Highly enantio- and diastereoselective inverse electron demand hetero-Diels-Alder reaction using 2-alkenoylpyridine N-oxides as oxo-heterodienes

Article abstract of DOI:10.1002/adsc.200800606

A general catalytic inverse electron demand hetero-Diels Alder reaction for 2-alkenoylpyridine N-oxides is presented. 2-Alkenoylpyridine N-oxides react very efficiently with alkenes in the presence of bisoxazolidine-copper(II) [BOX-Cu(II)] complexes to gi

Full text of DOI:10.1002/adsc.200800606

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DOI: 10.1002/adsc.200800606
Highly Enantio- and Diastereoselective Inverse Electron Demand
Hetero-Diels–Alder Reaction using 2-Alkenoylpyridine N-Oxides
as Oxo-Heterodienes
Santiago Barroso,a Gonzalo Blay,a,* M. Carmen MuÇoz,b and Josꢀ R. Pedroa,*
a
Departament de Quꢁmica Orgꢂnica, Facultat de Quꢁmica, Universitat de Valꢃncia, C/Dr. Moliner, 50, 46100 Burjassot
(Valꢃncia), Spain
Fax : (+34)-9-6354-4328; phone: (+34)-9-6354-4336; e-mail: gonzalo.blay@uv.es or jose.r.pedro@uv.es
Departament de Fꢁsica Aplicada, Universitat Politꢃcnica de Valꢃncia, 46071 Valꢃncia, Spain
b
Received: October 3, 2008; Published online: December 19, 2008
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800606.
Abstract: A general catalytic inverse electron
demand hetero-Diels Alder reaction for 2-alkenoyl-
pyridine N-oxides is presented. 2-Alkenoylpyridine
N-oxides react very efficiently with alkenes in the
presence of bisoxazolidine-copper(II) [BOX-
Cu(II)] complexes to give chiral dihydropyrans
bearing a pyridine ring at the 6-position with very
high yields and excellent diastereo- and enantiose-
lectivity. These heterodienes exhibited higher reac-
tivity and enantioselectivity than the corresponding
non-oxidized 2-alkenoylpyridines.
which are key structural elements in many bioactive
natural products and important pharmaceuticals.[2]
Most of the recent developments for this reaction
have mainly focused on enantioselective procedures
leading to optically active compounds.[3] The reaction
between carbonyl compounds and conjugated dienes,
the direct electron demand HDA reaction, has re-
ceived much attention and a good number of catalytic
enantioselective systems have been developed for this
reaction, especially with aldehydes[3,4] and, to a lesser
extent, with ketones.[3,5] In contrast, the catalytic
enantioselective inverse electron demand HDA, that
is the reaction between unsaturated carbonyl com-
pounds and electron-rich alkenes, has not been inves-
tigated to a great extent, and only few effective cata-
lytic systems have been reported. Tietze et al.[6] pub-
lished the first example of this class of reaction, an in-
tramolecular cycloaddition catalyzed by a diacetone
glucose derived-titanium(IV) Lewis acid. Later, Wada
Keywords: asymmetric catalysis; cycloaddition;
hetero-Diels–Alder reaction; oxygen heterocycles;
pyridines
The hetero-Diels–Alder (HDA) reaction with carbon- et al.[7] reported the first intermolecular catalytic
yl compounds (Scheme 1) is a powerful procedure for enantioselective inverse electron demand HDA reac-
the construction of six-membered oxygenated hetero- tion with a’-arylsulfonyl enones as heterodienes cata-
cycles, i.e., dihydropyrans and dihydropyranones,[1] lyzed by a bulky TADDOL-Ti(IV) complex deriva-
tive. The chiral BOX-Cu(II) complexes have been
found by Evans et al.[8] to catalyze the enantioselec-
tive HDA reaction of a,b-unsaturated acyl phospho-
nates with vinyl ethers. Simultaneously, the same cata-
lytic system was applied by Jørgensen et al.[9] to HDA
reactions of a,b-unsaturated acyl esters with different
electron-rich alkenes. In both cases the resulting dihy-
dropyrans were obtained in high yields and with ex-
cellent stereoselectivities. An HDA reaction with
enals as heterodienes has been described by Jacobsen
et al.[10] using a chiral Schiff base-Cr
ACHTNUTRGNEUNG(III) complex as
catalyst. This catalyst system has been extended by
the groups of Carreaux and Hall[11] to 3-boronacrolein
in a three-component [4+2]/allylboration sequence.
Scheme 1. HDA reaction with carbonyl compounds.
Adv. Synth. Catal. 2009, 351, 107 – 111
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Santiago Barroso et al.
COMMUNICATIONS
for bipyridines,[17] as well as flexible isosters of biolog-
ically active compounds.[18]
Our investigation started by checking the reaction
between alkenoylpyridine 2a (R=Ph) and ethyl vinyl
ether (1a) in the presence of the 6-CuACTHNURTGNENG(U OTf)2 complex
in dichloromethane. As anticipated from our previous
experience, a sluggish reaction took place to give
compound 4a as an 85:15 endo:exo mixture with low
enantioselectivity for both diastereomers (Table 1,
entry 1). Therefore, we studied the reaction with alke-
noylpyridine N-oxide 3a (R=Ph). As expected, under
almost identical reaction conditions (lower tempera-
ture) the pyridine oxide experienced a faster reaction
to give the HDA adduct 5a (R=Ph) with very good
diastereo- and enantioselectivity (entry 2). Copper(II)
triflate gave better results than zinc(II) triflate and
magnesium triflate. Other solvents were tested, which
gave similar or worse results than dichloromethane.
The use of other BOX ligands (7 and 8) did not im-
Scheme 2. Inverse electron demand HDA reaction with 2-al-
kenoylpyridine derivatives as heterodienes.
Finally, the concept of the HDA reaction has been prove the enantioselectivity, although ligand 7 provid-
also extended to organocatalyzed reactions via the ed the opposite enantiomer to that obtained with 6.
participation of enamine intermediates as dienophiles Finally, with ligand 6 in dichloromethane, the reaction
by using proline derivatives as catalysts.[12]
temperature could be lowered to À408C bringing
2-Alkenoylpyridines 2 (Scheme 2) have been used about further improvement on the diastereo- and
as dienophiles in catalytic enantioselective DA reac- enantioselectivity of the reaction.
tions with a number of catalysts.[13] However, in a pre-
A preliminary study of the substrate scope for the
vious work,[14] we found that these substrates failed to heterodiene has been carried out using the Cu(II)-6
give the DA reaction in the presence of the “privi- complex as catalyst (Table 2). The R group on the
leged” BOX-Cu(II) complexes (Figure 1).[15] This heterodiene was amenable to variation. Substrates
problem could be overcome by using the related 2-al- bearing an aromatic ring (3a–d) attached to the
kenoylpyridine N-oxides 3 as dienophiles.[16] These double bond reacted with ethyl vinyl ether (1a) to
substrates provided higher reactivity and enantiose- give the corresponding dihydropyrans 5a–d, almost as
lectivity than the corresponding non-oxidized 2-alke- a single diastereomer, with high yields, diastereo- and
noylpyridines with the same catalyst, allowing one to
obtain the expected cycloadducts with good yields
and excellent enantioselectivities (up to 96% ee).
Compounds 2 have been also used as heterodienes in
Table 1. Enantioselective inverse-electron demand HDA reac-
tion of ethyl vinyl ether (1a) and 2-alkenoylpyridine N-oxide 3a
(R=Ph) according to Scheme 2. Screening of ligands and con-
ditions.[a]
a racemic HDA reaction catalyzed by yttriumACTHNUGTRNEUNG(III)
hexafluoroacetone.[17] However, an asymmetric ver-
sion of this reaction has not been described so far.
In this communication, we present a new catalytic
enantioselective inverse electron demand HDA reac-
tion of compounds 3 as heterodienes, leading to opti-
cally active dihydropyrans bearing a pyridine ring at
the 2-position of the oxygenated heterocycle. These
kinds of compounds are useful synthetic precursors
Entry
M
L
Solv.
T
t
endo:exo ee
[8C] [h]
[%][b]
1[c]
2
3
4
5
6
7
8
9
Cu
Cu
Cu
Cu
Cu
Cu
Cu
Zn
N
6
6
7
8
6
6
6
6
6
6
6
CH2Cl2 r.t. 90 85:15
16 (15)[d]
CH2Cl2
CH2Cl2
CH2Cl2
Tol
MeNO2
THF
0
0
0
0
0
0
0
0
0.5 99:1
0.5 94:6
91
À75[e]
79
88
46
87
51
0
1
3
5
98:2
>99:1
96:4
2.5 >99:1
120 >99:1
77 >99:1
CH2Cl2
CH2Cl2
MgACHTUNTGRENNUNG
Cu
Cu
10
11
CH2Cl2 À20
4
>99:1
94
96
CH2Cl2 À40 20 >99:1
[a]
Full conversion of the heterodiene in all the cases.
The ee for the major endo 5a isomer determined by HPLC.
Reaction carried out with 2a.
In brackets, ee for exo-4a.
The opposite enantiomer was obtained
[b]
[c]
[d]
[e]
Figure 1. BOX ligands used in this study.
108
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Highly Enantio- and Diastereoselective Inverse Electron
COMMUNICATIONS
Table 2. Enantioselective inverse electron demand HDA re-
action of ethyl vinyl ether (1a) and compounds 3 catalyzed
by 6-CuACHTUNGTRENNUNG
(OTf)2 according to Scheme 2.[a]
Entry
3
R
t
Yield endo:exo ee
[h] [%][b]
[%][c]
1
2
a
b
c
d
e
f
Ph
20 5a, 99 @99:1
96
94
96
96
4-MeOC6H4 48 5b, 80 @99:1
3
4-BrC6H4
4-NO2C6H4
2-furyl
3-fury
t-Bu
14 5c, 99 @99:1
5d, 85 @99:1
4[d]
5
3
70 5e, 99 99:1
70 5f, 98 99:1
18 5g, 99 95:5
96
6
7
96
g
96 (37)[e]
[a]
All reactions carried out at À408C, unless otherwise
stated.
[b]
[c]
[d]
[e]
Isolated yield after column chromatography.
The ee for the major endo isomer determined by HPLC.
Reaction carried out at À208C.
In brackets, ee for exo-5g.
Figure 2. Structure of alkenes 1 and HDA products (endo)
in Table 3.
enantioselectivities, regardless of the nature of the
substituent on the phenyl ring (Table 2, entries 1–4).
Compounds 3e and f, bearing a heteroaromatic furan
ring, reacted slower although very efficiently, afford-
ing the expected products 5e and f with high diaster-
eo- and enantioselectivity (entries 5 and 6), without
any interference of the furan ring. The heterodiene
also tolerates an alkyl group attached to the double
bond. Thus, the reaction with the tert-butyl- substitut-
ed 3g afforded the major endo-adduct in 96% ee to-
gether with a small amount of the exo-adduct
(entry 7).
Next we carried out a preliminary study of the reac-
tion with other electron-rich alkenes using compound
3a as heterodiene (Figure 2, Table 3). Vinyl ethers
(entries 1 and 2), N-vinyllactams (entry 3) and vinyl
sulfides (entries 4 and 5) were found to be very effi-
cient dienophiles in this reaction. In almost all the
cases the HDA products 5h–l were obtained with very
high yields, diastereo- and enantioselectivities; only in
the case of 2-methoxypropene (1c) was the product
obtained with low diastereoselectivity, probably be-
cause of the methyl group exerting steric hindrance to
the endo approach. The efficiency of the catalytic
Table 3. Enantioselective inverse-electron demand HDA re-
action of different alkenes
1 with 3a catalyzed by
6-Cu
AHCTUNGTRENNUNG
Entry
1
t
5
Yield
[%][b]
endo:exo
ee
[h]
[%][c]
1
2
3
b
c
d
e
e
f
7
7
25
7
5
h
i
j
k
l
99
92
93
98
99
99
@99:1
66:34
@99:1
@99:1
98:2
99
94 (95)[d]
>99
96
4
5[e]
6[f]
97
77
21
m
>97:3
[a]
All reactions carried out in dichloromethane at À408C,
unless otherwise stated.
Isolated yield after column chromatography.
The ee for the major endo isomer determined by HPLC.
In brackets, ee for exo-5i.
[b]
[c]
[d]
[e]
[f]
Reaction carried out with heterodiene 3c.
Reaction carried out at room temperature.
To show an example of potential transformations
system was also demonstrated by the reaction with on the HDA adducts we subjected compound 5a to
less reactive alkenes. Thus, 4-methoxystyrene (1f) re- catalytic hydrogenation in the presence of Pd/C
acted with 3a to give quantitatively the HDA adduct (Scheme 3). Deoxygenation of the pyridine ring took
5m with high diastereoselectivity and a meritorious place concomitantly with the stereoselective hydroge-
77% ee for the major endo-adduct.
nation of the double bond to give a 92:8 mixture of
The absolute stereochemistry of endo-5l (Table 2, diastereomers from which, the all-cis-trisubstituted
entry 5) was elucidated by X-ray crystallographic tetrahydropyran 9 bearing three stereogenic centers
analysis (Figure 3)[19] and for the rest of the products was obtained without variation of ee, after chromatog-
it was assigned on the assumption of a uniform mech- raphy.
anistic pathway. The stereochemistry of the products
indicates the preference of the alkene to approach the heterodienes for the Cu(II)-BOX-catalyzed enantio-
heterodiene from the si face of the double bond. selective HDA reaction of enones with alkenes. 2-Al-
In summary, we have presented here a new type of
Adv. Synth. Catal. 2009, 351, 107 – 111
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109

Santiago Barroso et al.
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Experimental Section
Procedure for the Catalytic Enantioselective HDA
Reaction
CuACTHUNTRGENNG(U OTf)2 (9.0 mg, 0.025 mmol) contained in a dry Schlenk
tube was heated at 908C under vacuum for 1 h. After this
time (S,S)-Ph-BOX 6 (8.4 mg, 0.025 mmol) and CH2Cl2
(1.5 mL) were added under a nitrogen atmosphere and the
mixture was stirred for 1 h at room temperature. Then, alke-
noylpyridine N-oxide 3a (56 mg, 0.25 mmol) was added and
the mixture stirred for 0.5 h. After this time, the solution
was cooled to À408C and the ethyl vinyl ether (72 mL,
0.75 mmol) was added. After completion of the reaction
(TLC), flash chromatography on silica gel eluting with hexa-
ne:EtOAc (3:7) afforded product 5a (73.7 mg, 99%). Chiral
HPLC analysis [Chiralpak AD-H, hexane:IPA (85:15),
1.0 mLminÀ1]: exo tR =10.3 min, exo tR =11.6 min, (+)-endo
tR =16.5 min (major), (À)-endo tR =24.7 min (minor); endo/
exo>99.9:0.1; ee (endo)=96%; [a]2D5: +98.8 (c 1.5, MeOH,
Figure 3. ORTEP plot for the X-ray structure of compound
5l. Flack parameter 0.005(9).
1
96% ee); H NMR (300 MHz, CDCl3): d=8.23 (1H, dd, J=
0.8 Hz, J=6.5 Hz), 7.80 (1H, dd, J=2.1 Hz, J=8.2 Hz), 7.40
(1H, dd, J=1.1 Hz, J=2.9 Hz), 7.24 (6H, m), 7.12 (1H,
ddd, J=2.1 Hz, J=6.6 Hz, J=7.4 Hz), 5.24 (1H, dd, J=
1.9 Hz, J=9.0 Hz), 4.07 (1H, qd, J=7.1 Hz, J=9.5 Hz), 3.93
(1H, ddd, J=2.9 Hz, J=6.9 Hz, J=10.1 Hz), 3.71 (1H, qd,
J=7.1 Hz, J=9.5 Hz), 2.40 (1H, dddd, J=1.3 Hz, J=1.9 Hz,
J=6.9 Hz, J=13.2 Hz), 1.99 (1H, ddd, J=9.0 Hz, J=
10.8 Hz, J=13.2 Hz), 1.29 (3H, t, J=7.1 Hz); 13C NMR
(75.5 MHz, CDCl3): d=143.56 (s), 143.05 (s), 141.58 (s),
141.06 (d), 128.43 (d), 127.36 (d), 126.48 (d), 125.01 (d),
124.25 (d), 123.43 (d), 112.73 (d), 100.31 (d), 64.67 (t), 38.87
(d), 36.93 (t), 15.12 (q); MS (FAB): m/z (%)=298 (M+ +1,
100), 226 (47), 191 (23), 161 (58); HR-MS: m/z=298.1457,
calcd. for C18H20NO3: 298.1443.
Scheme 3. Hydrogenation of compound 5a.
kenoylpyridine N-oxides show not only increased re-
activity, but also higher levels of enantioselectivity
than the corresponding non-oxidized 2-alkenoylpyri-
dines. The reaction enhances the scope of the catalytic
enantioselective inverse electron demand HDA reac-
tion, which has been limited to a small number of het-
erodienes, so far. High conversions and ees are ob-
Acknowledgements
Financial support from the Ministerio de Educaciꢀn y Cien-
cia and FEDER (CTQ 2006–14199/BQU) is gratefully ac-
knowledged. S. B. thanks the U.V. for a pre-doctoral grant (V
tained regardless of the nature of the substituent on Segles program).
the double bond of the heterodiene, allowing either
aryl, heteroaryl or alkyl groups. The high efficiency of
the catalytic system is also demonstrated by the reac-
tion with less reactive alkenes. We have shown that
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scope of the HDA reaction with these heterodienes is
under development.
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