Efficient and rapid regioselective deprotection of isopropylidene ketals

Article abstract of DOI:10.1515/znb-2005-0909

A simple and efficient protocol is described for the regioselective hydrolysis of terminal isopropylidene ketal protection in carbohydrate derivatives 1a-11a. It uses either CoCl₂·2H₂O in acetonitrile or InCl₃ in methanol

Full text of DOI:10.1515/znb-2005-0909

Efficient and Rapid Regioselective Deprotection of Isopropylidene Ketals
Sakkarapalayam M. Mahalingam and Indrapal Singh Aidhen
Department of Chemistry, Indian Institute of Technology Madras, Chennai - 600 036, India
Reprint requests to Prof. Dr. I. S. Aidhen. Fax +91-44-22574202. E-mail: isingh@iitm.ac.in
Z. Naturforsch. 60b, 962 – 966 (2005); received March 2, 2005
A simple and efficient protocol is described for the regioselective hydrolysis of terminal isopropy-
lidene ketal protection in carbohydrate derivatives 1a – 11a. It uses either CoCl2 ·2H2O in acetonitrile
or InCl3 in methanol at temperatures ranging from 50 to 60 ◦C. The low cost of CoCl2 · 2H2O along
with its requirement in catalytic quantities offers a great advantage for the multi-gram scale reaction.
Key words: Isopropylidene Ketals, Regioselectivity, Carbohydrates, Deprotection, Hydrolysis
Isopropylidene ketals, commonly called acetonides, undesired degradation. This could be probably due
have been extensively used [1], particularly in the do- to higher acidic pH of the medium. At the first in-
main of carbohydrate chemistry for the protection of stance, this tempted the use of its congener in the
1,2- and 1,3-diols. In a multi-step synthetic sequence, same group, namely cadmium. However, the poor sol-
particularly with substrates having multiple acetonide ubility of CdSO4 · 8H2O in acetonitrile as compared
protections, one often needs a selective hydrolysis of to Zn(NO3)2 · 6H2O possibly precluded any reaction
one acetonide over the other. Towards this end both at room temperature. For promoting the solubility of
Brønsted and Lewis acid based reagents have been re- the cadmium salt, it appeared that aqueous condi-
ported wherein the less hindered terminal isopropy- tion would be promising. High miscibility of water in
lidene ketals have been selectively hydrolysed in the THF at low concentration drew our attention to the
presence of an internal one. In the Brønsted acid class, THF/H2O (9:1) system. However, once again no hy-
aq HCl [2a], aq HBr [2b], aq AcOH [2c], 0.8% H2SO4 drolysis occurred at room temperature even with 1
in MeOH [2d] and Dowex-H+ in MeOH:H2O (9:1) equivalent of CdSO4 ·8H2O in THF/H2O. Interestingly
[2e] have been the frequent choice of reagents. In a clean reaction ensued as the temperature was raised
◦
contrast to this, the use of Lewis acid based reagents to 50 C. The efficacy of CdSO4 · 8H2O in THF/H2O
has been scarce and limited to FeCl3 · 6H2O/SiO2 [3], was general as can be seen from the results in the Table,
CuCl2 · 2H2O in ethanol [4], CeCl3 · 7H2O [5] and but the known toxicity of cadmium justified a search
Zn(NO3)2 ·6H2O [6] or Yb(OTf)3 ·H2O [7] in acetoni- for a safer alternative.
trile. While using the Brønsted acids strict control of
the reaction parameters like pH of the medium and re-
action periods becomes extremely crucial for high re-
gioselectivity. Any negligence in reaction periods leads
to further hydrolysis and subsequent problems of pu-
rification and diminished yield. In case of Lewis acids
the situation is better. Besides the Lewis acidity of
the metal ions, apparent in situ hydrolysis of the salt
also provides the necessary protic medium for a valu-
able and useful synergistic effect at least in the case of
cupric [4] and zinc ions [6].
In this pursuit our attention was drawn to cobalt
chloride. Cobalt(II) chloride in particular has attracted
the attention of synthetic organic chemists in the last
few years [8] because of the fact that it essentially op-
erates under near neutral conditions. It has been re-
cently used to catalyze acylation [9], tosylation [10],
chemoselective acetal formation [11] and thioacetal-
ization of aldehydes [12]. Realizing the fact that mild-
ness of the reaction conditions is extremely impor-
tant during selective removal of terminal isopropyli-
dene protections particularly in substrates containing
At few occasions we found that long hours of stir- other functionalities and obtained through multi-step
ring of substrate 1a with Zn(NO3)2 · 6H2O in ace- synthetic sequence, we decided to explore the possible
tonitrile for regioselective hydrolysis also caused hy- use of cobalt(II) chloride. We found no precedence in
drolysis of the second ketal function and also some the literature towards such an attempt.
c
0932–0776 / 05 / 0900–0962 $ 06.00 ꢀ 2005 Verlag der Zeitschrift fu¨r Naturforschung, Tu¨bingen · http://znaturforsch.com
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S. M. Mahalingam – I. S. Aidhen · Efficient and Rapid Regioselective Deprotection of Isopropylidene Ketals
963
a
Table 1. Regioselective depro-
tection of isopropylidene ketals
using CdSO4 · 8H2O, CoCl2 ·
2H2O and anhydrous InCl3.
Compound
Substrate
1 – 11a
Product
1 – 11b
Hydrolysis Condition
Method Time
(h)
Yield
(%)b
A
B
8
6
80
91
1
2
3
4
5
6
7
C
10
85
A
B
10
8
76
89
C
12
85
A
B
6
8
76
91
C
10
80
A
B
8
6
75
86
C
12
80
A
B
7
6
78
93
C
12
82
A
B
8
6
78
90
C
10
82
A
B
10
8
78
92
C
12
80
A
B
10
8
75
82
8
9
C
12
78
A
B
8
6
76
90
C
12
80
A
B
8
7
75
90
10
C
12
78
A
B
10
7
75
88
11
C
12
76
a
Method A: CdSO4 ·8H2O in THF/H2O at 50 ◦C; Method B: CoCl2 ·2H2O in acetonitrile at 55 ◦C;
Method C: InCl3 in methanol at 60 ◦C; isolated yields of pure products with compatible IR,
b
1H NMR, 13C NMR and mass spectral data.
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964
S. M. Mahalingam – I. S. Aidhen · Efficient and Rapid Regioselective Deprotection of Isopropylidene Ketals
Commercially available magenta colored CoCl2 · of problematic situations encountered with the other
6H2O itself was found to be unsuitable for hydrolysis. known reagents in the literature.
Thermo-gravimetric analysis (TGA) of this sample re-
flected loss of the absorbed moisture along with four Experimental Section
molecules of H2O from the ◦crystal lattice when the
The starting substrates 2a [19], 3a [20], 4a [21] and 5a
[22] were prepared from 1a [23] as described in the litera-
ture. The aryl ketone 6a was prepared by an umpolung strat-
egy [24], wherein the acyl-anion equivalent reacts with di-
temperature was around 185 C. He◦ating of this com-
mercial hexahydrate sample at 200 C for 4 h consis-
tently afforded a blue colored salt whose composition
was CoCl2 · 2H2O. The confirmation to this compo-
isopropylidene protected arabinitol iodide [25]. Substrate 7a
sition comes from the powder XRD pattern [13]. We
were delighted to see that 10 mol% of this cobalt(II)
and 8a were prepared by sequential reduction and hydro-
genation of the corresponding p-fluorophenyl ketone 6a and
chloride dihydrate with respect to substrate in acetoni- phenyl ketone [26], respectively. Substrates 9a and 10a were
◦
prepared from D-fructose as described in the literature [27].
Silylation of 9a using TBDMSiCl/imidazole in DMF af-
forded 11a.
trile at 55 C was sufficient for a clean, rapid and ef-
ficient regioselective removal of terminal isopropyli-
dene ketal in all the substrates 1a – 11a (Table). A so-
lution of CoCl2 · 2H2O in acetonitrile (0.033 M) had
a pH of 4.4 and this itself was indicative of the mild-
ness of the protocol herein for selective removal of the
terminal acetonides, as compared to Zn(NO3)2 ·6H2O.
In the latter case the solution is more acidic, with pH
being 2.76. It was further noted that even anhydrous
CoCl2 in commercial grade acetonitrile was excellent
for the proposed objective. We speculate that the oc-
cluded moisture in the solvent was sufficient for the
observed clean removal of the terminal isopropylidene
protection.
The concept of transketalization is extremely useful,
when aqueous conditions are to be avoided. Towards
this end a combination of alcohol or thiol with anhy-
drous Brønsted acid such as PPTS, or TsOH have been
used [14, 15]. Thiols have also been used along with
Lewis acid such as BF3 · Et2O [16]. Although InCl3
has been used for the same purpose [17], its use for
deprotection of acetals in methanol is only a very re-
cent advent [18]. Our present study aiming at regiose-
lective removal of terminal acetonide prompted its ex-
ploration and to our satisfaction clean reaction ensued
with all the substrates 1a-11a using 10 mol% of InCl3
in methanol at 60 ◦C.
General experimental procedure using CdSO4 · 8H2O:
A mixture of isopropylidene ketals (1 – 11a) (1 mmol) and
CdSO4 · 8H2O (1.1 mmol, 0.846 g) in THF/H2O (9:1) was
stirred at 50 ◦C for 6 – 8 h. After completion of the reaction as
indicated by TLC, the THF was evaporated, the oily residue
was diluted with water and the crude product was extracted
by using CH2Cl2 (3 × 10 ml). The CH2Cl2 layer was dried
over anhydrous Na2SO4 and concentrated. The residue ob-
tained was subjected to a quick purification by a short filtra-
tion column on silica gel (100 – 200 mesh).
General experimental procedure using CoCl2 · 2H2O: A
mixture of isopropylidene ketals (1 – 11a) (1 mmol) and
10 mol% (0.016 g) of CoCl2 · 2H2O in acetonitrile (3 ml)
[0.33 M with respect to substrate] was stirred at 55 ◦C for 6 –
8 h. After completion of the reaction as indicated by TLC,
the solvent was evaporated, CH2Cl2 (3 × 10 ml) was added
to the oily residue and this solution was washed with water
(5 ml). The CH2Cl2 layer was dried over anhydrous Na2SO4
and concentrated. The residue obtained was subjected to a
quick purification by a short filtration column on silica gel
(100 – 200 mesh).
General experimental procedure using InCl3: To a mix-
ture of isopropylidene ketals (1 – 11a) (1 mmol) in methanol
was added 10 mol% (0.022 g) of anhydrous InCl3 and the
solution was heated at 60 ◦C for 10 – 12 h. After completion
of the reaction as indicated by TLC, the solvent was evapo-
rated, the oily residue was diluted with water, the crude prod-
In conclusion, the use CoCl2 · 2H2O and InCl3 has
offered a simple, efficient and clean procedure for re- uct was extracted by using CH2Cl2 (3×10 ml). The CH2Cl2
layer was dried over anhydrous Na2SO4 and concentrated.
The residue obtained was subjected to a quick purification
by a short filtration column on silica gel (100 – 200 mesh).
gioselective removal of terminal isopropylidene pro-
tection. The isolated yields are excellent and the re-
action is amenable to scale-up. The more attractive
among the two is CoCl2 · 2H2O, due to its extremely
low cost and requirement in catalytic quantities. This
is of great significance in the light of the fact that
Yb(OTf)3 · H2O recently used for the same purpose
is comparatively very expensive. These new protocols
Spectral data for the starting substrates 6a, 7a, 8a and 11a
6a: [α]D + 40.2◦ (c = 1, CHCl3). – IR (CHCl3):
1675 cm−1. – 1H NMR (400 MHz CDCl3): δ = 1.25 (s,
3H), 1.28 (s, 3H), 1.31 (s, 3H), 1.41 (s, 3H), 3.30 (d,
should therefore serve as valuable alternatives, in cases J = 8.2 Hz, 1H), 3.61 – 3.83 (m, 4H), 4.54 (dd, J = 8.4,
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S. M. Mahalingam – I. S. Aidhen · Efficient and Rapid Regioselective Deprotection of Isopropylidene Ketals
965
4.8 Hz, 1H), 7.0 – 7.1 (m, 2H), 7.91 – 7.99 (m, 2H). – 25.46, 41.27, 71.38, 74.07, 74.98, 78.94, 107.72, 113.92,
13C NMR (100 MHz, CDCl3): δ = 25.24, 25.31, 25.46, 129.45, 131.38, 165.54, 195.49. – HRMS (ESI): calcd. for
26.74, 41.27, 71.38, 74.07, 74.98, 78.94, 107.72, 109.61,
113.92, 129.45, 131.38, 165.54, 195.49. – HRMS (ESI):
calcd. for C18H23FO5 [M+Na]+: 361.1224; found 361.1216.
C
15H19FO5 [M+Na]+: 321.1224; found 321.1216.
7b: [α]D + 15.2◦ (c = 1, CHCl3). – IR (CHCl3):
3425 cm−1. – 1H NMR (400 MHz CDCl3): δ = 1.38
(s, 3H), 1.42 (s, 3H), 1.83 – 1.89 (m, 1H), 1.95 – 2.03 (m,
1H), 2.68 – 2.73 (m, 1H), 2.80 – 2.85 (m, 1H), 3.61 – 3.83
(m, 4H), 3.94 – 4.02 (m, 1H), 6.94 – 7.00 (m, 2H), 7.14 –
7.20 (m, 2H). – 13C NMR (100 MHz, CDCl3): δ = 27.03,
27.36, 31.47, 35.83, 63.82, 72.46, 78.38, 81.04, 108.92,
115.176, 129.779, 137.31, 160.05. – HRMS (ESI): calcd. for
C15H21FO4 [M+Na]+: 307.1431; found 307.1412.
7a: [α]D + 45.2◦ (c = 1, CHCl3). – H NMR (400 MHz
1
CDCl3): δ = 1.31 (s, 3H), 1.38 (s, 3H), 1.41 (s, 3H), 1.42
(s, 3H), 1.83 – 1.89 (m, 1H), 1.95 – 2.03 (m, 1H), 2.68 – 2.73
(m, 1H), 2.80 – 2.85 (m, 1H), 3.61 – 3.83 (m, 4H), 3.94 –
4.02 (m, 1H), 6.94 – 7.00 (m, 2H), 7.14 – 7.20 (m, 2H). –
13C NMR (100 MHz, CDCl3): δ = 25.46, 26.74, 27.03,
27.36, 31.47, 35.83, 63.82, 72.46, 78.38, 81.04, 107.70,
108.92, 115.176, 129.779, 137.31, 160.05. – HRMS (ESI):
calcd. for C18H25FO4 [M+Na]+: 347.1431; found 347.1412.
8b: [α]D − 27.3◦ (c = 1, CHCl3). – IR (CHCl3):
3424 cm−1. – 1H NMR (400 MHz CDCl3): δ = 1.32 (s, 3H),
1.36 (s, 3H), 1.79 – 1.82 (m, 1H), 1.93 – 2.03 (m, 1H), 2.64 –
2.73 (m, 1H), 2.79 – 2.85 (m, 1H), 3.60 – 3.70 (m, 4H), 3.92 –
4.02 (m, 1H), 7.09 – 7.23 (m, 5H). – 13C NMR (100 MHz,
CDCl3): δ = 27.06, 27.37, 32.30, 35.79, 63.80, 72.40, 78.48,
81.15, 108.92, 125.87, 128.37, 128.42, 141.76. – HRMS
(ESI): calcd. for C15H22O4 [M+Na]+: 289.1525; found
289.1053.
1
8a: [α]D − 27.3◦ (c = 1, CHCl3). – H NMR (400 MHz
CDCl3): δ = 1.25 (s, 3H), 1.28 (s, 3H), 1.32 (s, 3H), 1.36
(s, 3H), 1.79 – 1.82 (m, 1H), 1.93 – 2.03 (m, 1H), 2.64 – 2.73
(m, 1H), 2.79 – 2.85 (m, 1H), 3.60 – 3.70 (m, 4H), 3.92 –
4.02 (m, 1H), 7.09 – 7.23 (m, 5H). – 13C NMR (100 MHz,
CDCl3): δ = 25.31, 26.74, 27.06, 27.41, 32.30, 35.79, 63.80,
72.40, 78.48, 81.15, 108.90, 109.61, 125.87, 128.37, 128.42,
141.76. – HRMS (ESI): calcd. for C18H26O4 [M+Na]+
329.1525; found 329.1053.
9b: [α]D − 103.5◦ (c = 1, CHCl3). – IR (CHCl3):
3440 cm−1. – 1H NMR (400 MHz CDCl3): δ = 1.36 (s,
3H), 1.41 (s, 3H), 3.10 (br s, 1H), 3.46 (br s, 1H), 3.67 – 4.12
(m, 7H), 4.44 (br s, 1H). – 13C NMR (100 MHz, CDCl3):
δ = 26.23, 26.41, 64.15, 68.75, 69.34, 71.82, 71.89, 105.79,
111.95. – HRMS (ESI): calcd. for C9H16O6 [M+Na]+:
243.0954; found 243.0284.
11a: [α]D −126.5◦ (c = 1, CHCl3). – 1H NMR (400 MHz
CDCl3): δ = 1.25 (s, 3H), 1.28 (s, 3H), 1.32 (s, 3H), 0.86
(s, 3H), 0.90 (s, 3H), 1.67 (s, 9H), 2.09 (s, 3H), 2.16 (s,
3H), 3.11 (br s, 1H), 3.37 (br s, 1H), 4.30 – 5.10 (m, 7H). –
13C NMR (100 MHz, CDCl3): δ = −3.93, −3.08, 19.02,
25.31, 26.62, 26.74, 27.07, 27.46, 64.40, 70.59, 71.01, 72.36,
72.49, 106.80, 109.42, 112.55. – HRMS (ESI): calcd. for
C18H34O6Si [M+Na]+: 397.1819; found 397.1026.
10b: [α]D −35.30◦ (c = 1, CHCl3). – IR (CHCl3): 3392,
1744 cm−1. – 1H NMR (400 MHz CDCl3): δ = 1.32 (s, 3H),
1.47 (s, 3H), 3.10 (br s, 1H), 3.46 (br s, 1H), 3.75 – 4.81
(m, 6H), 4.44 (br s, 1H). – 13C NMR (100 MHz, CDCl3):
δ = 26.13, 26.28, 63.27, 69.50, 73.54, 74.16, 104.33, 113.52,
198.95. – HRMS (ESI): calcd. for C9H14O6 [M+Na]+:
241.0797; found 241.0147.
Data for the new compounds obtained after regioselective
deprotection
11b: [α]D −106.5◦ (c = 1, CHCl3). – 1H NMR (400 MHz
CDCl3): δ = 1.32 (s, 3H), 0.86 (s, 3H), 0.90 (s, 3H), 1.67 (s,
9H), 2.09 (s, 3H), 2.16 (s, 3H), 3.11 (br s, 1H), 3.37 (br s,
1H), 4.30 – 5.10 (m, 7H). – 13C NMR (100 MHz, CDCl3):
δ = −3.93, −3.08, 19.02, 26.62, 27.07, 27.46, 64.40, 70.59,
71.01, 72.36, 72.49, 106.80, 112.55. – HRMS (ESI): calcd.
for C15H30O6Si [M+Na]+: 357.1819; found 357.1026.
5b: [α]D + 57.3◦ (c = 1, CHCl3). – IR (CHCl3): 3456,
1737 cm−1. – 1H NMR (400 MHz CDCl3): δ = 1.31 (s,
3H), 1.33 (s, 3H), 2.58 (dd, J = 15.6, 8.8 Hz, 1H), 2.77
(dd, J = 15.9, 3.25 Hz, 1H), 3.59 (t, J = 7.8 Hz, 1H), 3.72
(s, 3H), 3.95 (dd, J = 8.8, 4.9 Hz, 1H), 4.34 (m, 1H), 4.53
(dd, J = 8.5, 6.1 Hz, 1H), 4.70 (td, J = 8.3, 2.9 Hz, 1H). –
13C NMR (100 MHz, CDCl3): δ = 26.96, 29.07, 38.97,
52.00, 63.92, 73.04, 76.10, 79.54, 172.0. – HRMS (ESI):
calcd. for C9H18O6 [M+Na]+: 245.1000; found 245.1012.
Acknowledgements
6b: [α]D + 12.2◦ (c = 1, CHCl3). – IR (CHCl3): 3456,
We thank DST New Delhi for the funding towards a
1676 cm−1. – 1H NMR (400 MHz CDCl3): δ = 1.28 (s, 400 MHz NMR instrument for the Department of Chemistry,
3H), 1.31 (s, 3H), 3.30 (d, J = 8.2 Hz, 1H), 3.61 – 3.83 (m, IIT-Madras under the IRPHA scheme. We are extremely
4H), 4.54 (dd, J = 8.4, 4.8 Hz, 1H), 7.0 – 7.1 (m, 2H), 7.91 – grateful to Professor Varadaraju U. V. for his constant sup-
7.99 (m, 2H). – 13C NMR (100 MHz, CDCl3): δ = 25.24, port during the XRD studies.
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S. M. Mahalingam – I. S. Aidhen · Efficient and Rapid Regioselective Deprotection of Isopropylidene Ketals
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