Anthraquinon-2-ylmethoxycarbonyl (Aqmoc): A new photochemically removable protecting group for alcohols

Article abstract of DOI:10.1021/ol015787s

(formula presented) Synthesis and photochemistry of a new photochemically removable protecting group for alcohols is described. Four carbonates of galactose derivatives (1-4) were synthesized from the corresponding arylmethanols via 4-nitrophenyl carbonate intermediates. Among them, photolysis of anthraquinon-2-ylmethoxycarbonyl (Aqmoc) galactose (1) proceeded with overall photolysis efficiency of 150 (quantum yield 0.10, and molar absorptivity 1500 M^-1 cm^-1) and rate constant of ～10⁶ s^-1. To demonstrate its application to a biologically related molecule, 5′-Aqmoc-adenosine (5) was synthesized and photolyzed to yield adenosine in 91% yield.

Full text of DOI:10.1021/ol015787s

ORGANIC
LETTERS
2001
Vol. 3, No. 12
1809-1812
Anthraquinon-2-ylmethoxycarbonyl
(Aqmoc): A New Photochemically
Removable Protecting Group for
Alcohols
Toshiaki Furuta,*,†,‡ Yuuki Hirayama,‡ and Michiko Iwamura‡
PRESTO, JST, and Department of Biomolecular Science, Toho UniVersity,
2-2-1 Miyama, Funabashi, 274-8510 Japan
furuta@biomol.sci.toho-u.ac.jp
Received March 5, 2001
ABSTRACT
Synthesis and photochemistry of a new photochemically removable protecting group for alcohols is described. Four carbonates of galactose
derivatives (1−4) were synthesized from the corresponding arylmethanols via 4-nitrophenyl carbonate intermediates. Among them, photolysis
of anthraquinon-2-ylmethoxycarbonyl (Aqmoc) galactose (1) proceeded with overall photolysis efficiency of 150 (quantum yield 0.10, and
molar absorptivity 1500 M-1 cm-1) and rate constant of ∼106 s-1. To demonstrate its application to a biologically related molecule, 5′-Aqmoc-
adenosine (5) was synthesized and photolyzed to yield adenosine in 91% yield.
Photochemically removable protecting groups for various
functional groups have great potential in both synthetic1 and
biological chemistry. The importance of this type of protec-
tion has been growing because of recent advances in solid-
phase synthesis in combinatorial chemistry2 and the need
for photochemically labile probe molecules known as caged
compounds in cell biology.3 There have been continuous
efforts to exploit new photolabile protecting groups, such
as benzoin-type,4 phenacyl-type,5 and coumarinylmethyl-
type,6 since 2-nitrobenzyl groups were introduced to make
caged biologically important phosphates.7 The functional
groups to be masked appear to have mainly been phosphates
or carboxylates. The photolabile hydroxy protecting groups,
other than the 2-nitrobenzyl-type, that have been reported
thus far are o-hydroxycinnamate,8 hydroxystyrylsilyl,9 3,5-
(4) Discovered by (a) Sheehan, J. C.; Wilson, R. M. J. Am. Chem. Soc.
1964, 86, 5277. For selected papers for the application, see: (b) Givens,
R. S.; Athey, P. S.; Kueper, L. W., III; Matuszewski, B.; Xue, J. J. Am.
Chem. Soc. 1992, 114, 8708. (c) Papageorgiou, G.; Corrie, J. E. T.
Tetrahedron 1997, 53, 3917. (d) Pirrung, M. C.; Fallon, L.; McGall, G. J.
Org. Chem. 1998, 63, 241. (e) Hansen, K. C.; Rock, R. S.; Larsen, R. W.;
Chan, S. I. J. Am. Chem. Soc. 2000, 122, 11567.
(5) Discovered by (a) Sheehan, J. C.; Umezawa, K. J. Org. Chem. 1973,
38, 3771. For application to caging chemistry, see: (b) Givens, R. S.; Weber,
J. F. W.; Jung, A. H.; Park, C.-H. Methods Enzymol. 1998, 291, 1 and
references therein. (d) Conrad, II, P. G.; Givens, R. S.; Weber, J. F. W.;
Kandler, K. Org. Lett. 2000, 2, 1545.
(6) Discovered by (a) Givens, R. S.; Matuszewski, B. J. Am. Chem. Soc.
1984, 106, 6860. For application to caging chemistry, see: (b) Furuta, T.;
Iwamura, M. Methods Enzymol. 1998, 291, 50 and references therein. (c)
Furuta, T.; Wang, S. S.-H.; Dantzker, J. L.; Dore, T. M.; Bybee, W. J.;
Callaway, E. M.; Denk, W.; Tsien, R. Y. Proc. Natl. Acad. Sci. U.S.A.
1999, 96, 1193. (d) Kaupp, U. B.; Dzeja, C.; Frings, S.; Bendig, J.; Hagen,
V. Methods Enzymol. 1998, 291, 415 and references therein. (e) Hagen,
V.; Bendig, J.; Frings, S.; Eckardt, T.; Helm, S.; Reuter, D.; Kaupp, U. B.
Angew. Chem., Int. Ed. 2001, 40, 1046.
† PRESTO, JST.
‡ Toho University.
(1) For reviews, see: (a) Pillai, V. N. R. Synthesis 1980, 1. (b) Pillai, V.
N. R. Organic Photochemistry; Padwa, A., Ed.; Marcel Dekker: New York,
1987; Vol. 9, pp 225-323.
(2) For example, see: Akerblom, E. B. Mol. DiVersity 1999, 4, 53.
(3) For example, see: (a) Adams, S. R.; Tsien, R. Y. Annu. ReV. Physiol.
1993, 55, 755. (b) Caged Compounds; Methods in Enzymology; Marriott,
G., Ed.; Academic Press: New York, 1998; Vol. 291. (c) Dorman, G.;
Prestwich, G. D. Trends Biotechnol. 2000, 18, 64.
(7) (a) Engels, J.; Schlaeger, E.-J. J. Med. Chem. 1977, 20, 907. (b)
Kaplan, J. H.; Forbush, G., III; Hoffman, J. F. Biochemistry 1978, 17, 1929.
10.1021/ol015787s CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/17/2001

dimethoxybenzoin carbonate,10 2-benzoylbenzoic acid,11 and
9-phenylxanthyl groups.12 However, the application of these
groups in caging chemistry has been somewhat limited.
In this study, we examined the synthesis and photochem-
istry of four photolabile protecting groups for alcohols and
found that the anthraquinon-2-ylmethoxy-carbonyl (Aqmoc)
group shows satisfactory and unique photoreactivity, which
could be favorable for a wide range of applications.
It is well-known that appropriately functionalized aryl-
methyl esters of carboxylates are photolyzed to produce
parent acids.13 By a structural analogy, one can easily imagine
that arylmethyl carbonates might be photolyzed to produce
parent alcohols. Thus, we designed four candidates as
photoremovable protecting groups for alcohols, [anthraquinon-
2-ylmethoxycarbonyl (Aqmoc), 7-methoxycoumarin-4-yl-
methoxy carbonyl (MCMoc), pyren-1-ylmethoxy carbonyl
(Pmoc), and phenanthren-9-ylmethoxy carbonyl (Phmoc)],
since anthraquinon-2-ylmethyl esters of carboxylate14 and
phosphate,15 7-methoxycoumarin-4-ylmethylesterofphosphate,6a
and pyren-1-ylmethyl esters of carboxylate16 and phosphate17
are known to be photolyzed.
Since the goal of our project is to develop novel caged
compounds having improved and unique photoreactivity that
could be applied to caging chemistry, we focused our
attention on the properties in aqueous solution. Thus, the
resulting galactose derivatives (1-4) were subjected to
photolysis in 50% THF-H2O (100 µM) at 350 nm.
All compounds produced a parent galactose derivative18
and photo byproducts, as indicated in Scheme 2. Several
Scheme 2. Photolysis of the Arylmethyl Carbonates
Four carbonates of galactose derivatives were synthesized
from the corresponding arylmethanols via 4-nitrophenyl
carbonates (Scheme 1). Although the 4-nitrophenyl carbonate
Scheme 1. Synthesis of the Arylmethyl Carbonates (1-4)a
photo byproducts other than the parent alcohol were detected
in the photolysis mixture of 1 in THF-H2O. While we have
not yet been able to isolate and thoroughly characterize all
of the byproducts, one of the major byproducts that stems
from the anthraquinone moiety was determined to be
anthraquinon-2-ylmethanol tetrahydro-furanyl ether on the
basis of LC-MS analysis (m/z 308).
Figure 1 shows that the time courses for the consumption
of the starting materials follow single-exponential decay,
suggesting that the photolysis reactions proceeded without
any interference by photo byproducts.
a (a) 4-Nitrophenylchloroformate/DMAP. (b) 1,2,3,4-di-O-iso-
propylidenegalactopyranoside/DMAP.
intermediates are stable enough to be isolated, we chose a
one-pot sequential addition procedure because of the simplic-
ity and achieved good to excellent yields to prepare the
desired carbonates.
The quantum yields for the disappearance of the starting
materials were determined and are summarized in Table 1
with selected absorption data. A higher efficiency of pho-
tolysis at around 350 nm would be desirable, especially for
cell biological applications, since we can reduce the light
intensity in an uncaging reaction to minimize cell damage
and chromophore bleaching. Among the four compounds
tested in this study, the Aqmoc group showed a satisfactory
high efficiency of photolysis. The Φꢀ value, which is a
(8) Porter, N. A.; Bruhnke, J. D. J. Am. Chem. Soc. 1989, 111, 7616.
(9) Pirrung, M. C.; Lee, Y. R. J. Org. Chem. 1993, 58, 6961.
(10) Pirrung, M. C.; Bradley, J.-C. J. Org. Chem. 1995, 60, 1116.
(11) Jones, P. B.; Pollastri, M. P.; Porter, N. A. J. Org. Chem. 1996, 61,
9455.
(12) Misetic, A.; Boyd, M. K. Tetrahedron Lett. 1998, 39, 1653.
(13) (a) Cristol, S. J.; Bindel, T. Organic Photochemistry; Marcel
Dekker: New York, 1983; Vol. 6, p 327. (b) Pincock, J. A. Acc. Chem.
Res. 1997, 30, 43.
(14) Kemp, D. S.; Reczek, J. Tetrahedron Lett. 1977, 18, 1031.
(15) Furuta, T.; Torigai, H.; Sugimoto, M.; Iwamura, M. J. Org. Chem.
1995, 60, 3953.
(16) Iwamura, M.; Ishikawa, Y.; Koyama, K.; Sakuma, K.; Iwamura,
H. Tetrahedron Lett. 1987, 28, 679.
(17) Furuta, T.; Torigai, H.; Osawa, T.; Iwamura, M. Chem. Lett. 1993,
1179.
(18) Although we did not do any product isolation, the production of
the parent galactose derivative and photo byproducts from each carbonate
was checked by HPLC and thin-layer chromatography, respectively.
1810
Org. Lett., Vol. 3, No. 12, 2001

Figure 2. Absorption spectra of 2.5 µM Aqmoc-Gal (1) in THF-
H2O upon 350 nm irradiation (RPR-350 nm × 4): (a) 0 min; (b)
0.5 min; (c) 1 min; (d) 4 min of irradiation, respectively.
Figure 1. Time course for the consumption of the arylmethyl
carbonates (1-4) in 50% THF-H2O upon 350 nm irradiation (RPR-
350 nm × 16).
production of strongly absorbing photo byproducts often
causes a problem known as a filter effect. Therefore,
disappearance of the strong absorption band in the UV region
concomitant with photolysis reflects an additional benefit of
the Aqmoc group, especially for photolysis in thick samples,
such as protein crystals and biological tissues.
product of the quantum yield and extinction coefficient, is
an indication of the efficiency of photolysis: a high value
reflects a high efficiency. Typical Φꢀ values for conventional
2-nitrobenzyl-type cages are 20-100, so a value of 150 for
Aqmoc-Gal is high enough for its application in caging
chemistry. Table 1 also shows the solubility in water
containing 1% DMSO.
The isolated yield of the deprotected alcohol from the more
concentrated solution of 1 should be noted. The photolysis
of 2 mM solution of 1 in 50% THF-H2O gave the desired
1,2,3,4-di-O-isopropylidene-D-galactopyranose in 68% iso-
lated yield together with bis(1,2,3,4-di-O-isopropylidene-D-
galactopyranosyl) carbonate (16% yield), which might be
the secondary product from the reaction between 1 and the
deprotected alcohol. The observed isolated yield for the
deprotection is not satisfactory from a synthetic viewpoint
and seems to be the drawback of Aqmoc group. However,
the trouble encounterd in the reaction is not so serious for
our purposes, because we could minimize such secondary
reactions by using more diluted solutions in caging chemistry
and by attaching the substrate on a solid support in solid-
phase synthetic chemistry.
Table 1. Selected Physical and Chemical Properties
λmax (ꢀb)
ꢀ350
Φ350
0.10
Φꢀ350
solubilityd
a
b
c
1
2
3
4
327 (4 100)
323 (13 100)
343 (30 800)
297 (9 530)
1 500
3 400
6 200
190
150
10
42
2 × 10-4
3 × 10-5
2 × 10-6
1 × 10-5
2.9 × 10-3
6.7 × 10-3
8.9 × 10-4
0.17
a Absorption spectra were taken in 50% THF-H2O. b Extinction coef-
ficient (cm-1 M-1). c Quantum yields for the disappearance of the starting
materials upon 350 nm irradiation. d The molar concentration (mol dm-3
of the saturated solution in deionized water containing 1% DMSO.
)
Next, we investigated the mechanism of the photolysis of
Aqmoc-Gal and found that the photolytic consumption of
the starting material was effectively quenched by trans-1,3-
pentadiene (ET ) 247 kJ mol-1) in a concentration-dependent
manner, suggesting that photolysis would occur via the triplet
excited state. On the basis of a Stern-Volmer analysis, the
lifetime of the triplet excited state was estimated to be 21.8
ns (kqτ ) 218 M-1, assuming kq ) kdiff ) 1010 s-1 M-1),
from which the rate constant of photolysis was calculated
to be 4.6 × 106 s-1.
Scheme 3. Synthesis and Photolysis of 5′-Aqmoc-Adenosine
(5)a
Figure 2 shows the difference in the absorption spectrum
of the Aqmoc moiety before and after photolysis. The
disappearance of a relatively strong 330-nm peak, which is
characteristic of the n-π* transition of the anthraquinone
chromophore, and the appearance of a weakly absorbing
long-wavelength peak at around 380 nm are noteworthy. The
a (a) 4-Nitrophenylchloroformate/DMAP. (b) 2′,3′-O-isopropy-
lydeneadenosine/DMAP. (c) TFA/THF-H2O.
Org. Lett., Vol. 3, No. 12, 2001
1811

Since one important and potential application of photo-
removable alcohol protecting groups is solid-phase oligo-
nucleotide array synthesis,19 we tested the Aqmoc group for
nucleoside protection. Thus, 5′-Aqmoc-adenosine (5) was
synthesized. Upon photolysis, free adenosine was obtained
from 5 (10 µM solution in 50% THF-KMOPS buffer) in
91% yield.20
In conclusion, we synthesized four arylmethyl carbonate-
type photolabile protecting groups and found that the Aqmoc
group may be useful in caging chemistry. Although the
Aqmoc group is still limited in its application to cell biology,
i.e., lower photolytic efficiency without THF (data not
shown), and the possible production of a reactive intermedi-
ate, the present concept of the combination of an arylmethyl
group and a hydroxyl functionality as a carbonate enables
us to make new photoremovable protecting groups with
various photochemical properties.
Acknowledgment. This work was supported by a grant-
in-aid for the scientific research from the Ministry of
Education, Science and Culture, Japan and by the Nishida
research fund for fundamental organic chemistry.
Supporting Information Available: Procedure for the
preparation and photolysis and spectroscopic data for com-
pounds 1-5 is provided. This material is available free of
charge via the Internet at http://pubs.acs.org.
(19) Lipshutz, R. J.; Fodor, S. P.; Gingeras, T. R.; Lockhart, D. J. Nat
Genet. 1999, 1(Suppl), 20 and references therein.
(20) The yield was determined from HPLC analysis of the photolysis
mixture. See Supporting Information.
OL015787S
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Org. Lett., Vol. 3, No. 12, 2001