The high-pressure [4+2]cycloaddition of 1-methoxybuta-1,3-diene to the glycolaldehyde-derived heterodienophiles, catalyzed by chiral metallosalen complexes

Article abstract of DOI:10.1016/j.tetasy.2005.07.031

A high-pressure (ca. 10 kbar) reaction of 1-methoxybuta-1,3-diene 1 with variously O-protected glycolaldehydes 2, catalyzed by the chiral (salen)Cr(III)Cl 4a-d and 5 or (salen)Co(II) 6a-f and 7 complexes, has been studied. The best results were obtained for tert- butyldimethylsilyloxyacetaldehyde 2a. The reaction afforded, in good yield (up to 90%) and with very good diastereoselectivity (up to 92%) and enantioselectivity (up to 93% ee), the [4+2]cycloadducts 3a, which are compounds of significant synthetic interest. The stereochemical model of the cycloaddition reaction is discussed.

Full text of DOI:10.1016/j.tetasy.2005.07.031

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 16 (2005) 2959–2964
The high-pressure [4+2]cycloaddition of
1-methoxybuta-1,3-diene to the glycolaldehyde-derived
heterodienophiles, catalyzed by chiral metallosalen complexes
Piotr Kwiatkowski,a Wojciech Chaładaj,a Małgorzata Malinowska,b
Monika Asztemborskac and Janusz Jurczaka,b,
*
aInstitute of Organic Chemistry, Polish Academy of Sciences, 01-224 Warsaw, Poland
bDepartment of Chemistry, Warsaw University, 02-093 Warsaw, Poland
cInstitute of Physical Chemistry, Polish Academy of Sciences, 01-224 Warsaw, Poland
Received 15 July 2005; accepted 26 July 2005
Available online 26 August 2005
Abstract—A high-pressure (ca. 10 kbar) reaction of 1-methoxybuta-1,3-diene 1 with variously O-protected glycolaldehydes 2, cat-
alyzed by the chiral (salen)Cr(III)Cl 4a–d and 5 or (salen)Co(II) 6a–f and 7 complexes, has been studied. The best results were
obtained for tert-butyldimethylsilyloxyacetaldehyde 2a. The reaction afforded, in good yield (up to 90%) and with very good dia-
stereoselectivity (up to 92%) and enantioselectivity (up to 93% ee), the [4+2]cycloadducts 3a, which are compounds of significant
synthetic interest. The stereochemical model of the cycloaddition reaction is discussed.
ꢀ 2005 Elsevier Ltd. All rights reserved.
1. Introduction
Jørgensen et al.7 and Oi et al.,8 by applying BINOL-
titanium(IV), bisoxazoline-copper(II), BINAP-palla-
dium(II) and -platinum(II) complexes, respectively. A
similar approach has also been presented by Wu et al.9
using the (salen)cobalt(II) complex as a catalyst in the
reaction of 1-alkyl-3-silyloxybuta-1,3-diene with ethyl
glyoxylate. However, the enantioselectivities were
moderate.
The application of chiral catalysts to the hetero-Diels–
Alder reactions of 1,3-dienes with carbonyl compounds
opens a useful route to optically active dihydropyranone
and dihydropyran derivatives, valuable intermediates in
the total synthesis of natural products.1 The enantiose-
lective version of this reaction has been intensively inves-
tigated since 1990.2 The attention of synthetic chemists
has focussed mostly on the reaction of simple aldehydes
with DanishefskyÕs diene, which led to the formation of
the dihydropyranone system.3 Chromium and cobalt
salen-type complexes were also applied in this reaction,
which was investigated by research groups of Jacobsen
et al.,4 and Yamada et al.5 However, the asymmetric
version of the [4+2]cycloaddition of simple 1,3-dienes,
for example, 1-methoxybuta-1,3-diene, 2,3-dimethylb-
uta-1,3-diene and cyclohexa-1,3-diene required the
application of activated carbonyl compounds such as,
for example, alkyl glyoxylates. This subject was mostly
investigated by the research groups of Mikami et al.,6
We have recently shown that the commercially available
(salen)chromium(III) 4a and cobalt(II) 6a complexes are
useful for the reaction of alkyl glyoxylates with simple
1,3-dienes.10 However, when the non-activated heterodi-
enophiles, such as acetaldehyde or benzaldehyde, were
used in the [4+2]cycloaddition even with activated 1-
methoxybuta-1,3-diene, carried out under ambient con-
ditions, the reaction failed. In such cases, the application
of high-pressure techniques led to a successful reaction.
This methodology has recently been applied to the
Lewis-acid-catalyzed [4+2]cycloaddition of 1-methoxy-
buta-1,3-diene 1 to non-activated heterodienophiles
derived from glycolaldehyde 2, leading to racemic 1-
methoxy-5,6-dihydro-2H-pyran derivatives in moderate
yields.11 The enantioselective version of this cycloaddi-
tion using tert-butyldimethylsilyloxyacetaldehyde as a
*
Corresponding author. Tel.: +48 22 632 0578; fax: +48 22 632
6681; e-mail: jurczak@icho.edu.pl
0957-4166/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2005.07.031

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P. Kwiatkowski et al. / Tetrahedron: Asymmetry 16 (2005) 2959–2964
OR
OR
OR
OR
H
10 kbar
H
H
H
H
OR
+
+
+
+
O
O
O
O
O
Lewis Acid
MeO
H
MeO
(2R,6R)-3a-h
H
MeO
H
MeO
H
OMe
1
2a
b
c
SiMe2But
SiPh2But
SiPh3
(2S,6S)-3a-h
R=
(2S,6R)-3a-h
(2R,6S)-3a-h
PPTS/MeOH
COMe
COPh
d
e
PPTS/MeOH
f
g
h
CH2Ph
CHPh2
CPh3
Scheme 1.
(salen)chromium(III) and cobalt(II) complexes in the
reaction of diene 1 with heterodienophile 2a resulted
in the formation of cis/trans diastereomeric mix-
tures of cycloadduct 3a in moderate to high
enantioselectivity.12
H
N
H
H
N
H
N
N
O
M
X
Cr
Cl
R2
O
R2
O
O
Initially, we decided to use the (salen)CrCl complex 4a
for the reaction of diene 1 with heterodienophile 2a (Ta-
ble 1, entries 1–4). The product 3a was obtained in good
yield (even in the case when 0.5 mol % of the catalyst
was used) along with very high endo-selectivity, with
the major cis-cycloadduct being formed with 84–87%
ee. Similar diastereoselectivities were obtained when
other silyloxyacetaldehydes 2b and 2c were used as hete-
rodienophiles; however, the yields and enantioselectivi-
ties were much lower (entries 5 and 6).
R1
R1
R1= R2= But
M= Cr, X= Cl
M= Cr, X= Cl
M= Cr, X= Cl
M= Cr, X= Cl
M= Co, X= Cl
M= Cr, X= BF4
4a
4b
5
R1= But, R2= Me
4c R1= But, R2= OMe
R1= Me, R2= But
R1= R2 = But
R1= R2 = But
4d
4e
4f
H
H
H
H
N
N
O
N
N
Application of the modified chromium(III) catalysts 4b–
d (entries 7–9) and 5 (entry 10) to the reaction of 1 with
2a afforded results similar in terms of yield, diastereo-
and enantioselectivity to the results shown in entries
1–4. The use of the cobalt chloride complex 4e (entry
11) gave a significantly lower yield as well as a slightly
lowered diastereo- and enantioselectivity.
Co
Co
R2
O
R2
O
O
R1
R1
R1= R2= But
6a
7
6b R1= But, R2= Me
R1= But, R2= OMe
R1= Me, R2= But
R1= But, R2= H
6c
6d
6e
6f
R1= But, R2= NO2
Next, we investigated the influence of catalyst 4f, in
which the counterion Clꢀ was replaced by BF4ꢀ, on the
reaction of diene 1 with two heterodienophiles 2a and
2d (Table 1, entries 12–15). The complex 4f, which pre-
viously worked very well in the reactions of Danishef-
skyÕs diene with simple aldehydes,4 turned out to be
very poor in terms of both yield and enantiomeric
excess. The major reaction product was the trans-diaste-
reomer as a result of acidic isomerization of the initial
diastereomeric mixture. The complex 4f having a less-
coordinated counterion is more active and acidic than
the chloride complex, causing a partial polymerization
of diene 1, which explains low yields.
Figure 1. The (salen)Cr(III), Co(II) and (III) complexes used.
heterodienophile and chiral (salen)chromium(III) and
cobalt(II) complexes as catalysts was also published.12
Herein, we report an extension of our high-pressure
approach to the [4+2]cycloaddition of 1-methoxybuta-
1,3-diene (1) to variously O-protected glycolaldehydes
2 (Scheme 1). The reaction was catalyzed by various
(salen)chromium(III) and cobalt(II) complexes, as
shown in Figure 1.
Next, we attempted to optimize the reaction of diene 1
with heterodienophile 2a using the neutral (salen)Co(II)
complex 6a (Fig. 1 and Table 2). We found that this
commercially available catalyst was stereochemically
highly effective, although the yield was lower when com-
pared to the cationic more active chromium–chloride
complex 4a.
2. Results and discussion
As mentioned above, we found that simple glycolalde-
hyde derivatives 2a–h react with 1-methoxybuta-
1,3-diene 1 in the presence of Eu(fod)3 under 10 kbar
pressure to afford the racemic cycloadducts 3a–h in
moderate to good yields, and with good cis-diastereo-
selectivities as a result of the predominance of the endo
addition.11 Moreover, the application of chiral
In the beginning, we investigated the influence of the
amount of catalyst and solvent on the course of the reac-

P. Kwiatkowski et al. / Tetrahedron: Asymmetry 16 (2005) 2959–2964
2961
Table 1. High-pressure [4+2]cycloaddition of diene 1 to heterodienophiles 2a–d in the presence of the complexes 4a–f and 5a
Entry
Catalyst
Mol %
Dienophile
Solvent
Yield (%)
cis/transb
ee for cis-3b (%)
ee for trans-3b (%)
1
2
3
4
5
6
7
8
4a
4a
4a
4a
4a
4a
4b
4c
4d
5
5
2
2
0.5
2
2
2
2
2
2
2
2
2
2
2
2a
2a
2a
2a
2b
2c
2a
2a
2a
2a
2a
2a
2a
2d
2d
CH2Cl2
CH2Cl2
Toluene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Toluene
CH2Cl2
Toluene
80
70
84
60
55
49
82
70
59
90
32
35
29
31
25
91:9
95:5
96:4
96:4
97:3
96:4
94:6
93:7
89:11
96:4
81:19
8:92
8:92
11:89
5:95
84
85
87
85
56
52
86
86
86
82
75
10
10
0
66
65
78
65
32
31
73
76
66
46
32
12
10
2
9
10
11
12
13
14
15
4e
4f
4f
4f
4f
4
4
a The reactions were carried out using 1 mmol of aldehyde 2 (0.5–1 mol/L) and 1.2 mmol of diene 1 under the pressure of 10–11 kbar at 20 ꢁC for
24 h.
b The cis/trans ratio and the enantiomeric excess were determined by GC on a capillary chiral column.
Table 2. High-pressure [4+2]cycloaddition of diene 1 to heterodienophiles 2a–h, in the presence of complexes 6a–f and 7a
Entry
Catalyst
Mol %
Dienophile
Solvent
Yield (%)
cis/transb
ee for cis-3b (%)
ee for trans-3b (%)
1
2
3
4
5
6
7
8
6a
6a
6a
6a
6a
6a
6a
6a
6a
6a
6a
6a
6a
6b
6c
6d
6e
6f
2
2
5
5
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2a
2a
2a
2a
2b
2b
2c
2c
2d
2e
2f
CH2Cl2
Toluene
CH2Cl2
Toluene
CH2Cl2
Toluene
CH2Cl2
Toluene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
47
38
61
48
25
30
32
38
29
26
34
45
50
27
32
49
38
35
15
95:5
83:17
93:7
80:20
98:2
98:2
97:3
94:6
81:19
79:21
89:11
92:8
95:5
93:7
93:7
95:5
93:7
86:14
88:12
93
74
93
74
85
66
65
38
15
20
56
62
61
86
85
91
85
70
58
79
59
73
60
66
48
31
18
5
9
10
11
12
13
14
15
16
17
18
19
8
33
28
30
72
69
75
71
33
18
2g
2h
2a
2a
2a
2a
2a
2a
7
a The reactions were carried out using 1 mmol of aldehyde 2 (0.5–1 mol/L) and 1.2 mmol of diene 1 under the pressure of 10–11 kbar at 20 ꢁC for
24 h.
b The cis/trans ratio and the enantiomeric excess were determined by GC on a capillary chiral column.
tion of 1 with 2a, catalyzed by 6a, and found that an in-
crease in concentration of the catalyst favourably influ-
enced the reaction yield slightly, whereas diastereo- and
enantioselectivity remained practically unchanged
(Table 2, entries 1–4). In turn, the change of the solvent
from methylene dichloride to toluene decreased the yield
of the reaction, diastereo- and enantioselectivity (cf.
entries 1–2 and 3–4). Similar effects in terms of enantio-
selectivity were observed for two other O-silyloxy-
acetaldehydes 2b and 2c (entries 5–8).
10). Application of the O-benzyl-type heterodienophiles
2f–h led again to moderate yields and enantioselectivi-
ties, whereas diastereoselectivity was very good (entries
11–13).
We also studied the effect of the ligand structure with
respect to the substituted salicylidene moieties (Table 2,
entries 1 and 14–18). These simple modifications had a
small influence on the diastereo- and enantioselectivity
1
2
of the process. When R are t-Bu or Me and R are alkyl
or methoxyl groups, the resulting enantioselectivities
were in the range of 85–93%. The presence of the NO2
group (entry 18) resulted in a drop in both diastereo-
and enantioselectivity. A change in the diamine part
resulted in a significant loss of both enantioselectivity
and yield in the case of the cobalt catalyst 7 (Table 2,
In the case of the acyl derivatives of glycolaldehyde 2d
and 2e, the yield and diasteroselectivity of their reaction
with diene 1 were substantially lower, when compared to
the heterodienophiles 2a–c; the enantioselectivity was
extremely poor for both diastereomers (entries 9 and

2962
P. Kwiatkowski et al. / Tetrahedron: Asymmetry 16 (2005) 2959–2964
OH
OH
OR
OH
OH
H2, Pd/C
MeOH
H
H
H
H
H
+
+
+
O
O
O
O
O
MeO
(2S,6S)-8
H
MeO
H
MeO
H
MeO
(2S,6R)-8
MeO
(2R,6S)-8
H
H
3b, c, g or h
(2R,6R)-8
Scheme 2.
RO
entry 19), while this was not observed in the case of the
RO
favored
cationic chromium catalyst 5 (Table 1, entry 10).
approach
OMe
G
H
O
G
G
O
O
N
The ratio of four [4+2]cycloadducts 3 obtained (Scheme
1) was measured by gas chromatography using chiral
columns. In the case of cycloadducts 3a and 3d–f, the
measurements were performed using crude reaction mix-
tures, after removal of the solvent and catalyst. In these
cases, a chiral column a-dex 120 was applied. The ste-
reoisomeric ratio of the four remaining cycloadducts
3b,c,g,h was measured after hydrogenation of initial
mixtures, catalyzed by 10% Pd/C, which resulted in a
mixture of four stereoisomeric alcohols 8 (Scheme 2).
After removal of the catalyst and solvent, the crude mix-
ture was treated with a commercial trimethylsilylating
reagent and subjected to gas chromatography on a chi-
ral b-dex 120 column.
O
Cr
G
Cr
N
(1R,2R)-complex
OR
H
O
MeO
H
(2S,6R)-3a-h
Scheme 3. Stereochemical model.
The absolute configuration of the major products was
determined by chemical correlation. The formation of
the diastereomers (2S,6R)-3 predominated in the pres-
ence of all (1R,2R)-chiral catalysts used. The post-reac-
Therefore, the approach of diene 1 to the complexed
dienophile 2 should occur from the outer side. The ob-
served direction of the asymmetric induction [(2S,6R)-
3 are always formed as the major products by the use
of (1R,2R)-catalysts] is in good agreement with our
tereochemical model.
tion mixture containing the cycloadducts
3 was
isomerized in the presence of a catalytic amount of
pyridinium p-toluenesulfonate (PPTS) in methanol
(Scheme 1), while the resulting mixtures of trans-3a–
c,f,g or h were hydrogenated using 10% Pd/C to yield
the enantiomeric mixture of trans-alcohols (2R,6R)-8
and (2S,6S)-8 (Scheme 2). In the case of cycloadducts
3d and 3e, following isomerization and hydrogenation,
the mixtures of trans-products were reduced with
LiAlH4 to afford the desired trans-alcohols 8. The spe-
cific rotations indicated unequivocally in all cases that
the major enantiomer was (2R,6R)-8, corresponding to
the cycloadducts (2S,6R)-3 as the major products of
the original post-reaction mixture.13
3. Conclusion
In conclusion, we have shown a convenient high-pres-
sure method of synthesis of 2-methoxy-5,6-dihydro-
2H-pyran derivatives 3 from 1-methoxybuta-1,3-diene
1 and non-activated aldehydes 2, carried out in the pres-
ence of 2 mol % of the commercially available chiral
(salen)Cr(III) 4a and (salen)Co(II) 6a complexes. In
the case of chromium catalysts, the yield (up to 90%)
and enantioselectivity (82–87% ee) are satisfactory.
The complex 6a catalyzes the reaction more effectively
as concerns enantioselectivity (up to 93% ee), although
the yields are substantially lower (25–61%). The best
results were obtained with tert-butyldimethylsilyloxy-
acetaldehyde 2a. This high-pressure approach reveals
higher enantioselectivity and diastereoselectivity of the
studied reaction, compared to the analogous reaction
of n-butyl glyoxylate under normal conditions cata-
lyzed by the same (salen)chromium and cobalt
complexes.10b,c
Rationalization of our results obtained in this work is
based on the stereochemical model shown in Scheme 3.
Our model originates from two sources: 1 from the
conformational analysis of metallosalen complexes and
their influence on the asymmetric induction of catalytic
epoxidation of olefins reported by Katsuki et al.,14 and 2
from the X-ray analysis of the (salen)Co(III)SbF6 com-
plex with two molecules of benzaldehyde in the axial
positions, published recently by Rawal et al.15 The cru-
cial conclusion given by Katsuki et al. concerns non-
planar, usually stepped conformation of the complex.
In turn, the Rawal et al. proposal based on the crystal
structure,15 confirmed by theoretical consideration by
Yamada et al.,16 points out that the aldehyde molecules
are not oriented perpendicularly to the complex plane
but this is slightly deformed as shown in Scheme 3.
The stereochemical results are well rationalized on the
basis of the model proposed. The present results open
up a rational and efficient way to the optically active
6-substituted 2-methoxy-5,6-dihydro-2H-pyrans, which
are valuable intermediates in the synthesis of modified
sugars and other natural products.

P. Kwiatkowski et al. / Tetrahedron: Asymmetry 16 (2005) 2959–2964
2963
4. Experimental
Compound 3a—(column a-dex 120, T = 120 ꢁC,
160 kPa): t(2S,6S) = 24.4, t(2R,6R) = 25.5, t(2S,6R) = 26.6,
t(2R,6S) = 27.1 min; (column b-dex 120, T = 130 ꢁC,
100 kPa): t(2S,6S) = 25.0, t(2R,6R) = 25.6, t(2R,6S) = 28.2,
t(2S,6R) = 29.1 min.
4.1. General
All chemicals were used as received unless otherwise
noted. The reagent-grade solvents (CH2Cl2, toluene,
hexane and AcOEt) were distilled prior to use. Flash
chromatography was performed on silica gel (Merck
Kieselgel 60, 230–400 mesh). Optical rotations were re-
corded using a JASCO DIP-360 polarimeter.
Compound 3d—(column a-dex 120, T = 120 ꢁC,
160 kPa): t(2S,6S) = 16.6, t(2R,6R) = 17.2, t(2S,6R) = 18.6,
t(2R,6S) = 19.2 min.
Compound 3e—(column a-dex 120, T = 160 ꢁC,
160 kPa): t(2S,6S) = 76.2, t(2R,6R) = 78.2, t(2S,6R) = 81.7,
t(2R,6S) = 82.9 min.
Enantiomeric excess of products was determined by gas
chromatography performed using a Hewlett–Packard
GC unit equipped with a capillary chiral column a- or
b-dex 120 (30 m · 0.25 mm I.D., Supelco, Bellefonte,
USA).
Compound 3f—(column a-dex 120, T = 160 ꢁC,
160 kPa): t(2S,6S) = 38.5, t(2R,6R) = 39.4, t(2S,6R) = 42.0,
t(2R,6S) = 42.6 min.
4.2. Materials
Compounds 3b,c,g,h were hydrogenated to 8 and the
alcohols converted to the trimethylsilyl ether and ana-
lyzed on GC. column b-dex 120, 120 ꢁC, 120 kPa:
1-Methoxy-1,3-butadiene 1, (R,R)-N,N0-bis(3,5-di-tert-
butylsalicylidene)-1,2-cyclohexanediaminochromium(III)
chloride 4a (if contains solvents should be dried before
use) and (R,R)-N,N0-bis(3,5-di-tert-butylsalicylidene)-
1,2-cyclohexanediaminocobalt(II) 6a were purchased
from Aldrich. Other chromium(III) 4b–d and 5 and
cobalt(II) 6b–f, 7 and 4e salen complexes were prepared
according to the known procedures, starting from an
appropriate salen ligand and CrCl2 or Co(OAc)2 salts.17
Salen ligands were synthesized according to the method
described by Larrow and Jacobsen.18 The O-protected
glycolaldehydes (2) were prepared from the correspond-
ing allyl ethers for 2a–c and 2f–h or esters for 2d, e via
ozonolysis.
t(2S,6S) = 12.9, t(2R,6R) = 13.4, t(2R,6S) = 16.3, t(2S,6R)
=
16.8 min.
4.5. Chemical correlation
A mixture of cycloadducts cis:trans-3, obtained in the
presence of chiral metallosalen complexes, was isomer-
ized in the presence of a catalytic amount of pyridinium
p-toluenesulfonate (PPTS) in methanol for ca. 24 h. The
resulting isomer, mainly trans-3, was subjected to reduc-
tion with H2–Pd/C. Following filtration and concentra-
tion, the mixture was chromatographed on a silica gel
column using hexane/AcOEt. The specific rotation
measurement indicated that the major enantiomer was
laevorotatory (ꢀ)-(2R,6R)-8 in all cases when the
4.3. General procedure for the catalytic high-pressure
[4+2]cycloaddition19
20
D
(1R,2R)-catalyst was used. Lit.13 (2S,6S)-8; ½aŠ
þ129.7 (c 4.3, benzene).
¼
A 2 ml Teflon ampoule was charged with the chromium
or cobalt catalyst (usually 2–5 mol %), a solution of the
appropriate O-protected glycolaldehyde 2 (1 mmol) in
CH2Cl2 or toluene, 1.2–1.5 equivof 1-methoxybutadi-
ene, and the ampoule filled with an appropriate solvent,
sealed and placed in a high-pressure vessel and the pres-
sure was then slowly increased to 10–11 kbar at 20 ꢁC.
After stabilization, the reaction mixture was kept under
these conditions for 24 h. After decompression, the mix-
ture was subjected to column chromatography.
References
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