Direct conversion of silyl ethers or tetrahydropyranyl ethers into the corresponding diphenylmethyl ethers

Article abstract of DOI:10.1081/SCC-100105322

Direct conversion of alcohol silyl ethers into the corresponding diphenylmethyl (DPM) ethers can be easily performed by the reaction with diphenylmethyl formate in the presence of a catalytic amount of trimethylsilyl trifluoromethanesulfonate. By the additional use of triethylsilane, tetrahydropyranyl ethers can be also converted into the corresponding DPM ethers.

Full text of DOI:10.1081/SCC-100105322

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DIRECT CONVERSION OF SILYL ETHERS OR
TETRAHYDROPYRANYL ETHERS INTO THE
CORRESPONDING DIPHENYLMETHYL ETHERS
Takeshi Suzuki a , Kumiko Kobayashi a , Kojiro Noda a & Takeshi Oriyama b
a Faculty of Science , Ibaraki University , Bunkyo, Mito, 310-8512, Japan
b Faculty of Science , Ibaraki University , Bunkyo, Mito, 310-8512, Japan
Published online: 15 Aug 2006.
To cite this article: Takeshi Suzuki , Kumiko Kobayashi , Kojiro Noda & Takeshi Oriyama (2001) DIRECT CONVERSION
OF SILYL ETHERS OR TETRAHYDROPYRANYL ETHERS INTO THE CORRESPONDING DIPHENYLMETHYL ETHERS, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 31:18, 2761-2766,
DOI: 10.1081/SCC-100105322
To link to this article: http://dx.doi.org/10.1081/SCC-100105322
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SYNTHETIC COMMUNICATIONS, 31(18), 2761–2766 (2001)
DIRECT CONVERSION OF SILYL
ETHERS OR TETRAHYDROPYRANYL
ETHERS INTO THE CORRESPONDING
DIPHENYLMETHYL ETHERS
Takeshi Suzuki, Kumiko Kobayashi, Kojiro Noda,
and Takeshi Oriyama*
Faculty of Science, Ibaraki University,
Bunkyo, Mito 310-8512, Japan
ABSTRACT
Direct conversion of alcohol silyl ethers into the correspond-
ing diphenylmethyl (DPM) ethers can be easily performed by
the reaction with diphenylmethyl formate in the presence of a
catalytic amount of trimethylsilyl trifluoromethanesulfonate.
By the additional use of triethylsilane, tetrahydropyranyl ethers
can be also converted into the corresponding DPM ethers.
Alkyl ethers such as benzyl and substituted benzyl ethers have been
widely used as a typical protecting group of the hydroxy function as well as
other-type protecting groups, such as silyl ether-, acetal-, and ester-types.1
The unique ability of benzyl ether and substituted benzyl ethers, such
as p-methoxybenzyl (PMB), diphenylmethyl (DPM), and triphenylmethyl
(Tr) ethers, to be selectively cleaved by simple hydrogenation under mild
*Corresponding author.
2761
Copyright & 2001 by Marcel Dekker, Inc.
www.dekker.com

ORDER
REPRINTS
2762
SUZUKI ET AL.
and neutral reaction conditions is the advantage over other-type protecting
groups. Generally, protection of alcohols into alkyl ethers is accomplished
by reaction with the corresponding alkyl halides under the influence of a
base such as sodium hydride,2 sodium hydroxide,3 and so on. Direct trans-
formation into alkyl ethers from other-type functional groups, not from a
parent alcohol, is very important from the standpoint of green chemistry
(efficiency and convenience).
In a series of our studies4 on the direct transformation of a protecting
group of the hydroxy function into another, we have previously demon-
strated that direct conversion of aryl trialkylsilyl ethers into the correspond-
ing aryl alkyl ethers can be conveniently carried out by reaction with alkyl
bromide under the influence of cesium fluoride.5 However, this reaction is
limited to aryl silyl ethers and does not extend to alkyl silyl ethers. On the
other hand, we have found that alcohol tetrahydropyranyl (THP) ethers
can be readily converted into the corresponding benzyl ethers in high
yields by the reaction with triethylsilane and benzaldehyde catalyzed by
trimethylsilyl trifluoromethanesulfonate (TMSOTf ) as shown in Scheme
1.6 However, an attempted direct conversion of THP ether into DPM
ether by the use of benzophenone instead of benzaldehyde gave an unsuc-
cessful result, and the corresponding DPM ether was obtained in low yield
(Scheme 2).
Scheme 1.
Scheme 2.
In this communication, we report a highly efficient and convenient
method for the direct conversion of alkyl silyl ethers or alkyl THP ethers
into the corresponding DPM ethers in a convenient one-pot procedure.
After a screening of various reaction conditions, we found that
diphenylmethyl acetate (AcODPM) reacted with alcohol t-butyldimethyl-
silyl (TBS) ether under the influence of TMSOTf, and the desired DPM

ORDER
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CORRESPONDING DPM ETHERS
2763
ether was obtained (Scheme 3). A diphenylmethyl cation generated from
AcODPM under Lewis acid conditions was presumed to be an active
species, and it reacted with alkyl silyl ether to provide the corresponding
alkyl DPM ether. Examination of other DPM alcohol esters in the reaction
of TBS ether of 3-phenylpropanol under similar reaction conditions (Table 1,
Runs 2–4) indicated that diphenylmethyl formate (HCO2DPM) is more
effective, and the combination with silica gel as an additive gave the best
result (Run 5). We have, therefore, tentatively chosen a combination of the
TBS ether (0.3 mmol), HCO2DPM, TMSOTf, (molar ratio; 1 : 1.5 : 0.05),
and silica gel (300 mg) as the optimum reaction conditions.
Scheme 3.
Representative examples of a one-pot conversion of alcohol TBS
7
ethers into the corresponding DPM ethers are collected in Table 2. As
can be seen, the reaction was successful with TBS ethers of secondary alco-
hols as well as a primary alcohol (Runs 2 and 3). In the presence of other-
type protecting groups, such as a benzyl ether and benzoate, TBS ethers are
chemoselectively transformed into the corresponding DPM ethers in high
yields (Runs 4 and 5).
Table 1. The Effect of DPM Ester and Silica Gel
0:05eq
TMSOTf
1:5eq
PhðCH2Þ3OTBS þ RCO2DPM ÀÀÀÀÀÀÀÀÀÀ! PhðCH2Þ3ODPM
CH3CN=0ꢀC=1h
Run
RCO2DPM
Additive
Yield (%)a
1b
2
3
4
AcODPM
AcODPM
PhCO2DPM
HCO2DPM
HCO2DPM
–
–
–
–
83
68
69
80
94
5
silica gel
aIsolated yield of purified product.
b2.5 equiv. of AcODPM was used. Reaction was carried out at r.t. for 12 h.

ORDER
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2764
SUZUKI ET AL.
Table 2. Synthesis of Various DPM Ethers from TBS Ethersa
TMSOTf
ROTBS þ HCO2DPM ÀÀÀÀÀÀÀÀÀÀÀÀ! RODPM
silica gel=CH3CN
ꢀ
0
C=1 h
Run
1
ROTBS
Yield (%)b
94
2
3
4
5
86
84
93
90
aMolar ratio of TBS ether : HCO2DPM : TMSOTf ¼ 1 : 1.5 : 0.05.
bIsolated yield of purified product.
Next, we examined the scope of trialkylsilyl ethers (Table 3). The
reaction was conducted with various silyl ethers of 3-phenylpropanol
using 5 mol% of TMSOTf. Triethylsilyl (TES) ether and triisopropylsilyl
(TIPS) ether could also be converted into the DPM ether in high yields
(Runs 2 and 3). However, in the case of t-butyldiphenylsilyl (TBDPS)
ether, the reaction was very slow and the DPM ether was obtained in
lower yield (Run 4).
As mentioned above, we recently reported that THP ethers can be
readily converted into the corresponding benzyl ethers by the reaction
with triethylsilane, benzaldehyde catalyzed by TMSOTf (Scheme 1). This
reaction proceeded via a formation of the corresponding TES ether as an
intermediate.6 This result suggested that using HCO2DPM in place of
benzaldehyde under the influence of silica gel would afford the correspond-
ing DPM ethers from THP ethers. We tested the direct conversion of THP
ethers into DPM ethers. As can be seen from Table 4, DPM ethers of pri-
mary and secondary alcohols were obtained in good yields as well as a
conversion from silyl ethers (Runs 1–6). The reaction was compatible with
the aromatic ketone (Run 7) which is reducible by exposure to triethylsilane
under acidic conditions.8

ORDER
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CORRESPONDING DPM ETHERS
2765
Table 3. Synthesis of DPM Ethers from Various Silyl Ethersa
TMSOTf
PhðCH2Þ3OSi þ HCO2DPM ÀÀÀÀÀÀÀÀÀÀÀ! PhðCH2Þ3ODPM
silica gꢀel=CH3CN
0
C=1 h
Run
Si
Yield (%)b
1
2
3
4
t-BuMe2Si (TBS)
Et3Si (TES)
i-Pr3Si (TIPS)
94
90
91
47
t-BuPh2Si (TBDPS)
aMolar ratio of silyl ether : HCO2DPM : TMSOTf ¼ 1 : 1.5 : 0.05.
bIsolated yield of purified product.
Table 4. Synthesis of Various DPM Ethers from THP Ethersa
Et3SiH=TMSOTf
ROTHP þ HCO2DPM ÀÀÀÀÀÀÀÀÀÀÀ! RODPM
silicaꢀgel=CH3CN
0
C=0:5À1 h
Run
1
ROTHP
Yield (%)b
90
2
3
89
83
4
5
6
7
80
78
89
79
aMolar ratio of THP ether : HCO2DPM : Et3SiH : TMSOTf ¼ 1 : 1.5 : 1.1 : 0.05.
bIsolated yield of purified product.

ORDER
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2766
SUZUKI ET AL.
In a series of our investigations on the one-step conversion between
typical protecting groups of the hydroxy function, we have added new direct
transformation of silyl ethers into the DPM ethers, and THP ethers into the
DPM ethers.
REFERENCES
1. (a) Greene, T.W.; Wuts, P.G.M. Protective Groups in Organic Synthesis,
2nd Ed., John Wiley and Sons: New York, 1991. (b) Kocienski, P.J.
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2. Czernecki, S.; Georgoulis, C.; Provelenghiou, C. Tetrahedron Lett.
1976, 3535.
3. Freedman, H.H.; Dubois, R.A. Tetrahedron Lett. 1975, 3251.
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885. (c) Oriyama, T.; Oda, M.; Gono, J.; Koga, G. Tetrahedron Lett.
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7. A typical experimental procedure for the direct conversion of alcohol
TBS ethers into the corresponding DPM ethers is as follows: To a
suspension of silica gel (300 mg), 2-t-butyldimethylsilyloxy-3-phenyl-
propane (75.1 mg, 0.300 mmol) and diphenylmethyl formate (95.5 mg,
0.450 mmol) in CH3CN (2 mL) was added a solution TMSOTf
(3.3 mg, 0.015 mmol) in CH3CN (0.5 mL) at 0ꢀC under argon atmos-
phere. The reaction mixture was stirred for 1 h at 0ꢀC and quenched
with saturated aqueous sodium hydrogen carbonate. The organic mate-
rials were extracted with Et2O and dried over anhydrous magnesium
sulfate. The solvent was evaporated and 1-diphenylmethyloxy-3-phenyl-
propane (85.2 mg, 94%) was isolated by thin-layer chromatography on
silica gel (ether : hexane ¼ 1 : 10).
8. (a) West, C.T.; Donnelly, S.J.; Kooistra, D.A.; Doyle, M.P. J. Org.
Chem. 1973, 38, 2675. (b) Fry, J.L.; Orfanopoulos, M.; Adlington,
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Received in Japan August 28, 2000

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