Novel photolabile protecting group for phosphate compounds

Article abstract of DOI:10.1055/s-0031-1290326

A novel photolabile protecting group, thiochromone S,S-dioxide, containing the diazomethyl group for protection of phosphate derivatives is described. Deprotection of the successfully protected phosphate derivatives proceeded smoothly under photoirradiation using an ultrahigh-pressure mercury lamp to recover the corresponding phosphates quantitatively, and the photoproduct derived from the thiochromone derivative showed high fluorescence intensity. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0031-1290326

LETTER
367
Novel Photolabile Protecting Group for Phosphate Compounds
P
hotolabile Protec
o
ting Group fo
u
r
Phosphate
l
C
ompo
a
unds i Zhang, Hiroki Tanimoto, Yasuhiro Nishiyama, Tsumoru Morimoto, Kiyomi Kakiuchi*
Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, Nara 630-0101,
Japan
Fax +81(743)726089; E-mail: kakiuchi@ms.naist.jp
Received 25 October 2011
Among the functional groups studied by photochemists,
Abstract: A novel photolabile protecting group, thiochromone S,S-
the phosphate group would appear to be the most unattrac-
tive. It has one of the weakest UV-visible absorptions and
is viewed as a relatively reactive functional group. De-
dioxide, containing the diazomethyl group for protection of phos-
phate derivatives is described. Deprotection of the successfully pro-
tected phosphate derivatives proceeded smoothly under
photoirradiation using an ultrahigh-pressure mercury lamp to recov- spite these apparent restrictions, the photochemistry of
er the corresponding phosphates quantitatively, and the photoprod-
molecules containing a phosphate group is of intense in-
uct derived from the thiochromone derivative showed high
fluorescence intensity.
terest to biochemists specializing in the study of the func-
tion of nucleotides and other organophosphates in
biological processes.4
Key words: thiochromone, diazomethyl, photolabile protecting
groups, phosphates, photochemistry
Recently, we developed a novel photolabile protecting
group, thiochromone S,S-dioxide derivative 1a (Figure 1)
and applied it preliminarily to the protection and photo-
Protection and deprotection of functional groups under
chemically selective conditions is a key technique in or-
deprotection for alcohols, amines, and carboxylic acids.5
ganic and bioorganic synthesis. In many cases, these reac-
O
O
tions are carried out under acidic or basic conditions or
require highly reactive reagents, leading to the limitation
of use of these protecting groups. Among various protect-
ing groups, photolabile protecting groups (PLPGs) have
valuable and unique features, because the photodeprotec-
tion process can typically take place under neutral and re-
agent-free conditions.1 Especially for biological and
medical applications, PLPGs provide indispensable meth-
ods for the introduction of biologically active compounds
to cell or tissue culture in a spatially and temporally con-
trolled manner, which is of utmost significance in time-
resolved biological studies and precisely controlled drug
release.2
N
OH
N
S
S
O
O
O O
1b
1a
Figure 1 Structures of new PLPGs
Herein, we report our progress in the development of new
PPG 1b based on the common skeleton possessing the di-
azomethyl group as linker to connect with different func-
tional groups, specifically, phosphate derivatives.
Diazoalkanes, as represented by diazomethane, are well
known as versatile reagents for various organic reactions
such as esterification of acids. The value of diazomethane
arises from its powerful reactivity and with nitrogen as
sole byproduct.6
In recent years, PLPGs that protect alcohols, amines,
amides, carboxylic acids, aldehydes, and ketones have
been well documented.3 However, few practically useful
protecting groups for phosphate derivatives are known.
O
O
S
O
PhB(OH)2
H3PO4
I
Pd(PPh3)2Cl2
O
O
I2, CAN
93%
P2O5
SeO2
61%
+
77%
OEt
96%
SH
O
S
Me
Me
S
Me
2
3
4
O
S
O
O
S
TsNHNH2
83%
10% NaOH
86%
MCPBA
90%
N
O
O
N
N
S
S
NHTs
5
O
O
O
O
O O
6
7
1b
Scheme 1 The synthetic route of new PLPG 1b
SYNLETT 2012, 23, 367–370
x
x
.
x
x
.
2
0
1
1
Advanced online publication: 27.01.2012
DOI: 10.1055/s-0031-1290326; Art ID: U59511ST
© Georg Thieme Verlag Stuttgart · New York

368
Y. Zhang et al.
LETTER
2-(Diazomethyl)-3-phenyl-4H-thiochromen-4-one 1,1- efficiency in photodeprotection leading to more than 99%
dioxide (1b) was prepared from thiophenol and ethyl ace- yields determined by 1H NMR spectroscopy.
toacetate (Scheme 1). Modified reaction using H3PO4 and
In the case of 9a and 9d, 8a and 8d were isolated after
photodeprotection in pure form in 97% and 96% yields,
respectively (Table 1, entries 1 and 4).
P2O5 instead of polyphosphoric acid improved the yield of
methylthiochromone (2).5 Then, iodination of 2 with
CAN and I2 followed by Suzuki–Miyaura coupling of the
resulting iodide 3 with phenylboronic acid gave the de-
sired compound 4 in 89% overall yield. Allylic oxidation
Photodeprotection of compound 9f was carried out using
the microreactor apparatus (Key-Chem Lumino; pro-
duced by YMC CO., LTD.) with 1.5 W UV-LED lamps
of 4 with SeO2 produced aldehyde 5 along with
hetarenoindanone.7 Furthermore oxidation of the sulfur
Table 1 Protection and Photodeprotection of Phosphates
atom of 5 with MCPBA furnished thiochromone S,S-diox-
ide 6 in 90% yield.8 Then 6 was treated with an equimolar
amount of tosylhydrazide to convert into tosylhydrazone
7, and the following Bamford–Stevens reaction gave tar-
get diazomethyl compound 1b in 71% overall yield
(Scheme 2).9,10
Entry
Phosphates Protection
Photodeprotection
Time Yield (%)c Time Yield (%)e
(h)a
(min)d
1
2
3
4
5
6
7
8
8a
8b
8c
8d
8e
8f
3
97
87
91
92
80
75
73
81
12
>99 (97)a
>99
2
11
The new PLPG (1b) was promising in protecting phos-
phate derivatives and was then further examined with var-
ious substrates for its application scope (Scheme 2),
including aliphatic phosphates 8b–e, aromatic phosphates
8a,h, cycle phosphates 8f,g, and dihydrogen phosphate
8h. Protection reactions of phosphates 8a–h were straight-
forward, typically in the following reaction conditions.
Treatment of phosphates with 1.25 equivalents of 1b in
the presence of a catalytic amount of 10% aqueous HCl in
chloroform at 60 °C afforded the products 9a–h in high
yield (Table 1). The products 9a–h were reasonably stable
and insensitive to the laboratory lighting.
3
11
>99
4
13
>99 (96)a
>99
4
13
5
12
>99
8g
8h
5
14
>99
6b
14
>99
a Reaction conditions: 1b (0.125 mmol), 8 (0.1 mmol), CHCl3 (2.0
mL), 10% aq HCl to adjust pH 4, 60 °C.
b Reaction conditions: 1b (0.25 mmol), 8h (0.1 mmol), CHCl3 (2.0
mL), 10% HCl to adjust pH 4, 60 °C.11
Deprotection of 9a–h proceeded smoothly under photoir-
radiation using a 500 W ultrahigh-pressure mercury lamp
filtered through Pyrex glass (>280 nm) to recover the cor-
responding phosphates quantitatively within 14 minutes
(Table 1). Although the substrates 9a–h were the different
c Isolated yield.
d Irradiated with a 500 W ultrahigh-pressure mercury lamp filtered
through Pyrex glass in a 1 mm pathlength quartz cell (0.01 M in
CDCl3) without excluding air.
phosphates protected compounds, they all showed similar e Determined by 1H NMR spectroscopy.
O
O
P
OR2
O
1b, 10% HCl
hv (>280 nm)
R1O
OH
R1O
P OH
O
O
CDCl3 (0.01M)
CHCl3, 60 °C
OR2
8a–h
S
P
OR2
8a–h
O
O
R1O
9a–h
protection
photodeprotection
O
O
O
HO
P(OEt)2
HO P(OPh)2
HO P(On-Bu)2
8a
8b
8c
O
O
O
HO P(OBn)2
HO P(O-2-ethylhexyl)2
HO
P
O
O
8d
8e
8f
O
O
HO
P
O
O
O
HO
P
OH
8g
8h
Scheme 2 Protection and photodeprotection with new PLPG
Synlett 2012, 23, 367–370
© Thieme Stuttgart · New York

LETTER
Photolabile Protecting Group for Phosphate Compounds
369
O
P
1'
1'
HO
O
O
O
S
8f
hνa,b
O
1
O
O
P
O
2
O
O
O
1
9f
S
O
O
10
C1'
C2
C1eq.
C1ax.
15 min
0 min
5
4
Figure 2 Sections of the 1H NMR spectra (d = 3.6–5.1 ppm) of 0.01
M CDCl3 solution of 9f recorded after 0, 1, 5, 10, and 15 min of irra-
diation. a Using 1.5 W UV-LED (365 nm) as a light source. b Using
0.48 W UV-LED (375 nm) as a light source.
(6 × 0.25 W, 365 nm), and the reaction was monitored by
1H NMR (Figure 2). The characteristic peaks at d = 3.83/
4.34 ppm for C1 protons and d = 4.99 ppm for C2 protons
of 9f disappeared as the reaction proceeded, and a new
doublet peak at d = 4.00 ppm for the C1¢ protons of 8f ap-
Figure 3 UV-Vis and fluorescence spectra upon excitation at 350
nm of 1.0×10–5 M MeOH solution of 9f recorded after 0, 1, 3, 5, 7, 9,
10, and 12 min of irradiation.
intensity than the starting material 9f, and the quantum
yield for the fluorescence emission is very high (F = ca.
0.85). Therefore the monitoring of the reaction progress is
possible by fluorescence spectroscopy.
1
peared. These H NMR spectra showed clearly that the
deprotection of the protected phosphates proceeded
smoothly under photoirradiation of 365 nm LED light to
recover the corresponding phosphates quantitatively. Af-
ter the photoreaction, a workup of the reaction mixture
gave the tetracyclic compound 10 as a yellow crystal as
reported previously.5
In summary, we developed a novel photolabile protecting
group for phosphates. Installation of this protecting group
to the different kinds of phosphates was achieved in excel-
lent yield. The photodeprotection proceeded smoothly un-
der UV light (>280 nm), especially under LED light with
long wavelength (375 nm). This result shows potential ap-
plications of the new PLPG in biological experiments.
To make this PLPG effective for the use in biological ex-
periments, the short wavelength, high-energy light re-
quired for deprotection of certain protecting groups may
result in cell damage or protein modification, and accord-
ingly, there has been interest in protecting groups that can
release the substrates under irradiation of longer wave-
lengths. The above photodeprotection with LED lamps
promoted us to examine the photodeprotection with an
LED lamp. Using a very weak light source, 0.48 W UV-
LED (375 nm, with 15 nm half-width of light), the photo-
deprotection of 9f in 1 mm width quartz cell was carried
out, monitoring by UV-vis and fluorescence spectroscopy
(Figure 3). As the reaction proceeded, the absorption at
around 365 nm increased, and a new fluorescence emision
at 440 nm was observed. Thus the new absorption and
emission peaks with similar tendency to that reported
previously5 can be attributed to the new product 10. Inter-
estingly, the compound 10 has much stronger fluorescent
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.
Acknowledgment
This research was 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 Advan-
ced Research Partnership Project, which are gratefully appreciated.
We also thank to Ms. Huan Zhang, Mr. Yannick Escudie, and Dr.
Hiroshi Nomura for kind help with syntheses of 2, Ms. Yoshiko
Nishikawa for HRMS measurement.
© Thieme Stuttgart · New York
Synlett 2012, 23, 367–370

370
Y. Zhang et al.
LETTER
dd, J = 7.3, 7.9 Hz), 7.56 (2 H, dd, J = 7.9, 7.3 Hz), 7.37 (2
H, d, J = 6.7 Hz). 13C NMR (125 MHz, CDCl3/TMS):
d = 185.3, 179.5, 148.7, 141.8, 141.2, 135.7, 133.4, 131.5,
131.1, 129.0, 128.7, 128.5, 128.1, 123.4. HRMS (EI): m/z
calcd for C16H10O4S [M+]: 298.0300; found: 298.0298.
(9) Analytical and Spectral Data of 7
References and Notes
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4900. (b) Igarashi, T.; Shimokawa, M.; Iwasaki, M.; Nagata,
K.; Fujii, M.; Sakurai, T. Synlett 2007, 1436. (c) Bochet,
C. G. J. Chem. Soc., Perkin Trans. 1 2002, 125.
Yellow solid; mp 165.5–167.8 °C. IR (KBr): 3420, 2359 cm–1.
1H NMR (500 MHz, CDCl3/TMS): d = 8.20 (3 H, m), 7.92
(1 H, dd, J = 7.9 Hz), 7.81 (2 H, d, J = 8.5 Hz), 7.78 (1 H, dd,
J = 7.9 Hz), 7.50 (3 H, m), 7.32 (2 H, d, J = 8.5 Hz), 7.19 (2
H, d, J = 7.0 Hz), 5.30 (1 H, s), 2.42 (3 H, s). 13C NMR (125
MHz, CDCl3/TMS): d = 178.1, 145.0, 144.3, 143.4, 142.2,
142.1, 136.8, 135.0, 134.5, 133.0, 130.8, 129.9, 129.8,
128.6, 128.6, 128.3, 128.2, 123.4, 21.7. HRMS–FAB: m/z
calcd for C23H18N2O5S2 [M + H]+: 467.0735; found:
467.0742.
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(10) Analytical and Spectral Data of 1b
Yellow solid; mp 122.3–123.5 °C. IR (KBr): 3433, 2359 cm–1.
1H NMR (500 MHz, CDCl3/TMS): d = 8.26 (1 H, d, J = 7.9
Hz), 8.06 (1 H, d, J = 7.9 Hz), 7.85 (1 H, dd, J = 7.9 Hz),
7.79 (1 H, dd, J = 7.9 Hz), 7.48 (3 H, m), 7.27 (2 H, d, J = 7
Hz), 4.95 (1 H, s). 13C NMR (125 MHz, CDCl3/TMS):
d = 175.0, 139.6, 133.5, 133.5, 132.5, 129.8, 129.6, 129.1,
128.9, 128.8, 128.7, 127.4, 122.7, 47.5. HRMS–ESI: m/z
calcd for C32H20O6S2Na [2 M – 2 N2 + Na]+: 587.0599;
found: 587.0600.
(7) In this step, we observed an interesting side reaction and for
details, see: Zhang, Y.; Tanimoto, H.; Nishiyama, Y.;
Morimoto, T.; Kakiuchi, K. Heterocycles 2011, 83, 2337.
(8) Analytical and Spectral Data of 6
(11) Treatment of 8h with 2.5 equiv of 1b in the presence of a
catalytic amount of 10% HCl aq in CHCl3 at 60 °C afforded
diprotection product in 81% yield. The complete experi-
mental procedures and analytical data are shown in the
Supporting Information.
Yellow solid; mp 213.7–214.2 °C. IR (KBr): 1685, 1310 cm–1.
1H NMR (500 MHz, CDCl3/TMS): d = 9.54 (1 H, s), 8.24 (1
H, d, J = 6.7 Hz), 8.15 (1 H, d, J = 7.9 Hz), 7.97 (1 H, dd,
J = 7.9, 7.9 Hz), 7.83 (1 H, dd, J = 6.7, 7.9 Hz), 7.63 (1 H,
Synlett 2012, 23, 367–370
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