Study of the Paternò–Büchi type photolabile protecting group and application to various acids

Article abstract of DOI:10.1016/j.tetlet.2016.09.065

An efficient photolabile protecting group, thiochromone S,S-dioxides with the diazomethyl group for phosphate derivatives, amino acids and sulfonic acids was developed. Protection and photodeprotection reactions proceeded smoothly under mild conditions without any catalyst.¹H NMR and HPLC spectra studies demonstrated the photolysis properties of the photolabile protecting group and gave an exact quantification of the released substrates. Specially, the photoproduct derived from the thiochromone derivatives following Paternò–Büchi type photo-cycloaddition showed high fluorescence intensity. This fluorescent characteristic demonstrated the photodeprotection progress also can be monitored by fluorescence spectra.

Full text of DOI:10.1016/j.tetlet.2016.09.065

Accepted Manuscript
Study of the Paternò-Büchi Type Photolabile Protecting Group and Application
to Various Acids
Youlai Zhang, Huan Zhang, Chi Ma, Junru Li, Yasuhiro Nishiyama, Hiroki
Tanimoto, Tsumoru Morimoto, Kiyomi Kakiuchi
PII:
S0040-4039(16)31235-7
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.09.065
TETL 48135
Reference:
Tetrahedron Letters
To appear in:
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Revised Date:
Accepted Date:
10 August 2016
13 September 2016
18 September 2016
Please cite this article as: Zhang, Y., Zhang, H., Ma, C., Li, J., Nishiyama, Y., Tanimoto, H., Morimoto, T., Kakiuchi,
K., Study of the Paternò-Büchi Type Photolabile Protecting Group and Application to Various Acids, Tetrahedron
Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.09.065
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Study of the Paternò-Büchi Type Photolabile
Protecting Group and Application to
Various Acids
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Youlai Zhanga, Huan Zhanga,*, Chi Maa, Junru Lia, Yasuhiro Nishiyamab, Hiroki Tanimotob, Tsumoru
Morimotob, Kiyomi Kakiuchib,*

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Study of the Paternò-Büchi Type Photolabile Protecting Group and Application to
Various Acids
Youlai Zhanga , Huan Zhanga,  , Chi Maa , Junru Lia , Yasuhiro Nishiyamab , Hiroki Tanimotob , Tsumoru
Morimotob , Kiyomi Kakiuchib, 
aSchool of Chemistry and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, P.R. China
bGraduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, Nara 630-0192, Japan
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
An efficient photolabile protecting group, thiochromone S,S-dioxides with the diazomethyl
group for phosphate derivatives, amino acids and sulfonic acids was developed. Protection and
1
photodeprotection reactions proceeded smoothly under mild condition without any catalyst. H
NMR and HPLC spectra studies demonstrated the photolysis properties of the photolabile
protecting group and gave an exact quantification of the released substrates. Specially, the
photoproduct derived from the thiochromone derivatives following Paternò-Büchi type photo-
cycloaddition showed high fluorescence intensity. This fluorescent characteristic demonstrated
the photodeprotection progress also can be monitored by fluorescence spectra.
Available online
Keywords:
Thiochromone
Photolabile protecting group
Amino acids
Sulfonic acids
Photochemistry
2016 Elsevier Ltd. All rights reserved.
Molecules capable of releasing active groups with light
activation, termed photolabile protecting groups (PLPGs), are
appealing protective tools for organic synthesis and life science.1
The photodeprotection process can typically take place under
neutral conditions without any chemical reagents.2 From the
synthetic chemists’ point of view, photochemical methods are
valuable alternatives to the conventional approaches, which
employ acids, bases, or reducing/oxidizing reagents.3 For
biological and medical research, this approach provides an
indispensable method for the introduction of biologically active
compounds to the cell or tissue culture when spatial and/or
temporal control is desired with clean photochemistry.4 Caged
compounds are synthetic molecules with biological functions that
are masked temporarily by covalently introduced photolabile
protecting groups. Spot illumination to an appropriately designed
caged compound enables control of fundamental biological
processes with high spatial and temporal resolution. The
applications of PLPGs are by no means limited to biochemical
kinetic studies and also include photolithography, DNA synthesis
and microarray fabrication, surface patterning and
photolithographic preparation of high-density biochips and solid
state synthesis.5 The past two decades have witnessed continuous
emergence of new PLPGs and their creative applications, such as
o-nitrobenzyl groups, coumarin groups, arylcarbonylmethyl
groups and arylmethyl groups.6
It is well-known that thiochromone derivatives have an
expanding conjugated system over the bicyclic skeleton.7
However, the study in PLPGs field was seldom reported.
Recently, we have designed and synthesized a series efficient
PLPGs, which possess photo-recognition ability based on
thiochromone as parent nucleus structure. 8-10
Thiochromone derivative 1 was prepared from thiophenol and
ethyl acetoacetate.11 The synthesis route is expected to be simple
and efficient. Allylic oxidation of 1 with SeO2 produced aldehyde
2 along with hetarenoindanone. Based on the mechanistic
proposal we have studied,12 in order to avoid the intramolecular
Friedel-Crafts reaction to form byproduct, MS3Å was added in
the reaction to remove H2O and improve the yield to 87%.
Furthermore oxidation of the sulfur atom of 2 with m-CPBA
furnished thiochromone S,S-dioxide 3 in 95% yield. Then 3 was
treated with an equimolar amount of tosylhydrazide to convert to
tosylhydrazone 4 and following Bamford-Stevens reaction gave
target PLPG with diazomethyl group in 71% yield (Scheme 1).10
The PLPG has some advantageous features.10 For example, it
is easily synthesized from commercially available inexpensive
materials, it has high protection/deprotection efficiencies, and it
possesses remarkable dark stability. In particular, mindful of the
unique structural features, we have developed
a neutral
protecting protocol and demonstrated for the first time that both
protection and deprotection reactions can be conducted under
———
 Corresponding author. e-mail: zhanghuan80887@163.com
 Corresponding author. e-mail: kakiuchi@ms.naist.jp

2
Tetrahedron Letters
neutral conditions without using any other chemical reagents.
amino acid, methods for the introduction of the protecting groups
as well as for their removal are discussed.
Herein we reported our effort in further enriching the PLPG and
applications in releasing various acids.
Table 1. Protection and photodeprotection of amino acids
Entry
Acids
Protection
Photo-
deprotectionb
Time Yield
(h)
Time
(min)
Yield
(%)a
(%)c
>99
(97)a
1
2
65
80
15
14
8a
8b
>99
Scheme 1. Synthesis of PLPG.
75
82
(98)a
Among various functional groups, we are particularly
interested in developing PLPG for phosphate derivatives, amino
acids and sulfonic acids. These acids are important functional
groups in organic chemistry and often need protection in the
course of multistep syntheses. Moreover, many biologically
important molecules also have these groups.
3
4
72
65
86
75
15
15
>99
>99
8c
The PLPG was promising in protecting phosphate derivatives
and then further examined with various substrates for its
application scopes, including aliphatic phosphates, aromatic
phosphates, cycle phosphates (Scheme 2).10 Protection reactions
of phosphates were straightforward, typically in the following
reaction conditions. Treatment of phosphates with 1.25
equivalent of PLPG in the presence of a catalytic amount of 10%
aqueous HCl in chloroform at 60 °C afforded the products in
high yield. The products were reasonably stable and insensitive
to the laboratory lighting. Deprotection proceeded smoothly
under photoirradiation using a 500 W ultra-high pressure
mercury lamp filtered through Pyrex glass (>280 nm) to recover
corresponding phosphates quantitatively within 14 minutes.
Although the substrates were the different phosphates protected
compounds, they all showed similar efficiency in
photodeprotection leading to more than 99% yields determined
by 1H NMR spectroscopy.10
8d
>99
5
6
68
65
88
85
13
14
8e
8f
(98)a
>99
(97)a
a Isolated yield.
b Irradiated with a 500 W ultra-high pressure mercury lamp filtered through
Pyrex glass in a 1mm pathlength quartz cell (0.01 M in CDCl3).
c Determined by 1H-NMR spectroscopy.
The data of protection and photodeprotection reactions for
amino acids are shown (Table 1). In order to apply the PLPG to
biological compounds, the protection reactions were carried out
under neutral conditions. The mixuture of the PLPG and amino
acids were stirred in cholroform at room temperature to get the
target protection products (75-88%). The photodeprotection
reactions were carried out with a 500 W ultra-high pressure
mercury lamp filtered through Pyrex glass in a 1mm pathlength
quartz cell (0.01 M in CDCl3), determined by 1H-NMR
spectroscopy. Deprotection proceeded smoothly under
photoirradiation to recover corresponding amino acids
quantitatively within 15 minutes. They all showed similar
efficiency in photodeprotection leading to more than 99% yield
determined by 1H NMR spectroscopy. In these reactions, not only
photodeprotections, but also the protections were run under
neutral conditions. The development of the protecting groups to
peptide chemistry is in progress.
Sulfonic acids have largely remained unexplored in the terms of
a functional group that is released from a photolabile protecting
group. One recent example has been reported,13 but the protection
yield was low (26%) and the quantum efficiency of 0.081 is
unreasonable when compared with other leaving groups that were
reported. From this view, we would like to apply the PLPG to the
protection and photodeprotection for sulfonic acids.
Scheme 2. Protection and photodeprotection of phosphates.
Besides phosphate derivatives, the PLPG was also applied for
some other functional groups. Amino acids are critical to life and
have many functions in metabiolism. Herein, we provide a
concise analysis of the protection of amino acids. The strategy
was to protect the carboxylic acid of functional group. For each

3
Table 2. Protection and photodeprotection of sulfonic acids
Entry
Acids
Protection
Photo-
deprotectionb
Time Yield
(h)
Time
(min)
Yield
(%)a
(%)c
>99
(97)a
1
2
12
85
14
15
10a
10b
>99
12
76
Figure 1. Sections of the 1H NMR spectra (3.5-5.1 ppm) of 0.01
M DMSO-d6 solution of 9a recorded after 0, 3, 6, 9, 12, 15 min
of irradiation.
>99
(96)a
3
4
6
5
87
70
13
15
10c
10d
>99
a Isolated yield.
b Irradiated with a 500 W ultra-high pressure mercury lamp filtered through
Pyrex glass in a 1mm pathlength quartz cell (0.01 M in CDCl3).
c Determined by 1H-NMR spectroscopy.
7
8b
9b
4000
3500
3000
2500
2000
1500
1000
500
The data of protection and photodeprotection reactions for
sulfonic acids are shown (Table 2). About the protection reaction,
as we know that, sulfonic acids are strong acid. So, in protection
reaction, no catalyst was added and the yield were obtained (70-
87%). The photodeprotection reactions proceeded smoothly under
photoirradiation to recover corresponding sulfonic acids
quantitatively within 15 minutes determined by 1H NMR
spectroscopy.
15 min
12 min
9 min
6 min
3 min
0 min
In order to study the detail of the photodeprotection reaction,
1.5 W UV-LED lamps was used and the reaction was monitored
by 1H NMR (Figure 1), HPLC (Figure 2,3), UV-vis and
fluorescence spectroscopies (Figure 4).
0
0.0
2.5
5.0
7.5
10.0
Time (min)
Photodeprotection studies monitored by 1H NMR spectroscopy
were carried out by irradiating substrate 9a in NMR tubes. 9a was
prepared to 0.01 M solution of DMSO-d6, the solution was remove
gas and transferred to a syringe pump with nitrogen atmosphere.
Figure 2. HPLC trace following the irradiation (0, 3, 6, 9, 12, 15
min) of the 9b in CH3OH (mobile phase CH3OH : H2O with
volume ratio 80 : 20, 0.1% HOAc ). Flow rate: 1.0 mL/min;
detection wavelength: 210 nm, 20 °C.
Before irradiation, the characteristic peaks at 4.97 ppm (Ha, s,
2H) and 3.68 ppm (Hb, sd, 2H) derived from the two methylenes
of 9a were observed. During irradiation, the two peaks
disappeared as the reaction proceeded and a new peak at 3.58
ppm (Hb’, sd, 2H) of 8a appeared (Figure 1). After 15 min, these
1H NMR spectra showed clearly that the protected amino acid 9a
released corresponding amino acid 8a completely. After the
photoreaction, a work-up of the reaction mixture gave the
injector (20 μL sample loop). The analysis was performed on an
Unitary C18 column (5 μm, 250 × 4.6 mm2, Huapu, China) and
Chromatography Data System N2000 (Surwit Technology,
Hangzhou, China). Mobile phase was CH3OH:H2O (80:20, v/v,
containing 0.1% HOAc) and flow rate was set at 1.0 mL/min.
The wavelength of detector was set at 210 nm with injection
volume 10 μL.
The HPLC chromatogram obtained during irradiation was
shown in Figure 2. The peak with a retention time of 10.1 min
corresponding to 9b was observed prior irradiation. During
irradiation, this peak disappeared as the reaction proceeded and
were replaced with two new peaks corresponding to amino acid
8b with a retention time of 5.4 min and photolysis product 7
which was observed at 6.8 min. After 15 min, the protected
amino acid 9b released corresponding amino acid 8b completely.
Importantly, the similar results were verified that these
photodeprotection reactions were none accompanied by
photochemical side reactions.
tetracyclic compound
7 as a yellow crystal as reported
previously.8,10 1H-NMR spectroscopy also suggested that, no
unexpected secondary effects interfere with photolysis
throughout the photo reaction.
Next, we used reversed-phase (RP) HPLC analysis to
investigate the photochemical properties after exposure to
irradiation at 365 nm. Here, substrate 9b (0.001M in CH3OH)
was chosen to study the progress of photodeprotection. The
chromatography system consisted of a LC-10ATVP pump and
SPD-10AVP UV–vis detector (Shimadzu, Kyoto, Japan) with an

4
Tetrahedron Letters
quantification of the released substrates under the same
conditions used for the photolysis.
100
90
80
70
60
50
40
30
20
10
0
Interestingly, the compound 7 which was obtained from
photodeprotection reaction has much stronger fluorescent intensity
and the quantum yield for the fluorescence emission is so high
(0.85).8 Then, we tried to study the UV-vis and fluorescence
spectra during reaction.
9b
8b
The photodeprotection of 9a in 1 mm width quarts cell was
carried out and monitored by UV-vis and fluorescence
spectroscopies (Figure 4). As the reaction proceeded, the
absorption at around 350 nm increased, and a new fluorescence
emision at 440 nm was observed. Thus the new absorption and
emission peaks were attributed to the new product 7. Specially, the
ratio of released substrate 8a and fluorescent compound 7 is 1:1
during the photodeprotection reaction. At the same time, the
reaction showed from non fluorescent to high fluorescent.
Therefore, we can monitor the reaction progress which was applied
by PLPG by fluorescence spectroscopy.
0
3
6
9
12
15
Time (min)
Figure 3. HPLC standard curves of the released corresponding
amino acid 8b (0%/0min, 36.6%/3min, 62.3%/6min,
82.5%/9min, 94.4%/12min, 99.9%/15min) and decreased starting
material 9b (100%/0min, 63.4%/3min, 37.7%/6min, 17.5%/9min,
5.6%/12min, 0.1%/15min) during photodeprotection.
Then, the proecesses of protection and photodeprotection were
discussed. As expected, the reaction of PLPG with acids produced
the corresponsing esters through a protonation derived SN2
mechanism (Scheme 3).14
0.25
UV/vis
15min
13min
0.2
0.15
0.1
11min
9min
7min
5min
1min
0min
0.05
0
250
300
350
400
450
Wavelength/nm
15min
13min
11min
9min
7min
5min
1min
0min
Fluorescence
500
Scheme 3. Process of protection reaction.
400
300
200
100
0
360
410
460
Wavelength/nm
510
560
610
Figure 4. UV-vis and fluorescence spectra upon excitation at 350
-5
nm of 1.0 × 10 M methanol solution of 9a recorded after 0, 1,
5, 7, 9, 11, 13 and 15 min of irradiation.
Then, we wanted to follow the release of protected substrates
during irradiation. For this purpose, we studied the progress of
reaction quantitatively by HPLC analysis. The approach was
accomplished by the use of the external calibration of starting
material (9b, peak at 10.1 min, Figure 2) and the released
corresponding amino acid (8b, peak at 5.4 min, Figure 2). The
decreasing amount of starting material 9b, in addition to the
released corresponding amino acid 8b, demonstrated the
photolysis properties of the photolabile protecting groups (Figure
3). These two curves are symmetrical with respect to the time
axis. This means that the promise of the concept to give an exact
Scheme 4. Proposed photodeprotection process.
A
possible mechanism following to account for the
photodeprotection is summarized (Scheme 4). First, the substrate
which including a carbonyl functionality is excited by UV
irradiation to afford the corresponding triplet state (a). Then, the
excited carbonyl forms a diradical (b) which attacks alkene to
form the furan ring through Paternò-Büchi type photocyclo-
addition. Then, aromatization of (c) produces the final tetracyclic
compound 7 and releases acids.

5
M.; Sakurai, T. Synlett 2007, 9, 1436. (c) Bochet, C. G. J. Chem.
Soc. Perkin Trans. 1 2002, 2, 125.
In summary, a photolabile protecting group with diazomethyl
group as linker group was developed to react with the free acid
form of phosphate derivatives, amino acids and sulfonic acids
under neutral conditions. The photodeprotection reactions
proceeded smoothly under photoirradiation to recover
corresponding acids quantitatively within 15 minutes determined
by 1H NMR spectra and HPLC. Based on the special
thiochromone structure, the photoproduct showed high
fluorescence quantum yield. This unique photochemical property
makes it is possible to monitor the photochemical reaction by
fluorescence spectra. These results show potential applications of
the PLPG as valuable alternative of usual protecting groups
under neutral conditions.
3.
4.
Greene, T. W.; Wuts, P. G. M. Protective Groups in Organic
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6.
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (21303120) and the Natural Science
Foundation of Tianjin (13JCYBJC42100, 16JCQNJC13700).
This research was also supported in part by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT) in Japan, for scientific
research from Nara Institute of Science and Technology (NAIST)
and NAIST Advanced Research Partnership Project, which are
gratefully appreciated. We also thank to Ms. Yoshiko Nishikawa
for HRMS measurement.
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9.
Supplementary data
Experimental procedures, compound characterization data and
copies of NMR spectra for all products are included in
Supplementary data. Supplementary data associated with this
article can be found in the online version.
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K. Synlett 2012, 23, 367.
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6
Tetrahedron
Highlights
An efficient photolabile protecting group was
developed for various acids.
Protections and photodeprotections proceeded
smoothly under neutral condition.
1H NMR and HPLC spectra studies demonstrated
the photolysis properties.
The photoproduct showed high fluorescence
quantum yield.
The photodeprotection progress was also monitored
by fluorescence spectra.