3165; (d) M. A. Kostiainen, D. K. Smith and O. Ikkala, Angew.
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Angew. Chem., Int. Ed., 2007, 46, 4290; (f) C. Park, J. Lim,
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2008, 47, 7527; (h) T. Muraoka, C.-Y. Koh, H. Cui and S. I. Stupp,
Angew. Chem., Int. Ed., 2009, 48, 5946.
6 (a) J. A. Barltrop and P. Schofield, Tetrahedron Lett., 1962, 697;
(b) J. A. Barltrop and P. Schofield, J. Chem. Soc., 1965, 4758;
(c) J. W. Chamberlin, J. Org. Chem., 1966, 31, 1658.
7 (a) H. E. Zimmerman and V. R. Sandel, J. Am. Chem. Soc., 1963,
85, 915; (b) H. E. Zimmerman and S. Somasekhara, J. Am. Chem.
Soc., 1963, 85, 922; (c) H. E. Zimmerman, J. Am. Chem. Soc., 1995,
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5616.
Scheme 4 Controlled deprotection.
8 C. Birr, W. Lochinger, G. Stahnke and P. Lang, Liebigs Ann.
Chem., 1972, 763, 162.
In the course of the photochemical studies, we observed that
the traditional Tr and MMTr groups, which are inert to
Pyrex-filtered irradiation (l 4 280 nm), can be photochemically
removed with the Pyrex-filtered light in methanol if
chloroform is present.14 Presumably, HCl generated from the
photoreaction of chloroform with a protic solvent promoted
the removal of the protecting groups.15 Thus, irradiation of 15
with Pyrex-filtered light in MeOH with 0.5% (v/v) CDCl3 for
15 min led to a quantitative removal of the MMTr group to
9 (a) It is known that the 9-phenylxanthen-9-yl (pixyl) group and
9-phenylthioxanthyl (S-pixyl) are photochemically removable to
release primary alcohols and nucleosides;9b–c; (b) A. Misetic and
M. K. Boyd, Tetrahedron Lett., 1998, 39, 1653; (c) M. P. Coleman
and M. K. Boyd, J. Org. Chem., 2002, 67, 7641.
10 Protective Groups in Organic Synthesis, ed. T. W. Greene and
P. G. M. Wuts, Wiley-Interscience, New York, 1999.
11 (a) Quantum yields were determined by chemical actinometry.
A. Defoin, R. Defoin-Straatmann, K. Hildenbrand, E. Bittersmann,
D. Kreft and H. J. Kuhn, J. Photochem., 1986, 33, 237;
(b) Monochromatic light was obtained by passing light from a
450 W medium pressure mercury lamp through three quartz
cuvettes containing 2M NiSO4 in 5% H2SO4 (cell 1), 0.8 M CoSO4
in 5% H2SO4 (cell 2), and 2.46 E-4 M BiCl3 in conc. HCl/H2O
(2 : 3) (cell 3). The monochromatic light centres at 285 nm and was
opaque above 325 nm and below 250 nm11c; (c) H. E. Zimmerman,
J. M. Nuss and A. W. Tantillo, J. Org. Chem., 1988, 53, 3792.
12 C.-T. Chen, J.-H. Kuo, V. D. Pawar, Y. S. Munot, S.-S. Weng,
C.-H. Ku and C.-Y. Liu, J. Org. Chem., 2005, 70, 1188.
13 Photoreactions of 8 have been carried out in different solvents such
as MeCN, MeCN/water, DMSO/water, acetone/water, benzene/
water, chloroform, chloroform/water, and methanol. The reactions
in methanol and MeCN/water (9 : 1) seemed to provide better
results.
14 Irradiation of 6 in MeOH (5 mM) with 0.5% (v/v) chloroform for
20 min with the Pyrex-filtered light resulted in complete removal of
the MMTr group. Under the same reaction conditions, the Tr
group could not be removed from 5, even after increasing chloro-
form in methanol to 10%. However, deprotection of Tr was
observed after 20 min irradiation with a 3-phenylpropanol/5 ratio
of 0.06 : 1 in MeOH/chloroform (1 : 1) and 0.15 : 1 in MeOH/
chloroform (1 : 9). When these irradiated reaction solutions
were kept in dark, the deprotection reactions gradually reached
completion overnight, which clearly indicates the existance of
photogenerated acidic substance(s) catalyzing the reactions.
Furthermore, in control experiments, there was no sign of photo-
reactions when 5 and 6 were irradiated with the Pyrex-filtered light
in MeOH, MeCN/water, chloroform, or MeCN/chloroform.
15 (a) Utilization of photogenerated acids from sensitizers in protecting
group installation have been recently reported, albeit the identity
of some acids are not clear;15b–c; (b) H. J. P. de Lijser and
C.-K. Tsai, J. Org. Chem., 2004, 69, 3057; (c) R. P. Oates and
P. B. Jones, J. Org. Chem., 2008, 73, 4743.
1
produce 16 based on H NMR analysis (Scheme 4).16
In conclusion, a new PPG (i.e. DMATr) for the protection
of alcohols has been developed. The PPG can be efficiently
removed with light and has high stability toward acid treat-
ment compared with traditional trityl type of protecting
groups. A new protocol for PPG installation has also been
developed. Primary and secondary alcohols can be protected
in high yields. The photogenerated acid by irradiation of
chloroform in a protic solvent proves to be an effective method
in removing acid labile protecting groups, which will be useful
in many important applications such as on-chip lithographic
DNA synthesis,17 photo-patterning organic conductive
18
materials and organic light-emitting materials.19
This work is supported by NSF (CHE-0848489) and the
University of Alabama at Birmingham.
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16 See details in Supporting Information.
17 X. Gao, P. Yu, E. LeProust, L. Sonigo, J. P. Pellois and H. Zhang,
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4 C. G. Bochet, J. Chem. Soc., Perkin Trans. 1, 2002, 125.
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This journal is The Royal Society of Chemistry 2010
1516 | Chem. Commun., 2010, 46, 1514–1516