Regioselective mono-deprotection of di-ferf-butylsilylene acetal derived from 1,3-diol with ammonium fluoride

Article abstract of DOI:10.1246/bcsj.20130237

Here we report a novel and efficient method for the regioselective mono-deprotection of di-terf-butylsilylene acetals derived from 1,3-diols consisting of primary and secondary alcohols. The ammonium fluoride-mediated reactions of pyripyropene A derivative, thymidine and uridine derivatives, methyl β-D-glucofuranoside, and pyranoside derivatives each gave the corresponding primary alcohol with high regioselectivity.

Full text of DOI:10.1246/bcsj.20130237

© 2014 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 87, No. 1, 113-118 (2014)
113
Regioselective Mono-Deprotection of Di-tert-butylsilylene
Acetal Derived from 1,3-Diol with Ammonium Fluoride
Masaki Ohtawa, Hiroshi Tomoda, and Tohru Nagamitsu*
Graduate School of Pharmaceutical Sciences, Kitasato University, 5-9-1 Shirokane, Minato-ku, Tokyo 108-8641
Received August 26, 2013; E-mail: nagamitsut@pharm.kitasato-u.ac.jp
Here we report a novel and efficient method for the regioselective mono-deprotection of di-tert-butylsilylene acetals
derived from 1,3-diols consisting of primary and secondary alcohols. The ammonium fluoride-mediated reactions of
pyripyropene A derivative, thymidine and uridine derivatives, methyl β-D-glucofuranoside, and pyranoside derivatives
each gave the corresponding primary alcohol with high regioselectivity.
Selective mono-silylation of the more hindered hydroxy
group of 1,3-diols is frequently required in organic synthesis,
but cannot be achieved by direct methods and generally requires
a two-step sequence, i.e., disilylation of a 1,3-diol followed by
selective mono-deprotection at the less hindered site,1,2 or
regioselective mono-deprotection of a di-tert-butylsilylene
acetal derived from a 1,3-diol. Although the use of a silylene
acetal in the latter case sometimes has the advantage of allow-
ing selective protection of a 1,2- or 1,3-diol unit in a polyol
compound,3 the regioselective mono-deprotection usually needs
strong acids (BF3¢OEt2 or BF3¢SMe2;4a,4b Scheme 1, eq 1)
or bases (n-BuLi4c or t-BuLi;4d Scheme 1, eq 2) as cleavage
reagents. The latter procedure is therefore problematic for acid-
or base-labile substrates. In the present paper, we describe a
novel method for the regioselective mono-deprotection of di-
tert-butylsilylene acetals derived from 1,3-diols consisting of
primary and secondary alcohols.
We found that the 7-p-cyanobenzoyl PPPA derivative 2 has
higher ACAT2 inhibitory activity than 1 (Figure 1).
We next focused on the synthesis of new 7-p-cyanobenzoyl
PPPA derivatives with chemical modifications at the 1- and 11-
positions, for further SAR studies. It was easy to synthesize
derivatives with the same acyl groups at the 1- and 11-positions,
but many attempts to prepare new derivatives with different acyl
groups at the 1- and 11-positions were unsuccessful because
O
O
N
O
H
O
N
13
HO
HO
H
O
O
O
O
O
O
O
O
H
1
7
O
O
H
11
O
O
CN
O
O
Results and Discussion
2
PPPA (1)
In recent years, structure-activity relationship (SAR) studies
of pyripyropene A (PPPA, 1), a potent and selective inhibitor
toward acyl-CoA:cholesterol acyltransferase 2 (ACAT2) and
a promising candidate for new cholesterol-lowering or anti-
atherosclerotic agents, have been in progress in our laboratory.5
IC50 /µM
IC50 /µM
0.07
0.0009
ACAT2
ACAT2
Figure 1. Structures of PPPA (1) and 7-p-cyanobenzoyl
PPPA derivative 2.
O
O
a
O
t-Bu Si
t-Bu
HO
(eq.1)
O
O
O
O
O
O
t-Bu SiF
t-Bu
n-Bu
t-Bu
Si
t-Bu
t-Bu
Si
O
OH
O
O
t-Bu
b
O
(eq.2)
BnO
t-Bu
t-Bu
BnO
BnO
OH
+
Si
n-Bu
5:95
Scheme 1. Known procedures for regioselective mono-deprotection of di-tert-butylsilylene acetal. Reagents and conditions:
(a) allyltrimethylsilane, BF3¢OEt2, toluene, 85 °C, 95%; (b) n-BuLi, N,N,N¤,N¤-tetramethylethylenediamine, ¹78 °C, 83%.
Published on the web January 15, 2014; doi:10.1246/bcsj.20130237

114
Bull. Chem. Soc. Jpn. Vol. 87, No. 1 (2014)
Mono-Deprotection of Silylene Acetal
O
O
N
O
H
O
N
13
HO
HO
H
O
O
O
a-c
O
H
1
7
AcO
OAc
O
t-Bu Si
t-Bu
H
11
AcO
O
CN
3
PPPA (1)
Scheme 2. Synthesis of di-tert-butylsilylene acetal-protected PPPA derivative 3. Reagents and conditions: (a) NaOMe, aq. MeOH,
rt, quantitative; (b) (t-Bu)2Si(OTf)2, 2,6-lutidine, DMF, 0 °C, quantitative; (c) p-cyanobenzoic acid, 1-ethyl-3-(3-dimethylamino-
propyl)carbodiimide, catalyst N,N-dimethylpyridin-4-amine, CH2Cl2, rt, 81%.
Table 1. Optimization of Reaction Conditions for Regioselective Mono-Deprotection of Di-tert-butylsilylene Acetal-
Protected PPPA Derivative 3
O
O
N
O
O
N
HO
13
HO
O
O
O
O
O
O
H
H
1
R1O
O
H
H
t-Bu
OR2
CN
Si
11
O
CN
t-Bu
4 : R1 = SiF(t-Bu)2, R2 = H (desired)
5 : R1 = H, R2 = SiF(t-Bu)2
6 : R1 = H, R2 = H
3
Yield/%
Entry
Conditionsa)
Recovered
4
5
6
3
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
TBAF (2.5 equiv), AcOH (2.5 equiv), THF, 10 min
TBAF (1.0 equiv), AcOH (1.0 equiv), THF, 2 h
Et3N¢3HF (1 equiv), THF, 10 min
Et3N¢3HF (0.3 equiv), THF, 10 min
CsF (10 equiv), 10% aq. MeCN, 24 h
KF (10 equiv), 10% aq. THF, 24 h
NH4F (l equiv), MeOH, 5 h
NH4F (5 equiv), MeOH, 5 h
NH4F (10 equiv), MeOH, 3 h
NH4F (10 equiv), DMF, 3 h
NH4F (10 equiv), MeOH/DMF (1:1), 3 h
NH4F (10 equiv), DMSO, 18 h
NH4F (10 equiv), Et2O, 24 h
NH4F (10 equiv), THF, 24 h
NH4F (10 equiv), MeCN, 8 h
®
traceb)
®
®
®
®
®
®
®
1
quant.
41
quant.
41
®
®
47
®
44
89
92
49
3
®
®
®
®
95
75
®
®
®
®
®
18
50
81
83
80
15
®
®
2
21
11
®
®
8
5
12
6
®
®
®
7
62
®
10
63
75
9
21
9
NH4F (10 equiv), acetone, 6 h
10
a) All reactions were carried out at room temperature. b) Detected by TLC and ESI-MS.
regioselective protection or acylation was difficult. Finally, the
regioselective mono-deprotection of di-tert-butylsilylene acetal-
protected PPPA derivative 3,5b which was synthesized from
1 using silylene acetal, as described above (Scheme 2), was
investigated (Table 1).
as de-esterification and dehydration of the 13-hydroxy group
under all conditions proceeded, without giving the desired
corresponding primary alcohols. Therefore, we next embarked
on the development of a novel method using a fluoride anion.
Generally, the fluoride anion, with a high affinity toward sili-
con, is useful for desilylation reactions in organic chemistry.
However, regioselective mono-deprotection of dialkylsilylene
acetals with the fluoride anion seems to be difficult and has
First, the di-tert-butylsilylene acetal 3 was subjected to basic
4b
conditions (n-BuLi4c) and acidic conditions (BF3¢SMe2
)
reported previously. Consequently, only side reactions such

M. Ohtawa et al.
Bull. Chem. Soc. Jpn. Vol. 87, No. 1 (2014)
115
yield, accompanied by a small amount of undesired secondary
alcohol 9a and diol 10a (Entry 1). The reactions of uridine
derivative 7b,12 which is a ribonucleoside, and methyl β-D-
glucofuranoside 7c12 with NH4F in MeOH at 0 °C also
furnished the primary alcohols 8b and 8c in yields of 83%
and 86%, respectively, with trace amounts of diols 10b and
10c13 (Entries 2 and 3). The number of equivalents of NH4F in
Entries 1-3 was optimized to afford the corresponding primary
alcohols in high yields.14 In contrast, the reactions of methyl α-
D-glucopyranoside 7d12 and methyl β-D-galactopyranoside 7e15
with NH4F (10 equiv) in MeOH at room temperature afforded
the desired primary alcohols 8d and 8e in moderate yields, with
about 40% of recovered starting materials16 (Entries 4 and 6).
The starting materials were not consumed completely because
the mono-deprotected 8d and 8e could be recyclized under
the reaction conditions to provide the corresponding starting
materials 7d and 7e with high thermodynamic stability.17 To try
to prevent recyclization, Dess-Martin periodinane (DMP) was
added to the reaction mixtures to oxidize the resulting primary
alcohols to the corresponding aldehydes (Entries 5 and 7).
DMF was used as the solvent instead of MeOH. The corre-
sponding aldehyde products were not obtained, but, surpris-
ingly, the yields of the desired 8d and 8e increased, and only
small amounts of the starting materials 7d and 7e, respectively,
were recovered. It seems likely that temporary protection of
the resulting hydroxy group with an active species originating
from hypervalent iodine prevented recyclization; mechanistic
studies of the effect of DMP in the NH4F-mediated regiose-
lective mono-deprotection reaction are currently in progress
in our laboratory. In contrast, the reaction of 3-deoxyglucose
derivative 7f12 with NH4F (10 equiv) in MeOH at room tem-
perature for 0.5 h gave the desired 8f in 39% yield, accom-
panied by diol 10f in 14% yield and recovered starting material
in 42% yield (Entry 8). Prolonging the reaction time increased
the yield of the corresponding diol 10f and decreased the yields
of the desired 8f and recovered starting material 7f (Entry 9).
Moreover, the reactions of di-tert-butylsilylene acetals 7g4b and
7h,12 derived from simpler 1,3-diols, with NH4F (10 equiv)
in MeOH or DMF at room temperature afforded only trace
amounts of the desired 8g and 8h, regardless of the reaction
time (Entries 10-13), and the yields of diols 10g and 10h
and recovered starting materials 7g and 7h followed the same
trends as those shown by 10f and 7f. These results indicate that
moderate steric bulkiness around the secondary hydroxy group
is important for avoiding over-deprotection of the desired
primary alcohol with the fluorosilyl ether.
O
H
O
N
HO
H
O
O
O
O
t-Bu
t-Bu
Si
CN
O
F–
+
NH4
Figure 2. Proposed transition-state in regioselective mono-
deprotection of di-tert-butylsilylene acetal.
not been reported so far.6 We expected that hydrogen-bond
formation by the fluoride anion in the reaction would reduce its
nucleophilicity to silicon and result in regioselective mono-
deprotection. Initially, tetra-n-butylammonium fluoride, the
most typical desilylating reagent, in the presence of acetic acid
was examined (Entries 1 and 2). Consequently, triol 6 was
obtained as the major product, without the desired primary
alcohol 4. Furthermore, treatment with Et3N¢3HF7 also gave
the same results (Entries 3 and 4). The use of inorganic fluoride
reagents such as CsF8 and KF9 in aqueous solution led to no
reaction (Entries 5 and 6). Subsequent treatment with NH4F
(1 equiv) in MeOH afforded the desired 4 (Entry 7), but in poor
yield (18%), with recovered 3 (49%). Therefore, optimization
of the reaction conditions using NH4F was conducted. An
increase in the number of equivalents of NH4F improved
the yield of the desired 4, and the best yields were obtained
by treatment with 10 equiv of NH4F (Entries 8 and 9). Next,
various solvents were investigated in the regioselective mono-
deprotection of 3. The use of dimethylformamide (DMF) and a
1:1 mixture of MeOH and DMF instead of MeOH gave almost
the same results (Entries 10 and 11). In contrast, the reaction
in dimethyl sulfoxide (DMSO) gave only a low yield of 4,
with a large amount of the undesired 1,3-diol 6 (Entry 12).
Moreover, the use of ethers such as tetrahydrofuran (THF) and
Et2O led to no reaction and a very low yield (10%) of 4 with
recovered 3 (75%) (Entries 13 and 14). Although the reactions
in MeCN and acetone afforded the desired 4 in good yield
regioselectively, the reactions proceeded a little more slowly
than those in MeOH and DMF. As a result, MeOH and DMF
proved to be the most suitable solvents for the regioselective
mono-deprotection of di-tert-butylsilylene acetal.
The regioselectivity observed in these reactions probably
comes from kinetically controlled ring cleavage, in which the
ammonium ion is located beside the more sterically accessible
oxygen through Coulombic forces, prior to delivery of fluoride,
as shown in Figure 2. In addition, reduction of the nucleophi-
licity of the fluoride anion toward silicon, caused by formation
of strong hydrogen-bonds with ammonium ions in the reac-
tion,10 will contribute to the regioselective mono-deprotection.
We next investigated the scope of this reaction (Table 2).
First, the regioselective mono-deprotection reactions of di-tert-
butylsilylene acetal-protected D-glucofuranosides with NH4F
were examined. The thymidine derivative 7a,11 which is a
deoxyribonucleoside, was treated with NH4F in MeOH at
0 °C, and provided the desired primary alcohol 8a in 81%
Next, we focused on the regioselective mono-deprotection of
other types of di-tert-butylsilylene acetals (Table 3). Although
treatment of di-tert-butylsilylene acetal 7i,12 consisting of
primary and tertiary alcohols, with NH4F (10 equiv) in MeOH
at room temperature provided the desired primary alcohol 8i,
the reaction proceeded very slowly and gave 8i in only low
yield (35%), accompanied by unreacted 7i (31%) and the diol
10i (30%) (Entry 1). Formation of the primary alcohol 8i, i.e.,
the di-tert-butylfluorosilyl ether of the tertiary alcohol, would
probably be difficult because of its steric bulkiness. In contrast,
the reaction of di-tert-butylsilylene acetal 7j, derived from a
1,4-diol with NH4F (10 equiv) in MeOH at room temperature,
led to no reaction (Entry 2), even though the reaction of di-tert-

116
Bull. Chem. Soc. Jpn. Vol. 87, No. 1 (2014)
Mono-Deprotection of Silylene Acetal
Table 2. Investigation of Scope of Regioselective Mono-Deprotection of Di-tert-butylsilylene Acetals
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
Si
Si
Si
O
O
O
OH
OH
O
F
OH OH
F
7a-h
8a-h
9a-h
10a-h
t-Bu
Si
t-Bu
t-Bu
Si
O
t-Bu
O
O
O
O
R3
R1
R2
O
AcO
OMe
O
7d R3 = OAc
7f R3 = H
7g
t-Bu Si
t-Bu
O
t-Bu
Si
7a R1 = thymine, R2 = H
7b R1 = uracil, R2 = OAc
7c R1 = OMe, R2 = OAc
t-Bu
t-Bu
t-Bu
O
Si
O
O
O
O
OMe
AcO
AcO
7e
7h
Yield/%
Entry
7
Conditions
8a-8h
9a-9h
10a-10h
7a-7h
1
2
3
4
5
6
7
8
9
10
11
12
13
a
b
c
d
d
e
e
f
NH4F (5 equiv), MeOH, 0 °C, 3 h
NH4F (7 equiv), MeOH, 0 °C, 2 h
NH4F (5 equiv), MeOH, 0 °C, 3.5 h
NH4F (10 equiv), MeOH, rt, 6 h
NH4F (10 equiv), DMP (2 equiv), DMF, rt, 2 h
NH4F (10 equiv), MeOH, rt, 6 h
NH4F (10 equiv), DMP (2 equiv), DMF, rt, 2 h
NH4F (10 equiv), MeOH, rt, 0.5 h
NH4F (10 equiv), MeOH, rt, 1 h
NH4F (10 equiv), DMF, rt, 2 h
NH4F (10 equiv), DMF, rt, 24 h
NH4F (10 equiv), MeOH, rt, 2 h
NH4F (10 equiv), MeOH, rt, 20 h
81
83
86
58
85
54
82
39
28
trace
trace
5
4
13
trace
7
®
®
®
®
14
32
9
®
®
®
37
7
®
®
®
®
®
®
®
®
®
®
®
®
44
9
42
18
82
33
77
35
f
g
g
h
h
45
4
51
7
butylsilylene acetal 7k, derived from a 1,3-diol, provided
the desired primary alcohol 8k in modest yield (Entry 3).
Furthermore, the reaction of di-tert-butylsilylene acetal 7l,
derived from a simple 1,2-diol, with NH4F (10 equiv) in MeOH
at room temperature gave rise to the undesired diol 10l in
high yield, as expected (Entry 4); the reason could be the same
as that used to explain the reactions of 7f, 7g, and 7h. These
results indicate that suitable substrates for NH4F-mediated
regioselective mono-deprotection reactions are di-tert-butyl-
silylene acetals derived from 1,3-diols consisting of a primary
alcohol and a secondary alcohol with moderate steric bulkiness.
This is one of the limitations of this reaction.
Although they have some limitations, NH4F-mediated
regioselective mono-deprotection reactions of suitable di-tert-
butylsilylene acetals are very useful for preparation of the
corresponding primary alcohols bearing fluorosilyl ethers under
mild conditions. Furthermore, the primary alcohols are rela-
tively stable, except under basic conditions,4c and are useful
compounds for further chemical transformations. Pagenkopf
reported that the simple fluorosilane derivatives are stable
under weakly acidic conditions and in several transformations,
including Jones oxidation, PCC oxidation, and the Finkelstein
reaction.4b In our previous SAR studies, the syntheses of
various new PPPA derivatives were achieved by chemical
modifications of primary alcohol 4.5b
To investigate further applications of the NH4F-mediated
regioselective mono-deprotection reaction, chemical transfor-
mations of the sugar 8d, which is one of the products easily
prepared by the reaction, were attempted in various reactions
such as glycosylation, phenyl sulfenylation by an SN2 reaction,
and Dess-Martin oxidation (Scheme 3). The glycosylation of
8d as an acceptor with donor 11 in the presence of AgOTf6 as
an activator proceeded smoothly to give the desired disaccha-
ride 12 in 87% yield (β only). Phenyl sulfenylation of 8d
was also achieved by treatment with diphenyl disulfide and
tributylphosphine,18 producing 13 in 90% yield. Finally, 8d
was converted to aldehyde 14 in 72% yield by Dess-Martin
oxidation.19

M. Ohtawa et al.
Bull. Chem. Soc. Jpn. Vol. 87, No. 1 (2014)
117
Table 3. Investigation of Scope of Regioselective Mono-Deprotection of Di-tert-butylsilylene Acetals
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
Si
Si
Si
O
O
OH OH
O
OH
OH
O
F
F
7i
8i
9i
10i
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
Si
Si
Si
O
O
O
OH
F
OH
O
F
OH OH
n
n
n
n
R
R
R
R
7j R = isopropyl
n = 2
7k R = isopropenyl
n = 1
8j R = isopropyl
n = 2
8k R = isopropenyl
n = 1
9j R = isopropyl
10j R = isopropyl
n = 2
10k R = isopropenyl
n = 1
n = 2
9k R = isopropenyl
n = 1
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
Si
Si
O
t-Bu
O
F
OH
OH
Si
O
OH
F
O
OH
7l
8l
9l
10l
Yield/%
Entry
7
Conditions
8i-8l
9i-9l
10i-10l
7i-7l
1
2
3
4
i
j
k
l
NH4F (10 equiv), MeOH, rt, 20 h
NH4F (10 equiv), MeOH, rt, 24 h
NH4F (10 equiv), MeOH, rt, 4 h
NH4F (10 equiv), DMF, rt, 14 h
35
®
®
®
®
®
30
5
trace
88
31
91
35
®
56
trace
OBz
OBz
O
BzO
BzO
O
BzO
BzO
O
SPh
OH
BzO
11
F
t-Bu Si
t-Bu
F
F
BzO
O
AcO
Br
O
O
O
PhSSPh, Bu3P
AgOTf, MS4Å
O
AcO
t-Bu Si
t-Bu
O
AcO
t-Bu Si
t-Bu
toluene, rt
90%
CH2Cl2, rt
87%
AcO
AcO
AcO
OMe
OMe
OMe
13
8d
12
DMP, CH2Cl2, rt
72%
F
OHC
O
t-Bu Si
t-Bu
O
AcO
AcO
OMe
14
Scheme 3. Chemical transformations of primary alcohol 8d obtained by regioselective mono-deprotection of di-tert-butylsilylene
acetal.
ing primary alcohol bearing a fluorosilyl ether. Investigations
of further applications of this reaction using other di-tert-
Conclusion
We have developed a novel and efficient method for the
regioselective mono-deprotection of di-tert-butylsilylene ace-
tals derived from 1,3-diols consisting of a primary alcohol
and a secondary alcohol with moderate steric bulkiness. This
NH4F-mediated reaction allows easy access to the correspond-
butylsilylene acetals are in progress in our laboratory.
This research was partly supported by a Grant-in-Aid for
Young Scientists from the ministry of Education, Culture,
Sports, Science and Technology (MEXT) of Japan (MO). We

118
Bull. Chem. Soc. Jpn. Vol. 87, No. 1 (2014)
Mono-Deprotection of Silylene Acetal
isopropyl-1,3-disiloxane-1,3-diyl-protected pyranoside by treat-
ment with HF¢pyridine was reported. T. Ziegler, R. Dattmann,
M. Duszenko, Carbohydr. Res. 2000, 327, 367.
thank Ms. N. Sato and Dr. K. Nagai (Kitasato University) for
kindly measuring NMR and MS spectra.
Supporting Information
7
M. C. Pirrung, S. W. Shuey, D. C. Lever, L. Fallon, Bioorg.
Med. Chem. Lett. 1994, 4, 1345.
The experimental procedure and spectroscopic data of
newly synthesized compounds are included in the Supporting
Information. This material is available free of charge on the
web at http://www.csj.jp/journals/bcsj/.
8
9
P. F. Cirillo, J. S. Panek, J. Org. Chem. 1990, 55, 6071.
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5
a) M. Ohtawa, H. Yamazaki, S. Ohte, D. Matsuda, T.
17 The retreatment of compound 8d with NH4F (10 equiv) in
MeOH at room temperature gave the recyclized acetal 7d in 19%
yield, accompanied by diol 10d (44%) and starting material (23%).
Recyclization of 8d and 8e in the regioselective mono-deprotection
reaction of di-tert-butylsilylene acetals 7d and 7e could therefore
also proceed.
Ohshiro, L. L. Rudel, S. Ōmura, H. Tomoda, T. Nagamitsu,
Bioorg. Med. Chem. Lett. 2013, 23, 1285. b) M. Ohtawa, H.
Yamazaki, D. Matsuda, T. Ohshiro, L. L. Rudel, S. Ōmura, H.
Tomoda, T. Nagamitsu, Bioorg. Med. Chem. Lett. 2013, 23, 2659.
c) M. Ohtawa, H. Yamazaki, S. Ohte, D. Matsuda, T. Ohshiro,
L. L. Rudel, S. Ōmura, H. Tomoda, T. Nagamitsu, Bioorg. Med.
Chem. Lett. 2013, 23, 3798.
18 F. S. González, H. H. Baer, Carbohydr. Res. 1990, 202, 33.
19 D. B. Dess, J. C. Martin, J. Org. Chem. 1983, 48, 4155.
6
The regioselective mono-deprotection reaction of 4,6-tetra-