Metal alkoxide promoted regio- and stereoselective Ci=O and Ci=C metathesis of allenoates with aldehydes

Article abstract of DOI:10.1002/anie.201310183

The reaction of 2,3-allenoates and aldehydes in the presence of an alkoxide affords alkyl 4,5-diaryl-3-oxo-2-propylpent-4(E)-enoates and cis-3,4-diaryloxetanes through a formal Ci=O and Ci=C metathesis. A mechanism for this reaction has been proposed. All'ene' all: The reaction of aldehydes with 2,3-allenoates in the presence of tBuOM (M=Li or Na) affords the stereodefined, highly functionalized γ,δ-unsaturated β-ketoesters 1 in good to excellent yields with E stereoselectivity. The reactivity of 5-hydroxy-2,3-allenoates and the isolation of 3,4-diaryloxetanes led to a rationale for the mechanism of the reaction.

Full text of DOI:10.1002/anie.201310183

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Angewandte
Communications
DOI: 10.1002/anie.201310183
Synthetic Methods
=
=
Metal Alkoxide Promoted Regio- and Stereoselective C O and C C
Metathesis of Allenoates with Aldehydes**
Minyan Wang, Zhao Fang, Chunling Fu, and Shengming Ma*
In memory of Run Run Shaw
Abstract: The reaction of 2,3-allenoates and aldehydes in the
presence of an alkoxide affords alkyl 4,5-diaryl-3-oxo-2-
propylpent-4(E)-enoates and cis-3,4-diaryloxetanes through
During our study on the reactions of 2,3-allenoates,[7] we
noticed that the reaction of p-chlorobenzaldehyde (1a) and
ethyl 2-propyl-4-phenylbuta-2,3-dienoate (2a) in the pres-
ence of tBuOLi at 508C unexpectedly afforded the stereo-
defined g,d-unsaturated b-ketoester (E)-3aa in 73% (NMR)
and 71% (isolated) yield (Scheme 1 and see entry 1 in
=
=
a formal C O and C C metathesis. A mechanism for this
reaction has been proposed.
=
I
t has been well established that 2,3-allenoates may readily
Table 1). For this synthetic operation, the C O bond of the
accept the attack of nucleophiles[1,2] and electrophiles,[3] thus
forming intermediates A–C and D, respectively (Scheme 1).
In addition, they may also undergo cyclic oxametalation to
form intermediate E,[4] and the treatment of 2-substituted 2,3-
allenoates with a base such as TBAF may generate the
corresponding alk-3-yn-2-enolate F, which could undergo 1,2-
addition with aldehydes or conjugated enals or enones.[5,6]
aldehyde is cleaved and the O atom is connected to the
middle carbon atom of the allene moiety, while the remaining
“R C ” is bonded to the C4 carbon atom of the allenoate. In
addition, exclusive E selectivity of the C C bond is observed.
Thus, with such an original observation, different solvents,
such as DCE, MeCN, and Et2O, were screened but they led to
a decrease in the yield (entries 2–4, Table 1). Replacement of
1
=
=
Table 1: Optimization of the reaction conditions.[a]
Entry
Solvent
Base
Yield [%][b]
Recovery [%]
(E)-3aa
1a
2a
1
2
3
4
5
6
7
8
THF
DCE
MeCN
Et2O
THF
THF
THF
THF
THF
THF
THF
THF
tBuOLi
tBuOLi
tBuOLi
tBuOLi
tBuONa
K2CO3
73 (71)[c]
–
0
–
–
–
72
11
24
–
–
–
–
36
–
–
–
74
15
8
–
–
0
55
59
45
0
0
0
42
72
64
28
LiOH.H2O
TBAF
9[d,e]
10[e]
11[f]
12[e,g]
tBuOLi
tBuOLi
tBuOLi
tBuOLi
Scheme 1. Reactivities of 2,3-allenoates.
–
19
0
[a] Used 0.3 mmol of allenoate, 0.45 mmol of aldehyde, and 0.6 mmol of
base in this reaction. [b] The yields were determined by 1H NMR analysis
with CH2Br2 as the internal standard. [c] Yield of isolated product.
[d] Added 1.5 equiv of 12-crown-4. [e] Added 1.5 equiv of base. [f] Added
1.2 equiv of aldehyde 1a. [g] The reaction was conducted at 108C.
DCE=1,2-dichloroethane, THF=tetrahydrofuran.
[*] M. Wang, Dr. Z. Fang, Prof. Dr. C. Fu, Prof. Dr. S. Ma
Laboratory of Molecular Recognition and Synthesis
Department of Chemistry, Zhejiang University
310027 Hangzhou, Zhejiang (P.R. China)
E-mail: masm@sioc.ac.cn
[**] Financial support from the National Basic Research Program of
China (2011CB808700) and National Natural Science Foundation of
China (Nos. 21232006 and 31172192) is greatly appreciated. S.M. is
a Qiu Shi Adjunct Prof. at Zhejiang University. We thank Weilong Lin
in this group for reproducing the preparation of (Z)-3ca in Table 2,
(E)-3af in Table 3, and (E)-4aa in Table 4.
tBuOLi with a stronger base (tBuONa) or a weaker base was
less effective (entries 5–8, Table 1). Notably, the presence of
12-crown-4, which is proposed to enhance the basicity of the
lithium salt, led to unsatisfactory results (entry 9, Table 1).
Lowering the amount of base to 1.5 equivalents showed no
influence on the yield whereas a reduced amount of 1a
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201310183.
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Table 3: The reaction of different 2,3-allenoates with 1a.[a]
proved to be deleterious (entries 10 and 11, Table 1). When
the reaction was conducted at 108C, only 28% of (E)-3aa was
formed with recovery of 19% of 2a (entry 12, Table 1). Thus,
1.5 equivalents of tBuOLi and 1.5 equivalents of 1a in THF at
508C for 1.5 hours were established as the standard reaction
conditions for further study.
With the optimized reaction conditions in hand, the scope
of the aromatic aldehydes 1 was firstly investigated as shown
in Table 2. Reactions proceeded smoothly under the standard
Entry
R1
Ar
Yield [%]
1
2
3
4
5
6
7
Me
Et
C6H5 (2b)
C6H5 (2c)
52[b] [(E)-3ab]
49 [(E)-3ac]
69 [(E)-3ad]
63 [(E)-3ae]
75 [(E)-3af]
58 [(E)-3ag]
77 [(E)-3ah]
nPr
nPr
nPr
nPr
nPr
p-MeC6H4 (2d)
p-MeOC6H4 (2e)
p-BrC6H4 (2 f)
m-ClC6H4 (2g)
p-FC6H4 (2h)
Table 2: The reaction of different aldehydes with 2a.[a]
[a] The reaction was carried out with 1.0 mmol of allenoate, 1.5 equiv of
aldehyde, and 1.5 equiv of tBuOLi at 508C in 6.0 mL of THF for 1.5 h.
Yield is that of isolated product. [b] The purity of (E)-3ab is 82%, as
Entry
Ar
Yield [%]
1
determined by H NMR analysis with CH2Br2 as the internal standard.
1
p-ClC6H4 (1a)
p-ClC6H4 (1a)
p-BrC6H4 (1b)
p-FC6H4 (1c)
o-BrC6H4 (1d)
m-ClC6H4 (1e)
p-NCC6H4 (1 f)
p-MeOOCC6H4 (1g)
C6H5 (1h)
m-MeOC6H4 (1i)
furan-2-yl (1j)
N-Me-pyrrol-2-yl (1k)
pyridin-4-yl (1l)
68 [(E)-3aa]
74 [(E)-3aa]
59 [(E)-3ba]
62 [(E)-3ca]
65 [(E)-3da]
64 [(E)-3ea]
64 [(E)-3 fa]
64 [(E)-3ga]
52 [(E)-3ha]
48 [(E)-3ia]
61 [(E)-3ja]
51 [(E)-3ka]
45 [(E)-3la]
2[b]
3
exclusive E selectivity of the double bond (see Figure S1 in
the Supporting Information).[8]
4
5
6
7
To unveil the mechanism, some control experiments were
conducted. When sodium hydroxide, a relatively weaker base,
was used for the reaction of 1h and 2a, (E)-3ha was afforded
8
1
9[c]
10[c]
11[c]
12[c]
13[c]
in a 34% yield as determined by H NMR analysis, and in
addition 23% yield of the substituted oxetane cis-4ha was
observed (Scheme 2). The stereochemistry of cis-4ha was
secured through the information from the vicinal 1H-1H
coupling constant (J = 4.0 Hz) and 2D NMR data (COSY,
HSQC, HMQC, and phase-sensitive NOESY; see the Sup-
porting Information). This compound could be further trans-
formed into the corresponding final product (E)-3ha under
the standard reaction conditions in 74% yield. Although the
[a] The reaction was carried out with 1.0 mmol of allenoate, 1.5 equiv of
aldehyde, and 1.5 equiv of tBuOLi at 508C in 6.0 mL of THF for 1.5 h.
Yield is that of isolated product. [b] The reaction was conducted on
a gram scale using 1.0012 g 2a. The reaction proceeded for 2.0 h.
[c] tBuONa was used instead of tBuOLi and the reactions proceed for
1.0 h.
reaction conditions, thus affording differently substituted
ethyl 3-oxo-4,5-diaryl-2-propylpent-4(E)-enoates [(E)-3] in
moderate to good yields. Substituents, such as a halogen at the
para, ortho, or meta positions of the aromatic ring in the
aldehyde furnished the corresponding products (E)-3aa-ea in
decent yields (entries 1–6, Table 2). Both CN and COOMe
groups, which are active under basic conditions, were also well
tolerated (entries 7 and 8, Table 2). When benzaldehyde and
aromatic aldehydes bearing electron-donating groups were
introduced, tBuONa instead of tBuOLi was used (entries 9
and 10, Table 2). In addition, aldehydes having a heteroaro-
matic ring such as furan, pyrrole, and pyridine are also
suitable substrates for the reaction. It is noteworthy that the
reaction can be easily conducted using 1a with 1.5 equivalents
of 2a on a gram scale, thus affording (E)-3aa in 74% yield
(entry 2, Table 2).
Next, we examined the reaction of different ethyl 2-alkyl-
4-arylbuta-2,3-dienoates (2) with 1a (Table 3). R1 could be an
alkyl group such as methyl or ethyl (entries 1 and 2, Table 3),
while Ar could be substituted with both electron-donating
and electron-withdrawing groups (entries 3–7, Table 3). The
structures of the products were unambiguously established by
the X-ray diffraction study of (E)-3ab, which displayed the
Scheme 2. Some control reactions for mechanistic studies. TBAF=te-
tra-n-butylammonium fluoride.
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Table 4: The reaction of different aldehydes (1) with 2a to afford cis-4.[a]
formation of ethyl 5-hydroxy-4,5-diphenyl-2-propylpenta-2,3-
dienoate (5ha) was not observed, 5ha prepared from
a reported procedure[5a] reacted with tBuONa in THF at
508C and afforded (E)-3ha in 71% yield.
Furthermore, the reaction of the optically active 2,3-
allenoate (Sa)-2i, having a defined Sa axial chirality in the
allene moiety,[9] with 1a under the optimized reaction
conditions afforded racemic (E)-3ai [Eq. (1)].
Entry
Ar
t [h]
Yield [%]
1
2
3
4
5
p-ClC6H4 (1a)
p-BrC6H4 (1b)
p-NCC6H4 (1 f)
C6H5 (1h)
3
3
4
2
8
42 (cis-4aa)
43 (cis-4ba)
36 (cis-4 fa)
45 (cis-4ha)
30 (cis-4la)
Pyridin-4-yl (1l)
[a] The reaction was carried out with 1.0 mmol of allenoate, 1.5 equiv of
aldehyde, 1.5 equiv of KOH, and 10 mol% of CuI at 508C in 6.0 mL of
1,4-dioxane in open air. Yield is that of isolated product.
Based on these results, we envisioned that anion cis-IM3
may be the key intermediate for this transformation as
depicted in Scheme 3. In the presence of tBuOÀ, the
deprotonation of the g-hydrogen atom of 2 would occur,
thus affording the intermediate IM1 or its equivalent.[5a,6]
Then g-addition of the aromatic aldehyde 1 to IM1 affords
the 5-hydroxy-2,3-allenoate anion IM2, which could be
followed by intramolecular nucleophilic oxygen attack at
the b-position to form the four-membered ring cis-4. Finally,
À
cis-4 undergoes ring cleavage at the C O bond, thus
generating IM3 which is then isomerized to the acyclic
products, highly functionalized a,b-enones E-3. The stereo-
selectivity is believed to be determined by the cis orientation
of Ar1 and Ar2 in cis-IM3.
Scheme 4. Transformations of (E)-3aa. DMF=N,N-dimethylform-
amide, LB-Phos=2,4,6-trimethoxyphenyldi(cyclohexyl)phosphine.
sterically bulky ligand LB-Phos (Scheme 4).[15] As usual, the
reaction of (E)-3aa with 3-iodoprop-1-ene using NaH as
a base in DMF or 3-bromoprop-1-yne in the presence of
K2CO3 afforded the synthetically attractive diene (E)-7aa[16]
and enyne (E)-8aa,[17] respectively, without the formation of
any O-alkylation products. Interestingly, the ester function-
ality in (E)-3aa may be selectively reduced even in the
presence of the ketone, thus affording (E)-9aa.[18]
In conclusion, we have developed a metal alkoxide
promoted condensation between aromatic aldehydes and
alka-2,3-dienoates, a reaction which provides a highly effi-
cient synthetic approach to 4,5-diaryl-3-oxoalken-4(E)-
enoates and the 2-alkylideneoxetanes cis-4. Considering the
efficiency and the observed stereoselectivity, the mechanism,
the highly functionalized nature of the products 3 and 4, and
the easy availability of the starting materials (see the
Supporting Inforamtion for details), this transformation will
be of high importance to organic chemists. Additional studies
are being carried out in our laboratory.
Scheme 3. Proposed mechanism.
Because of the existence of the reactive oxetane[10] and the
exocyclic double bond,[11] the in situ generated oxetanes cis-4
may be widely employed as useful building blocks in the
construction of complicated organic molecules. However, the
literature on the synthesis of cis-4 is limited.[12–14] Herein, after
studying the effect of solvent, base, and catalyst, the
optimized reaction conditions for affording cis-4a were
identified (see Tables S1–S3). The scope of the aromatic
aldehydes 1 was investigated as shown in Table 4.
In addition, g,d-unsaturated b-ketoesters have emerged as
useful synthons in organic synthesis (Scheme 4). The C Cl
bond in (E)-3aa can easily be converted into the biphenyl
product (E)-6aa in 78% yield by using the electron-rich and
À
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Chem. Res. 2003, 36, 701 – 712; d) A. Hoffmann-Rçder, N.
Received: November 23, 2013
Revised: December 23, 2013
Published online: February 19, 2014
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Keywords: aldehydes · alkali metals · allenes ·
.
reaction mechanisms · synthetic methods
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