Chemoselective Two-Directional Reaction of Bifunctionalized Substrates: Formal Ketal-Selective Mukaiyama Aldol Type Reaction

Article abstract of DOI:10.1055/s-0035-1560478

In the presence of an acidic zwitterion bearing a highly stabilized carbanion, reactions of ω,ω-dialkoxy carbonyl compounds with ketene silyl acetals (KSA) resulted in an unusual molecular transformation; substitution reaction with the KSA at the ketal mo

Full text of DOI:10.1055/s-0035-1560478

SYNLETT
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© Georg Thieme Verlag Stuttgart · New York
2015, 26, 2457–2461
letter
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H. Yanai et al.
Letter
Synlett
Chemoselective Two-Directional Reaction of Bifunctionalized Sub-
strates: Formal Ketal-Selective Mukaiyama Aldol Type Reaction
Hikaru Yanai*
OTBS
Yuichi Sasaki
OEt
TBSO OMe
O
Yuki Yamamoto
Takashi Matsumoto*
R2
zwitterion A (1 mol%)
R2
R1
R1
Et2O, 0 °C
OMe
MeO OMe
EtO2C
School of Pharmacy, Tokyo University of Pharmacy and Life
Sciences, 1432-1 Horinouchi, Hachioji, Tokyo 192-0392, Japan
yanai@toyaku.ac.jp
R1 = alkyl, aryl, or H
R2 = Me, Ph
H
H3O+
tmatsumo@toyaku.ac.jp
N
O
Me
R2
Tf2C
R1
Me
OMe
EtO2C
zwitterion A
Received: 12.07.2015
R
I
I
Accepted after revision: 12.08.2015
Published online: 09.09.2015
DOI: 10.1055/s-0035-1560478; Art ID: st-2015-u0537-l
I(coll)2ClO4
ROH
O
RO
H
O
Ph
O
O
O
O
CH2Cl2
O
Ph
Ph
Abstract In the presence of an acidic zwitterion bearing a highly stabi-
lized carbanion, reactions of ω,ω-dialkoxy carbonyl compounds with ke-
tene silyl acetals (KSA) resulted in an unusual molecular transformation;
substitution reaction with the KSA at the ketal moiety and simultane-
ous silylative acetalization of the ketone moiety.
Ph
Ph
Ph
Scheme 1 Reaction of alkenyl acetals with iodonium sources
Key words acid catalysis, zwitterion, carbanion, chemoselectivity,
Mukaiyama aldol reaction
instead of the alkene structure, using carbon nucleophiles
(Scheme 2).
LA
O
R3SiO
Chemoselective molecular transformations of multi-
functionalized substrates bearing two or more reactive
groups are key methodologies for organic synthesis. From
the viewpoints of atom and step economies, the numbers of
protection/deprotection steps should be as small as possi-
ble in chemical synthesis.1 Such requirement in organic
synthesis encourages highly chemoselective reactions. An
alternative strategy in order to increase the efficiency of
synthesis would be the two-directional synthesis using
symmetric or pseudosymmetric bifunctionalized sub-
strates.2 We were interested in a combination of the chemo-
selectivity and the two-directional strategy, in other words,
the highly chemoselective C–C bond-forming reaction at
one reactive group of the substrates along with simultane-
ous transformation of another reactive group.
As an informative example, Kita and Fujioka reported a
reaction of alkenyl acetals with iodonium sources in the
presence of alcohols as nucleophiles (Scheme 1).3 They pro-
posed that the intramolecular iodoetherification-type reac-
tion giving rise to the bicyclic oxonium and the following
alcoholysis proceeded to give the ring-expanded product.
This inspired us to study the transformation of ω,ω-di-
alkoxy ketones 1, which had the polar carbonyl group
O
R1
R1
Me
R1
OMe
OMe
R2
OMe
R3Si Nu
Lewis acid
O
OMe
R2
OMe
Nu
R2
1
2
Scheme 2 This work
Here some difficulties were easily predictable in our
case. First, chemoselective activation of the carbonyl group
over the ketal functionality would be required. However,
the use of typical Lewis acids was at risk to activate not
only the carbonyl but also the ketal without the selectivity.
For instance, in a typical Mukaiyama aldol reaction, both
carbonyl and ketal substrates served as good substrates.4,5
For this issue, a number of researches revealed that well-
considered choice of the Lewis acid promoters brought
about carbonyl-selective activation.6 The second point was
the background reaction with the nucleophiles at the ini-
tially activated carbonyl group. To overcome these prob-
lems, we focused on the zwitterion-induced addition reac-
tion of ketene silyl acetals (KSA) to lactone substrates.7 Re-
cently, we reported that acidic zwitterion 38 nicely
promoted the addition reaction of KSA 5a to isocoumarin 4
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 2457–2461

2458
H. Yanai et al.
Letter
Synlett
to give the corresponding silyl acetal 6 without decomposi-
tion of some acid-sensitive functionalities including pyrane
and silyl acetal moieties in the product (Scheme 3). In this
reaction system, in situ generation of loosely contacting ion
pair A as a catalytically active species was assumed.9
Table 1 Reaction of 4,4-Dimethoxycyclohexanone 1a with KSA 5a
OTBS
OEt
O
5a (1.2 equiv)
MeO
MeO
acid catalyst (1 mol%)
0 °C, 10 min
1a
CO2Et
O
OTBS
OTBS
OMe
TBSO
zwitterion 3
(1 mol%)
OTBS
OEt
O
O
CO2Et
MeO
MeO
MeO
EtO2C
CH2Cl2, 0 °C
Ph
Ph
7a
2a
I
I
4
5a
6 87%
Entry Acid catalyst
Solvent
Yield of Yield of dr of
7a (%)a 2a (%)a 2a
R
1b
2
zwitterion 3d
CH2Cl2
Et2O
DME
Et2O
Et2O
Et2O
Et2O
Et2O
24
0
61
94
87
0
3.5:1
2.9:1
3.9:1
–
H
N
N
R
zwitterion 3d
zwitterion 3d
TfOH
Tf
5a
R3Si
O
–
R
Tf
C–
C
R
3
0
Tf
OEt
'R3Si+' equivalent A
H
Tf
4
0
3 (R = n-C5H11
)
5c
6c
7
Tf2NH
0
0
–
carbon acid 8d
TBSOTf
0
34
21
0
3.0:1
2.1:1
–
Scheme 3 Zwitterion-induced addition reaction of KSA to lactones
<5
0
8
4-pyrrolidinylbenzoic acid
That is, this species worked as a highly Lewis acidic for-
mal ‘R3Si+’equivalent to activate 4 through the silyl transfer
reaction from A to the carbonyl group of 4.10 Our observa-
tion suggested that, under the zwitterion-mediated condi-
tions, the desired selective activation of the carbonyl group
over the ketal functionality in substrate 1 would be possi-
ble. In this paper, we describe the chemoselective two-di-
rectional reaction of the ω,ω-dialkoxy ketones under the
zwitterion-induced conditions. This reaction realized the
unusual ketal-selective Mukaiyama aldol type reaction11
along with the protection of the substrate carbonyl group
by a single reaction operation.
At first, we examined the reaction of 4,4-dimethoxycy-
clohexanone (1a) with KSA 5a. The selected results are
summarized in Table 1. Upon treatment with 1.2 equiva-
lents of 5a and 1 mol% of zwitterion 3, the substrate 1a was
rapidly consumed (Table 1, entry 1). After usual extractive
workup followed by chromatographic purification, simple
Mukaiyama aldol product 7a was isolated in 24% yield. The
desired methyl silyl ketal 2a (two diastereomers in a ratio of
3.5:1) was obtained in 61% yield as well. This fact supported
our hypothesis. By optimizing the reaction conditions, we
successfully found that ethereal solvents such as diethyl
ether (Et2O) and 1,2-dimethoxyethane (DME) were better
options to effect the desired reaction (Table 1, entries 2 and
3). For example, the reaction in Et2O gave 2a in 94% yield,
exclusively. On the other hand, the use of more polar THF
did not meet good consumption of 1a. It should be noted
that the present transformation realized the C–C bond for-
mation at the ketal functionality and the protection of the
carbonyl group in a single reaction operation. In this reac-
tion system, the use of TfOH and Tf2NH instead of 3 yielded
a complex mixture (Table 1, entries 4 and 5). Carbon acid
a Isolated yield.
b Reaction was carried out for 1.5 h at –40 °C.
c A complex mixture was obtained.
d
Tf
H
N
Tf
HO
OH
CHTf2
Tf
Me
–
Tf
Me
OH
3
8
Tf
Tf
812,13 and TBSOTf were also applicable, while the yields of
2a were dissatisfactory (Table 1, entries 6 and 7). Further-
more, 4-(pyrrolidin-1-yl)benzoic acid was ineffective for
this transformation (Table 1, entry 8).
With methyl silyl ketal 2a in hands, its hydrolysis was
studied (Scheme 4).14 In a mixed solvent of AcOH–H2O–ace-
tone (1:1:1 v/v), 2a was easily hydrolyzed to give ketone 9a
in 88% yield. Moreover, the same transformation was
achieved by treating 2a with tetrabutylammonium fluoride
(TBAF). Overall chemistry (the zwitterion-induced reaction
with KSA followed by ketonization) was equivalent to the
unusual ketal-selective Mukaiyama aldol reaction.
OTBS
O
aq AcOH
OMe
MeO
EtO2C
2a (dr = 2.9:1)
MeO
EtO2C
acetone, r.t., 1 h
88%
9a
TBAF (3.0 equiv)
THF, 0 °C, 8 h
75%
Scheme 4 Conversion of silyl acetal 2a to ketone 9a
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 2457–2461

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H. Yanai et al.
Letter
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Next, we conducted the reaction of benzene-fused sub-
strate 1b (Scheme 5). However, in this case, the reaction at
the carbonyl group was faster than that at the ketal moi-
ety.15 This would be ascribable to lower conformational
flexibility of the substrates.
yielded the desired methyl silyl ketal in 87% yield
(dr = 1:1.0) without formation of any side products. After
acidic hydrolysis of thus obtained crude material in a one-
pot manner, ketone 9c was isolated in 87% yield from 1c
(Table 2, entry 1). According to this one-pot procedure, the
reaction of diphenyl substrate 1d was also conducted. Al-
though preferential formation of the desired 9d was ob-
served in the reaction in Et2O, the chemoselectivity was
moderate (Table 2, entry 2). This problem was successfully
solved by using DME instead of Et2O; 9d and 10d were iso-
lated in 81%, 3% yields, respectively, after acid hydrolysis
(Table 2, entry 3). By the reaction of phenyl ketone 1e in
Et2O, 9e was obtained in 69% yield along with formation of
a small amount of 10e (Table 2, entry 4).
OTBS
5a (1.2 equiv)
OEt
OTBS
CO2Et
O
zwitterion 3 (1 mol%)
MeO
MeO
MeO
MeO
Et2O, 0 °C, 10 min
1b
7b 78%
Scheme 5 Reaction of 1b with 5a
Indeed, several acyclic substrates gave excellent results
under the optimized conditions. Selected results are sum-
marized in Table 2. The reaction of 1c with KSA 5a (R = H)
Table 2 Reaction of ω,ω-Dialkoxy Carbonyl Compounds 1 with KSA 5
R4
R4
OTBS
OEt
1)
2)
5 (1.2 equiv)
O
R4
O
R4
TBSO
R1
R2
OR3
R4
zwitterion 3 (1 mol%)
Et2O, 0 °C, 15–60 min
R2
CO2Et
R1
R1
n
R2
n
+
R3O OR3
EtO2C
n
aq AcOH, acetone
r.t., 30 min
R4
O
10
1
9
Entry
1
9
Yield of 9 (%)a
Yield of 10 (%)
1
O
87
58
81
–
O
1c (R = Me)
1d (R = Ph)
1d
9c
9d
9d
R
R
R
2
31
3
R
OMe
MeO OMe
EtO2C
O
3b
O
Me
Me
4
5
69
83
7
1e
1f
Ph
9e
9f
Ph
OMe
MeO OMe
EtO2C
O
O
Ph
Ph
–
Me
Me
OMe
MeO OMe
EtO2C
O
6
7
8
9
92
91
73
71
–
1g (R = Et)
9g
9h
9i
O
–
1h (R = i-Bu)
1i (R = c-C3H5)
1j (R = H)
Ph
Ph
R
R
18
–
OMe
MeO OMe
EtO2C
O
9j
O
Ph
Ph
10
1k
1f
9k
9f'
9m
92
85
–
–
Et
Et
OEt
EtO OEt
EtO2C
O
O
O
Ph
Ph
11c
Me
O
Me
OMe
Me
MeO OMe
EtO2C
Me
12d
35
61
20
15
EtO2C
MeO OMe
Ph
OMe
Ph
1m
13b,d
Me
Me
a Isolated yield.
b The reaction was carried out by using 1.6 equiv of 5 in DME as a solvent.
c 5 mol% of carbon acid 8 were used.
d 1-Phenylhexane-1,5-dione was also obtained.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 2457–2461

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H. Yanai et al.
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In contrast, methyl ketone 1f, which differed from 1e in
the positions of phenyl and methyl groups, specifically pro-
duced 9f in 83% yield (Table 2, entry 5). Under similar con-
ditions, ethyl ketone 1g and isobutyl ketone 1h gave selec-
tively 9g and 9h in excellent yields (Table 2, entries 6 and 7).
The reaction of cyclopropyl ketone 1i remained to proceed
in a ketal-selective manner, and 9i was isolated in 73% yield
(Table 2, entry 8). Surprisingly, aldehyde 1j was also con-
verted into 9j in 71% yield (Table 2, entry 9). Diethyl ketal
1k could be used as a suitable substrate (Table 2, entry 10).
When the reaction of 1f with disubstituted KSA 5b (R = Me)
was addressed in the presence of zwitterion 3, no reaction
was observed. However, by using stronger acid 8 instead of
3, the desired product 9f′ was obtained in 85% yield (Table
2, entry 11). Although the reaction of δ,δ-dimethoxy ketone
1m with 5a in Et2O gave 9m in 35% yield due to the compet-
itive formation of 10m, the reaction in DME gave 9m in an
acceptable yield (Table 2, entries 12 and 13).
alkoxy groups was observed in this reaction. The present
result implied that, in each substrate, the 1,4-migrations of
the alkoxy group during the reaction strictly occurred in an
intramolecular manner.
On the basis of observed chemoselectivity and the
crossover study, we propose a reaction pathway as shown
in Scheme 7. At first, the in situ generated ‘R3Si+’ equivalent
A reacts with the ketone moiety of 1 to give silyl carboxoni-
um B. In cases of conformationally flexible substrates, B is
smoothly captured by the methoxy group in an intramolec-
ular manner (formation of C). The following ring opening
giving rise to D causes the 1,4- or 1,5-migration of the me-
thoxy group. In such reaction system, the site-selectivity of
nucleophilic attack by KSA would reflect thermodynamic
distribution between B and D. As mentioned above, ethere-
al solvents such as Et2O and DME were effective to obtain 2.
This suggested that stabilization of the cationic intermedi-
ates B and D by solvation played an important role.
To gain insights into the reaction pathway, we also con-
ducted a crossover study; the reaction using an equimolar
mixture of 1f and 1k (Scheme 6).
Compared with the silyl carboxonium B equipped with
sterically bulky TBS group near the cationic center, less hin-
dered methyl carboxonium D would be strongly stabilized
by effective solvation.16 In fact, the reaction outcomes of a
pair of substrates 1e (R1 = Ph, R2 = Me) and 1f (R1 = Me,
R2 = Ph) also supported these cationic intermediates; phe-
nyl ketone 1e gave a mixture of 9e and 10e, whereas, meth-
yl ketone 1f produced 9f in a one-sided manner. In the for-
mer, since the silyl carboxonium B (R1 = Ph, R2 = Me) was
exceptionally stabilized by the resonance effect of the phe-
nyl group, a small amount of 10e was formed.
In summary, we successfully developed a novel chemo-
selective Mukaiyama aldol type reaction of ω,ω-dialkoxy
ketones.17 From two points of view, the present transforma-
tion has advantages. First, this is an example of the unusual
ketal-selective Mukaiyama aldol type reaction. Second, the
present reaction realized the C–C bond formation to con-
struct the molecular architecture and simultaneous protec-
tion of ketone functionality in a single reaction operation.
O
O
OTBS
OEt
5a (0.60 mmol)
TBSO OEt
Et
Ph
Ph
Me
Et
MeO OMe
EtO OEt
1f (0.25 mmol)
1k (0.25 mmol)
TBSO OMe
zwitterion 3 (1 mol%)
Ph
Ph
Me
Et2O, 0 °C, 15 min
OMe
OEt
EtO2C
2f 82%
EtO2C
2k 81%
Scheme 6 Crossover study using 1f and 1k
When this mixture was treated with KSA 5a (1f, 0.25
mmol; 1k, 0.25 mmol; 5a 0.60 mmol) in the presence of 1
mol% of 3, two silyl acetals 2f and 2k were isolated in 82%
and 81% yields, respectively. Indeed, no scrambling of the
TBS
O
TBS
TBS
O
O
O
OMe
R1
n
A
R2
R2
R2
R1
R1
R1
O
n
n
n
R2
Me
Me
MeO OMe
O
OMe
O
O
Me
Me
1
B
C
D
OTBS
OEt
OTBS
OEt
5a
5a
O
CO2Et
CO2Et
TBSO OMe
TBSO
R1
TBSO
R1
H3O+
H3O+
R2
R2
R2
R2
R1
R1
n
n
n
n
OMe
OMe
MeO OMe
EtO2C
EtO2C
O
10
7
2
9
Scheme 7 Possible reaction pathway for the present reaction
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 2457–2461

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Acknowledgment
(10) For selected examples on in situ generation of silicon Lewis
acids, see: (a) García-García, P.; Lay, F.; García-García, P.;
Rabalakos, C.; List, B. Angew. Chem. Int. Ed. 2009, 48, 4363.
(b) Yamamoto, H.; Boxer, M. B. J. Am. Chem. Soc. 2006, 128, 48.
(c) Inagawa, K.; Takasu, K.; Ihara, M. J. Am. Chem. Soc. 2005, 127,
3668.
This work was partially supported by a Grant-in-Aid for Scientific Re-
search on Innovative Areas ‘Advanced Molecular Transformations by
Organocatalysts’ from the MEXT, Mitsubishi Gas Chemical Co. Ltd.,
and the Kurata Memorial Hitachi Science and Technology Foundation
(11) Li, W.-D. Z.; Zhang, X.-X. Org. Lett. 2002, 4, 3485.
(12) (a) Yanai, H.; Ogura, H.; Fukaya, H.; Kotani, A.; Kusu, F.; Taguchi,
T. Chem. Eur. J. 2011, 17, 11747. (b) Zhang, M.; Sonoda, T.;
Shiota, Y.; Mishima, M.; Yanai, H.; Fujita, M.; Taguchi, T. J. Phys.
Org. Chem. 2015, 28, 181.
(13) Other examples on carbon acid catalysis, see: (a) Ishihara, K.;
Hasegawa, A.; Yamamoto, H. Angew. Chem. Int. Ed. 2001, 40,
4077. (b) Hasegawa, A.; Naganawa, Y.; Fushimi, M.; Ishihara, K.;
Yamamoto, H. Org. Lett. 2006, 8, 3175. (c) Saadi, J.; Akakura, M.;
Yamamoto, H. J. Am. Chem. Soc. 2011, 133, 14248.
(14) Stern, A.; Swenton, J. S. J. Org. Chem. 1987, 52, 2763.
(15) Under similar conditions, perfect carbonyl selectivity was
observed in the Mukaiuyama aldol reaction of benzaldehyde
and its dimethyl acetal with 5a. Likewise, the reaction of 1,4-
dioxaspiro[4.5]decan-8-one, which was a cyclic ketal derivative
of 1a, proceeded in a carbonyl selective manner.
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0035-1560478.
S
u
p
p
ortiInfogrmoaitn
S
u
p
p
ortioInfgrmoaitn
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(17) Typical Procedure (Table 2, Entry 1)
To a solution of 5,5-dimethoxyhexan-2-one (1c, 80.1 mg, 0.501
mmol) and zwitterion 3 (2.6 mg, 5.0 μmol) in Et2O (1.5 mL) was
added a solution of tert-butyl[(1-ethoxyvinyl)oxy]dimethylsi-
lane (5a, 121 mg, 0.598 mmol) in Et2O (0.5 mL) at 0 °C. After
being stirred for 20 min at 0 °C, the reaction was quenched by
treatment with Et3N (0.3 mL), then it was concentrated under
reduced pressure. The resulting residue was dissolved in a
mixed solvent of acetone, H2O, and AcOH (1:1:1 v/v, 3.0 mL).
This mixture was stirred for 30 min at r.t. After usual extractive
workup, the obtained residue was purified by column chroma-
tography on silica gel (hexane–EtOAc, 3:1) to give ethyl 3-
methoxy-3-methyl-6-oxoheptanoate (9c) in 87% yield (94.1 mg,
0.436 mmol); colorless oil. IR (neat): ν = 2980, 2947, 2910,
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NMR (100 MHz, CD3CN): δ = 13.5, 22.1, 29.1, 31.0, 37.3, 42.5,
48.6, 60.0, 74.9, 170.5, 208.2. MS (ESI-TOF): m/z = 239 [M + Na]+.
HRMS: m/z calcd for C11H20O4 [M + Na]+: 239.1259; found:
239.1259. Anal. Calcd for C11H20O4: C, 61.09; H, 9.32. Found: C,
60.91; H, 9.29.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 2457–2461