498
R. Dalpozzo et al.
SHORT PAPER
Er(OTf)3 can be used several times without significant
loss of activity. After work-up, the aqueous phase can be
evaporated under reduced pressure to furnish Er(III) salt
which can be reused after drying overnight over P2O5.
References
(1) (a) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory
and Practice; Oxford Science Publications: Oxford, 1998.
(b) Collins, T. C. Green Chemistry: Frontiers in Chemical
Synthesis and Processes; Oxford University Press: Oxford,
1998. (c) Tundo, P.; Anastas, P.; St Clair Black, D.; Breen,
J.; Collins, T.; Memoli, S.; Miyamoto, J.; Polyakoff, M.;
Tumas, W. Pure Appl. Chem. 2000, 72, 1207.
(2) (a) Bartoli, G.; Cupone, G.; Dalpozzo, R.; De Nino, A.;
Maiuolo, L.; Marcantoni, E.; Procopio, A. Synlett 2001,
1897. (b) Bartoli, G.; Cupone, G.; Dalpozzo, R.; De Nino,
A.; Maiuolo, L.; Procopio, A.; Sambri, L.; Tagarelli, A.
Tetrahedron Lett. 2002, 43, 5945. (c) Dalpozzo, R.; De
Nino, A.; Maiuolo, L.; Procopio, A.; Tagarelli, A.; Sindona,
G.; Bartoli, G. J. Org. Chem. 2002, 67, 9093. (d) Dalpozzo,
R.; De Nino, A.; Maiuolo, L.; Procopio, A.; Nardi, M.;
Bartoli, G.; Romeo, R. Tetrahedron Lett. 2003, 44, 5621.
(3) (a) Greene, T. W.; Wuts, P. G. M. Protective Groups in
Organic Synthesis, 3rd Ed.; John Wiley and Sons: New
York, 1999. (b) Kocienski, P. J. Protecting Groups, 1st Ed.;
Georg Thieme Verlag: Stuttgart, 1994.
Er(OTf)3 has be proven to be a very versatile agent for
deprotection of acetals and ketals under almost neutral
conditions; a 0.1 M solution of Er(OTf)3 in water is only
weakly acidic (pH ca. 5.9), and the aqueous layer from
workup was less acidic (pH ca. 6.6). Furthermore, the use
of Er(OTf)3 as a Lewis acid catalyst in the deprotection of
acetals and ketals is, also an improvement with respect to
the use of Ce(OTf)3 that we recently reported.2c Er(OTf)3
is much more active than analogous cerium (III) deriva-
tive and amounts 6–50 times smaller are sufficient to hy-
drolyse the same substrates. Finally, Er(OTf)3 is non toxic
and commercially available at a lower price than other ac-
tive lanthanide triflates;10 it presents still some other ad-
vantages in terms of many green chemistry principles: it
is of general use in the cleavage of dialkyl as well as cyclic
acetals and ketals giving very high yields of carbonyl
compounds with good selectivity and very smooth reac-
tion conditions, only catalytic amounts of promoter are re-
quired, the reactions are conducted at room temperature,
and it is easy to recover and reuse the catalyst.
(4) (a) Steel, P. G. J. Chem. Soc., Perkin Trans. 1 2001, 2727.
(b) Kobayashi, S.; Manabe, K. Acc. Chem. Res. 2002, 35,
209. (c) Kobayashi, S.; Sugiura, M.; Kitagawa, H.; Lam, W.
W.-L. Chem Rev. 2002, 102, 2227.
(5) (a) Marcantoni, E.; Nobili, F.; Bartoli, G.; Bosco, M.;
Sambri, L. J. Org. Chem. 1997, 62, 4183. (b) Carrigan, M.
D.; Sarapa, D.; Smith, R. C.; Wieland, L. C.; Mohan, R. S. J.
Org. Chem. 2002, 67, 1027.
(6) Kobayashi, S.; Nagayama, S.; Busujima, T. J. Am. Chem.
Soc. 1998, 120, 8287.
(7) Tsuruta, H.; Yamaguchi, K.; Imamoto, T. Chem. Commun.
1999, 1703.
(8) Castell, P.; Galia, M.; Serra, A.; Salla, J. M.; Ramis, X.
Polymer 2000, 41, 8465.
(9) All the chemicals were purchased from commercial sources
besides the protected ketals 16–19 in Table 2, which were
synthesized according the trivial literature methods.2c,
(10) Er(OTf)3 can be purchased from Aldrich €41.00 for 5g
(0.008 mol) meanwhile, the prices of Sc(OTf)3 and
Yb(OTf)3 sold by Aldrich are €206.4 for 5 g (0.01 mol) and
€79 for 5 g (0.008 mol), respectively.
Deprotection with Er(OTf)3; Typical Procedure
A solution of 1,1-diethoxycyclohexane (172.3 mg, 1.0 mmol) and
Er(OTf)3 (6.14 mg, 0.01 mmol) in CH3NO2 saturated with water (20
mL) was stirred at r.t. After 30 min, the solvent was removed under
reduced pressure and the residue was extracted with Et2O. The or-
ganic layer was washed with water and dried over anhyd. Na2SO4.
The yield of cyclohexanone was determined by GC-MS using the
standard addition method (>99%). All compounds reported in
Table 2 were isolated by flash chromatography and their full NMR
and IR data were compared with those of a pure sample.
Synthesis 2004, No. 4, 496–498 © Thieme Stuttgart · New York