rings4 can be modified using photolabile protecting groups.
Protection of nucleobases is advantageous when the comple-
mentary double-strand formation must be controlled. The
collection of photoremovable protective groups designed for
preparing caged compounds has been growing,5 but reported
caging groups available for caged nucleobases are of only
two types: o-nitrobenzyl (NB-type) and 2-(o-nitrophenyl)-
ethyl (2-NPE-type) groups.2
Although the reported compounds are useful for photo-
control of DNA or RNA structures and functions, NB- and
2-NPE-type phototriggers present inherent drawbacks. Both
groups have their absorption maxima at around 290 nm,
which results in poor absorption of uncaging light, typically
of 330-385 nm. To overcome this shortage, introducing
alkoxy substituents into the phenyl ring can give a longer
absorption maximum with higher molar absorptivity. The
introduction of alkoxy substitutions somehow reduces the
photolysis quantum yield; for that reason, the overall
photolysis efficiency of the alkoxylated o-nitrobenzyl group
is almost identical to that of the parent compounds. Further-
more, both NB- and 2-NPE-types need introduction of
substituents on the carbon atom R or â to the leaving groups
to achieve improved photolysis quantum yields. Conse-
quently, the carbon atoms form an asymmetric center and
produce a pair of diastereomers when the group is used to
produce a caged compound of a chiral molecule. Chromato-
graphic separation of the diastreomers is necessary to avoid
the resultant complexity.3
ological solution (pH 7.2). Second, results of a previous study
showed that the photolysis quantum yield of the Bhc group
was larger than that of other coumarin cages including the
(7-diethylaminocoumarin-4-yl)methyl (DEACM) group (R1
) H, R2 ) Et2N in Figure 1).8a Third, the Bhc group has no
stereocenters. In addition, we have reported Bhc-caged
compounds of neurotransmitters,7 second messengers,8 and
mRNAs.3c,d,9 All have improved photochemical properties
for both one- and two-photon excitation conditions; they were
used in either live cells, brain slices, or the whole body. We
added Bmc group, which is the 7-methoxy analogue of the
Bhc group, into an entry to see if free hydroxyl group in the
Bhc group is necessary for photolysis efficiency. In addition,
if we were able to add a substituent in this position without
sacrificing any photolysis efficiency, the synthetic scheme
would be simplified, and an additional function would be
tethered.
For this study, three types of caging groups were intro-
duced into C-4 exocyclic amine in deoxycytidine as car-
bamates to give four caged dCs (Figure 2). Photochemical
The purpose of this study is to develop new caged
compounds of nucleobases having improved physical and
chemical properties. To achieve that goal, we investigated
the utility of coumarin-type protecting groups as photo-
triggers for nucleobases. Among the coumarin-based caging
groups reported6 were chosen (6-bromo-7-hydroxycoumarin-
4-yl)methyl (Bhc) group (R1 ) Br, R2 ) OH in Figure 1)7
Figure 2. Structures of the caged nucleosides tested in this study.
properties of the caged dCs were measured under the same
reaction conditions, so that a direct comparison of their
photolysis reactivity could be made.
Figure 1. Phototriggers for caged compounds. X denotes a leaving
group.
Two coumarin-type protecting groups, (6-bromo-7-hydroxy-
coumarin-4-yl)methoxycarbonyl (Bhcmoc) and (6-bromo-
7-methyoxycoumarin-4-yl)methoxycarbonyl (Bmcmoc), were
introduced into 2′-deoxycytidine (dC) using the temporal
for the following reasons. First, Bhc group has its absorption
maximum at around 370 nm with large molar absorptivity
(ꢀmax ) 17,000-19,000 M-1cm-1) under a simulated physi-
(8) (a) Suzuki, A. Z.; Watanabe, T.; Kawamoto, M.; Nishiyama, K.;
Yamashita, H.; Ishii, M.; Iwamura, M.; Furuta, T. Org. Lett. 2003, 5, 4867.
(b) Furuta, T.; Takeuchi, H.; Isozaki, M.; Takahashi, Y.; Kanehara, M.;
Sugimoto, M.; Watanabe, T.; Noguchi, K.; Dore, T. M.; Kurahashi, T.;
Iwamura, M.; Tsien, R. Y. ChemBioChem 2004, 5, 1119. (c) Nishigaki,
T.; Wood, C. D.; Tatsu, Y.; Yumoto, N.; Furuta, T.; Elias, D.; Shiba, K.;
Baba, S. A.; Darszon, A. DeV. Biol. 2004, 272, 376. (d) Wood, C. D.;
Nishigaki, T.; Furuta, T.; Baba, S. A.; Darszon, A. J. Cell Biol. 2005, 169,
725.
(4) (a) Chaulk, S. G.; MacMillan, A. M. Nucleic Acids Res. 1998, 26,
3173. (b) Pitsch, S.; Weiss, P. A.; Wu, X.; Ackermann, D.; Honegger, T.
HelV. Chim. Acta 1999, 82, 1753.
(5) Mayer, G.; Heckel, A. Angew. Chem., Int. Ed. 2006, 45, 4900.
(6) Furuta, T. In Dynamic Studies in Biology: Phototriggers, Photo-
switches and Caged Biomolecules; Goeldner, M., Givens, R. S., Eds.; Wiley-
VCH: New York, 2005.
(7) Furuta, T.; Wang, S. S.; 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.
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Org. Lett., Vol. 9, No. 23, 2007