α,β-unsaturated aldehydes as substrates for asymmetric C-C bond forming reactions with thiamin diphosphate (ThDP)-dependent enzymes

Article abstract of DOI:10.1002/adsc.200700550

The enzymes benzaldehyde lyase (BAL) from Pseudomonas fluorescens, benzoylformate decarboxylase (BFD) from Pseudomonas putida and pyruvate decarboxylase (PDC) from Saccharomyces cerevisiae provide different C-C bond forming possibilities of α,β-unsaturated aldehydes with aliphatic and aromatic aldehydes. Structure elucidation and determination of the absolute configuration of the products, which were obtained with high regio- and stereoselectivity were carried out. Selective 1,2-reactivity with yields of 75% and >98% ee, for one single isomer (A) were obtained, by choosing the suitable enzyme in combination with the appropriate substrates. By varying enzymes or substrates the regioisomeric hydroxy ketones C, with up to >99% ee, can be obtained. The application of these new chiral building blocks in the synthesis of natural products or biological active substances is considerably facilitated by applying the different ThDP-dependent enzymes as catalysts.

Full text of DOI:10.1002/adsc.200700550

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DOI: 10.1002/adsc.200700550
a,b-Unsaturated Aldehydes as Substrates for Asymmetric
À
C C Bond Forming Reactions with Thiamin Diphosphate
(ThDP)-Dependent Enzymes
ACHTREUNG
Anabel Cosp,a,e Carola Dresen,b,e Martina Pohl,c Lydia Walter,b Caroline Rçhr,d
and Michael Müllerb,*
a
Institute of Biotechnology 2, Research Center Jülich, 52425 Jülich, Germany
Institute of Pharmaceutical Sciences, Albert-Ludwigs-Universität Freiburg, Albertstr. 25, 79104 Freiburg, Germany
b
Fax : (+49)-761-203-6351; e-mail: michael.mueller@pharmazie.uni-freiburg.de
Institute of Molecular Enzyme Technology, Heinrich-Heine University Duesseldorf, Research Center Jülich, 52426 Jülich,
c
Germany
d
Institute of Inorganic and Analytical Chemistry, Albert-Ludwigs-Universität Freiburg, Albertstr. 21, 79104 Freiburg,
Germany
e
These authors contributed equally to this work
Received: November 21, 2007; Published online: March 17, 2008
Supportinginformation for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: The enzymes benzaldehyde lyase (BAL) buildingblocks in the synthesis of natural products
from Pseudomonas fluorescens, benzoylformate de- or biological active substances is considerably facili-
carboxylase (BFD) from Pseudomonas putida and tated by applyingthe different ThDP-dependent en-
pyruvate decarboxylase (PDC) from Saccharomyces zymes as catalysts.
À
cerevisiae provide different C C bond formingpossi-
bilities of a,b-unsaturated aldehydes with aliphatic
Abbreviations: BAL, benzaldehyde lyase; BFD,
and aromatic aldehydes. Structure elucidation and benzoylformate decarboxylase; PDC, pyruvate de-
determination of the absolute configuration of the carboxylase; His, hexahistidine; 2-HPP, 2-hydroxy-1-
products, which were obtained with high regio- and phenylpropan-1-one; PAC, phenylacetylcarbinol;
stereoselectivity were carried out. Selective 1,2-reac- NTA, nitrilotriacetic acid; ThDP, thiamin diphos-
tivity with yields of 75% and >98% ee, for one phate; wt, wild-type.
single isomer (A) were obtained, by choosingthe
suitable enzyme in combination with the appropriate
substrates. By varyingenzymes or substrates the re-
Keywords: benzaldehyde lyase; benzoylformate de-
gioisomeric hydroxy ketones C, with up to >99% ee, carboxylase; biocatalysts; pyruvate decarboxylase;
can be obtained. The application of these new chiral stereochemistry
Introduction
donor and aliphatic aldehydes as acceptor give rise to
2-hydroxy-1-phenyl-1-propanone derivatives with S
Thiamin diphosphate (ThDP)-dependent enzymes are absolute configuration in the case of BFD catalysis[5]
well-known to catalyse a broad range of asymmetric and R for BAL.[6] If aromatic aldehydes are employed
reactions.[1] Enzymes of this class like benzaldehyde as donor and acceptor, (R)-benzoins are optained
lyase (BAL) from Pseudomonas fluorescens,[2] ben- with both enzymes.[7,8] (R)-Phenylacetylcarbinol is
zoylformate decarboxylase (BFD) from Pseudomonas formed in the PDC-catalysed reaction of decarboxy-
putida[3] and pyruvate decarboxylase (PDC) from Sac- lated pyruvate with benzaldehyde as acceptor alde-
charomyces cerevisiae[4] enable various modes of C C hyde.[4,9]
bond formingreactions. Besides the physiological re-
À
Here we describe the use of a,b-unsaturated alde-
À
activity, several of these enzymes can catalyse, for ex- hydes as substrates for C C bond ligation in BAL-,
À
ample, the C C bond formation between two alde- BFD- and PDC-catalysed reactions. These substrates
hydes, giving chiral 2-hydroxy ketones with high enan- might act as donor and as acceptor for the 1,2- (A, C)
tioselectivity. The reactions of aromatic aldehydes as and the 1,4-addition (B, D) (Scheme 1).
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Anabel Cosp et al.
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Scheme 1. ThDP-dependent catalytic reaction possibilities of a,b-unsaturated aldehydes.
In reactions accordingto Equation (1) the a,b-un- protected hydroxy ketones in the presence of sodium
saturated aldehyde is bound to ThDP and reacts as hydride and butanol.[18] a,b-Unsaturated-a’-acetoxy
donor resultingin the formation of hydroxy ketone A ketones have been submitted to lipase-catalysed race-
or hydroxy enone B. In reactions accordingto Equa-
tion (2) the a,b-unsaturated aldehyde is attacked by
mic resolution.[19]
It is our aim to extend the donor-acceptor concept
an “active aldehyde” and reacts as acceptor resulting developed[8] and to broaden the scope of reactivity by
in the formation of hydroxy ketone C or 1,4-diketone usingaromatic and aliphatic a,b-unsaturated alde-
D.
hydes as substrates in ThDP-dependent enzymatic
The aim of the work presented here was to evaluate transformations.
the regio- and stereoselectivity of the enzymes BAL,
BFD and PDC when using a,b-unsaturated aldehydes
in combination with a second aldehyde – pyruvate in Results and Discussion
case of PDC – as substrates. The resultingenantioen-
riched hydroxy ketones A and C, which are difficult a,b-Unsaturated Aldehydes as Donor Substrates
to synthesise by other means, are valuable building [Scheme 1, Equation (1)]
blocks for organic synthesis.[10] These structural moiet-
ies are found in diverse natural products.[11] Moreover, We started evaluatingthe reaction between different
they can be easily converted into other building a,b-unsaturated aldehydes (1a–i) and acetaldehyde
blocks by functional group transformations, such as (2) catalysed by BAL and wild-type BFD (BFDwt).
reduction of or nucleophilic addition to the carbonyl In addition to the wild-type enzymes the variant
function and oxidative cleavage of or addition to the BFD-H281A,[20] which showed a broader substrate
double bond.[12,13]
range in earlier studies, was used (Table 1).[8] We as-
Hydroxy ketones of type A and C have been syn- sumed that in the presence of acetaldehyde the a,b-
thesised amongothers by direct oxidation of enolates,
unsaturated aldehydes 1a–i would act as donors in
although with low levels of stereoinduction,[14] by this kind of condensation, in a similar fashion as aro-
treatment of chiral hydroxy acids with a vinyllithium matic aldehydes do. Nevertheless, the ambident
reagent,[15] by addition of 1-lithio-1-methoxyallene to nature of the umpoled a,b-unsaturated aldehydes[21]
a chiral ketone followed by hydrolysis,[16] by addition could cause some regioselectivity problems since the
of an umpoled a,b-unsaturated acyl anion to alde- electrophilic addition of acetaldehyde can occur in a
hydes,[17] or by aldol reaction between aldehydes and 1,2- or 1,4-fashion. In fact, only one regioisomer aris-
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a,b-Unsaturated Aldehydes as Substrates for Asymmetric C C Bond FormingReactions
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Table 1. Reaction of the a,b-unsaturated aldehydes 1a–i and acetaldehyde (2) catalysed by BAL, BFDwt or BFD-H281A.
Entry
R
R’
Aldehyde
Enzyme
Product
Isolated Yield
ee
Abs. config.
1
2
3
4
5
6
7
8
Ph
H
1a
BAL
(R)-3a
3a
80%[a]
<1%
<1%
75%[a]
<1%
<1%
<1%
<1%
<1%
48%[a]
23%[a]
94%
80%
-
-
> 96%
-
-
-
R
-
-
R
-
-
-
-
-
R
R
BFDwt
BFD-H281A
BAL
BFDwt
BFD-H281A
BAL
BFDwt
BFD-H281A
BAL
Ph
CH3
Br
1b
1c
(R)-3b
3b
Ph
(R)-3c
3c
-
-
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Furyl
H
1d
1e
(R)-3d
(R)-3e
39%[b]
<1%
3f
50%
> 98%
S
-
-
87%
24%
27%
77%
23%
-
-(CH2)4-
BAL
(S)-3e
BFDwt
BFD-H281A
1f
1g
-
-
H
CH2CH3
H
BAL
BAL
41%[c]
80%[d]
79%[d]
63%[d]
63%[d]
26%[d]
<1%
A
3g
n.d.[e]
n.d.[e]
n.d.[e]
n.d.[e]
n.d.[e]
-
BFDwt
BFD-H281 A
BAL
BFDwt
BFD-H281A
BAL
A
H
1h
1i
3h
3i
CH2CH3
CH3
71%[d]
<1%
>98%
-
-
-
BFDwt
[a]
Reaction performed in preparative scale (100 mL) with 5–10 mM a,b-unsaturated aldehyde (1a, b, d, e), 450–500 mM 2
and 150–180 mglyophilised powder containing30% of pure BAL alongwith phosphate buffer, ThDP and MgSO
Reaction performed in 15 mL reaction volume.
Accordingto GC-MS. Reaction performed on an analytical scale (1.5 mL).
Conversion (mol%) accordingto NMR. Reaction performed on an analytical scale (1.5 mL).
Accordingto chiral phase HPLC the same absolute configuration.
.
4
[b]
[c]
[d]
[e]
ingfrom the attack at carbon C-1 of compounds 1a–i over, hexenal (1g) and octenal (1h) were also donor
was observed resultingsolely in the formation of
products 3 (Table 1).
substrates for BFDwt in the presence of acetaldehyde
(entry 16 and 19). The only substrate for BFD-H281A
BAL catalyses the reaction of 2 and several a,b-un- in the ligation with acetaldehyde was hexenal (1g)
saturated aldehydes 1 to give products of type (R)-3, with moderate conversion to give 3g with poor enan-
with yields ranging from very good (entries 1 and 4) tioselectivity (entry 17).
to poor (entry 11), and enantioselectivities spanning
The enantioselectivity of the products obtained was
from excellent (entries 4 and 11) to moderate (en- determined by chiral phase HPLC and preparation of
tries 1 and 10). All tested unsaturated aldehydes, the Mosher ester derivative by reaction of (R)-3b [or
except a-bromocinnamaldehyde (1c, entry 7) were ac- (S)-3e, respectively] with Mosherꢁs acid chloride.
cepted as a donor substrate by this enzyme. This
The absolute configuration was assigned according
should not be greatly affected by variation of the sub- to Mosherꢁs method,[24] showingthat R-configured
strate ratio, as shown for other substrate combinations products arise from BAL catalysis (see entries 1, 4, 10
in BAL catalysis. Nevertheless, influence of substrate and 11) while the S enantiomer is gained in one case
ratio on donor/acceptor properties are well known for with BFDwt as a catalyst (entry 12). Surprisingly,
other ThDP-dependent enzymes.[22]
usinghexenal
(
1g, entry 16) and octenal (1h,
With BFDwt as a catalyst cyclohex-1-enecarbalde- entry 19), both BAL and BFDwt yielded products
hyde (1e) was converted to (S)-3e with moderate with the same absolute configuration as was deter-
yield and good enantioselectivity (entry 12).[23] More- mined by chiral phase HPLC.
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Table 2. Reduction of (R)-3a, (R)-3b, (R)-4a, (R)-4b to the correspondingdiols.
Entry
Substrate
Reductant
NaBH4
Yield of diol
anti:syn
1
2
3
4
5
(R)-3a (R’=H, R’’=H)
84%
50:50
78:22
12:88
93:7
Zn
PhMe2SiH/TBAF
Zn(BH4)2
PhMe2SiH/TBAF
A
70%
(R)-4a (R’=H, R’’=OAc)
(R)-3b (R’=CH3, R’’=H)
(R)-4b (R’=CH3, R’’=OAc)
46%[a]
80%
AHCTREUNG
65%[a]
7:93
[a]
Yield over two steps.
These results were confirmed by chiroptical correla- turned our attention to extend the scope of this reac-
tions. The optical rotation of (R)-4a (ee=80%), the tion to other acceptor substrates. The a-heteroatom-
O-acetylated derivative of (R)-3a, was determined as substituted acetaldehydes[28,29] 8, 9 are promisingsub-
[a]25: + 18.68 (CHCl3), while the optical rotation of strates as the putative products 11, 12 of the reaction
theD known E-(S)-2-acetoxy-5-phenyl-4-penten-3-one would open a wide range of further transformations
[(S)-4a] (ee >95%) is described to be [a]2D5: À298 towards more complex structures. The use of formal-
(CHCl3).[12]
dehyde (7)[30] seemed attractive, because the elongat-
Diastereoselective reduction of the carbonyl func- ed products are not easy to obtain selectively by
tion of (R)-3a and (R)-3b should give access to anti- chemical methods.[31]
diols (5a or 5b) or syn-diols (6a or 6b), respectively
(Table 2).
The reactions were run on an analytical scale and
the results are summarised in Table 3. The product
For the preparation of the anti-diols 5a and 5b hy- yields arisingfrom the condensation with formalde-
droxy ketones 3a and 3b were reduced with Zn- hyde are very high (entries 1, 4, 10 and 13) with the
ACHTREUNG(BH4)2, since this reductant is known to transform 2- noticeable exception of the condensation with 1d
hydroxy ketones into the anti-diols predominantly.[25] (entry 7). Compounds 10a, 10b and 10e were synthes-
Accordingly, the corresponding anti-diols were isolat- ised on a preparative scale and isolated in 51%, 82%
ed with good yields and moderate (5a) to good (5b) and 56% yield, respectively.
diastereoselectivities (Table 2). In contrast to Zn-
Acetaldehyde derivatives 8, 9 behave differently
ACHTREUNG(BH4)2, chelation with NaBH4 is not sufficient to dependingon which unsaturated aldehyde is used as a
induce high stereocontrol.
donor substrate. For instance, cinnamaldehyde (1a),
Comparison of the optical rotations of 5a and 5b a-methylcinnamaldehyde (1b), furylacrolein (1d), and
with literature data[26,27] corroborated the (2R,3S) con- 2-methylenebutanal (1f) gave poor to moderate yields
figuration of the predominant anti-diols, confirming (entries 2, 3, 5, 6, 8, 9, 14, 15), whereas the carbocyclic
the stereochemistry at C-2 of compounds 3a and 3b aldehyde 1e offers good results with respect to yield
to be R.
Diastereoselective syn reduction of (R)-4a and (R)-
4b was performed usingthe fluoride ion-catalysed hy-
(entries 11 and 12).
The BAL-catalysed condensation of 1e with 8 or 9,
(entries 11 and 12, Table 3) were performed on a 50-
drosilane reduction, a methodology developed by mL scale (Scheme 2). Moderate yields (24% of 11e,
Hiyama and co-workers.[18,19] After acetylating( R)-3a and 29% of 12e) were obtained. The ee measured by
and (R)-3b and treatingthe resulting a-acetoxyenones preparation of the Mosher ester derivative was 90%
(R)-4a and (R)-4b with the aforementioned reductant for 11e and 93% for 12e, showingthat these kinds of
the syn-diols 6a and 6b were obtained with moderate reactions occur with good stereochemical control. Ac-
to good yields (Table 2).
cordingto the analysis of the proton NMR of the
Since BAL proved to be a powerful biocatalyst for Mosher ester derivatives, the absolute configuration
the carboligation reaction of a,b-unsaturated alde- of the new created stereocentre is R.
hydes as a donor and acetaldehyde as an acceptor, we
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Table 3. Reaction between a,b-unsaturated aldehydes 1a, 1b, 1d–f and formaldehyde (7) or acetaldehyde derivatives 8 and 9
catalysed by BAL.
Entry
R
R’
Aldehyde
X
Product
Conversion[a]
1
2
3
4
5
6
7
8
Ph
H
1a
H
10a
11a
12a
10b
11b
12b
10d
11d
12d
10e
11e
12e
10f
11 f
12 f
92%
10%
60%
95%
25%
<1%
<1%
<1%
<1%
>99%
80%
76%
79%
58%
<1%
CH2OCH3
CH
H
CH2OCH3
CH
H
CH2OCH3
CH
H
ACHTRE(UNG OCH3)2
Ph
CH3
H
1b
1d
ACHTRE(UNG OCH3)2
Furyl
-(CH2)4-
H
9
ACHTRE(UNG OCH3)2
10
11
12
13
14
15
1e
CH2OCH3
CHACHTRE(UNG OCH3)2
1f
CH2CH3
H
CH2OCH3
CH(OCH3)2
AHCTREUNG
[a]
Accordingto GC-MS. Reactions performed on an analytical scale (1.5 mL) with 10 mM a,b-unsaturated aldehyde (1a–f),
60 mM 7, 8 or 9 and 1 mglyophilised powder containing30% of pure BAL alongwith phosphate buffer, ThDP and
MgSO4.
Scheme 2. Reaction of cyclohex-1-enecarbaldehyde (1e) and the acetaldehyde derivatives 8 and 9 catalysed by BAL.
In summary, a,b-unsaturated aldehydes act as cross-condensation, 16 and 17 arisingfrom self-con-
donors in the presence of acetaldehyde, acetaldehyde densation.
derivatives, or formaldehyde under BAL or BFDwt
Remarkably, the BAL-catalysed reaction of 1a or
catalysis, giving rise to condensation products of type 1b with different aromatic aldehydes (13a–d) produ-
3, 10, 11, and 12. The acceptor aldehyde adds in all ces only 2 of the 4 possible products, namely those of
cases to the carbonyl carbon of the a,b-unsaturated type 14 and 17. Products 15, 16 for which the a,b-un-
aldehydes (1,2-addition).
saturated aromatic aldehydes would have acted as the
To further broaden the scope of this condensation acceptor substrate,[32] were not observed under the
reaction the aliphatic acceptor aldehyde has been re- conditions tested.
placed by substituted aromatic aldehydes. Up to 4 dif-
The yields of the cross-condensation product 14 are
ferent compounds (each of them in two possible en- moderate as summarised in Table 4. The ee and abso-
antiomeric forms) are feasible, if selective 1,2-addi- lute configurations were determined by analysis of
tion is assumed (Table 4): 14 and 15 obtained by the proton NMR of the Mosher ester derivatives.
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Table 4. Reaction between a,b-unsaturated aromatic aldehydes 1a, b and benzaldehyde derivatives 13a–d catalysed by
BAL.[a]
Entry a,b-Unsaturated aldehyde Aromatic aldehyde Yield of 14 Yield of 17 ee
Absolute Configuration
1
2
3
4
5
6
7
8
1a, R’=H
13a, Y=H
13b, Y=2-F
13c, Y=2-Cl
13d, Y=3,5-CH3O
13a, Y=H
13b, Y=2-F
13c, Y=2-Cl
13d, Y=3,5-CH3O
14a: <1%
14b: 56%
14c: 40%
14d: 33%
14e: <1%
14f: <1%
17a: <1%
17b: 8%
-
-
-
-
-
-
17c: 18%
17d: 20%
17a: 50%
17b: <1%
14d: 72% 14d: R
-
-
1b, R’=CH3
-
-
14g:[b] 43% 17c:[b] 17% 14g: 99% 14g: R
14h: <1%
17d: 25%
-
-
[a]
Reactions performed on a preparative scale (50 mL) with 10 mM a,b-unsaturated aldehyde (1a, b), 10 mM aromatic alde-
hyde (13a–d) and 30 mglyophilised powder containing30% BAL.
Reaction performed in 30 mL reaction volume with 15 mglyophilised powder containing30% BAL.
[b]
Thus, we have shown that the donor-acceptor con-
Servi et al. reported that commercially available
cept described for aromatic aldehydes,[8] can be ex- (Sigma) purified PDC from Saccharomyces cerevisae
tended to the BAL-catalysed condensation of a,b-un- (ScPDC) was able to condense benzaldehyde (13a)
saturated aldehydes like 1a, b with substituted benzal- with pyruvate (18), but not a,b-unsaturated alde-
dehydes like 13a–d.
hydes.[35] However, it is known that bakerꢁs yeast cat-
alyses the transformation depicted in Scheme 3.[36,37]
This transformation is thought to occur as follows:
pyruvate, obtained by degradation of glucose, is con-
densed with the a,b-unsaturated aldehyde (1a–c) cata-
lysed by PDC to the hydroxy ketone 19 which is re-
a,b-Unsaturated Aldehydes as Acceptor Substrates
[Scheme 1, Equation (2)]
Pyruvate decarboxylase (PDC)[4] is known to catalyse duced in situ to the isolated diol ent-5a–c by means of
the reaction between benzaldehyde (13a) (acceptor an alcohol dehydrogenase (Scheme 3).[38]
substrate) and pyruvate (18) or acetaldehyde, respec-
Crude extract of E. coli cells overexpressingthe
tively, (activated acetaldehyde as donor substrate) to PDC gene from Zymomonas mobilis (ZmPDC) was
yield (R)-phenylacetylcarbinol [(R)-PAC].[33] We con- used to test the type of reactivity depicted in
ducted analoguous transformations by reacting a,b- Scheme 3. Compounds 1a–c were investigated as pos-
unsaturated aldehydes and pyruvate in the presence sible acceptors versus pyruvate (18) as donor. Forma-
of PDC (Table 5). Substrates 1a and 1g–i were expect- tion of (R)-phenylacetylcarbinol arisingfrom the con-
ed to act as the acceptor substrates. Although the am- densation of pyruvate (18) and benzaldehyde (13a)
bident nature of 1 could also give rise to 1,4-addition served as a positive control. Although the crude ex-
products[34] in PDC-catalysed reactions only 1,2-regio- tract catalyses the (R)-PAC-production, no condensa-
selectivity was observed (see below).
tion products were detected with 1a, 1b, or 1c. Thus,
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Table 5. Reaction of a,b-unsaturated aldehydes 1g–i and pyruvate (18) catalysed by ScPDC.
Entry
R
R’
Aldehyde
Product
Conversion[a]
ee[b]
1
2
3
C
H
H
Me
1g
1h
1i
19g
19h
19i
58
50
<2%
>98%
>98%
n.d.
[a]
Conversion (mol%) accordingto NMR. Reaction performed on an analytical scale (1.5 mL).
Accordingto chiral phase HPLC.
[b]
Scheme 3. Bakerꢁs yeast-catalysed transformation of a,b-unsaturated aldehydes.[36,37]
it can be assumed that ZmPDCwt is not suitable as aldehyde (2), a-heteroatom-substituted acetaldehydes
catalyst to perform this kind of transformations. (8 and 9), and formaldehyde (7) as well as additional
As already stated, bakerꢁs yeast is known to trans- aromatic aldehydes were the acceptor substrates. All
form a,b-unsaturated aldehydes 1a–c to chiral diols of them resulted in the hydroxy enones of type A.
ent-5a–c (Scheme 3),[35,36] and PDC from Saccharomy-
To find a possibility for gaining access to the iso-
ces cerevisiae (ScPDC) is thought to be one of the en- meric hydroxy enone C, we tested the reactivity of
zymes responsible for this transformation. E. coli the small a,b-unsaturated aliphatic aldehydes acrolein
BL21ACHTREUNG(DE3) expression strain was transformed with (1j), methylacrolein (1k) and crotonaldehyde (1l),
the pET22bPDC1 vector and enzyme expression was with aromatic aldehydes such as benzaldehyde (13a)
induced with IPTG. The cell-free extract was used for and 3,5-dimethoxybenzaldehyde (13b) with BAL as
the followingtests. The putative reaction between dif-
biocatalyst (Table 6).
ferent a,b-unsaturated aldehydes (1a and 1g–i) and
The reactions with 3,5-dimethoxybenzaldehyde
pyruvate was assayed (Table 5). Again, the reaction (13b) as donor gave the best conversion in all cases
between pyruvate (18) and benzaldehyde (13a) to tested, with methylacroleine (1k) showingthe best ac-
give (R)-PAC served as positive control experiment.
ceptor capacity. The enantiomeric excess >99% of
Whereas cinnamaldehyde (1a) and 2-methyl-2-pen- 20e was determined by chiral phase HPLC and by
tenal (1i) were poor substrates for ScPDC, the a,b-un- comparison of the Mosher esters prepared by reaction
saturated aldehydes hexenal (1g) and octenal (1h) act with (R)- and (S)-Mosherꢁs acid chloride, respectively.
as acceptor substrates with ScPDC. The resultinghy-
The absolute configuration was assigned according to
droxy enones 19g and 19h (C) are formed with excel- the Mosherꢁs method,[24] showingthe R configuration
lent enantioselectivity (>98% ee). Under BAL cataly- of 20e. Single crystal X-ray structure analysis was car-
ses, usingthe same substrates, the a,b-unsaturated al- ried out (Figure 1). Measurements of the torsion
À
À
À
dehydes 1g and 1h act as donors with acetaldehyde angle O2 C2 C1 O1 resulted in 12.5(2)8 and the
and give access to the isomeric hydroxy enones 3g angle O1 H2’ O2 was calculated to be 117(2)8. The
and 3h (A). distance between O2 and H2’ was calculated to be
À
À
Until now all tested a,b-unsaturated aldehydes 1a, 0.98(2) Š and between O1 an H2’ 1.99(2) Š, showing
1b and 1d–i act as donor, if BAL is the catalyst. Acet- that a hydrogen bond between the carbonyl oxygen
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Table 6. Reaction of benzaldehyde derivatives 13a, d and a,b-unsaturated aliphatic aldehydes 18j, k, l catalysed by BAL.
Entry
Aldehyde
Aldehyde
Product
Conversion[a]
ee
1
2
3
4
5
6
13a
1j
1k
1l
1j
1k
1l
20a
1%
12%
2%
36%
55%
25%
n.d.
>98%
n.d.
n.d.
>99%
n.d.
20b[b]
20c
13d
20d
20e[c]
20f
[a]
Accordingto GC-MS.
Reaction performed on a semipreparative scale (12 mL).
Reaction performed on a preparative scale (100 mL) with 20 mM 13b, 200 mM 1k and 80 mglyophilised powder contain-
[b]
[c]
ing30% BAL.
droxy ketones[8] (Figure 2) the results of the Mosher
method are confirmed.
Conclusions
In summary, we have demonstrated, that a,b-unsatu-
rated aldehydes are accepted as substrates by BAL,
BFD and ScPDC.
In BAL and BFDwt catalysis a,b-unsaturated aro-
matic aldehydes act as donors when combined with
various aldehydes such as formaldehyde, acetalde-
hyde, acetaldehyde derivatives or benzaldehyde deriv-
atives, affordinghihgly functionalised condensation
products with high regio- and stereoselectivity. Never-
theless, the same a,b-unsaturated aldehyde can act as
acceptor substrate as well, if other enzyme or sub-
Figure 1. Molecular structure of 20e showingthe conforma-
tion.
strate combinations are used.
Moreover, one enzyme can use selectively a,b-un-
saturated aldehydes as donor and as acceptor sub-
and the hydroxy hydrogen is present, typical for 2-hy- strates, which is shown through experiments with
droxy ketones. The single crystal X-ray structure anal- BAL. Small a,b-unsaturated aliphatic aldehydes react
ysis presents the rigid conformation of 20e. In combi- regio- and stereoselectively as acceptors in the BAL-
nation with the UV and circular dichroism of 20e catalysed reaction with an aromatic aldehyde as
compared with those of other (R)-configured 2-hy- donor.
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À
a,b-Unsaturated Aldehydes as Substrates for Asymmetric C C Bond FormingReactions
FULL PAPERS
Figure 2. CD spectra of (R)-1-(3,5-dimethoxyphenyl)-2-hydroxy-3-methylbut-3-en-1-one (20e, solid line) in comparison to
(R)-2-(2-iodophenyl)-1-(3,5-dimethoxyphenyl)-2-hydroxyethanone[8] (dashed line).
In the BAL- and BFD-catalysed reactions with Experimental Section
acetaldehyde a,b-unsaturated aliphatic aldehydes
react selectively as donor. In PDC-catalysed transfor- General Remarks
mations with pyruvate as substrate, which forms by
The characterisation data are available in the SupportingIn-
decarboxylation the activated acetaldehyde, the same
formation.
a,b-unsaturated aliphatic aldehydes react selectively
as acceptor.
In all cases the 1,2-regioselectivity is observed
soley. Thus, if a,b-unsaturated aldehydes are submit-
ted to C C bond formingreactions with different
ThDP-dependent enzymes, 1,2-addition products can
be chemically and stereoselectively synthesised.
Enzyme engineering should improve the scale-up po-
tential with these non-physiological substrates. This
opens the possibility for new retrosynthetic disconnec-
Representative Example for the Synthesis of (R)-2-
Hydroxy-4-penten-3-ones (R)-3
À
(R)-2-Hydroxy-5-phenylpent-4-en-3-one [(R)-3a]: Cinnam-
AHCTREaUNG ldehyde (1a) (126 mL, 132 mg, 1 mmol) and acetaldehyde
(2) (2.5 mL, 45 mmol) were added into a mixture of DMSO
(20 mL) and buffer A (75 mL) under an N2 atmosphere.
After addition of BAL (50 mglyophilised powder in 5 mL
tions in synthetic strategies. In future, the access to buffer A) the reaction mixture was slowly stirred at 258C.
Reaction process was monitored by GC-MS. Additional
amounts of BAL were added, after 24 h (25 mglyophilised
powder in 2 mL buffer A), after 3 days (50 mglyophilised
powder in 5 mL buffer A) and after 4 days (25 mglyophi-
lised powder in 2 mL buffer A). After 7 days, the mixture
was extracted with ethyl acetate (3”125 mL), the organic
layer was washed with water and brine and dried over
Na2SO4. Evaporation of the solvent and purification of the
crude product by column chromatography (SiO2, isohexane/
ethyl acetate, 5:1) afforded (R)-3a as a pale yellow oil;
yield: 139 mg(80%); ee=80%.
chiral buildingblocks like A and C for the synthesis
of natural products or biological and pharmacological
active substances should be considerably facilitated.
Furthermore, the still unexplored possibility of reac-
tions towards products B and D demonstrates the po-
tential of diversity-oriented approaches in biocatalytic
syntheses.
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(R)-2-Hydroxy-4-methyl-5-phenylpent-4-en-3-one
[(R)-
tracted with CHCl3 (3”10 mL). The organic layer was
washed with a saturated solution of NaHCO3 and brine and
dried over Na2SO4. Evaporation of the solvent afforded (R)-
4a; yield: 20 mg(98%); ee=80%.
(R)-4-Methyl-3-oxo-5-phenylpent-4-en-2-yl acetate [(R)-
4b]: (R)-3b (40.9 mg, 0.22 mmol) was submitted to the same
acylatingreaction conditions as for ( R)-3a affording( R)-4b;
yield: 43 mg(86%).
3b]: Reaction of a-methylcinnamaldehyde (1b) (140 mL,
146 mg, 1 mmol) with 2 under the same procedure as above
afforded after purification of the crude product by column
chromatography (SiO2, isohexane/ethyl acetate=5:1) (R)-3b
as a pale yellow oil; yield: 143 mg(75%); ee >96% [(R)-
and (S)-Mosher ester of (R)-3b, NMR] .
(R)-5-(Furan-2-yl)-2-hydroxypent-4-en-3-one [(R)-3d]: 3-
(Furan-2-yl)acrylaldehyde (1d) (123 mg, 1 mmol) and acetal-
dehyde (2) (2.8 mL, 50 mmol) were added into a mixture of
DMSO (20 mL) and buffer A (75 mL) under an N2 atmos-
phere. After addition of BAL (100 mglyophilised powder in
3 mL buffer A) the reaction mixture was slowly stirred at
258C. Reaction process was monitored by GCMS. Addition-
al amounts of BAL were added, after 24 h (50 mglyophi-
lised powder in 2 mL buffer A) and after 3 days (30 mg
lyophilised powder in 1 mL buffer A). After 5 days, the mix-
ture was extracted with ethyl acetate (3”125 mL), the or-
ganic layer was washed with water and brine and dried over
Na2SO4. Evaporation of the solvent and purification of the
crude product by column chromatography (SiO2, isohexane/
ethyl acetate=5:1) afforded (R)-3d as a brown oil; yield:
79 mg(48%); ee=50%.
AHCTREUNG
AHCTREUNG
a stirred solution of (R)-3a (33.5 mg, 0.19 mmol) in anhy-
drous Et2O at 08C under N2 atmosphere. After 1 hour dilut-
ed acetic acid (25% (v/v), 1 mL) was added and the mixture
was extracted with ethyl acetate (3”10 mL). The organic
layer was washed with water and brine and dried over
Na2SO4. Evaporation of the solvent and purification of the
crude product by column chromatography (SiO2, CH2Cl2/
MTBE=5:1) afforded 5a; yield: 24 mg(70%); ( dra:s =78:22
accordingto 1H NMR from the crude product).
AHCTRE(UNG 2R,3S)-4-Methyl-5-phenylpent-4-ene-2,3-diol (5b): (R)-3b
was submitted to identical reductive reaction conditions as
for (R)-3a. Work-up, evaporation of the solvent and purifi-
cation of the crude product by column chromatography
(SiO2, CH2Cl2/ethyl acetate=9:1) afforded 5b; yield:
32.2 mg(80%); ( dra:s =93:7 accordingto 1H NMR from the
crude product).
(R)-3-Cyclohexenyl-2-hydroxypropan-3-one [(R)-3e]: Re-
action of cyclohexen-1-carbaldehyde (1e) (51 mL, 49 mg,
0.45 mmol) with 2 under the same conditions as above af-
forded after purification of the crude product by column
chromatography (SiO2, isohexane/ethyl acetate=5:1) (R)-3e
as a colourless oil; yield: 16 mg(23%); ee> 98% (HPLC).
(S)-3-Cyclohexenyl-2-hydroxypropan-3-one [(S)-3e]: Cy-
clohexen-1-carbaldehyde (1e) (17 mL, 16 mg, 0.15 mmol)
and acetaldehyde (2) (420 mL, 7.5 mmol) were added into a
mixture of DMSO (3 mL) and buffer B (10 mL) under N2
atmosphere. After addition of BFDwt (11 mglyophilised
powder in 2 mL buffer B) the reaction mixture was slowly
stirred at 308C. Reaction process was monitored by GCMS.
Additional amounts of BFDwt were added, after 1 and 2
days. After 12 days, the mixture was extracted with ethyl
acetate (3”30 mL), the organic layer washed with water and
brine and dried over Na2SO4. Evaporation of the solvent
and purification of the crude product by column chromatog-
raphy (SiO2, isohexane/ethyl acetate=5:1) afforded (S)-3e;
AHCTRE(UNG 2R,3R)-5-Phenylpent-4-ene-2,3-diol (6a): To a hexame-
thylphosphoric triamide (545 mL) solution of (R)-4a
(23.3 mg, 0.107 mmol) and PhMe2SiH (20.5 mL, 0.13 mmol)
was added TBAF (1M in THF, 5.2 mL, 0.005 mmol) at 08C
and the mixture was stirred for 2.5 h at this temperature.
NaOH (1M in MeOH, 2 mL) was added and the reaction
mixture stirred for 30 min. After treatment with H2O
(2 mL), the solution was extracted with Et2O (3”15 mL)
and the organic layer was dried over Na2SO4. Evaporation
of the solvent and purification of the crude product by
column chromatography (SiO2, CH2Cl2/MTBE=6:1) afford-
ed 6a; yield: 9 mg(47%);
( dra:s =12:88 accordingto
1H NMR).
AHCTRE(UNG 2R,3R)-4-Methyl-5-phenylpent-4-ene-2,3-diol (6b): To a
hexamethylphosphoric triamide (600 mL) solution of (R)-4b
(40.4 mg, 0.17 mmol) and PhMe2SiH (33 mL, 0.21 mmol) was
added TBAF (1M in THF, 8.5 mL, 0.0085 mmol) at 08C and
the mixture was stirred for 4 h at this temperature. The reac-
tion was monitored by TLC and since no reaction was ob-
served, PhMe2SiH (33 mL, 0.21 mmol) and TBAF (1M in
THF, 8.5 mL, 0.0085 mmol) were added. After 16 h at 08C,
NaOH (1M in MeOH, 2 mL) was added and the reaction
mixture stirred for 30 min. After treatment with H2O
(2 mL), the solution was extracted with Et2O (3”15 mL)
and the organic layer was dried over Na2SO4. Evaporation
of the solvent and purification of the crude product by
column chromatography (SiO2, petroleum ether/ethyl ace-
tate=2:1) afforded 6b; yield: 25 mg(76%); ( dra:s =7:93 ac-
cordingto 1H NMR).
yield: 9 mg(39%); ee=94% (Lit.: ee=94%)[5]
.
2-Hydroxyoct-4-en-3-one (3g), 2-hydroxydec-4-en-3-one
(3h) and 2-hydroxy-4-methylhept-4-en-3-one (3i): 1 mg
lyophilised powder of BAL was incubated in 1.5 mL of
buffer A with 5% MTBE in presence of 30 mM hexenal
(1g), octenal (1h) or 2-methyl-2-pentenal (1i) and 450 mM
acetaldehyde (2) at 258C and 300 rpm. After 48 h the reac-
tions were stopped and extraction with CDCl3 was per-
formed.
1 mglyophilised powder of BFDwt or BFD-H281A was
incubated in 1.5 mL of buffer B with 5% MTBE in presence
of 30 mM hexenal (1g), octenal (1h) or 2-methyl-2-pentenal
(1i) and 450 mM acetaldehyde (2) at 258C and 300 rpm.
After 42 h the reactions were stopped and extraction with
CDCl3 was performed.
Reaction Conditions for 10–12a, 10–12b, 10–12d, 10–
12e, 10–12f
(R)-3-Oxo-5-phenylpent-4-en-2-yl acetate [(R)-4a]: To a
stirred solution of (R)-3a (16.8 mg, 0.095 mmol) in anhy-
drous pyridine (1 mL) acetyl chloride (23 mL, 0.32 mmol)
was added at room temperature under N2 atmosphere. After
3 h saturated NH4Cl solution was added and the mixture ex-
1 mglyophilised powder of BAL was incubated in 1.5 mL of
buffer A with 20% DMSO in presence of 10 mM donor al-
dehyde 1a, b, d–f and 60 mM formaldehyde (7), methoxy-
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À
a,b-Unsaturated Aldehydes as Substrates for Asymmetric C C Bond FormingReactions
FULL PAPERS
acetaldehyde (8) or dimethoxyacetaldehyde (9) at 238C and
300 rpm. Conversion was controlled by GC-MS.
1-(2-Fluorophenyl)-1-hydroxy-4-phenylbut-3-en-2-one
(14b): Cinnamaldehyde (1a) (63 mL, 66 mg, 0.5 mmol) and
2-fluorobenzaldehyde (13b) (53 mL, 62 mg, 0.5 mmol) were
added into a mixture of DMSO (10 mL) and buffer A
(38 mL) under an N2 atmosphere. After addition of BAL
(30 mglyophilised powder in 2 mL buffer A) the reaction
mixture was slowly stirred at 288C. Reaction process was
monitored by GC-MS. After 42 h, the mixture was extracted
with ethyl acetate (3”50 mL), the organic layer washed
with water and brine and dried over Na2SO4. Evaporation of
the solvent and purification of the crude product by column
chromatography (SiO2, isohexane/ethyl acetate=10:1) af-
forded 14b (yield: 72 mg, 56%) and 17b (yield: 10 mg, 8%).
1-(2-Chlorophenyl)-1-hydroxy-4-phenylbut-3-en-2-one
[14c]: Cinnamaldehyde (1a) (63 mL, 66 mg, 0.5 mmol) and 2-
chlorobenzaldehyde (13c) (57 mL, 70 mg, 0.5 mmol) were
added into a mixture of DMSO (10 mL) and buffer A
(38 mL) under an N2 atmosphere. After addition of BAL
(30 mglyophilised powder in 2 mL buffer A) the reaction
mixture was slowly stirred at room temperature. Reaction
process was monitored by GC-MS. After 2 days, the mixture
was extracted with ethyl acetate (3”50 mL), the organic
layer washed with water and brine and dried over Na2SO4.
Evaporation of the solvent and purification of the crude
product by column chromatography (SiO2, isohexane/ethyl
acetate=10:1) afforded 52 mgof a mixture of cinnamalde-
hyde (1a), 2,2’-dichlorobenzoin (17c) and 14c (accordingto
1H NMR) and 35 mgof a mixture of 1a, 17c and 14c (ac-
cordingto 1H NMR). The calculated yield of 14c is 40% (ac-
cordingto 1H NMR).
1-Hydroxy-4-phenylbut-3-en-2-one (10a): Cinnamalde-
hyde (1a) (63 mL, 66 mg, 0.5 mmol) and formaldehyde (7)
(37% in H2O, 225 mL, 3 mmol) were added into a mixture of
DMSO (10 mL) and buffer A (38 mL) under N2 atmos-
phere. After addition of BAL (10 mglyophilised powder in
2 mL buffer A) the reaction mixture was slowly stirred at
room temperature. Reaction process was monitored by GC-
MS. Additional amounts of BAL were added after 1 day
(10 mglyophilised powder in 2 mL buffer A). After 5 days,
the mixture was extracted with ethyl acetate (3”50 mL), the
organic layer was washed with water and brine and dried
over Na2SO4. Evaporation of the solvent and purification of
the crude product by column chromatography (SiO2, isohex-
ane/ethyl acetate=5:1) afforded 10a as a pale yellow oil;
yield: 42 mg(51%).
1-Hydroxy-3-methyl-4-phenylbut-3-en-2-one (10b): Reac-
tion of methylcinnamaldehyde (1b) (70 mL, 73 mg,
0.5 mmol) with formaldehyde (7) under the same conditions
as above (10a) afforded after purification of the crude prod-
uct by column chromatography (SiO2, isohexane/ethyl ace-
tate=5:1) 10b, as a pale yellow oil; yield: 73 mg(82%).
2-Cyclohexenyl-1-hydroxyethanone (10e): Reaction of cy-
clohex-1-enecarbaldehyde (1e) (57 mL, 55 mg, 0. 5 mmol)
with formaldehyde (7) was performed under the same con-
ditions as above (1a to 10a), but with a reaction time of 11
days. After purification of the crude product by column
chromatography (SiO2, isohexane/ethyl acetate=5:1) 10e
was obtained as a colourless oil; yield: 39 mg(56%).
(R)-3-Cyclohexenyl-2-hydroxy-1-methoxypropan-3-one
[(R)-11e]: Cyclohex-1-enecarbaldehyde (1e) (57 mL, 55 mg,
0. 5 mmol) and methoxyacetaldehyde[39] (8) (0.45M,
3.37 mL, 1.5 mmol) were added into a mixture of DMSO
(10 mL) and buffer A (38 mL) under an N2 atmosphere.
After addition of BAL (30 mglyophilised powder in 2 mL
buffer A) the reaction mixture was slowly stirred at room
temperature. Reaction process was monitored by GC-MS.
After 7 days, the mixture was extracted with ethyl acetate
(3”50 mL), the organic layer washed with water and brine
and dried over Na2SO4. Evaporation of the solvent and pu-
rification of the crude product by column chromatography
(SiO2, isohexane/ethyl acetate=5:1) afforded (R)-11e as a
colourless oil; yield: 22 mg(24%); ee accordingto ( R)-
Mosher ester of (R)-11e: 90% (GC-MS, NMR).
(R)-3-Cyclohexenyl-2-hydroxy-1,1-dimethoxypropan-3-
one [(R)-12e]: Cyclohex-1-enecarbaldehyde (1e) (57 mL,
55 mg, 0. 5 mmol) and dimethoxyacetaldehyde (9) (226 mL,
1.5 mmol) were added into a mixture of DMSO (10 mL)
and buffer A (38 mL) under an N2 atmosphere. After addi-
tion of BAL (30 mglyophilised powder in 2 mL buffer A)
the reaction mixture was slowly stirred at room temperature.
Reaction process was monitored by GCMS. After 7 days,
the mixture was extracted with ethyl acetate (3”50 mL), the
organic layer washed with water and brine and dried over
Na2SO4. Evaporation of the solvent and purification of the
crude product by column chromatography (SiO2, from iso-
hexane/ethyl acetate=10:1 to isohexane/ethyl acetate=5:1)
afforded (R)-12e as a colourless oil; yield: 31 mg(29%); ee
accordingto ( S)-Mosher ester of (R)-12e: 93% (GC-MS,
NMR).
(R)-1-(3,5-Dimethoxyphenyl)-1-hydroxy-4-phenylbut-3-
en-2-one [(R)-14d]: Cinnamaldehyde (1a) (63 mL, 66 mg,
0.5 mmol) and 3,5-dimethoxybenzaldehyde (13d) (83 mg,
0.5 mmol) were added into a mixture of DMSO (10 mL)
and buffer A (38 mL) under an N2 atmosphere. After addi-
tion of BAL (30 mglyophilised powder in 2 mL buffer A)
the reaction mixture was slowly stirred at room temperature.
Reaction process was monitored by GC-MS. After 24 h, the
mixture was extracted with ethyl acetate (3”50 mL), the or-
ganic layer washed with water and brine and dried over
Na2SO4. Evaporation of the solvent and purification of the
crude product by column chromatography (SiO2, isohexane/
ethyl acetate=5:1) afforded (R)-14d as a pale yellow oil
(yield: 50 mg, 33%) and 17d (yield: 34 mg, 20%); ee accord-
ingto ( R)-Mosher ester of (R)-14d: 72% (GC-MS, NMR)
(R)-1-(2-Chlorophenyl)-1-hydroxy-3-methyl-4-phenylbut-
3-en-2-one [(R)-14g]: Methylcinnamaldehyde (1b) (42 mL,
44 mg, 0.3 mmol) and 2-chlorobenzaldehyde (13c) (34 mL,
42 mg, 0.5 mmol) were added into a mixture of DMSO
(6 mL) and buffer A (22 mL) under an N2 atmosphere.
After addition of BAL (15 mglyophilised powder in 2 mL
buffer A) the reaction mixture was slowly stirred at room
temperature. Reaction process was monitored by GC-MS.
After 7 days, the mixture was extracted with ethyl acetate
(3”50 mL), the organic layer washed with water and brine
and dried over Na2SO4. Evaporation of the solvent and pu-
rification of the crude product by column chromatography
(SiO2, isohexane/ethyl acetate=10:1) afforded (R)-14g as a
pale yellow oil (yield: 37 mg, 43%) and 17c (yield:14 mg,
17%); ee accordingto ( R)-and (S)-Mosher ester of (R)-14g:
99% (GC-MS, NMR)
Adv. Synth. Catal. 2008, 350, 759 – 771
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Anabel Cosp et al.
FULL PAPERS
À
À
À
1.428(2); O3 C8 1.3723 (0.0021); O3 C10 1.420(2); C1 O1
1.219(2); C1 C6 1.502(2); C1 C2 1.519(3); C2 C3 1.527(3);
Reaction Conditions for 19g–i
À
À
À
20 mL ScPDC (cell-free extract, decarboxylase activity: 173
U/mL) were incubated in 1.5 mL of 0.1M citrate buffer,
pH 6.2, containing1.5 mM ThDP, 20 mM MgSO
ethanol in the presence of 30 mM hexenal (1g), octenal (1h)
or 2-methylpentenal (1i), respectively, and 50 mM sodium
pyruvate (2) at 258C and 300 rpm. After 20 h the reactions
were stopped by extraction with CDCl3.
À
À
À
À
C3 C4 1.317(3); C3 C5 1.485(3); C6 C7’ 1.394(2); C6 C7
À
À
À
1.396(3); C7 C8 1.391(2); C7’ C8’ 1.3995(2); C8 C9
with 10%
4
À
À
1.397(2); C8’ O3’ 1.369(2); C8’ C9 1.377(3).
Acknowledgements
Reaction Conditions for 20a–f
We express grateful thanks to Dr. Stephan Kçnig in Halle for
providing the pET22bPDC1 plasmid. Dr. Marion Wendorff
and Dr. Thorsten Eggert from the IMET are thanked for
transforming the plasmid in E. coli BL21ACHTRE(UNG DE3) cells and ex-
pression of ScPDC. We thank Volker Brecht for numerous
NMR measurements.
1 mglyophilised powder of BAL was incubated in buffer A
(1.5 mL) with 5% MTBE in presence of 10 mM benzalde-
hyde (13a) or 3,5-dimethoxybenzaldehyde (13d) and
100 mM acrolein (1j), methylacrolein (1k) or crotonalde-
hyde (1l), respectively, at 308C and 300 rpm. After 50 h the
reactions were stopped by extraction with ethyl acetate.
(R)-1-(3,5-Dimethoxyphenyl)-2-hydroxy-3-methylbut-3-
en-1-one [(R)-20e]: 3.5-Dimethoxybenzaldehyde (13d)
(332 mg, 2 mmol) and methylacrolein (1k) (1.6 mL,
20 mmol) were added into a mixture of MTBE (5 mL) and
buffer A (90 mL) under an N2 atmosphere. After addition
of BAL (40 mglyophilised powder in 5 mL buffer A) the re-
action mixture was slowly stirred at 308C. Reaction process
was monitored by GCMS. Additional amount of BAL
(40 mglyophilised powder in 5 mL buffer A) was added
after 8 h. After 48 h, the mixture was extracted with ethyl
acetate (3”125 mL), the organic layer was washed with
water and brine and dried over Na2SO4. Evaporation of the
solvent and purification of the crude product by column
chromatography (SiO2, cyclohexane/ethyl acetate=5:1) af-
forded (R)-20e; yield: 160 mg(34%); ee >99%. After crys-
tallisation from diethyl ether clear crystals were obtained;
mp 96.18C.
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Crystal Structure of (R)-20e
Crystals suitable for X-ray analysis were obtained by recrys-
tallisation from diethyl ether at 48C. Crystallographic data
for (R)-1-(3,5-dimethoxyphenyl)-2-hydroxy-3-methylbut-3-
en-1-one, (R)-20e, C16O4H16: Mr =236.26, orthorhombic,
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culated H-atom positions were treated usingthe riding
model. CCDC 668323 contains the supplementary crystallo-
graphic data which can be obtained free of charge from
The Cambridge Crystallographic Data Centre via
ACHTREUNGwww.ccdc.cam,ac,uk/data_request/cif, or from The Cam-
bridge Crystallographic Data Centre 12 Union Road, Cam-
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À
À
1.219(2); O2 C2 1.426(2); O3’ C8’ 1.369(2); O3’ C10’
770
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