Aromatic aldehyde-selective aldol addition with aldehyde-derived silyl enol ethers

Article abstract of DOI:10.1039/c7cc08936h

The aldol reaction using aldehyde-derived silyl enolates as nucleophiles with aromatic aldehydes chemoselectively proceeded in the presence of silyl triflate and 2,2′-bipyridyl to produce β-siloxy aldehydes, while the aliphatic aldehydes were completely recovered. The unprecedented chemoselectivities depend on the reactivities of the pyridinium-type intermediates derived from the aromatic and aliphatic aldehydes.

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Aromatic Aldehyde-Selective Aldol Addition with Aldehyde-
Derived Silyl Enol Ethers
Takahiro Kawajiri,a Reiya Ohta,b Hiromichi Fujioka,b Hironao Sajiki*a and Yoshinari Sawama*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
[a] General cross aldol reaction of aldehydes:
The aldol reaction using aldehyde-derived silyl enolates as
nucleophiles with aromatic aldehydes chemoselectively
proceeded in the presence of silyl triflate and 2,2’-bipyridyl to
produce β-siloxy aldehydes, while the aliphatic aldehydes were
completely recovered. The unprecedented chemoselectivities
depend on the reactivities of the pyridinium-type intermediates
derived from the aromatic and aliphatic aldehydes.
SiO
dehydration
CHO
4'
R
R
2
OH
4
or
Lewis acid
CHO
(eq. 1)
CHO
3'
R
R
OH OH
over-reaction
1
(R = Aryl)
3
or
CHO
or1' (R = Alkyl)
n
5 or 5'
[b] Resolution methods:
R1
OSiCl3
Cl2
Si
OH OMe
OMe
R1
O
O
cat. Lewis base
CHO
(eq. 2)
R
R
Cl
R
-78 oC
R1
1
1'
Carbon-carbon bond formations represented by the aldol
reaction are fundamental and important in organic synthesis.1
The cross-aldol reaction among different carbonyl substrates
can be traditionally accomplished by the preliminary formation
of a particular carbonyl substrate into the corresponding
or
then MeOH
chlorohydrine intermediates
OTMS
R4
TMS
O
OH
R4
OH
OH OH
R3 R4
O
R2O
cat. TiCl4
CHO
(eq. 3)
(eq. 4)
H
-78 to -40 o
C
R3
R2O
R3
R2O
OH
oxocarbenium
OTTMSS
enolate as
a nucleophile to avoid the undesired self-
R5
cat. HNTf2
-78 oC
TMS
TMS
condensation.2 The Mukaiyama aldol reaction using silyl enol
ethers as easily-handled nucleophiles is well-known to be a
powerful and practical synthetic tool,3 while the cross aldol
Si
OTTMSS
CHO
TMS
CHO
R
R
tris(trimethylsilyl)silyl
(TTMSS)
R5
1 or 1'
This work
[c]
:
reaction of an aldehyde (
from acetaldehydes ( ) as a nucleophile is still challenging (eq.
1). Namely, 1’ and can be coupled in the presence of a
Lewis acid to give the β-hydroxy aldehydes ( or 3’), which
comparatively easily undergo the dehydration to form
undesired α,β-unsaturated aldehydes ( or 4’). Furthermore,
the generated aldehydes ( 3’ and 4’) are also reactive and
the suppression control of the oligomerization to the
oligomers (e.g., or 5’) by the addition of the remaining is
generally difficult.4 The reaction of an aldehyde (
or 1’) using
1 or 1’) with silyl enol ethers derived
Chemoselective cross aldol reaction of aromatic aldehydes via pyridinium salts
2
SiO
SiO
Ar
SiO SiO
Ar
SiOTf
2,2'-bipyridyl
OTf
OTf
OSi
H2O
2
1/
2
N
R
N
R
(eq. 5)
1
CHO
oC
Ar
0
3
A
B
3
Si = TMS, TES, TBS
4
3/
4/
hemiacetal by the intramolecular nucleophilic attack of the
hydroxy group to give the carbohydrate (eq. 3).6 Furthermore,
the use of tris(trimethylsilyl)silyl (TTMSS) enol ethers
possessing a bulky silyl moiety could also solve the problematic
over-reaction to produce the β-siloxy aldehyde derivatives,
which easily underwent further chemical modifications (eq.
4).7 In addition, organocatalytic aldol reactions8 and
asymmetric aldol reactions using boron enol ethers9 have been
reported. However, there are no reports accomplishing the
chemoselectivity between the aromatic and aliphatic
5
2
1
trichlorosilyl enol ether gave the proposed chlorohydrine
intermediates to avoid the over-reaction, and the following
addition of MeOH provided the corresponding acetal (eq. 2).5
The in situ-generated aldehyde resulting from the aldol
reaction between the β-hydroxy aldehyde as the starting
material and the trimethylsilyl enol ether could be trapped as a
aldehydes. We now demonstrate the aromatic aldehyde (1: Ar-
a.Laboratory of Organic Chemistry, Gifu Pharmaceutical University
1-25-4, Daigaku-nishi, Gifu, 501-1196, (Japan)
E-mail: sawama@gifu-pu.ac.jp
CHO)-selective Mukaiyama aldol reaction with the silyl enol
ether as a nucleophile in the presence of the silyl triflate and a
pyridine derivative via two kinds of pyridinium salt
b.Graduate School of Pharmaceutical Sciences, Osaka University
1-6, Yamada-oka, Suita, Osaka, 565-0871, (Japan)
intermediates (
A and B) (eq. 5), and the further modifications
Electronic
DOI: 10.1039/x0xx00000x
Supplementary
Information
(ESI)
available:
See
of the obtained β-siloxy aldehydes (
3) were carried out.
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We initially investigated the aldol reaction between 4-
methoxybenzaldehyde (1a) and trimethylsilyl (TMS) enol ether
2a) derived from acetaldehyde in CH2Cl2 at 0 oC (Table 1). The
(
use of 2 equiv. of trimethylsilyl triflate (TMSOTf) as a Lewis
acid gave a complex mixture including the undesired 4-
methoxycinnamaldehyde (4aa; 48%), unchanged 1a (6%) and
unidentified oligomers like
5, and the desired β-siloxy
aldehyde (3aa) was never detected (entry 1). The reaction
under basic conditions in the presence of an excess amount (3
equiv.) of pyridine derivatives together with TMSOTf was next
examined. While pyridine, N,N-dimethyl-4-aminopyridine
(DMAP) and 2-picoline rather interfered with the reaction
progress (entries 2-4), 2,6-lutidine and 2,4,6-collidine gave the
desired 3aa in the moderate yields (57 and 45%, respectively;
entries 5 and 6). 2-Phenylpyridine and 2,2’-bipyridyl were
much better additives to give 3aa in excellent yields (entries 7
and 8), while 2,4’-bipyridyl was totally ineffective (entry 9). The
Scheme 1. Chemoselective Mukaiyama aldol reaction.
detailed optimization results of the solvent, TMSOTf/2,2’- remained unchanged (Scheme 1, middle). The present
bipyridyl ratio, Lewis acid and base are described in the chemoselectivity could be achieved by the use of triethylsilyl
Electronic Supplementary Information (ESI).
Meanwhile, aliphatic aldehydes 1’a and 1’b
enol ether and tert-butyldimethylsilyl enol ether as other silyl
(
)
and enolates (See ESI). Furthermore, the substrate (1b) bearing
benzophenone (6a) were inert under the above reaction aromatic and aliphatic aldehydes within the same molecule
conditions in the presence of TMSOTf and 2,2’-bipyridyl, and was also converted into the corresponding β-siloxy aldehyde
the starting compounds were completely recovered (Scheme 1,
(3ba) resulting from the chemoselective transformation of the
top). [The reaction of acetophenone (6b) as a substrate gave aromatic aldehyde (Scheme 1d, bottom).
The reaction mechanism was proposed based on NMR studies
75% of silyl enol ether (6b’) together with 25% of recovered
6b.] Therefore, the aromatic aldehyde-selective aldol reaction
coexisting with aliphatic aldehydes has been accomplished.
When using a 1:1 mixture of 1a and 1’a as substrates, 1a was
chemoselectively transformed into 3aa in 97% yield and 1’a
(Spectra are indicated in ESI). The formation of the pyridinium
salt intermediate bearing an N,O-acetal structure from an
acetal in the presence of silyl triflate (SiOTf) via the
nucleophilic substitution of an alkoxy group of the acetal by a
pyridine derivative has been reported.10 The pyridinium salt
intermediate possesses an electrophilicity and reacts with
various nucleophiles. In a similar way, both aromatic and
aliphatic aldehydes can be transformed into the corresponding
Table 1. Effect of additional pyridine derivatives.
TMSOTf (2 equiv.)
TMSO
pyridinium salt (N,O-acetal) intermediates (A and C) by the
R
2a
OTMS
CHO
nucleophilic attack of 2,2’-bipyridyl on the activated aldehyde
by TMSOTf (Scheme 2). The proton peaks of each aldehyde
[9.87 (1a) and 9.72 (1’a) ppm] disappeared and the novel
(3 equiv.) (2 equiv.)
N
CHO
CHO
+
CH2Cl2, 0 o
C
2 h
Ar
Ar
Ar
1a
4aa
3aa
then H2O
30 min.
Ar = 4-MeOPh
characteristic peaks corresponding to each N,O-acetal [7.70 (
and 6.72 ppm] developed. Furthermore, was
chemoselectively converted into a diastereomixture of theβ-
siloxy aldehydes 3ca 3ma). Benzyl and silyl ethers as
protected hydroxyl groups (1h and 1i) could be tolerated the
intermediates (6.58/6.84 ppm for each of the N,O-acetal
isomers) in the presence of 2a, which was worked up with H2O
A)
(C
)
A
entry
pyridine derivative
yield [%][a]
(
-
1a
6
3aa
4aa
1
2
3
4
5
6
7
8
9
—
0
48
0
B
pyridine
98
99
97
40
46
2
0
DMAP
0
0
2-picoline
2,6-lutidine
2,4,6-collidine
2-phenylpyridine
2,2’-bipyridyl
2,4’-bipyridyl
1
0
57
45
94
0
0
0
4
96(90[b]
)
0
90
0
0
[a] The yield was determined by 1H NMR using 1,2-methylenedioxybenzene as an
internal standard. [b] Isolated yield.
Scheme 2. 1H NMR studies of the reaction intermediates. (Spectra of
A, B and C are
included in the ESI.)
2 | J. Name., 2012, 00, 1-3
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TMSO
Ar
OTMS TMSO
CN
Ar
OTMS
N3
Table 2. Scope of (hetero)aromatic aldehydes.
TMSO
2a
(2 equiv.)
7a, 86%
dr = 60:40 (28 h) dr = 67:33 (24 h)
TMSN3
7b, 42%
TMSOTf (2 equiv.)
2,2'-bipyridyl (3 equiv.)
CH2Cl2, 0 oC, 30 min.
TMSCN
1a
2 h
TMSO
OH
TMSO
Ar
then nucleophile
(5 equiv.)
CN
CN
Ar
allyl
7c, 94%
dr = 64:36 (6 h)
allyl-Bpin[a]
7d, 77% (12 h)
NC CN
Scheme 3. One-pot four component condensation. [a] 2 equiv. of allyl-Bpin was
used.
presence of the corresponding SiOTf (Table 3). Triethylsilyl
(TES) and tert-buthyldimethylsilyl (TBS) enol ethers (2b and 2c
generated from acetaldehyde or cyclohexylaldehyde (2d) were
applicable to give the corresponding β-siloxy aldehydes (3ab
3ad). The use of β–substituted (Z)-silyl enol ethers (2e 2i) gave
α-substituted β-siloxy aldehydes (3ae 3ai) in high yields and
moderate anti-selectivities. On the other hand, the syn-adduct
3ag’) was obtained as the main isomer from the (E)-silyl enol
ether (2g’). 1-Siloxy-1,3-diene (2j) could also be used as a
)
-
[a] The reaction was performed using 2,2’-bipyridyl (6 equiv.) and TMSOTf (4
equiv.).
-
-
to give the desired 3aa (9.76 ppm for the aldehyde peak). The
results shown in Table 1 indicated that the effect of steric
hindrance around the nitrogen atom of the pyridine derivative
is quite important to facilitate the present reaction. For
(
nucleophile to give the δ-siloxy α,β-unsaturated aldehyde (3aj
)
in 71% yield.
The pyridinium salt intermediate
example, the pyridinium salt intermediate (A) might be more
B (Scheme 2) via the
reactive toward the nucleophilic attack of 2a based on the
nucleophilic attack by 2a could be applicable for further
chemical modification in a one-pot manner (Scheme 3). The
continuous addition of TMSCN, TMSN3 or allyl-Bpin into the
reaction mixture gave the corresponding 1,3-diol derivatives
higher leaving-group ability of the pyridinium moiety bonded
to the benzylic carbon in comparison to the non-benzylic
B and
C
. Furthermore, the steric repulsion by both aromatic rings
7a,
7b and 7c) in moderate to excellent yields.11 Furthermore,
derived from the starting aldehyde (1a, Ar) and the relatively
bulky substituent (the pyridine ring of 2,2’-bipyridyl) at the 2-
position of the pyridine ring also encourage the reaction
(
the 1,1-dicyano alkene product (7d) could be obtained in 77%
yield by the use of malononitrile via the nucleophilic attack
and subsequent dehydration. Based on these reactions, the
progress. Therefore, the undesired over-reaction of
could be preferentially suppressed.
B with 2a
four components (1a, 2a, TMS group of TMSOTf and the
corresponding nucleophile) could be continuously condensed
to construct highly functionalized compounds in a one-pot
approach.
Various aromatic aldehydes
(1) were applicable for the
reaction with 2a (Table 2). Benzaldehyde derivatives bearing
electron donating and withdrawing groups on the aromatic
The four-component condensation reaction could also be
ring (1c
stated reaction conditions. Furthermore, cinnamaldehyde (1n
and hetero aromatic aldehyde derivatives (1o 1p and 1q) were
-1m) were efficiently converted to the corresponding
accomplished using benzaldehyde dimethyl acetal (
of aromatic aldehydes (Scheme 4, top). was treated with
TMSOTf and 2,2’-bipyridyl in CH2Cl2 at 0 oC, and TES enol ether
2b) and TMSCN were sequentially added to the reaction
mixture. Consequently, the TMS-protected alcohol ( ) was
obtained as the sole product. During the transformation of
derived from an aromatic aldehyde ( ) into , two possible
8) instead
)
8
,
also applicable to give the corresponding β-siloxy aldehydes in
excellent yields.
(
9
A
Various silyl enol ethers derived from aliphatic aldehydes were
next investigated as nucleophiles of the reaction with 1a in the
1
B
Table 3. Scope of silyl enolates.
[a] TMSOTf was used. [b] TESOTf was used. [c] TBSOTf was used. [d] 59% of 1a was
recovered.
Scheme 4. Proposed reaction mechanism for the reaction between aromatic
aldehydes ( ) and silyl enolates ( ) via two kinds of pyridinium salts ( and ).
1
2
A
B
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1
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Scheme 5. Further chemoselective transformation of the obtained benzyl silyl
ethers.
4
5
reaction paths via a β-siloxy aldehyde (
carbocation intermediate ( ; route ) are proposed. If the
reaction proceeds through , the corresponding TES-protected
alcohol (10) should be obtained by the reaction of using the
TES-derived silyl enol ether (2b). Because we have never
detected 10 in the reaction mixture, route might be the
probable pathway. In the present reaction using an aromatic
aldehyde as the substrate, the product could be produced as
short-lived intermediate in the reaction media and
instantaneously masked (protected) by the SiOTf and 2,2’-
bipyridyl to give without any over-reaction with
3; route a) and a
D
b
a) S. E. Denmark, and K. Ghosh, Angew. Chem. Int. Ed., 2001,
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8
a
6
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2.
B. Brady and H. Yamamoto, Angew. Chem. Int. Ed., 2012, 51
,
The products, obtained by the present reaction, possesses the
benzyl silyl ether moiety within the molecule, which was easily
and chemoselectively activated by FeCl3 or FeBr3.12 The siloxy
moiety of 3aa could be transformed by iron-catalyzed
nucleophilic substitutions in the presence of TMSN3 or
allylTMS to give the corresponding products (11a and 11b) in
good yields (Scheme 5). 7a bearing two kinds of siloxy
functionalities was chemoselectively converted by the
treatment with TMSN3 to the azido product (11c) via the FeBr3-
catalyzed chemoselective transformation of only the benzylic
siloxy moiety.
1942–1946; e) A. Izumiseki and H. Yamamoto, J. Am. Chem.
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In conclusion, we have developed the aromatic aldehyde-
selective aldol reaction using silyl enol ethers derived from
aliphatic aldehydes via two kinds of pyridinium salt
intermediates. The different electrophilicities between the
aromatic and aliphatic pyridinium salts intermediates enable
the suppression of the over-reaction. The obtained reaction
intermediates and products could be continuously and easily
modified into the highly-functionalized compounds. The
unprecedented and chemoselective transformations are useful
to develop a novel strategy for synthesis of the target
molecules.
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11 The pyridinium-type salt intermediate derived from decanal
(intermediate
allyl-Bpin.
C, Scheme 2) also reacted with TMSCN and
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,
This study was partially supported by a Grant-in-Aid for JSPS
Research Fellows from the Japan Society for the Promotion of
Science (JSPS, Number: 17J08551 for T. K.).
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Conflicts of interest
There are no conflicts to declare.
Notes and references
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