2H). 19F-NMR (376
MHz, CDCl3): δ −76.3 (d, J = 6.4 Hz). 13C NMR (100 MHz,
(m, 2H), 1.92 (p, 2H, J = 5.3 Hz). 19F-NMR (376 MHz, CDCl3): δ
−76.3 (d, J = 6.2 Hz). 13C NMR (100 MHz, CDCl3): δ 160.1, 157.8,
151, 147.0, 141.5, 134, 130.1, 128.3, 126.7, 124.9, 122.7, 122.6 (q, 1JCF
1
2
CD3OD): δ 141.5, 130.1, 128.4, 126.8, 122.5 (q, JCF = 281 Hz), 78.3
= 281 Hz), 117.8, 116.1, 93.5, 78.5 (q, JCF = 34 Hz), 56.7, 50, 49.9,
2
(q, JCF = 34 Hz), 53.2, 27.2, 21.6, 2.1. HRMS (EI) m/z: [M + Na]+
49.7, 27.4, 22.1, 21.6, 20.7. HRMS (EI) m/z: [M + H]+ Calcd for
C28H30F3N4O6S: 607.1838, found: 607.1841.
Calcd for C12H14F3IO3SNa: 444.9558, found: 444.9520.
3-[7-(tert-Butyl-dimethyl-silanyloxy)-3,4-dihydro-2H-quinolin-1-
yl]-propane-1-sulfonic acid 2,2,2-trifluoro-1-p-tolyl-ethyl Ester 13.
7-(t-Butyldimethylsilyloxy)-1,2,3,4-tetrahydroquinoline 116 (200 mg,
0.76 mmol) was dissolved in DMF (0.8 mL). Compound 10 (641 mg,
1.5 mmol) dissolved in DMF (0.8 mL) and potassium carbonate (120
mg, 0.91 mmol) were added to the solution and the reaction was
heated to 70 °C and stirred for 24 h. The reaction was poured into
water and acidified to pH 1 with 1 M HCl. The product was extracted
with ethyl acetate and the combined organic phases were washed with
water and brine and dried over sodium sulfate. The solvent was
removed by rotary evaporation and the crude material was purified by
flash column chromatography (0−5% acetone/hexanes) to yield a
1,11-Bis-[3-(2,2,2-trifluoro-1-p-tolyl-ethoxysulfonyl)-propyl]-
3,4,8,9,10,11-hexahydro-2H-13-oxa-6,11-diaza-1-azonia-penta-
cene Tetrafluoroborate 16. Compound 13 (44 mg, 79 μmol) and
compound 15 (55 mg, 91 μmol) were dissolved in a solution of
ethanol/water/hydrochloric acid (500 μL:50 μL:25 μL) and heated to
80 °C with stirring for 2 h. Solvent was removed via rotary evaporation
to give a fluorescent blue residue. The crude material was purified by
flash column chromatography (0−10% methanol/dichloromethane)
yielding the tetrafluoroborate salt as a blue solid (36 mg, 47%). mp
1
220−222 °C (dec.); H NMR (400 MHz, CD3OD): δ 7.47 (s, 2H),
7.43 (d, 4H, J = 8.1 Hz), 7.21 (d, 4H, J = 8 Hz), 6.81 (s, 2H), 6.07 (q,
2H, JHF = 6.5 Hz), 3.74−3.61 (m, 4H), 3.55 (t, 4H, J = 5.5 Hz), 3.45−
3.33 (m, 4H), 2.90 (t, 4H, J = 6 Hz), 2.28 (s, 6H), 2.15−2.07 (m, 4H),
1.99 (p, 4H, J = 5.5 Hz). 19F-NMR (376 MHz, CD3OD): δ −78.0 (d, J
1
viscous orange oil (337 mg, 80%). H NMR (400 MHz, CDCl3): δ
7.37 (d, 2H, J = 8.1 Hz), 7.22 (d, 2H, J = 7.9 Hz), 6.78 (d, 1H, J = 8.0
Hz), 6.12 (dd, 1H, J = 2.2 Hz, 8.0 Hz), 6.01 (d, 1H, J = 1.9 Hz), 5.73
(q, 1H, J = 6.4 Hz), 3.33−3.21 (m, 2H), 3.10 (t, 2H, J = 5.5 Hz),
3.12−2.99 (m, 2H), 2.65 (t, 2H, J = 6.3 Hz), 2.38 (s, 3H), 2.07 (p, 2H,
J = 7.2 Hz), 1.88−1.82 (m, 2H), 0.98 (s, 9H), 0.19 (s, 6H). 19F-NMR
(376 MHz, CDCl3): δ −76.4 (d, J = 6.4 Hz). 13C NMR (100 MHz,
−
= 6.5 Hz), −154.58 (s), −154.6 (s) (BF4 counterion). 13C NMR
(100 MHz, CD3OD): δ 154.6, 148.7, 141.1, 134.4, 130.8, 129.5, 129.4,
1
2
128.2, 127.4, 122.9 (q, JCF = 280 Hz), 95.0, 77.7 (q, JCF = 34 Hz),
50.64, 50.6, 27.2, 20.6, 20.5, 20.1. HRMS (EI) m/z: [M]+ Calcd for
C42H44F6N3O7S2: 880.2525, found: 880.2511.
1
CD3OD): δ 155.2, 145.7, 141.3, 130, 129.9, 128.3, 127, 122.6 (q, JCF
1,11-Bis-(3-sulfo-propyl)-3,4,8,9,10,11-hexahydro-2H-13-oxa-
6,11-diaza-1-azonia-pentacene Tetrafluoroborate 1 by TFA Cleav-
age of 16. Compound 16 (12 mg, 12 μmol) and water (20 μL) was
dissolved in trifluoroacetic acid (1 mL) and stirred at room
temperature for 2 h. TFA was removed via rotary evaporation and
the residue was dissolved in water (60 mL). The aqueous phase was
extracted with ethyl acetate (1 × 40 mL, 4 × 20 mL) and lyophilized
to give the pure blue compound as a tetrafluoroborate salt (8.1 mg,
99%). mp 365−367 °C (dec.); 1H NMR (400 MHz, D2O): δ 6.99 (s,
2H), 6.6 (s, 2H), 3.5 (br s, 8H), 2.86 (t, 4H, J = 7.4 Hz), 2.62 (br s,
4H), 1.99 (p, 4H, J = 7.0 Hz), 1.82 (br s, 4H). 19F-NMR (376 MHz,
D2O): δ −150.9 (s), −151.0 (s) (Tetrafluoroborate counterion).
HRMS (EI) m/z: [M]+ Calcd for C24H30N3O7S2: 536.1525, found:
536.1507.
3-[7-(tert-Butyl-dimethyl-silanyloxy)-3,4-dihydro-2H-quinolin-1-
yl]-propane-1-sulfonic acid monohydrate (17). 7-t-Butyldimethylsi-
loxy-1,2,3,4-tetrahydroquinoline 116 (214 mg, 0.812 mmol) and 1,3-
propanesultone (200 mg, 1.6 mmol) were dissolved in methanol (1.6
mL) and stirred overnight at room temperature. The solvent was
removed via rotary evaporation and the crude material was purified by
flash column chromatography (10% methanol/dichloromethane) to
yield a yellow-white solid (119 mg, 36%). mp 202−204 °C (dec.); 1H
NMR (400 MHz, CD3OD): δ 6.73 (d, 1H, J = 8.0 Hz), 6.14 (br s,
1H), 6.05 (dd, 1H, J = 1.8 Hz, 8 Hz), 3.35 (t, 2H, J = 7.2 Hz), 3.27 (t,
2H, J = 5.5 Hz), 2.88 (t, 2H, J = 7.5 Hz), 2.64 (t, 2H, J = 6.3 Hz), 2.07
(p, 2H, J = 7.6 Hz), 1.9 (p, 2H, J = 6.3 Hz), 0.97 (s, 9H), 0.17 (s, 6H).
13C NMR (100 MHz, CD3OD): δ 155, 154.8, 145.5, 129.4, 116.3,
2
= 281 Hz), 116, 108.2, 103.1, 78.2 (q, JCF = 34 Hz), 50.3, 49.7, 49.6,
27.5, 26, 22.5, 21.6, 21.1, 18.5, −4.1. HRMS (EI) m/z: [M + H]+
Calcd for C27H39F3NO4SSi: 558.2321, found: 558.2305.
3-(7-Methoxy-3,4-dihydro-2H-quinolin-1-yl)-propane-1-sulfonic
acid 2,2,2-trifluoro-1-p-tolyl-ethyl Ester 14. 7-Methoxy-1,2,3,4-
tetrahydroquinoline 126 (100 mg, 0.61 mmol) was dissolved in
DMF (0.5 mL). Compound 10 (517 mg, 1.23 mmol) dissolved in
DMF (0.8 mL) and potassium carbonate (96 mg, 0.74 mmol) were
added to the solution and the reaction was heated to 70 °C and stirred
for 24 h. The reaction was poured into water and acidified to pH 1
with 1 M HCl. The product was extracted with ethyl acetate and the
combined organic phases were washed with water and brine and dried
over sodium sulfate. The solvent was removed by rotary evaporation
and the crude material was purified by flash column chromatography
(0−10% ethyl acetate/hexanes) to yield a viscous, pale yellow oil (217
mg, 78%). 1H NMR (400 MHz, CD3OD): δ 7.37 (d, 2H, J = 8.1 Hz),
7.2 (d, 2H, J = 7.9 Hz), 6.76 (d, 1H, J = 8.1 Hz), 6.12 (dd, 1H, J = 2.4
Hz, 8.1 Hz), 6.07 (d, 1H, J = 2.4 Hz), 5.96 (q, 1H, JHF = 6.5 Hz), 3.69
(s, 3H), 3.23−3.02 (m, 4H), 3.01 (t, 2H, J = 5.6 Hz), 2.58 (t, 2H, J =
6.1 Hz), 2.32 (s, 3H), 1.95−1.82 (m, 2H), 1.79−1.73 (m, 2H). 19F-
NMR (376 MHz, CD3OD): δ −78.0 (d, J = 6.5 Hz). 13C NMR (100
MHz, CD3OD): δ 159.5, 145.7, 141.1, 129.5, 128.2, 127.3, 122.9 (q,
2
1JCF = 280 Hz), 115.3, 100.9, 97.2, 77.8 (q, JCF = 34 Hz), 54.4, 49.3,
49.1, 49, 27.2, 22.4, 20.5, 20.2. HRMS (EI) m/z: [M + H]+ Calcd for
C22H27F3NO4S: 458.1613, found: 458.1596.
3-[7-Methoxy-6-(4-nitro-phenylazo)-3,4-dihydro-2H-quinolin-1-
yl]-propane-1-sulfonic acid 2,2,2-trifluoro-1-p-tolyl-ethyl Ester 15.
Compound 14 (105 mg, 0.23 mmol) was dissolved in methanol (2.3
mL). 4-Nitrobenzenediazonium tetrafluoroborate (54 mg, 0.23 mmol)
was suspended in 10% sulfuric acid (2.3 mL) with vigorous stirring.
The organic solution was added to the aqueous mixture and stirred at
ambient conditions for 1 h. The solution was then neutralized with
ammonium hydroxide, resulting in a deep red precipitate. The mixture
was filtered and the filtrand washed with water to give a blood red
solid (140 mg, quantitative). This compound was dried in vacuo and
used in the next reaction without further purification. NMR of the
crude material revealed a contaminant of ammonium tetrafluoroborate
salt. A small sample of the compound was therefore desalted by flash
column chromatography (0−50% ethyl acetate/hexanes) to obtain
103.4, 50.5, 49.2, 49.1, 27.2, 25.2, 22.1, 21.9, 18, −5.2. HRMS (EI) m/
z: [M + H]+ Calcd for C18H32NO4SSi: 386.1821, found: 386.1821.
1,11-Bis-(3-sulfo-propyl)-3,4,8,9,10,11-hexahydro-2H-13-oxa-
6,11-diaza-1-azonia-pentacene Trifluoroacetate 1. Compound 17
(43 mg, 110 μmol) and 3-[7-Methoxy-6-(4-nitro-phenylazo)-3,4-
dihydro-2H-quinolin-1-yl]-propane-1-sulfonic acid 196 (48 mg, 110
μmol) were dissolved in a solution of ethanol: water: hydrochloric acid
(1 mL: 100 μL: 50 μL) and heated to 80 °C with stirring for 2 h. The
solvent was removed via rotary evaporation to give a fluorescent blue
residue. The crude material was purified to 99% purity by HPLC over
a C18 column to yield a blue solid (22 mg, 29%). Solvent A and B
were respectively aqueous 0.1% trifluoroacetic acid and acetonitrile/
0.1% trifluoroacetic acid. The flow rate during purification was 5 mL/
min. The absorbance detector was set at 294, 306, 383, 607, and 660
nm. Elution of the compound was obtained via the following method:
15% B for 5 min, 15−50% B over 35 min, 50−100% B over 5 min,
1
NMR. mp 153−155 °C (dec.); H NMR (400 MHz, CDCl3): δ 8.27
(d, 2H, J = 9.0 Hz), 7.86 (d, 2H, J = 9.0 Hz), 7.58 (s, 1H), 7.37 (d, 2H,
J = 7.8 Hz), 7.24 (d, 2H, J = 7.8 Hz), 6.17 (s, 1H), 5.76 (q, 1H, JHF
=
1
6.3 Hz), 3.98 (s, 3H), 3.61−3.47 (m, 2H), 3.31 (t, 2H, J = 5.4 Hz),
3.16−3.05 (m, 2H), 2.72 (t, 2H, J = 5.9 Hz), 2.37 (s, 3H), 2.23−2.11
100% B for 10 min. mp 365−367 °C (dec.); H NMR (400 MHz,
D2O): δ 6.72 (s, 2H), 6.44 (s, 2H), 3.38 (br s, 8H), 2.81 (t, 4H, J = 6.8
715
dx.doi.org/10.1021/jo302065u | J. Org. Chem. 2013, 78, 711−716

The Journal of Organic Chemistry
Note
Hz), 2.48 (br s, 4H), 1.91 (br s, 4H), 1.73 (br s, 4H). 19F-NMR (376
MHz, D2O): δ −76.1 (s) (TFA counterion). 13C NMR (100 MHz,
D2O): δ 153.9, 147.9, 132.4, 129.4, 129.2, 95.2, 51.4, 50.7, 48.0, 26.9,
21.7, 20.2. HRMS (EI) m/z: [M − 2H]− Calcd for C24H28N3O7S2:
534.1368, found: 534.1348.
ASSOCIATED CONTENT
* Supporting Information
NMR spectra for all compounds; Supporting Tables and HPLC
Figures. This material is available free of charge via the Internet
at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*Email: Stephen.miller@umassmed.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Institutes of Health
(R01GM087460). We thank GL Synthesis (Worcester, MA)
for use of their melting point apparatus.
REFERENCES
■
(1) Mujumdar, R. B.; Ernst, L. A.; Mujumdar, S. R.; Lewis, C. J.;
Waggoner, A. S. Bioconjugate Chem. 1993, 4, 105−111.
(2) Panchuk-Voloshina, N.; Haugland, R. P.; Bishop-Stewart, J.;
Bhalgat, M. K.; Millard, P. J.; Mao, F.; Leung, W. Y.; Haugland, R. P. J.
Histochem. Cytochem. 1999, 47, 1179−1188.
(3) Li, L.; Han, J.; Nguyen, B.; Burgess, K. J. Org. Chem. 2008, 73,
1963−1970.
(4) Niu, S. L.; Ulrich, G.; Ziessel, R.; Kiss, A.; Renard, P.-Y.; Romieu,
A. Org. Lett. 2009, 11, 2049−2052.
(5) Morgan, M. T.; Bagchi, P.; Fahrni, C. J. J. Am. Chem. Soc. 2011,
133, 15906−15909.
(6) Pauff, S. M.; Miller, S. C. Org. Lett. 2011, 13, 6196−6199.
(7) Greene, T. W.; Wuts, P. G. M. Protective Groups in Organic
Synthesis, 3rd ed.; John Wiley & Sons: New York, 1999.
(8) Miller, S. C. J. Org. Chem. 2010, 75, 4632−4635.
(9) Xie, M.; Widlanski, T. S. Tetrahedron Lett. 1996, 37, 4443−4446.
(10) Musicki, B.; Widlanski, T. S. J. Org. Chem. 1990, 55, 4231−4233.
(11) Simpson, L. S.; Widlanski, T. S. J. Am. Chem. Soc. 2006, 128,
1605−1610.
(12) Ali, A. M.; Hill, B.; Taylor, S. D. J. Org. Chem. 2009, 74, 3583−
3586.
(13) Roberts, J. C.; Gao, H.; Gopalsamy, A.; Kongsjahju, A.; Patch, R.
J. Tetrahedron Lett. 1997, 38, 355−358.
(14) Seeberger, S.; Griffin, R. J.; Hardcastle, I. R.; Golding, B. T. Org.
Biomol. Chem. 2007, 5, 132−138.
(15) Rusha, L.; Miller, S. C. Chem. Commun. (Camb.) 2011, 47,
2038−2040.
(16) Allen, A. D.; Ambidge, I. C.; Che, C.; Micheal, H.; Muir, R. J.;
Tidwell, T. J. Am. Chem. Soc. 1983, 105, 2343−2350.
(17) Krishnamurti, R.; Bellew, D. R.; Prakash, G. K. S. J. Org. Chem.
1991, 56, 984−989.
(18) Allen, A. D.; Fujio, M.; Tee, O. S.; Tidwell, T. T.; Tsuji, Y.;
Tsuno, Y.; Yatsugi, K. J. Am. Chem. Soc. 1995, 117, 8974−8981.
(19) Heiskanen, J. P.; Hormi, O. E. O. Tetrahedron 2009, 65, 518−
524.
(20) Song, J. J.; Tan, Z.; Reeves, J. T.; Gallou, F.; Yee, N. K.;
Senanayake, C. H. Org. Lett. 2005, 7, 2193−2196.
716
dx.doi.org/10.1021/jo302065u | J. Org. Chem. 2013, 78, 711−716