A real time reaction monitoring using fluorescent dansyl group as a solid-phase leaving group

Article abstract of DOI:10.1016/S0040-4039(03)01406-0

A real time monitoring method to monitor the progress of the solid-phase reaction has been developed. The substitution reaction on the solid-phase was clearly monitored as a change in the color depth using the fluorescent dansyl group as the leaving group. This methodology is applicable to both parallel and split combinatorial synthesis.

Full text of DOI:10.1016/S0040-4039(03)01406-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 5799–5801
A real time reaction monitoring using fluorescent dansyl group
as a solid-phase leaving group
Toshiro Suenaga,* Caroline Schutz and Tadashi Nakata*
RIKEN (The Institute of Physical and Chemical Research), Hirosawa 2-1, Wako, Saitama 351-0198, Japan
Received 18 April 2003; revised 4 June 2003; accepted 6 June 2003
Abstract—A real time monitoring method to monitor the progress of the solid-phase reaction has been developed. The
substitution reaction on the solid-phase was clearly monitored as a change in the color depth using the fluorescent dansyl group
as the leaving group. This methodology is applicable to both parallel and split combinatorial synthesis.
© 2003 Elsevier Ltd. All rights reserved.
Organic reactions on a solid support have become very
important for constructing a small organic molecule
library. Though many types of reactions on solid sup-
ports have been extensively developed, relatively limited
numbers of monitoring methods of the solid-phase
reaction are known.1
reactions on the solid support by its bulkiness, and (5)
having a functional group to convert or attach leaving
group or protecting group. Unfortunately, we were not
able to find an appropriate dye with all the properties
shown above, but we found the dansyl (5-dimethyl-
amino-1-naphthalenesulfonyl) group to be promising
because of its chemical stability, bright fluorescent color
upon irradiation, relatively small molecular weight
compared to normal dye molecules and its commercial
availability as dansyl chloride (1) (Fig. 1).
Several methods such as the Kaiser test,2 bromophenol
blue test,3 picric acid test4 and other colorimetric assays
on a solid support have been developed mainly for
detecting solid supported amines.5 A few methods have
been reported to detect other functional groups such as
hydroxyl group,6 carbonyl group,7 carboxyl group,6a,8
thiols,6b,c,9 some protective groups.10,11 We now report
development of a very simple and rapid real time
method for monitoring the progress of the solid-phase
substitution reaction using stained leaving group.
Though dansyl chloride is well known as a reagent for
the fluorescent labeling of amines, amino acids,
proteins, and phenols, the dansyl group has not been
extensively investigated as a leaving group, neverthe-
less, it has a structural similarity to the tosyl group.12,13
If the dansyl group could be used in place of the tosyl
or mesyl group, many kinds of substitution reactions
would be easily monitored on a solid support. There-
fore, we initially investigated the reactivity of the dansyl
group in the solution phase by comparing it with the
reactivity of the tosyl group. The sulfonylation with
dansyl chloride and tosyl chloride is shown in Scheme
1. The dansylation of a primary alcohol 2 is slightly
If leaving groups or protective groups are stained, the
substitution or deprotection process would be clearly
monitored as a change in the color depth without using
any spectroscopic apparatus, which guarantee rapid
analysis for parallel syntheses and also one bead analy-
sis for split synthesis. First, we investigated commer-
cially available dyes. The essential properties of the
‘dye’ should be: (1) deep color other than yellow in
order to distinguish the dye color from the color of the
resin itself or remaining impurity on the resin, (2)
chemically stable, (3) soluble in standard solvents in
order to be easily washed off from the resin after the
reaction, (4) small molecule so as not to disturb the
* Corresponding authors. Tel.: +81-48-467-9374; fax: +81-48-462-
4666; e-mail: suenaga@riken.go.jp; nakata@riken.go.jp
Figure 1. Dansyl chloride.
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01406-0

5800
T. Suenaga et al. / Tetrahedron Letters 44 (2003) 5799–5801
comparable rates to 4. The iodination of the secondary
dansylate 6 was faster than that of 7. In all cases, the
yields are comparable to each other. From these reac-
tivity results, it is clear that the dansyl group can be
used in place of the tosyl group.
We then examined the substitution reaction of the
dansylated substrate on a solid support (Scheme 2). The
mono dansylated 1,8-octanediol 12 was prepared from
the diol 11 and 1.1 equiv. of dansyl chloride in 45%
yield. 12 Was then attached to the succinic acid linker
by treating with succinic anhydride to give the acid 13
in 88% yield. The acid 13 was loaded onto Tenta Gel
with DIC. The completion of the reaction was con-
firmed by the standard Kaiser test.2 An intense light
green yellow fluorescence was observed for the resulting
dansylated resin 14 upon irradiating with 365 nm UV
light. The dansylate 14 was treated with sodium azide
in DMF at 70°C for the azidation. At first, we tried the
direct observation of the reaction progress for maxi-
mum convenience. The solution began to immediately
show a light blue fluorescence and the fluorescence of
the beads became weak and finally was hidden by the
intense background fluorescence of the solution upon
irradiation. Though it was clear that the azidation
proceeded and was nearly completed, it was difficult to
judge the exact completion of the reaction. To precisely
monitor the progress of the reaction, a small portion of
the resin was taken out from the reaction vessel and
placed into a vial. The resin in a vial was washed with
DMF (2×) and CH2Cl2 (2×) and then observed under
365 nm UV light. As shown in Figure 2, the strong
fluorescence gradually became weak in 7 min. Only a
very weak fluorescence was observed in 10 min. No
fluorescence was observed in 20 min. Thus, the progress
of the reaction was monitored as the transition of
fluorescence as initially expected. To confirm the com-
pletion of the reaction, the azide was cleaved from resin
with K2CO3 in methanol at room temperature. The
crude solution contains only one compound based on
the TLC analysis. The solution was neutralized with
Dowex 50W-X2 and purified by silica gel chromatogra-
phy to give pure 8-azidooctan-1-ol (16) in 87% based
on the initial loading amount of 13 on the Tenta Gel.
Scheme 1. Dansylation and tosylation of various alcohols.
Table 1. Substitution reactions of dansylates and tosylates
Substrate Conditiona Time
Product
Yield (%)
3
4
3
4
3
4
A
A
B
B
C
C
2.5 h
1.5 h
1.5 h
1.0 h
4.0 h
3.5 h
3-Iodopropyl-
benzene
3-Iodopropyl-
benzene
3-Azidopropyl-
benzene
3-Azidopropyl-
benzene
4-Phenylbutyro-
nitrile
4-Phenylbutyro-
nitrile
87
94
87
90
91
92
3
4
3
4
6
D
D
E
E
A
16 h
16 h
Propylbenzene
Propylbenzene
Heptylbenzene
Heptylbenzene
83b
80b
92
92
77
30 min
30 min
2.5 days 2-Iodopropyl-
benzene
7
A
3.5 days 2-Iodopropyl-
benzene
82
a A: NaI, acetone, reflux. B: NaN3, DMF, reflux. C: NaCN, DMSO,
90°C. D: LiBH4, THF, reflux. E: Bu2CuLi, ether, −20°C.
b Without any further purification.
slower than the tosylation. The dansylation of a sec-
ondary alcohol 5 needs more than twice as much time
as the tosylation. Competitive sulfonylation of p-
methoxyphenylpropanol (8) with dansyl chloride (3
equiv.) and tosyl chloride (3 equiv.) gave 34% of the
dansylate 9 and 60% of the tosylate 10.
Table 1 shows the results of various nucleophilic substi-
tution reactions of the dansylates and tosylates. The
iodination, azidation and cyanation of the primary
dansylate 3 were slightly slower than those of the
tosylate 4. The reduction and alkylation of 3 had
Scheme 2. Azidation of dansylated substrate on the resin.
Reagents and conditions: (a) DsCl (1.1 equiv.), Py, CH2Cl2,
45%; (b) succinic anhydride, Py, DMAP, CH2Cl2, 88%; (c)
DIC, CH2Cl2; (d) NaN3, DMF, 70°C, 1 h; (e) K2CO3,
MeOH, 87% (two steps).

T. Suenaga et al. / Tetrahedron Letters 44 (2003) 5799–5801
5801
Figure 2. Monitoring azidation reaction by fluorescence.
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Figure 3. Substitution rate and fluorescence intensity.
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It is important to quantitatively establish the sensitivity
of this method. Tosylated acid 17 was prepared as
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solid-phase substitution reaction using the dansyl group
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Acknowledgements
This work was supported by Special Coordination
Funds for Promoting Science and Technology from the
Ministry of Education, Culture, Sports, Science and
Technology.