Highly enantioselective alk-2-enylation of aldehydes through an allyl-transfer reaction

Article abstract of DOI:10.1002/anie.200390327

Enantiopure homoallylic alcohols such as 1 (conveniently prepared from alk-2-enyl metal reagents with (-)- or (+)-menthone) serve as alk-2-enyl donors to aldehydes. The allyl-transfer reaction, which proceeds via a chairlike six-membered-ring transition s

Full text of DOI:10.1002/anie.200390327

Angewandte
Chemie
stereogenic centers [Eq. (1)].[1,2d] Not only can this type of
reaction provide homoallylic alcohols with high enantiomeric
purity, it can also provide modified chiral building blocks after
ꢀ
suitable functionalization of the C C double bond in the
introduced allylic unit.
Highly enantiopure homoallylic alcohols have been
prepared by using allylic organometallic reagents
¼
(MCH2CH CH2; M = metal) and a stoichiometric amount
of a chiral auxiliary[3] or a catalytic amount of a chiral
promoter.[4] Furthermore, allylation reactions with g-substi-
tuted organometallic reagents 4 in the presence of a chiral
auxiliary or catalyst afford the g adduct 5 diastereo- and
enantioselectively.[5] This C C-bond-formation reaction is
ꢀ
particularly useful for the construction of vicinal stereogenic
centers in a flexible hydrocarbon chain [Eq. (2)].
However, to the best of our knowledge, no asymmetric
alk-2-enylation reaction (e.g. crotylation) of aldehydes to give
4-substituted homoallylic alcohols 6 (a adduct) has yet been
reported [Eq. (3)]. This is because the allylation reactions by
alk-2-enyl metal reagents 4 proceed, without exception, with
allylic transposition via a six-membered cyclic transition state,
affording exclusively the g adduct 5.[6]
Allyl-Transfer Reactions
Highly Enantioselective Alk-2-enylation of
Aldehydes through an Allyl-Transfer Reaction**
Junzo Nokami,* Kenta Nomiyama, Seiji Matsuda,
Nobuyuki Imai, and Kazuhide Kataoka
The enantioselective allylation of aldehydes to prepare
optically pure homoallylic alcohols is one of the most
attractive and popular methods for the construction of
Recently we discovered an efficient and stereoselective
nucleophilic alk-2-enylation reaction of aldehydes that pro-
duces the desired a adduct 6. In this procedure, no allylic
metal nucleophiles are involved, and a homoallylic alcohol 7
acts as an allyl donor. This unusual allylation reaction appears
to proceed through a 2-oxonium [3,3] sigmatropic rearrange-
ment, as shown in Scheme 1.[2a] The acid-catalyzed reaction of
aldehyde 1 with the homoallylic alcohol (g adduct) 7 stereo-
selectively gave the homoallylic alcohol a adduct 6 by an allyl
transfer from 7 via hemiacetal (H1) and oxonium cations T1
and T2.
More importantly, the Sn(OTf)2-catalyzed (10 mol%)
reaction of 3-phenylpropanal (1a) with optically pure
allyl donor (3R,4S)-1-phenyl-4-methylhex-5-en-3-ol (5a;
> 99% ee) gave (3S)-1-phenylhept-5-en-3-ol (6a) in 85%
yield with > 98% ee (Scheme 2).[2b,c]
[*] Prof. Dr. J. Nokami, K. Nomiyama, S. Matsuda, Dr. N. Imai,
K. Kataoka
Department of Applied Chemistry
Okayama University of Science
1-1 Ridai-cho, Okayama 700-0005 (Japan)
Fax: (+81)86-252-6891
E-mail: nogami@dac.ous.ac.jp
[**] We gratefully acknowledge Prof. I. E. Markó, Belgium, for his kind
discussion during this work. This work was financially supported by
the Nagase Science and Technology Foundation, a Grant for
Cooperative Research administered by the Japan Private School
Promotion Foundation, and a Grant in Aid for Scientific Research
from the Ministry of Education, Science, Culture, and Sports, Japan
(No. 12450357).
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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R
R1
OH
RCHO
1
O
–OH–
γ
+
α
OH
Sn(OTf)2
O
H1
hemiacetal
+
7 (crotyl donor)
OH
R1
MgCl
8
R1
+
O
+
O
R
R
OH
+ OH–
R1 = Me
9
OH
E
α
– acetone
R
Scheme 3. Stereoselctive reaction of (ꢀ)-menthone with alk-2-enylmag-
T1
stable C+
T2
6
more stable C+
less-hindered structure
& more-stable olefin
nesium chloride.
Scheme 1. Crotylation of aldehydes by an allyl-transfer reaction from
crotyl donor 7 to aldehyde 1.
RCHO
H
H
RCHO 1a
(R)-8a
+
+
OH
OH
pTSA•H2O
(cat.)
(10 mol%)
O
O
R1
R1
R
R
S
S
R
R
R
Sn(OTf)2
R
R = CH2CH2Ph
R1= Me
(10 mol%)
5a
6a
R = CH2CH2Ph
OH
H2O
R1
Scheme 2. Conversion of a g adduct into the a adduct through an
allyl-transfer reaction.
– menthone
R
6a
This result suggested that more broadly applicable chiral
alk-2-enyl donors could be prepared from cheap and readily
available optically pure ketones such as (ꢀ)- or (þ)-men-
thone. Thus, reaction of (E)-but-2-enylmagnesium chloride
4a (R1 = Me) with (2S,5R)-(ꢀ)-menthone gave a mixture of 1-
substituted menthols in good yield. Based on the reported
stereochemistry of the addition of Grignard reagents to
menthone,[7,8] the major product, isolated in 70–77%, was
determined to be one of the diastereomers of axial alcohols
(R)- or (S)-8a (Scheme 3).
RCHO
H
H
(S)-8a
+
+
pTSA•H2O
(cat.)
O
O
R1
R
R
S
R1
S
R = CH2CH2Ph
R1= Me
no reaction
Scheme 4. Stereospecific allyl-transfer reaction from (R)-8a to aldehyde
1a.
Both the diastereomers of 8a were readily separated and
independently used as crotyl donors in the allyl-transfer
reaction with 3-phenylpropanal 1a in the presence of p-
toluenesulfonic acid monohydrate (pTSA·H2O) as the cata-
lyst. Remarkably, whilst the major
homoallylic alcohol g adducts 8b–e selectively. Moreover,
stereochemically pure 8b–d were readily isolated in good
yields by chromatography on silica gel. The structures of the
minor by-products were assigned as the corresponding
isomer gave (5E,3R)-1-phenyl-
Table 1: Preparation of chiral allyl donors 8 from (ꢀ)-menthone.
hept-5-en-3-ol 6a in good yield
with > 99% ee, the minor isomer
did not react at all. Therefore, the
configuration of the major isomer
could be assigned as (R)-8a as
shown in Scheme 4.[2d]
To investigate this asymmetric
allylation further, we prepared
other chiral alk-2-enyl-donors,
from (ꢀ)-menthone[9] as shown in
Table 1. These chiral alk-2-enyl-
donors have substituents R1 (R1 =
γ
α
R1
M
M = MgCl, ZnBr, Ti(OiPr)3
O
(–)-menthone
R1
H
α
H
γ
γ
R1
R1
OH
OH
OH
α adduct 9
(R)-8
(S)-8
γ adduct
Entry
Allylic nucleophile
M
T [8C]
Yield [%][a]
(R)-8 (S)-8
R1
[equiv]
9
1[b]
2[b]
3[c]
4[d]
Et
MgCl
MgCl
ZnBr
Ti(OiPr)3
1.5
1.5
2.0
2.0
0
0
0
ꢀ78
b
c
d
e
79
82
66
11
11
0
trace
1.3
21
nPent
Cl(CH2)3
CH2 CH
¼
Et, nPent, (CH2)3Cl, CH CH2 in 8
in place of the methyl substituent
70[e]
5.0
¼
(R1 = Me) in crotyl donor 8a.
[a] Yield of isolated product. [b] The reaction was performed with (ꢀ)-menthone (10 mmol) and alk-2-
enylmagnesium chloride (derived from magnesium (15 mmol) and 1-chloroalk-2-ene (15 mmol)) in
THF at 08C for 2 h. [c] The reaction was performed with (ꢀ)-menthone (0.5 mmol) and 1-bromo-6-
chlorooct-2-ene (1 mmol), Zn (1 mmol), NH4Ac (1 mmol), in THF (2 mL) at 08C for 4 h. [d] The reaction
was performed with( ꢀ)-menthone (5 mmol) and pentadienyltitanium reagent (derived from KOtBu
(10 mmol), nBuLi (10 mmol), penta-1,3-diene (15 mmol), and ClTi(OiPr)3 (10 mol) in hexane) in THF at
ꢀ788C. [e] The allylic carbon atom is not stereogenic.
In all cases, the addition of the
allylic metal reagent to the carbon-
yl of (ꢀ)-menthone involved an
equatorial attack (favoring the car-
bonyl face trans to the isopropyl
group) to give the corresponding
1274
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Angew. Chem. Int. Ed. 2003, 42, No. 11

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Chemie
Table 2: Homoallylic alcohol a adducts 6 by allyl-transfer reaction from allyl-donors 8.[a]
pTSA·H2O-catalyzed allyl-transfer
R1
reaction in good yield and with
> 99% ee. To the best of our
knowledge, this is the first report
of such an asymmetric alk-2-enyla-
tion of aldehydes.[3] The chiral allyl
donors are conveniently prepared
from simple alk-2-enyl metal
reagents (e.g. Grignard reagents)
and inexpensive (ꢀ)- and (þ)-men-
thone. Finally, the absolute stereo-
chemistry of the final adducts can
be readily predicted from the con-
formational analysis of the six-
membered-ring chair-like transi-
tion state depicted in Scheme 4.
OH
RCHO
R1
H
α
γ
α
pTSA•H2O
(cat.)
R
OH
(R)-8
6
menthone
Entry
Allyl donor 8
R1
Aldehyde
R
Product 6[b]
Yield [%][c]
[equiv]
ee [%][d]
1
2
3
4
5
6
7
8
a
a
b
b
b
b
b
b
b
c
c
c
c
d
d
d
e
e
e
Me
Me
Et
Et
Et
Et
Et
Et
2
1
1
2
1
2
1
2
1
1
2
1
1
1
1
1
2
2
2
Ph(CH2)2
Ph(CH2)2
Ph(CH2)2
Ph
BnO(CH2)5
PhCHMe
PhS(CH2)2
Et2CH
a
a
b
c
d
e
f
g
h
i
83 (70)
75
85
61
88
68
90
71
75
92 (89)
72
93
88
92
96
95
83
83
>99 (97.0)
>99
>99
>99
>99
>99
>99
>99[e]
>99[e]
>99 (99.2)
99.4
>99
>99
>99
>99
>99
>99
>99
>99
¼
9
Et
CH2 CH(CH2)8
10
11
12
13
14
15
16
17
18
19
nPent
nPent
nPent
nPent
Cl(CH2)3
Cl(CH2)3
Cl(CH2)3
CH2 CH
CH2 CH
CH2 CH
Ph(CH2)2
Ph
j
Received: December 2, 2002 [Z50682]
BnO(CH2)5
PhS(CH2)2
Ph(CH2)2
BnO(CH2)5
PhS(CH2)2
Ph(CH2)2
BnO(CH2)5
PhS(CH2)2
k
L
m
n
o
p
q
r
Keywords: alcohols · aldehydes ·
.
allylation · asymmetric synthesis ·
homogeneous catalysis
¼
¼
¼
63
[1] For reviews on the reaction using
allyl(ic) metals, see: a) Y. Yama-
moto, N. Asao, Chem. Rev. 1993,
93, 2207 – 2293; b) J. A. Marshall
in Lewis Acids in Organic Syn-
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and related reactions, see: c) S. E.
Denmark, N. G. Almstead in
[a] The reactions were performed with allyl donor 8, (1 mmol, 2 equiv or 0.5 mmol, 1 equiv), aldehyde
(0.5 mmol), and pTSA·H2O (10 mol%) in CH2Cl2 (1 mL), at 208C for 20 h, unless otherwise noted.
[b] Yield and ee value of the corresponding product derived from the crude 8 are shown in parentheses.
[c] Yield of isolated product. [d] Determined by HPLC analysis (CHIRALCEL OD, iPrOH (5%) in hexane
as eluent) unless otherwise noted. >99 means that no signal of the corresponding enantiomer.
[e] Determined by HPLC analysis (CHIRALPAK AD, iPrOH (5%) in hexane as eluent) of the
corresponding MTPA ester derived from (þ)-MTPA. MTPA=a-methoxy-a-(trifluoromethyl)benzenea-
cetyl chloride.
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Chemistry
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Asymmetric 2-alkenylation of aldehydes by the allyl
transfer reaction from these chiral allyl donors 8b–e was
successfully carried out using pTSA·H2O as the catalyst. In all
cases, the desired, optically pure a adducts were obtained in
excellent yields.[2d] These results are summarized in Table 2.
Notably, allyl-transfer reactions using a crude mixture of 8
(containing all the other stereoisomers) and 3-phenylpropa-
nal gave the desired product in good yield and with high
ee values (Table 2, entries 1, 10). This result suggests that the
major isomer is far more reactive than the other stereo-
isomers.
Finally, highly enantioselective 2,4-pentadienylation of
aldehydes with 8e were carried out (Table 2, entries 17–19).[10]
Although the allylic carbon atom of 8e is not stereogenic,
only the E isomer of the final adduct is obtained. These results
showthat only one of the diastereotopic vinyl groups of 8e is
transferred. This observation reinforces even further our
proposed transition state, implying the R absolute stereo-
chemistry at the g-stereogenic center of allyl-donors 8.
In summary, we have discovered a novel asymmetric alk-
2-enylation reaction, using (ꢀ)- and (þ)-menthone as the
chiral auxiliaries, which gives the homoallylic alcohols in a
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Although almost all these reactions gave selectively g adduct,
there are a fewexceptions. For example, reaction of aldehydes
with a penta-2,4-dienylsilyl compound with Lewis acids gave the
corresponding a adduct. We assumed that the products will be
prepared by an allyl-transfer reaction from the g adduct to the a
adduct. This will be discussed elsewhere.
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[8] Commercially available (ꢀ)-menthone (90% purity, containing
ca. 5% of isomenthone) was not suitable for our purpose,
because it reacted with crotylmagnesium chloride to give a
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