An atom-efficient and powerful method for direct esterification of silyl ethers catalyzed by HClO4-SiO2

Article abstract of DOI:10.1016/j.tet.2010.12.028

An efficient and convenient procedure for direct esterification of alkyl and aryl silyl ethers with Ac₂O and a catalyst system of perchloric acid immobilized on a silica gel (HClO₄-SiO₂) has been developed. The silyl protecting groups are directly replaced by acetyls and the protecting groups themselves are transformed into acetates as the sole byproducts, which can be readily recovered and converted back to silylchlorides, the original protecting agents, thus minimizing wastes.

Full text of DOI:10.1016/j.tet.2010.12.028

Tetrahedron 67 (2011) 1096e1101
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
An atom-efficient and powerful method for direct esterification
of silyl ethers catalyzed by HClO4eSiO2
*
Ti-Jian Du, Qin-Pei Wu , Hai-Xia Liu, Xi Chen, Yi-Nan Shu, Xiao-Dong Xi, Qing-Shan Zhang, Yun-Zheng Li
Department of Applied Chemistry and Pharmaceutics, Beijing Institute of Technology, Beijing 100081, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 August 2010
Received in revised form 2 December 2010
Accepted 10 December 2010
Available online 17 December 2010
An efficient and convenient procedure for direct esterification of alkyl and aryl silyl ethers with Ac2O and
a catalyst system of perchloric acid immobilized on a silica gel (HClO4eSiO2) has been developed. The
silyl protecting groups are directly replaced by acetyls and the protecting groups themselves are
transformed into acetates as the sole byproducts, which can be readily recovered and converted back to
silylchlorides, the original protecting agents, thus minimizing wastes.
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Silyl ether
Esterification
Acetic anhydride
Protection
1. Introduction
Moreover, the high cost and susceptibility to moisture would be
a major concern in the industrial applications of a metal salt. The
Green chemistry advocates the need for environmentally benign
synthesis, which incorporates high atom efficiency, diminishes or
eliminates in the use or generation of hazardous reagents or wastes
and the cyclic use of reagents and catalysts.1 In most cases, pro-
tecting groups are indispensable in the multistep synthesis of
natural products and medicinal compounds but are the antithesis
of atom economy.2 Silyl groups are the most frequently employed
hydroxy protecting group. In the presence of Lewis or protic acids,
however, they are labile.2a,3 On the other hand, esters are quite
stable to Lewis and protic acids. The different stability of these two
kinds of protecting group often necessitates changing one or more
protecting groups.4 Direct transformation presents an obvious ad-
vantage for this purpose as it replaces two steps: cleavage of the
primary protecting group(s) and the subsequent installation of the
new protecting group(s). Therefore, studies in synthetic methods
for direct transforming protecting groups are considered valuable.5
A number of methods have been reported for the trans-
formation of tert-butyldimethylsilyl (TBDMS) ethers to acetates,
using FeCl3eAc2O,6 Cu(OTf)2eAc2O,7 In(OTf)3eAc2O,8 AcBreSnBr2,9
AcCleZnCl2,10 TiCl4eAc2O,11 ZrCl4eAc2O,12 or BF3$Et2OeNaIe
Ac2O.13 Each of these methods is incompatible with some func-
tional groups or delicate structures, limiting their application. This
laboratory has found that the FeCl3eAc2O combination induces
undesired pentofuranose and pentofuranoside chain-opening.14
presence of a metal halide also leads to a tedious workup when
conducted on large scales.15 Therefore, a mild, effective, and envi-
ronmental benign procedure for this purpose would be of value and
is still required for green organic synthesis. This report describes an
effective and environmental-friendly protocol for direct conversion
of silyl ethers into acetates in the presence of HClO4eSiO2
(Scheme 1), a catalyst, which is readily recovered and reused.16
RO-Ac
RO-P
HClO4-SiO2
CH2Cl2
Ac2O
POAc
P= TBDMS, TES, TBDPS,TIPDS
Scheme 1. Direct transformation of silyl ethers to acetates.
2. Results and discussion
2.1. Direct esterification of alkyl silyl ethers
With the concept of waste minimization in mind, we have de-
velopeda method fordirecttransformation ofacetonideintoacetates
usinganAc2OeHClO4eSiO2 system.17 Underthesameconditions, the
* Corresponding author. Tel.: þ86 10 68918511; fax: þ86 10 68914009; e-mail
address: qpwu@bit.edu.cn (Q.-P. Wu).
0040-4020/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.12.028

T.-J. Du et al. / Tetrahedron 67 (2011) 1096e1101
1097
primaryTBDMS groupin1 wasreadilydisplacedbyanacetylgroupto
give 2 in 94% isolated yield (entry a, Table 1). Furthermore, under
reflux in CH2Cl2 solution, both TBDMS groups were simultaneously
substituted to afford 3 in 89% isolated yield (entry b, Table 1). En-
couraged by these results and mild reaction conditions without the
use of metal salts, the scope of this protocol was examined further.
The steric hindered secondary ether 4 was transformed smoothly
underrefluxconditions toafford thedesiredproduct 5in87%isolated
yield. Both primary TBDMS ethers 6 and 7 were also successfully
transformed at rt (entry d and e, Table 1). In all cases as described
above, the anomeric acetonide group was transformed into an open-
chain isopropylidene acetal similar to the results obtained as trans-
formation of acetonides.14,17a As anticipated, displacement of the TES
ether in 9 was more readily achieved as the triethylsilyl (TES) ethers
are much more sensitive to acid than TBDMS ethers.2a,3a
Moreover, using this one-pot procedure, selective trans-
formation of TBDMS ethers of 11 was successfully achieved without
affecting the acid labile dimethyl acetal group (entry g, Table 1).
Table 1
Direct transformation of alkyl silyl ethers into acetates catalyzed by HClO4eSiO2
Entry
Substrate
TBDMSO
Product
Time (h)
0.5
Yielda (%)
9417a
AcO
OAc
OAc
O
O
TBDMSO
TBDMSO
a
O
O
O
O
O
2
1
TBDMSO
TBDMSO
AcO
AcO
OAc
OAc
O
O
b
c
2 (Reflux)
2 (Reflux)
0.5
8917a
8717a
8917a
O
O
O
3
1
BzO
BzO
AcO
OAc
OAc
O
O
O
TBDMSO
O
O
5
4
TBDMSO
AcO
AcO
AcO
OAc
OAc
O
d
O
O
3
6
OTBDMS
OTES
OAc
Ph
e
f
Ph
0.75
0.5
91
89
8
7
OAc
Ph
Ph
9
8
OMe
OMe
OTs
MeO
MeO
AcO
TBDMSO
OTs
g
h
i
2
85
81
91
11
12
O
OMe
MeO
TBDMSO
OTBDMS
AcO
OAc
3
13
14
OAc
OTBDMS
1 (reflux)
15
16
AcO
AcO
OAc
O
O
Si
O
j
5 (reflux)
87
O
O
Si
OAc
OAc
O
O
O
3
17
AcO
O
O
O
Si
O
OMe
k
4 (reflux)
8418
Si
O
AcO
19
18
a
Isolated yields and reproduced at least twice for each cases.

1098
T.-J. Du et al. / Tetrahedron 67 (2011) 1096e1101
Under the same conditions, however, the dimethyl acetal group in
13 was hydrolyzed during esterification of the TBDMS groups to
give the ketone 14 (entry i, Table 1). In view of the single difference
between the substrates 11 and 13, it is reasonable to speculate that
the tosylate group gives rise to the different products.19 The steri-
cally hindered secondary TBDMS ether 15 was also directly con-
verted into the menthol acetate 16 in excellent yield under reflux
(entry j, Table 1). Markiewicz’s reagent,20 a useful bifunctional
protecting agent, was readily displaced to give the desired products
with reasonable yields in both cases (entries j and k, Table 1). The
anomeric methoxy group in compound 18 was also substituted by
acetate.
4-Acetylnaphthalen-1-ol TBDMS ether, however, was formed
and further converted into 4-acetylnaphthalene acetate 26 as the
final product when naphthalen-1-ol TBDMS ether 25 was esterified
under these conditions as above (entry f, Table 2). Similarly,
4-acetylphenyl TBDMS ether 28 was isolated and identified as an
intermediate (Scheme 2) when phenyl TBDMS ether 27 underwent
this procedure. It was further transformed with longer reaction
times into 4-acetylphenyl acetate 29 in 82% yield. Therefore, both
acyl products 26 and 29 might suggest that acetic anhydride is
activated by perchloric acid to generate the acylium ion, a very
reactive species, acetylizing the aryl ring. Moreover, we observed
that, in the absence of perchloric acid, SiO2eAc2O proved in-
effective for this purpose. Thus a plausible mechanism involves
attack by the oxygen atom of the silyl ethers on the acylium ion
2.2. Direct transformation of aryl silyl ethers
26,28
rather than protonation of the silyl ether by HClO4
leading to
breaking the ROeSi bond to give the corresponding acetate,
a thermodynamically more stable product (Fig. 1). Based on this
proposal, the ease of esterification of alkyl silyl ethers over aryl
ethers could be attributed to the higher electron density on the
alkyl oxygen.
Compared with alkyl silyl ethers, aryl silyl ethers are more
sensitive to base hydrolysis as phenols are better leaving groups
than alcohols.2a This means most methodologies for deprotection
use more than stoichiometric amounts of fluoride sources (TBAF,21
KFeAl2O322), bases (NaOH,23 Carbonates,24 and LiOAc25) or strong
acids (HF26). The protocol described here provides an alternative
means of acetylative deprotection of phenol silyl ethers under quite
weak acid conditions. To the best of our knowledge, this direct
transformation is unprecedented.
Under similar conditions, o-methylphenyl TBDMS ether 20 was
directly transformed into corresponding acetate in 91% isolated yield
(entry a, Table 2). Substituent effects were observed for this direct
conversion reaction. Electron donating groups, such as alkyl and
alkoxy groups promote the reaction with high yields (entries a and b,
Table 2) and electronwithdrawinggroups led to much longer reaction
times (entries cee, Table 2). These trends agree with the lower nu-
cleophilicity of electron deficient phenols leading to a lower reaction
activity. Additionally, the aldehyde group in 24 was unaffected.27
Scheme 2. Direct transformation ofsilyl ethersto acetates along with acetylizing aryl ring.
Table 2
Direct esterification of aryl silyl ethers catalyzed by HClO4eSiO2
Entry
a
Substrate
Product
Time (h)
Yielda (%)
91
OTBDMS
Fig. 1. Catalytic cycle and plausible transition state.
4
20
The green and atom-economical features of this method are also
exhibited by the sole byproduct, trialkylsilyl acetate. Previous work
in this field has isolated the desired acetate products but has failed
to find direct evidence of the silyl acetates, namely byproducts.11,12
Here, TBDPSOAc was isolated in 86% yield after the complete es-
terification of compound 30 and workup with a saturated aqueous
solution of NaHCO3. TBDPSOAc could also be converted into
TBDPSOMe (88% yield) shown on the TLC plate when methanol
instead of aqueous solution of NaHCO3 was employed in the
workup (Scheme 3).29 Furthermore, TBDPSOAc was conveniently
transformed back into its initial form, TBDPSeCl in 95% yield, when
treated with concd HCl at rt. It is well known that both TBDPS
ethers and acetates are more stable than the corresponding com-
pounds of other common silyl protecting groups, such as TES, TMS,
TBDMS, and TIPS, under acid conditions.2a,3a More significantly,
methyl silyl ethers can also be conveniently converted back to the
protecting agent R3SiCl in excellent yields by simply stirring with
concd HCl without solvent (Scheme 4).30 This is in contrast to
fluoride-mediated desilylation, which affords silylfluorides, a non-
recoverable waste.5b Thus our protocol makes silyl protecting
groups recyclable agents with all the commensurate advantages of
atom economy.
MeO
F
b
c
1
93
84
OTBDMS
21
10
OTBDMS
22
Cl
d
e
10
16
87
86
OTBDMS
OMe
23
OHC
OTBDMS
24
OAc
OTBDMS
f
8
78
25
Ac
26
The practical and green aspects of this methodology are
exhibited in the workup as well. In contrast to fluoride-mediated
a
Isolated yields and reproduced at least twice for each cases.

T.-J. Du et al. / Tetrahedron 67 (2011) 1096e1101
1099
ethers, such as phenyl TMS, phenyl TES and phenyl TIPS ethers.31
This excellent selectivity also encouraged us to examine the se-
lective potentiality between aryl and alkyl ethers. The reaction of
a mixture of the relatively active 4-methoxyphenyl ether 21 and
phenylethanol TBDMS ether 7 with Ac2O at rt in half one hour gave
4-methoxyphenyl acetate and phenylethanol acetate 8 in 91% and
92% yields, respectively, without preference (Scheme 5). When ei-
ther the more sluggish 4-fluorophenyl ether 22 or 4-chlorophenyl
TBDMS ether 23 was mixed with 7, 8 was isolated as the sole
products in 93% and 91% yield and substrates 22 and 23 were
recovered in 91% and 90%, respectively. These results indicate
electron withdrawing substituents inactivate esterification of
phenolic silyl ethers under this protocol and lead to good selectivity
between aryl and alkyl silyl ethers.
Scheme 3. Products and byproducts generated by the direct esterification of silyl ethers.
Moreover, displacement of the TBDMS group in glycerol de-
rivative 34 was also achieved selectively in quite good yield without
affecting the TBDPS ether group.
To ascertain the selectivity between different TBDPS ethers,
equimolar mixtures of 31 and either 32 or 4-chlorophenyl TBDPS
ether 36 proceeded at rt in 1 h to afford 8 in 93% and 89% yield
and the substrates 32 and 36 were recovered in 95% and 94%,
respectively. This remarkable preference could be the result of
the different electronic density at the different ether oxygen
atoms.
Scheme 4. Recyclability of the sole byproducts back to silyl protecting agents.
desilylation, our method is feasible with a non-aqueous workup.
This is a remarkable advantage as aqueous workup tends to be
tedious and leads to large volumes of waste. For large scale
reactions, excess Ac2O, the solvent, and the byproducts R3SiOAc or
R3SiOMe can be recovered by fractional distillation after the cata-
lyst HClO4eSiO2 has been recovered via simple filtration.
3. Conclusions
Good chemoselectivity in the cleavage of silyl groups between
two different aryl silyl ethers is not easy to achieve.3a Our meth-
odology might provide some advantages in this aspect in view of
the proposed mechanism (Fig. 1). Treatment of an equimolar mix-
ture of (i) para-methoxyphenyl TBDMS ether 21 and TBDPS ether
32 for 1 h or (ii) para-fluorophenyl TBDMS ether 22 and TBDPS
ether 33 for 10 h afforded the desired acetates as sole products in
92% and 87% without the formation of TBDPS acetate in either case
and the TBDPS ethers 32 and 33 were recovered in 97% and 92%,
respectively (Scheme 5). Conceivably, similar selectivity should be
established between phenyl TBDPS ether and other phenylsilyl
In conclusion, silyl protecting groups are commonly used and
changed into esters in synthetic chemistry and we have developed
a simple, metal, and fluoride-free one-pot procedure for the effi-
cient esterification of silyl ethers in an environmentally benign
way. Phenolic silyl ethers could also be direct esterified with
excellent selectivity. This method uses HClO4eSiO2, a cheap, non-
toxic, and reusable catalyst previously reported by Chakra-
borti.16,32 Significantly, silyl protecting groups were transformed
into acetates as the sole byproducts, which were readily recover-
able and converted back to silyl protecting agents. Solvent and
Scheme 5. Selective esterification of different silyl ethers with various electronic environment.

1100
T.-J. Du et al. / Tetrahedron 67 (2011) 1096e1101
excess Ac2O could also be easily recovered via distillation and
wastes were minimized or eliminated. This direct transformation
procedure including mild conditions, atom economy, and envi-
ronmentally benign recycling should provide protecting groups
with a new era of usefulness.
170.6; HRMS(ESI), calcd for C14H20O7S, m/z 332.0930; found, m/z
355.0921 (MþNa)þ.
4.2.5. 1,3-Diacetoxy acetone 14. Colorless oil, 1H NMR (CDCl3):
dH¼2.18 (s, 6H), 4.76 (s, 4H); 13C NMR (CDCl3): dC¼20.6, 66.5, 170.3,
198.1; HRMS(ESI), calcd for C7H10O5, m/z 174.0528; found, m/z
197.0519 (MþNa)þ.
4. Experimental
4.1. General
4.2.6. 4-Acetoxy-3-methoxybenzaldehyde. 1H NMR (CDCl3): dH¼2.36
(s, 3H), 3.91(s, 3H), 7.25(d, J¼8.0 Hz,1H), 7.52 (m, 2H), 9.98(s,1H); 13C
NMR (CDCl3): dC¼20.7, 56.1, 110.8, 123.4, 124.8, 135.2, 144.9, 151.9,
168.4,191.1; HRMS(ESI), calcd for C10H10O4, m/z 194.0579; found, m/z
217.0569 (MþNa)þ.
Dichloromethane was dried over P2O5 and distilled. Acetic
anhydride was distilled before being used. All reactions were
monitored by thin layer chromatography (TLC) on gel F254
plates. 1H NMR and 13C NMR spectra were recorded on a Varian
Mercury 400 spectrometer or a Bruker AV500 spectrometer in
CDCl3 or DMSO-d6 and using TMS as internal standard. Exact
mass measurements were performed on a LCMS2010 spec-
trometer equipped with a standard electrospray ionization (ESI)
interface.
4.2.7. (R)-3-Acetoxy-2-((tert-butyldiphenylsilyl)oxy)propyl benzoate
36. 1H NMR (CDCl3): dH¼1.08 (s, 9H),1.93 (s, 3H), 4.17 (dd, J¼1.2 and
4.0 Hz, 2H), 4.23 (m, 1H), 4.34 (m, 2H), 7.34e7.46 (m, 8H), 7.57 (t,
J¼6.0 Hz, 1H), 7.68 (t, J¼6.0 Hz, 4H), 7.94 (d, J¼6.0 Hz, 2H); 13C NMR
(CDCl3): dC¼20.7, 26.8, 29.7, 65.4, 65.7, 69.4, 27.7, 128.4, 129.7, 129.9,
133.0, 135.8, 166.2, 170.7; HRMS(ESI), calcd for C28H32O5Si, m/z
476.2019; found, m/z 499.2026 (MþNa)þ.
4.2. General procedure for direct transformation
General procedure: HClO4eSiO2 (0.5 mmol/g; 50 mg) was added
to a stirred solution of substrate (1.0 mmol) and Ac2O (0.45 mL,
5.0 mmol) in CH2Cl2 (5 mL) at rt or heated under reflux. After
complete conversion and filtration to remove the catalyst, satu-
rated aqueous solution of NaHCO3 (10 mL) was added and sepa-
rated. The aqueous solution was extracted with CH2Cl2 (2ꢀ10 mL).
The organic layer was combined, washed with brine (10 mL), dried
(MgSO4), and concentrated under reduced pressure. The residual
was isolated through short column chromatography on silica gel,
which was eluted with ethyl acetateepetroleum (bp 60e90 ꢁC) to
give the target products.
Acknowledgements
Financial support of this work from the Ministry of Science and
Technology of China (2006AA100216) is appreciated.
Supplementary data
Supplementary data related with this article can be found online
version at doi:10.1016/j.tet.2010.12.028. These data include MOL
files and InChiKeys of the most important compounds described in
this article.
All new products were characterized by 1H NMR, 13C NMR and
HRMS (ESI) spectra and the NMR data for known compounds
matched that reported in literature.
References and notes
ꢀ
1. V. García, E. Pongracz, R. Keiski, http://www.oulu.fi/resopt/wasmin/garcia.pdf.
4.2.1. Aceyl 5-O-acetyl-3-O-TBDMS-2-O-(20-acetoxyisopropyl)-
b-D-
2. (a) Kocienski, P. J. Protecting Groups; Georg Thieme: Stuttgart: New York, NY,
2004; (b) Somoza, A. Chem. Soc. Rev. 2008, 37, 2668e2675; (c) Stamatov, D. S.;
Stawinski, J. Org. Biomol. Chem. 2010, 8, 463e477.
3. (a) Crouch, R. D. Tetrahedron 2004, 60, 5833e5836; (b) Wu, Q.-P.; Wang, Y.;
Chen, W.; Wang, H.; Liu, H. Lett. Org. Chem. 2006, 3, 13e15.
4. Kunz, H.; Unverzagt, C. Angew. Chem., Int. Ed. Engl. 1988, 27, 1697e1698.
5. (a) Stamatov, S. D.; Stawinski, J. Org. Biomol. Chem. 2007, 5, 3787e3800;
(b) Stamatov, S. D.; Kullberg, M.; Stawinski, J. Tetrahedron Lett. 2005, 46,
6855e6859.
6. Ganem, B.; Small, V. R. J. Org. Chem. 1974, 39, 3728e3730.
7. Chandra, K. L.; Saravanan, P.; Singh, V. K. Tetrahedron Lett. 2001, 42, 5309e5312.
8. Mineno, T. Tetrahedron Lett. 2002, 43, 7975e7978.
xylofuranoside 2. 1H NMR (CDCl3): dH¼0.13 (s, 6H, CH3), 0.9 (s, 9H,
CH3), 1.48 (s, 6H, CH3), 2.08 (m, 9H, CH3), 4.05 (t, J¼4.0 Hz, 1H), 4.22
(dd, J¼3.6 and 12.0 Hz, 1H), 4.28 (m, 1H), 4.35 (dd, J¼3.6 and
12.0 Hz, 1H), 5.19 (t, J¼4.0 Hz, 1H), 6.33 (d, J¼2.4 Hz, 1H); 13C NMR
(CDCl3): dC¼ꢂ0.5, 18.2, 21.0, 21.3, 21.4, 25.7, 25.8, 25.8, 26.8, 69.7,
72.0, 81.9, 96.8, 113.4, 170.5, 170.8.
4.2.2. Aceyl 3,5-di-O-acetyl-2-O-(20-acetoxyisopropyl)-
b-D-xylofur-
anoside 3. 1H NMR (CDCl3): dH¼1.45 (s, 3H, CH3), 1.46 (s, 3H, CH3),
2.05 (s, 3H, CH3), 2.06 (s, 3H, CH3), 2.07 (s, 3H, CH3), 2.10 (s, 3H,
CH3), 4.10 (dd, J¼3.6 and 8.0 Hz, 1H), 4.35 (dd, J¼10.4 and 18.4 Hz,
1H), 5.33 (t, J¼3.6 Hz, 2H), 6.16 (d, J¼1.6 Hz, 1H); 13C NMR (CDCl3):
dC¼13.6, 13.7, 13.8, 14.1, 19.9, 22.7, 54.9, 62.4, 62.5, 73.3, 89.4, 106.8,
162.9, 162.9, 162.9, 163.5.
9. Oriyama, T.; Oda, M.; Gono, J.; Koga, G. Tetrahedron Lett. 1994, 35, 2027e2030.
10. Kim, S.; Lee, W. J. Synth. Commun. 1986, 16, 659e664.
11. Chandrasekhar, S.; Ramachander, T.; Reddy, M. V.; Takhi, M. J. Org. Chem. 2000,
65, 4729e4731.
12. Reddy, C. S.; Smitha, G.; Chandrasekhar, S. Tetrahedron Lett. 2003, 44,
4693e4695.
13. Brar, A.; Vankar, Y. D. Tetrahedron Lett. 2006, 47, 5207e5210.
14. Wu, Q.-P.; Zhou, M.-X.; Xi, X.-D.; Song, D.; Wang, Y.; Liu, H.-X.; Li, Y.-Z.; Zhang,
Q.-S. Tetrahedron Lett. 2008, 49, 2714e2718.
4.2.3. Acetyl 3-O-acetyl-5-O-benzoyl-2-O-(20-acetoxyisopropyl)-
b-D-
15. (a) Sharma, U. Synlett 2009, 3219e3220; (b) Sartori, G. Chem. Rev. 2004, 104,
199e250.
xylofuranoside 5. 1H NMR (CDCl3): dH¼1.48 (s, 3H, CH3), 1.51 (s, 3H,
CH3), 2.06 (s, 3H, CH3), 2.09 (s, 3H, CH3), 2.14 (s, 3H, CH3), 4.45 (m,
2H), 4.55 (dd, J¼4.0 and 12.0 Hz, 1H), 5.45 (dd, J¼4.0 and 12.0 Hz,
1H), 5.53 (m, 1H), 6.22 (d, J¼2.0 Hz, 1H), 7.46 (m, 2H, C6H5), 7.57 (m,
1H, C6H5), 8.02 (m, 2H, C6H5); 13C NMR (CDCl3): dC¼20.9, 21.1, 21.4,
26.9, 27.2, 29.9, 62.9, 69.8, 69.8, 80.6, 96.6, 114.1, 128.7, 129.9, 133.6,
170.1.
16. (a) Chakraborti, A. K.; Gulhane, R. J. Chem. Soc., Chem. Commun. 2003,1896e1897;
(b) Das, B.; Venkateswarlu, K.; Suneel, K.; Majhi, A. Tetrahedron Lett. 2007, 48,
5371e5374 The recovered catalyst was reused three times with only a little
variation in the yields of the products. For example, the catalyst was used in four
consecutive runs for direct conversion of compound 1 to 2 in the yields of 94%,
92%, 91%, and 90%, respectively (reaction time: 30 min in each case)..
17. (a) Liu, H.; Wu, Q.-P.; Chen, X.; Xi, X.-D.; Zhang, Q.-S.; Li, Y.-Z. Carbohydr. Res.
2009, 344, 2342e2348; (b) Yu, J.-L.; Wu, Q.-P.; Zhang, Q.-S.; Li, Y.-Z.; Liu, Y.;
Zhou, Z.-M. Bioorg. Med. Chem. Lett. 2010, 20, 240e243.
18. Harry, V.; Helmut, Z. Chem. Ber. 1960, 93, 137e138.
19. Kumar, A.; Doddi, V. R.; Vankar, Y. D. J. Org. Chem. 2008, 73, 5993e5995.
20. (a) Rees, C. B.; Wu, Q.-P. Org. Biomol. Chem. 2003, 1, 1553e1561; (b) Wu, Q.-P.;
Simons, C. Synthesis 2004, 1533e1555.
21. Corey, E. J.; Venkateswarlu, A. J. Am. Chem. Soc. 1972, 94, 6190e6192.
22. Blass, E. B.; Harris, C. L.; Portlock, E. D. Tetrahedron Lett. 2001, 42, 1611e1613.
4.2.4. 1-Acetoxy-3-tosyloxy-2,2-dimethoxypropane 12. Colorless oil,
1H NMR (CDCl3): dH¼1.94 (s, 3H), 2.45 (s, 3H), 3.17 (s, 6H), 3.99 (s,
2H), 4.05 (s, 2H), 7.35 (d, J¼8.0 Hz, 2H), 7.81 (d, J¼8.4 Hz, 2H); 13C
NMR (CDCl3): dC¼21.8, 29.9, 48.6, 59.7, 64.5, 98.6, 128.3, 130.1, 145.9,

T.-J. Du et al. / Tetrahedron 67 (2011) 1096e1101
1101
23. Jiang, Z.-Y.; Wang, Y.-G. Chem. Lett. 2003, 32, 568e569.
28. Dalpozzo, R.;Bartoli, G.; Sambri, L.; Melchiorre, P. Chem. Rev. 2010,110, 3501e3551.
29. (a) Morton, D. R.; Thompson, J. L. J. Org. Chem. 1978, 43, 2102e2106;
(b) McCarthy, P. A. Tetrahedron Lett. 1982, 23, 4199e4202.
30. Shin, M.; Banno, T.; Mitsuo, I. J. Organomet. Chem. 2006, 691, 174e181.
31. Cunico, R. F.; Bedell, I. J. Org. Chem. 1980, 45, 4797e4798.
32. Chakraborti, K. A.; Singh, B.; Chankeshwara, V. S.; Patel, R. A. J. Org. Chem. 2009,
74, 5967e5974.
24. Jiang, Z.-Y.; Wang, Y.-G. Tetrahedron Lett. 2003, 44, 3859e3863.
25. Wang, B.; Sun, H. X.; Sun, Z. H. J. Org. Chem. 2009, 74, 1781e1784.
26. (a) Kumar, G. D. K.; Baskaran, S. J. Org. Chem. 2005, 70, 4520e4523; (b) Kendall,
M. P.; Johnson, V. J.; Cook, C. E. J. Org. Chem. 1979, 44, 1421e1424.
27. Kamble, V. T.; Jamode, V. S.; Joshi, N. S.; Biradar, A. V.; Deshmukh, R. Y. Tetra-
hedron Lett. 2006, 47, 5573e5576.