Perchloric acid adsorbed on silica gel (HClO4-SiO2) as an inexpensive, extremely efficient, and reusable dual catalyst system for acetal/ketal formation and their deprotection to aldehydes/ketones

Article abstract of DOI:10.1055/s-2006-958948

Perchloric acid adsorbed on silica gel (HClO₄-SiO₂) is reported as extremely efficient, inexpensive, and reusable catalyst for dual role for protection of aldehydes/ketones (with trialkyl orthoformates) as acetals/ketals and deprotection (with water-alcohol) to regenerate the carbonyl compounds in high yields at room temperature and in short times. Acetalization/ketalization of electrophilic aldehydes/ketones was carried out under solvent-free conditions. Weakly electrophilic aldehydes/ketones and aldehydes having a substituent that can coordinate with the catalyst, required the corresponding alcohol as solvent. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2006-958948

PAPER
299
Perchloric Acid Adsorbed on Silica Gel (HClO4–SiO2) as an Inexpensive,
Extremely Efficient, and Reusable Dual Catalyst System for Acetal/Ketal
Formation and Their Deprotection to Aldehydes/Ketones
H
ClO –SiO2 Cataalyst Systemj Kumar, Dinesh Kumar, Asit K. Chakraborti*
4
National Institute of Pharmaceutical Education and Research (NIPER), Sector 67, S. A. S. Nagar, Punjab 160062, India
Fax +91(172)2214692; E-mail: akchakraborti@niper.ac.in
Received 6 October 2006; revised 18 October 2006
er, the efficiency of the method depends upon the
coordination property of the catalyst to activate the tri-
alkyl orthoformate and/or the carbonyl substrate, and a
metal salt derived from the strongest protic acid is an ideal
Abstract: Perchloric acid adsorbed on silica gel (HClO4–SiO2) is
reported as extremely efficient, inexpensive, and reusable catalyst
for dual role for protection of aldehydes/ketones (with trialkyl
orthoformates) as acetals/ketals and deprotection (with water–
alcohol) to regenerate the carbonyl compounds in high yields at choice. The large negative H0 value of –14.1 of TfOH5
room temperature and in short times. Acetalization/ketalization of
makes TfOH the strongest protic acid and thus metal tri-
flates drew the attention.6 However, TfOH, liberated from
electrophilic aldehydes/ketones was carried out under solvent-free
conditions. Weakly electrophilic aldehydes/ketones and aldehydes
metal triflates may be the actual catalytic agent7 and might
having a substituent that can coordinate with the catalyst, required
be the reason for the potential side reactions (e.g. dehydra-
the corresponding alcohol as solvent.
tion, rearrangement etc.) with acid-sensitive substrates or
Key words: acetals, ketals, aldehydes, ketones, trimethyl orthofor-
deactivation of the substrates due to salt formation. Thus,
mate (TMOF), triethyl orthoformate (TEOF), protection/deprotec-
metal triflate-catalyzed acylation reactions are often car-
tion, HClO4–SiO2, dual catalyst
ried out at low temperatures (–8 to –60 °C) and in the
presence of excess of the acetylating agents8 and the metal
triflate catalyzed a-aminophosphonate synthesis require
Functional group protection and deprotection are un-
stoichiometric amounts of additional reagents such as mo-
avoidable transformations in organic synthesis of any
lecular sieves, MgSO4, etc.9 Further, Bi(OTf)3 is not com-
length and frequently excercized for the preparation of
mercially available and require additional efforts and is a
drug candidates accounting for >20% of all chemical
costly reagent to prepare,10 and TMSOTf is highly mois-
transformations and two transformations per molecule.1
ture sensitive and difficult to handle. This brings the atten-
The scope of synthetic manipulation makes aldehyde/
tion to triflimides,11 as HNTf2 is a weaker Brønsted acid
ketone functionalities widely present in synthons and sub-
than TfOH12 and ligand exchange does not take place with
targets. However, one often faces the challenge to carry
triflimides.13 However, metal triflimides are costly, very
out a desired transformation without affecting the alde-
few are available commercially and require additional ef-
hyde/ketone carbonyl groups and needs to mask their
forts and are costly reagents to prepare.14 Hence we fo-
chemical reactivity. Acetal/ketal formation by the reac-
cused our attention to catalysts derived from HClO4 as it
tion of aldehyde/ketone with alcohol is the most common
is weaker than TfOH; metal perchlorates do not undergo
practice to protect carbonyl groups2 and there are contin-
ligand exchange reaction and have been reported as effi-
uous efforts for the development of newer methods.3 The
cient electrophilic activation catalysts.15 However, most
recent trends involve the reaction with trialkyl orthofor-
metal perchlorates are available as hydrates and the anhy-
mates [trimethyl orthoformate (TMOF) or triethylortho-
drous perchlorates often loose their catalytic property on
formate (TEOF)] in the presence of various catalysts.1,4
exposure to air.15c,16 The sensitivity of acetal/ketal forma-
However, one or more drawbacks such as long reaction
tion to trace amount of water does not make metal per-
times, high temperatures, use of costly reagents/catalysts,
chlorates effective catalysts for this convertion. Herein we
use of additional reagents, requirement of stoichiometric
amount of the catalysts, use of excess of trialkyl orthofor-
mates (1.5–3 equiv), the need to use special apparatus,
moderate yields, and side reactions associated with some
17
report HClO4–SiO2 as an inexpensive, extremely effi-
cient, and reusable dual catalyst for interconversion of
aldehydes/ketones and their acetals/ketals.
The acetal formation is effective under two different ex-
perimental conditions: (i) Method A involves the treat-
ment of the substrate with TMOF/TEOF under neat
condition at room temperature and (ii) Method B requires
MeOH/EtOH as solvent when the substrate is treated with
TMOF/TEOF at room temperature. No acetal formation
took place in using aqueous HClO4. Substituted benzalde-
hydes with Me, Cl, Br, NO2, and OCOPh groups (Table 1,
entries 1–8), 1-naphthaldehyde (entry 14), 9-anthralde-
hyde (entry 15), arylalkyl aldehydes (entries 16,17) and
of these reported procedures necessiates the development
of an improved method.
The use of trialkyl orthoformates as the acetalization
agent in the presence of a more effective electrophilic ac-
tivation agent offers a better and milder method. Howev-
SYNTHESIS 2007, No. 2, pp 0299–0303
x
x
.
x
x
.2
0
0
7
Advanced online publication: 14.12.2006
DOI: 10.1055/s-2006-958948; Art ID: Z19706SS
© Georg Thieme Verlag Stuttgart · New York

300
R. Kumar et al.
PAPER
Table 1 HClO4–SiO2-Catalyzed Dimethyl Acetal Formation
(continued)
saturated cyclic ketones (entry 18) afforded excellent re-
sults after 2–45 minutes on treatment with TMOF at ~25–
30 °C under neat conditions (Table 1). For benzaldehyde
(entry 9), acetophenone (entry 23), 1,3-diphenylprop-2-
enone (entry 24) and aldehydes bearing substituents that
are capable of coordinating with the metal ion (entries 19–
22), the acetal formation was slow under neat conditions
but proceeded well using MeOH as solvent affording ex-
cellent yields. In the case of 9-anthraldehyde (entry 15),
three equivalents of TMOF was required to ensure com-
pletion of the reaction as the product was solid.
Entry Aldehyde/Ketone
Methoda Time
(min)
Yield
(%)b,c
RO
MeO
CHO
19
20
21
R = Me
B
B
B
25
25
15
91
86
94
R = c-C5H9
Table 1 HClO4–SiO2-Catalyzed Dimethyl Acetal Formation
CHO
S
Entry Aldehyde/Ketone
Methoda Time
(min)
Yield
(%)b,c
22
23
B
B
12
93
89
CHO
O
O
1 h
R3
CHO
R2
R1
24
B
2 h
85
O
1
2
R1 = R2 = H, R3 = Me
R1 = R2 = H, R3 = Cl
R1 = R2 = H, R3 = NO2
R1 = Br, R2 = H, R3 = H
R1 = Cl, R2 = H, R3 = H
R1 = R3 = H, R2 = NO2
R1 = NO2, R2 = H, R3 = H
R1 = R2 = H, R3 = OCOPh
R1 = R2 = R3 = H
A
A
A
A
A
A
A
A
B
B
B
B
B
A
10
5
93
91
93
90
95
92
95
85
91
88
96
95
80d
82
3
2
a Method A: The aldehyde/ketone (2.5 mmol) was treated with the
TMOF (2.75 mmol, 1.1 equiv) in the presence of HClO4–SiO2 (0.5
mol%) at r.t. (~25–30 °C) under solvent-free conditions; Method B:
The aldehyde/ketone (2.5 mmol) was treated with the TMOF (2.75
mmol, 1.1 equiv) in the presence of HClO4–SiO2 (0.5 mol%) at r.t.
(~25–30 °C) in anhyd MeOH (1 mL).
4
10
10
5
5
6
b Isolated yield of the corresponding acetal.
c All products were characterized by IR and NMR spectroscopy.
d The product was obtained as a 60:40 mixture of the acetal and the
starting aldehyde (NMR).
7
5
8
45
5
e The reaction was carried out in 3 equiv of TMOF.
f The product was obtained as a 77:33 mixture of the acetal and the
starting ketone (NMR).
9
10
11
12
13
14
R1 = R2 = H, R3 = F
R1 = R2 = H, R3 = CN
R1 = R2 = H, R3 = OBn
R1 = R2 = H, R3 = NMe2
CHO
8
10
10
The limited reports for diethyl acetal formation,3b,c,4a–c,e,g,6a
encouraged us to explore the catalytic efficiency of
HClO4–SiO2 during the reaction of a few representative
aldehydes and ketones with TEOF (Table 2).
8 h
10
Although the reactions were monitored by IR, visual mon-
itoring was possible for reactions under neat conditions:
immediately after the addition of the catalyst to the mix-
ture of the aldehyde/ketone and TMOF/TEOF, an exo-
thermic reaction takes place and the reaction mixture
becomes homogeneous (for solid aldehydes) indicating
completion of acetal formation. The products were isolat-
ed from the reaction mixture following an easy (filtration)
and non-aqueous workup (experimental). The catalyst
was recovered and reused after reactivation.
15
A
15
80e
CHO
16
17
A
A
2
85
90
CHO
Ph
10
CHO
O
The requirement of the use of MeOH/EtOH as solvent
during the dimethyl/diethyl acetal formation in case of
benzaldehyde, cinnamaldehyde, acetophenone and alde-
hydes bearing substituents such as OR, F, CN, NMe2, etc.
was the effect of the less electrophilic property of the al-
dehyde/ketone due to resonance that inhibited the effec-
tive formation of coordinate bond with the catalyst. The
18
A
5
85f
Synthesis 2007, No. 2, 299–303 © Thieme Stuttgart · New York

PAPER
HClO4–SiO2 Catalyst System
301
long reaction time (8 h) required for 4-dimethylaminobenz- vantages such as (i) the use of a cheap and easy to handle
aldehyde (Table 1, entry 13) is indicative of the decrease catalyst, (ii) room-temperature and solvent-free reaction
of the electrophilicity of the carbonyl carbon due to reso- conditions, (iii) short reaction times, (iv) easy and non-
nance. However, it is also possible that substituents such aqueous work-up, (v) high yields, (vi) no additional effort
as OR, F, CN, NMe2, etc. may retard the overall reaction to purify the product, (vii) feasibility of large scale opera-
due to complex formation with the catalyst. Thus, al- tion, and (viii) catalyst reuse fulfill the requirement of the
though the presence of the CN group increases the electro- ‘triple bottom-line philosophy’19 of the green chemistry.
philicity of the carbonyl carbon similar to that of the NO2
Table 3 HClO4–SiO2-Catalyzed Deprotection of Acetalsa
group, the comparison of acetal formation of 4-nitrobenz-
aldehyde (Table 1, entry 3) with that of 4-cyanobenzalde-
hyde (Table 1, entry 11), highlighted the coordinating
Entry Acetal
R
Time Yield
(min) (%)b,c
effect of the CN group. The role of MeOH/EtOH may be
explained by the fact that initial reaction of the aldehyde
with MeOH/EtOH leads to the formation of a hemiacetal
which undergoes nucleophilic attack on TMOF/TEOF
complexed with HClO4–SiO2 and results in acetal forma-
tion.4c
OR
OR
R3
R2
R1
1
2
R1 = R2 = H, R3 = H
Me
Et
10
12
10
15
15
30
92
91
90
90
92
86
88
90
95
90
92
85
91
90
90
90
92
Table 2 HClO4–SiO2-Catalyzed Diethyl Acetal Formationa
R1 = R2 = H, R3 = H
Yield (%)b,c
3
R1 = R2 = H, R3 = F
Me
Me
Me
Me
Et
Entry
Aldehyde/Ketone
Time (min)
4
R1 = R2 = H, R3 = CN
R1 = R2 = H, R3 = Cl
R1 = R2 = H, R3 = NO2
R1 = R2 = H, R3 = NO2
R1 = Br, R2 = H, R3 = H
R1 = Cl, R2 = H, R3 = H
R1 = NO2, R2 = H, R3 = H
R1 = R3 = H, R2 = NO2
R1 = R2 = H, R3 = OCOPh
R1 = R2 = H, R3 = OBn
R1 = R2 = H, R3 = Me
R1 = R2 = H, R3 = Me
R1 = H, R2 = R3 = OMe
R
CHO
5
1
2
3
4
5
6
R = H
5
10
4
93d
95
6
R = Me
R = NO2
7
1 h
95
8
Me
Me
Me
Me
Me
Me
Me
Et
10
15
20
25
30
25
10
20
10
15
10
25
1 h
83
CHO
Ph
Ph
9
84d
82d
CHO
O
10
11
12
13
14
15
16
17
a The aldehyde/ketone (2.5 mmol) was treated with the TEOF (2.75
mmol, 1.1 equiv) in the presence of HClO4–SiO2 (0.5 mol%) at r.t.
(~25–30 °C).
b Isolated yield of the corresponding acetal.
c All products were characterized by IR and NMR spectroscopy.
d The reaction was carried out in anhyd EtOH (1 mL).
Me
R1 = H, R2 = OMe, R3 = O-c-C5H9 Me
To explore the potential of HClO4–SiO2 for regeneration
of the parent carbonyl compounds from the acetals, vari-
ous dimethyl/diethyl acetals were treated with MeOH/
RO
OR
EtOH–H2O (1:1) in the presence of HClO4–SiO2 (0.5 18
mol%) at room temperature and to our satisfaction the
deprotection took place after 10 min – 1 h in excellent
yields (Table 3). This methodology is advantageous to the
Me
Me
15
30
85
82
RO
OR
recently reported procedures3g,4d,18 because it avoids the
19
use of costly catalyst, stoichiometric amount of the cata-
lyst, high temperature/pressure etc. There are limited
examples3a,h,4d when the same catalyst serves the dual pur-
pose of protection and deprotection. The deprotection also
took place on using aqueous HClO4 but we preferred to
use HClO4–SiO2 for ease of handling.
OR
20
Me
Me
10
10
92
91
S
OR
In conclusion, we have described herein HClO4–SiO2 as
an extremely efficient and new catalyst for dual purpose
of acetal/ketal formation and their deprotection. The ad-
OR
OR
21
O
Synthesis 2007, No. 2, 299–303 © Thieme Stuttgart · New York

302
R. Kumar et al.
PAPER
Large Scale Formation Dimethyl Acetal under Solvent-Free
Table 3 HClO4–SiO2-Catalyzed Deprotection of Acetalsa
(continued)
Conditions; 4-Methylbenzaldehyde Dimethyl Acetal; Typical
Procedure (Method A)
Entry Acetal
R
Time Yield
(min) (%)b,c
To a magnetically stirred mixture of 4-methybenzaldehyde (1.2 g,
10 mmol) and TMOF (1.16 g, 11 mmol, 1.1 equiv) was added
HClO4–SiO2 (0.1 g, 0.05 mmol, 0.5 mol%) and the mixture was
stirred at 25–30 °C. After complete consumption of the aldehyde
(10 min, TLC), the mixture was diluted with EtOAc (5 mL), filtered
through a plug of cotton (to separate the catalyst), and the cotton
plug was washed with EtOAc (2 × 2.5 mL). The combined EtOAc
filtrates were concentrated on a rotary evaporator to afford 4-meth-
ylbenzaldehyde dimethyl acetal (1.49 g, 90%). The cotton plug re-
taining the recovered catalyst was put on a round-bottomed flask
(25 mL) and dried in a rotary evaporator whereupon the catalyst
separated out from the cotton (87 mg, 87%). The catalyst was acti-
vated on heating under reduced pressure (10 mm Hg) at 80 °C for
24 h. The repetition of the reaction of 4-methylbenzaldehyde (0.6 g,
5 mmol) with TMOF (0.58 g, 5.5 mmol, 1.1 equiv) in the presence
of the recovered HClO4–SiO2 (0.05 g, 0.025 mmol, 0.5 mol%) af-
forded 4-methylbenzaldehyde dimethyl acetal (0.75 g, 90%). Dur-
ing a larger-scale operation, the treatment of 4-methybenzaldehyde
(3 g, 25 mmol) with TMOF (2.9 g, 27.5 mmol, 1.1 equiv) in the
presence of HClO4–SiO2 (0.25 g, 0.125 mmol, 0.5 mol%) afforded
4-methylbenzaldehyde dimethyl acetal (3.8 g, 92%) and the recov-
ered catalyst (0.22 g, 90%) after the usual work-up mentioned
above.
OR
22
23
24
Me
Et
25
25
85
85
80
OR
RO
OR
OR
Me
1 h
25
Ph
26
Me
Et
8
15
20
83
86
81
OR
OR
27
Et
Ph
OR
OR
OR
28
29
Me
Me
10
5
88
80
OR
OR
Formation of Dimethyl Acetal in Anhydrous MeOH; Benzalde-
hyde Dimethyl Acetal; Typical Procedure (Method B)
a The dimethyl/ethyl acetal (2.5 mmol) was treated with MeOH–H2O/
EtOH–H2O (1:1, 2.5 mL) in the presence of HClO4–SiO2 (0.5 mol%)
at r.t. (~25–30 °C).
To a magnetically stirred mixture of benzaldehyde (0.267 g, 2.5
mmol) and TMOF (0.290 g, 2.75 mmol, 1.1 equiv) in anhyd MeOH
(1 mL) was added HClO4–SiO2 (25 mg, 0.012 mmol, 0.5 mol%).
The mixture was stirred at 25–30 °C until complete consumption of
the aldehyde (5 min, TLC). The mixture was diluted with EtOAc (2
mL), filtered through a plug of cotton (to separate the catalyst), and
the cotton plug was washed with EtOAc (2 × 1 mL). The combined
EtOAc filtrates were concentrated on a rotary evaporator to afford
benzaldehyde dimethyl acetal;6a yield: 0.35 g (91%).
b Isolated yield of the corresponding aldehyde/ketone.
c All products were characterized by IR and NMR spectroscopy.
The NMR spectra were recorded on 300 MHz spectrometer in
CDCl3 using TMS as internal standard. The IR spectra were record-
ed as KBr pellets for solid samples and neat for liquid samples. The
reactions were monitored by TLC (silica gel-G). Evaporation of sol-
vents was performed at reduced pressure, using a rotary evaporator.
Formation of Diethyl Acetal in Anhydrous EtOH; Benz-
aldehyde Diethyl Acetal; Typical Procedure
To a magnetically stirred mixture of benzaldehyde (0.267 g, 2.5
mmol, 1 equiv) and TEOF (0.407 g, 2.75 mmol, 1.1 equiv) in anhyd
EtOH (1 mL) was added HClO4–SiO2 (25 mg, 0.012 mmol, 0.5
mol%) and the mixture was stirred at 25–30 °C until completion
consumption of the aldehyde (5 min, TLC). The mixture was dilut-
ed with EtOAc (2 mL), filtered through a plug of cotton (to separate
the catalyst), and the cotton plug was washed with EtOAc (2 × 1
mL). The combined EtOAc filtrates were concentrated on a rotary
evaporator to afford benzaldehyde diethyl acetal (0.39 g, 93%)
identical (spectral data) with an authentic sample.6a
Perchloric Acid Adsorbed on Silica Gel (HClO4–SiO2)
The preparation of HClO4–SiO2 was carried out following the re-
ported procedure.17a To a suspension of silica gel (23.75 g, 230–400
mesh) in Et2O (50 mL), was added HClO4 (1.25 g, 12.5 mmol, 1.78
mL of a 70% aq solution of HClO4) and the mixture was stirred
magnetically for 30 min at r.t. The Et2O was removed under reduced
pressure (rotary evaporator) and the residue heated at 100 °C for 72
h under vacuum to afford HClO4–SiO2 (0.5 mmol g–1) as a free-
flowing powder.
Deprotection of Acetal; Benzaldehyde; Typical Procedure
To a magnetically stirred mixture of benzaldehyde dimethyl acetal
(0.38 g, 2.5 mmol) in MeOH–H2O (1:1, 2.5 mL) was added HClO4–
SiO2 (25 mg, 0.012 mmol, 0.5 mol%) and the mixture was stirred at
25–30 °C until complete consumption of the acetal (10 min, TLC).
The mixture was diluted with EtOAc (2 mL), filtered through a plug
of cotton (to separate the catalyst), and the cotton plug was washed
with EtOAc (2 × 1 mL). The combined EtOAc filtrates were dried
(Na2SO4), filtered and concentrated on a rotary evaporator to afford
benzaldehyde (0.243 g, 92%) identical with an authentic sample.4d
Dimethyl Acetal under Solvent-Free Conditions; 4-Methylbenz-
aldehyde Dimethyl Acetal; Typical Procedure (Method A)
To a magnetically stirred mixture of 4-methybenzaldehyde (0.3 g,
2.5 mmol) and TMOF (0.29 g, 2.75 mmol, 1.1 equiv) was added
HClO4–SiO2 (25 mg, 0.012 mmol, 0.5 mol%) and the mixture was
stirred at 25–30 °C until complete consumption of the aldehyde (10
min, TLC). The mixture was diluted with EtOAc (2 mL), filtered
through a plug of cotton (to separate the catalyst), and the cotton
plug was washed with EtOAc (2 × 1 mL). The combined EtOAc fil-
trates were concentrated on a rotary evaporator to afford 4-methyl-
benzaldehyde dimethyl acetal (0.39 g, 93%) identical (spectral data)
with an authentic sample.6a
Large-Scale Deprotection of Dimethyl Acetal; 4-Methylbenz-
aldehyde; Typical Procedure
To a magnetically stirred mixture of 4-methylbenzaldehyde dimeth-
yl acetal (1.66 g, 10 mmol) in MeOH–H2O (1:1, 10 mL) was added
Synthesis 2007, No. 2, 299–303 © Thieme Stuttgart · New York

PAPER
HClO4–SiO2 Catalyst System
303
HClO4–SiO2 (0.1 g, 0.05 mmol, 0.5 mol%) and the mixture was
stirred at 25–30 °C until complete consumption of the acetal (10
min, TLC). The mixture was diluted with EtOAc (10 mL), filtered
through a plug of cotton (to separate the catalyst), and the cotton
plug was washed with EtOAc (2 × 5 mL). The combined EtOAc fil-
trates were dried (Na2SO4) and concentrated on a rotary evaporator
to afford 4-methylbenzaldehyde; colorless oil; yield: 1.01 g (85%).
(10) Répichet, S.; Zwick, A.; Vendier, L.; Roux, C. L.; Dubac, J.
Tetrahedron Lett. 2002, 43, 993.
(11) Ishihara, K.; Karumi, Y.; Kubota, M.; Yamamoto, H. Synlett
1990, 839.
(12) Foropoulos, J.; DesMarteau, D. D. Inorg. Chem. 1984, 23,
3720.
(13) Chakraborti, A. K.; Shivani J. Org. Chem. 2006, 71, 5785;
and references cited therein.
The remaining reactions were carried out following these general
procedures. In each case, the spectral data (IR and NMR) of known
compounds were found to be identical with those reported in the lit-
erature.
(14) Ishihara, K.; Kubota, M.; Yamamoto, H. Synlett 1996, 265.
(15) (a) Garg, S. K.; Kumar, R.; Chakraborti, A. K. Tetrahedron
Lett. 2005, 46, 1721. (b) Garg, S. K.; Kumar, R.;
Chakraborti, A. K. Synlett 2005, 1370. (c) Chakraborti, A.
K.; Sharma, L.; Gulhane, R.; Shivani Tetrahedron 2003, 59,
7661. (d) Chakraborti, A. K.; Gulhane, R.; Shivani Synlett
2003, 1805. (e) Chakraborti, A. K.; Bhagat, S.; Rudrawar, S.
Tetrahedron Lett. 2004, 45, 7641. (f) Kumar, R.;
Thilagavathi, R.; Gulhane, R.; Chakraborti, A. K. J. Mol.
Catal. A: Chem. 2006, 250, 227. (g) Shivani; Chakraborti,
A. K. J. Mol. Catal. A: Chem. 2006, in press.
References
(1) Carey, J. S.; Laffan, D.; Thomson, C.; Williams, M. T. Org.
Biomol. Chem. 2006, 4, 2337.
(2) (a) Meskens, F. A. J. Synthesis 1981, 501. (b) Greene, T.
W.; Wuts, P. G. M. Protecting Groups in Organic Synthesis,
3rd ed.; Wiley: New York, 1999.
(3) (a) I2: Basu, M. K.; Samajdar, S.; Becker, F. F.; Banik, B. K.
Synlett 2002, 319. (b) COCl2: Velusamy, S.;
(16) Bartoli, G.; Bosco, M.; Dalpozzo, R.; Marcantoni, E.;
Massaccesi, M.; Rinaldi, S.; Sambri, L. Synlett 2003, 39.
(17) HClO4–SiO2 is not commercially available and was first
prepared and used by Chakraborti et al.: (a) Chakraborti, A.
K.; Gulhane, R. Chem. Commun. 2003, 1896.
Punniyamurthy, T. Tetrahedron Lett. 2004, 45, 4917. (c)
RuCl3: De S, K.; Gibbs, R. A. Tetrahedron Lett. 2004, 45,
8141. (d) SO3H–SiO2: Shimizu, K.; Hayashi, E.; Hatamachi,
T.; Kodama, T.; Kitayama, Y. Tetrahedron Lett. 2004, 45,
5138. (e) [Hmim]BF4: Wu, H.-H.; Yang, F. Y.; Cui, P.;
Tang, J.; He, M.-Y. Tetrahedron Lett. 2004, 45, 4963. (f)
CAN–Na2CO3: Nair, V.; Rajan, R.; Balagopal, L.; Nair, L.
G.; Ros, S.; Mohanan, K. Indian J. Chem., Sect. B: Org.
Chem. Incl. Med. Chem. 2005, 44, 141. (g) Clays and
zeolites: Thomas, B.; Prathapan, S.; Sugunan, S. Micropor.
Mesopor. Mat. 2005, 80, 65. (h) Polymer-supported acid
catalysts in an electroosmotic flow reactor: Wiles, C.; Watts,
P.; Haswell, S. J. Tetrahedron 2005, 61, 5209.
(b) Chakraborti, A. K.; Gulhane, R. Indian Patent 266/DEL,
2003. Other uses: (c) Agarwal, A.; Rani, S.; Vankar, Y. D. J.
Org. Chem. 2004, 69, 6137. (d) Misra, A. K.; Tiwari, P.;
Madhusudan, S. K. Carbohydr. Res. 2005, 340, 325.
(e) Mukhopadhyay, B.; Russell, D. A.; Field, R. A.
Carbohydr. Res. 2005, 340, 1075. (f) Khan, A. T.; Parvin,
T.; Choudhury, L. K. Synthesis 2006, 2497. (g) Das, B.;
Venkateswarlu, K.; Majhi, A.; Reddy, M. R.; Reddy, K. N.;
Rao, Y. K.; Ravikumar, K.; Sridhar, B. J. Mol. Catal. A:
Chem. 2006, 246, 276. (h) Khan, A. T.; Ghosh, S.;
Choudhury, L. H. Eur. J. Org. Chem. 2006, 2226.
(i) Rudrawar, S.; Besra, R. C.; Chakraborti, A. K. Synthesis
2006, 2767. (j) Chakraborti, A. K.; Sunay, V.;
(4) (a) ZrCl4: Firouzabadi, H.; Iranpoor, N.; Karimi, B. Synlett
1999, 321. (b) NBS: Karimi, B.; Seradj, H.; Ebrahimian, G.-
R. Synlett 1999, 1456. (c) TBATB: Gopinath, R.; Haque, S.
J.; Patel, B. K. J. Org. Chem. 2002, 67, 5842. (d) B10H14:
Lee, S. H.; Lee, J. H.; Yoon, C. M. Tetrahedron Lett. 2002,
43, 2699. (e) LiBF4: Hamada, N.; Kazahaya, K.; Shimizu,
H.; Sato, T. Synlett 2004, 1074. (f) InCl3: Ranu, B. C.; Jana,
R.; Samanta, S. Adv. Synth. Catal. 2004, 346, 446. (g)
Cu(BF4)2: Kumar, R.; Chakraborti, A. K. Tetrahedron Lett.
2005, 46, 8319.
Chankeshwara, S. V. Org. Biomol. Chem. 2006, 4, 2769.
(18) (a) I2 in acetone: Sun, J.; Dong, Y.; Cao, L.; Wang, X.;
Wang, S.; Hu, Y. J. Org. Chem. 2004, 69, 8932. (b) H2O at
453 K and 1.02 MPa: Sato, K.; Kishimoto, T.; Morimoto,
M.; Saimoto, H.; Shigemasa, Y. Tetrahedron Lett. 2003, 44,
8623. (c) Er(OTf)3: Dalpozzo, R.; De Nino, A.; Maiuolo, L.;
Nardi, M.; Procopio, A.; Tagarelli, A. Synthesis 2004, 296.
(d) CAN: Ates, A.; Gautier, A.; Leroy, B.; Plancher, J.-M.;
Quesnel, Y.; Vanherck, J.-C.; Markó, I. E. Tetrahedron
2003, 59, 8989. (e) b-CD: Krishnaveni, N. S.; Surendra, K.;
Reddy, M. A.; Nageswar, Y. V. D.; Rama Rao, K. J. Org.
Chem. 2003, 68, 2018. (f) Ce(OTf)3: Dalpozzo, R.; De
Nino, A.; Maiuolo, L.; Procopio, A.; Tagarelli, A.; Sindona,
G.; Bartoli, G. J. Org. Chem. 2002, 67, 9093. (g)
(5) Olah G. A., Prakash G. K. S.; Superacids; Wiley: New York,
1985;.
(6) (a) Bi(OTf)3: Leonard, N. M.; Oswald, M. C.; Freiberg, D.
A.; Nattier, B. A.; Smith, R. C.; Mohan, R. S. J. Org. Chem.
2002, 67, 5202. (b) TMSOTf-(CH2OTMS)2: Hwu, J. R.;
Leu, L.-C.; Robl, J. A.; Anderson, D. A.; John, M. W. J. Org.
Chem. 1987, 52, 188. (c) TMSOTf-(CH2OTMS)2: Hwu, J.
R.; John, M. W. J. Org. Chem. 1985, 50, 3946.
(7) Dumeunier, R.; Markó, I. E. Tetrahedron Lett. 2004, 45,
825.
(8) Orita, A.; Tanahashi, C.; Kakuda, A.; Otera, J. J. Org. Chem.
Peroxymonosulfate on alumina: Bose, D. S.; Jayalakshmi,
B.; Narsaiah, A. V. Synthesis 2000, 67.
(19) Tundo, P.; Anastas, P.; Black, D. S.; Breen, J.; Collins, T.;
Memoli, S.; Miyamoto, J.; Polyakoff, M.; Tumas, W. Pure
Appl. Chem. 2000, 72, 1207.
2001, 66, 8926.
(9) Qian, C.; Huang, T. J. Org. Chem. 1998, 63, 4125.
Synthesis 2007, No. 2, 299–303 © Thieme Stuttgart · New York