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Table 5 Recovery and reuse of organophotoacids 2 and 5 in glycosyla-
tion of 1 and 8 under photoirradiation
range of applications in both academia and industry. The
development of several different types of organophotocatalysts,
including organophotoacids, and their application in environ-
mentally benign organic synthesis is now under investigation in
our laboratories.
This research was supported in part by the MEXT-supported
Program for the Strategic Research Foundation at Private Uni-
versities, 2012–2016, from the Ministry of Education, Culture,
Sports, Science and Technology of Japan (MEXT).
Recovery
Reaction yield using recovered
Entry Organophotoacid yielda (%) organophotoacida (%) (a : b)b
1
2
2
5
95
93
81 (51/49)
79 (54/46)
Notes and references
a
b
Yield of the isolated product. a : b Ratio was determined by 1H-NMR
1 (a) Carbohydrates in Chemistry and Biology, ed. B. Ernst, G. W. Hart
and P. Sina¨y, Wiley-VCH, Weinheim, Germany, 2000, vol. 1–4;
(b) Glycoscience, Chemistry and Chemical Biology, ed. B. O. Fraser-
Reid, K. Tatsuta and J. Thiem, Springer, Berlin, Germany, 2001,
vol. 1–3.
analysis.
the glycosyl donors 1, 22 and 23, we carried out glycosylation
of 8 using 24, which possesses a benzoyl protecting group at
the C2-position. In this case, high b-stereoselectivity was
observed due to the participation effect of the C2-protecting
group, and the b-glycoside 27 was selectively produced in high
yield (entries 5 and 6 in Table 4).
Finally, we investigated the reusability of organophotoacids
2 and 5 (Table 5). It was found that after completion of the
glycosylation reaction, both 2 and 5 were recovered in 490%
yield via column chromatography and could be reused without
any loss of efficiency. Furthermore, neither neutralization of
the reaction mixture nor extraction of the product was required
after completion of the reaction, because the removal of UV
light rendered the reaction mixture almost neutral.9,10 The
work-up for the reaction involved only evaporation of the
solvent, Et2O, which was also found to be reusable.
2 Functional Carbohydrates, ed. C. G. Biriaderis and M. S. Izydorczyck,
CPC Press, Boca Raton, FL, 2006.
3 (a) A. V. Demchenko, in Handbook of Chemical Glycosylation: Advances
in Stereoselectivity and Therapeutic Relevance, ed. A. V. Demchenko,
Wiley-VCH, Weinheim, Germany, 2008, pp. 1–27; (b) K. Toshima and
K. Sasaki, in Comprehensive Glycoscience, ed. J. P. Karmerling,
G.-J. Boons, Y. C. Lee, A. Suzuki, N. Taniguchi and A. G. J. Voragen,
Elsevier, Oxford, UK, 2007, pp. 261–311; (c) K. Toshima and
K. Tatsuta, Chem. Rev., 1993, 93, 1503.
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(c) W. J. Wever, M. A. Cinelli and A. A. Bowers, Org. Lett., 2013,
15, 30; (d) M. Nakanishi, D. Takahashi and K. Toshima, Org. Biomol.
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5 (a) R. S. Andrews, J. J. Becker and M. R. Gagne, Angew. Chem., Int.
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Ed., 2010, 49, 7274; (b) R. S. Andrews, J. J. Becker and M. R. Gagne,
Org. Lett., 2011, 13, 2406; (c) R. S. Andrews, J. J. Becker and
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M. R. Gagne, Angew. Chem., Int. Ed., 2012, 51, 4140.
6 L. M. Tolbert and K. M. Solntsev, Acc. Chem. Res., 2002, 35, 19.
7 Y. Nishikubo, S. Kanzaki, S. Matsumura and K. Toshima, Tetra-
hedron Lett., 2006, 47, 8125.
8 (a) R. R. Schmidt and J. Michael, Angew. Chem., Int. Ed., 1980,
19, 731; (b) R. R. Schmidt, Angew. Chem., Int. Ed., 1986, 25, 212.
9 W. Bartok, R. Hartman and P. Lucchesi, Photochem. Photobiol., 1965,
4, 499.
10 L. M. Tolbert and J. E. Haubrich, J. Am. Chem. Soc., 1994, 116, 10593.
11 A Blak-ray (B-100A) purchased from UVP, Inc. was used.
12 A radiometer sensor (UVX-36) purchased from UVP, Inc. was used.
UV light at 12 mW cmÀ2 was obtained using one lamp placed at
5 cm from the reaction mixture. UV light at 27 mW cmÀ2 was
obtained using two lamps placed at 3 cm from the reaction mixture.
13 It was confirmed that the other acid-sensitive glycosyl donors, such
as the corresponding glycal or glycosyl diethyl phosphite, could not
effectively be activated by the organophotoacids used.
Overall, the glycosylation reaction exhibited several environ-
mentally benign features: (1) non-harmful UV light (365 nm) is
used; (2) catalysts 2 and 5 are recoverable and reusable; (3) the
work-up involves only evaporation of the solvent.
In summary, we have developed a novel glycosylation
method, effective for several trichloroimidate glycosyl donors,
using an organophotoacid under photoirradiation. The reaction
is highly efficient and environmentally benign. As glycosylation
is a very important step in the synthesis of carbohydrate-
containing products, this useful method should find a wide
10698 | Chem. Commun., 2014, 50, 10695--10698
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