Enantioselective 1,3-dipolar cycloaddition reactions using complexes of (-)-sparteine

Article abstract of DOI:10.1002/jhet.5570450130

Regio- and stereoselective 1,3-dipolar cycloaddition of nitrile oxides to internal 2-pentenols and α,β-unsaturated esters catalyzed by (-) sparteine-lanthanide complexes affords corresponding 3-aryl-2-isoxazolines with enantioselectivities up to 68% ee.

Full text of DOI:10.1002/jhet.5570450130

Jan-Feb 2008
241
Enantioselective 1,3-Dipolar Cycloaddition Reactions Using
Complexes of (-)-Sparteine
Mirosꢀaw Gucma and W. Marek Goꢀꢁbiewski*
Institute of Industrial Organic Chemistry, 03-236 Warsaw, Poland
golebiewski@ipo.waw.pl
Receive June 27, 2007
-
O
+
CH2OH
+
N
C
R
O
O
N
N
(-)- Sparteine
Yb(OTf)3
HOCH2
R
+
R
CH2OH
R1
R1
R1
ee up to 68%
Regio- and stereoselective 1,3-dipolar cycloaddition of nitrile oxides to internal 2-pentenols and ꢀ,ꢁ-
unsaturated esters catalyzed by (-) sparteine-lanthanide complexes affords corresponding 3-aryl-2-
isoxazolines with enantioselectivities up to 68% ee.
J. Heterocyclic Chem., 45, 241 (2008).
A catalytic version of this reaction was also developed.
Sibi et al achieved excellent regio- and enantioselectivity
in the cycloaddition of nitrile oxide to crotonamide
derivatized as achiral pyrazolidinone using a chiral Lewis
acid obtained from bisoxazoline derivative of amino
indanol and magnesium iodide [9]. Coordination of amine
bases to Lewis acid was circumvented by application of
Amberlyst 21 as a base to generate nitrile oxides from
hydroximinoyl chlorides.
INTRODUCTION
1,3-Dipolar cycloaddition of nitrile oxides to alkenes is
the most useful method of preparation of 2-isoxazolines
[1], useful intermediates which can be easily transformed
by reductive N-O bond cleavage to several synthetically
important compounds such as ꢁ-hydroxy ketones, ꢁ-
hydroxy esters, ꢀ,ꢁ-unsaturated carbonyl compounds or
iminoketones [2]. The nitrile oxides can be formed either
by Huisgen method from aldoximes by chlorination and
base-induced dehydrochlorination [1] or by Mukayama
method from primary nitro compounds and phenyl
isocyanates [3]. While reactions with terminal alkenes
give almost exclusively 5-substituted 2-isoxazolines
problems of weak regio- and stereoselectivity appear for
RESULTS AND DISCUSSION
These results underlined the importance of creating a
chiral sphere around a metal center where by aggregation
of dipole and dipolarophile a good control of regio- and
stereoselectivity may be achieved. Such a sphere may
comprise a chiral diamine such as bisoxazoline applied as
a ligand for magnesium ion [9,10] or nickel ion [11,12],
isoxazolidinylpyridine derivative (PyBox) [13], Kemp’s
triacid derivative [14], and Rebeck benzoxazole [15].
It struck us that a similar chiral cavity as in the above
mentioned synthetic diamines could be provided by a
natural quinolizidine alkaloid (-)-sparteine (1). This
alkaloid in a free form shows a transoid conformation
with the C-ring in the boat form and trans C/D ring
juncture [16,17]. On protonation or formation of metal
complexes inversion of configuration on N-16 takes place
resulting in adoption of cisoid conformation with cis C/D
ring juncture [18,19] (Scheme 1), and a cavity between
two nitrogen atoms allowing sparteine to function as a
bidendate ligand. Following a pioneering contribution of
Hoppe [20] there has been recent upsurge of interest in
1,2-disubstituted
olefins.
Diastereoselective
and
enantioselective cycloadditions
have been studied
extensively with application of optically active substrates
and chiral catalysts [4]. Several obstacles such as
propensity of nitrile oxides to dimerize and form
unreactive complexes with metal catalysts as well as
interaction of tertiary amines used to generate nitrile
oxides with Lewis acids had to be overcome [5].
Cycloaddition of the nitrile oxides to the optically active
internal ꢀ-silyl allyl alcohols in the presence of
alkoxymagnesium bromides afforded trisubstituted 2-
isoxazolines without loss of chirality [6]. Ukaji et al
applied a related approach to the first asymmetric metal-
catalyzed 1,3-dipolar cycloaddition reaction of nitrile
oxides to ꢂ-substituted allylic alcohols with diethylzinc
and (R,R)-diisopropyl tartrate as the chiral auxiliary [7,8].

242
M. Gucma and W. M. Goꢀꢁbiewski
Vol 45
sparteine as a chiral ligand in asymmetric synthesis.
Initially organolithium compounds were applied in
asymmetric deprotonation, substitution and addition
reactions [21,22]. Later chiral complexes of sparteine with
other metal ions were used, such as complexes of
magnesium [23], tin [24], titanium [25,26], copper
[27,28], manganese [29], and palladium [30,31].
frequently because this group often enhances biological
activity of compounds.
Reaction run without any catalyst (entry 16) showed
low regioselectivity and yield. When nitrile oxide was
generated in the presence of one equivalent of (-)-
sparteine (entry 17) regioselectivity of the reaction was
much improved, although the yield was still
unsatisfactory.
Scheme 1
With increase of the catalyst amount (from 0.5 to 1.1
equivalent) direction of regioselectivity changed from
prevalence of regioisomer 4 (entries 1-2) to favoring of
isomer 5 (entries 3-5). Reactions carried out in diethyl
ether were more regioselective than those run in
dichloromethane, although the yields were lower.
Structure of the dipole influenced both the regio- and
enantioselectivity of the cycloaddition. Reaction of nitrile
oxides with electron-donating substituents (entries 12-15)
were more regioselective and enantioselective in
comparison with nitrile oxides bearing electron-with-
drawing substituents. In reaction 12, 13, and 15 only
regioisomer 5 was detected. Conversion of pentenols to
metal alkoxides with n-butyl lithium or Grignard reagent
increased the proportion of regioisomer 5 (entries 12, 14).
These experiments clearly demonstrate an effect of
sparteine and its complex with the lanthanide on
regioselectivity and enantioselectivity of the reaction.
This complex enables agglomeration of dipole and
dipolarophile in the chiral cavity of cisoid sparteine,
which results in facial selectivity. Electron-donating
substituents of the dipole increase the negative charge on
the oxygen atom, which seems to stabilize complex with
electro-positive lanthanide (entry 15) and sparteine,
resulting in enhanced regio- and stereoselectivity of the
reaction. Similar effect exert electron-donating
substituents of the dipole on interaction of sparteine with
lithium alkoxide (dipolarophile) and dipoles (entries 12
and 13), where excellent regioselectivity was achieved.
We tried also in one case application of the chiral
sparteine catalyst to the enantioselective cycloaddition
reaction of aryl nitrile oxides to unsaturated ester – methyl
crotonate (Scheme 3). It was gratifying to find
N
N
N
N
1 transoid
1 cisoid
In search of new chiral Lewis acid catalysts we
examined participation of lanthanides, metals of strong
oxophilicity which due to large ionic radii (1.1 – 0.9 Å)
show high coordination numbers (6-12) [32]. These
catalysts were successfully applied in asymmetric 1,3-
dipolar cycloaddition of nitrones and alkenes in the
presence of chiral amines and/or phenols [14,33]. We
have just achieved regio- and stereoselective 1,3-dipolar
cycloaddition of nitrile oxides to ꢀ,ꢁ-unsaturated esters
and internal 2-pentenols catalyzed by R-(+) BINOL-
lanthanide complexes affording corresponding 3-aryl-2-
isoxazolines with good enantioselectivities [34].
These data induced us to examine for the first time
application of chiral complexes of lanthanides with (-)-
sparteine in cycloaddition reaction of aryl nitrile oxides to
alkenols (Scheme 2). A catalyst formed from sparteine
and anhydrous ytterbium triflate showed the best
regioselectivity and yield of the reaction as compared to
praseodymium, scandium, and ytterbium triflate hydrate.
Nitrile oxides were generated by dehydrochlorination of
hydroximinoyl chlorides on Amberlyst A-21 column [9]
or in situ with triethylamine. The effects of catalyst
amount, addition order, solvent, structure of the dipole,
and presence of additional base on regio- and
stereoselectivity of the reaction with Z-2-pentol were
studied. Preliminary results are presented in Table 1.
Trifluoromethyl-substituted dipole was applied most
Scheme 2
-
O
N
C
CH2OH
Et
O
O
+
N
N
HOCH2
Et
(-)- Sparteine
Yb(OTf)3
+
+
Et
CH2OH
2
R1
R
R1
4
5
3
R = CF3, Et, i-Pr, OMe

Jan-Feb 2008
Enantioselective 1,3-Dipolar Cycloaddition Reactions Using Complexes of (-)-Sparteine
243
Table 1
Reactions of Z-2-penten-1-ol with nitile oxides in the presence of chiral (-)-sparteine catalyst
Entry
R
Sparte-
Yb(OTf)3
Addnl
base eq
Solvent
Yield
%
5:4 ratio
5 ee
%
4 ee
ine eq
0.5
0.6
1
1.1
1.1
2
0.5
0.7
1
1
1
1
1
1
1
-
1
eq
0.5
0.3
1
1.1
1.1
1
-
-
-
-
-
-
-
1
1
-
-
%
38
22
22
42
28
30
24
30
30
28
40
-
1a
2
3
4
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
i-Pr
Et
i-Pr
OMe
CF3
CF3
-
-
-
-
CH2Cl2
CH2Cl2
Et2O
Et2O
CH2Cl2
Et2O
CH2Cl2
CH2Cl2
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
CH2Cl2
CH2Cl2
CH2Cl2
36
48
17
42
61
20
40
33
51
21
35
39
37
62
55
20
18
1:2.1
1:1.5
3.8:1
1.9:1
1.7:1
1:5.7
1.2:1
1:1.3
1:1.6
1.6:1
4.9:1
0:100
0:100
1:1.4
0:100
1:1.2
1:6.1
24
18
38
19
34
28
40
26
30
49
42
50
48
68
60
-
b
5
-
6a
7
-
0.5 BuLi
0.7 BuLi
1 BuLi
1 BuLi
1 iPrMgBr
1 BuLi
1 BuLi
-
8a
9
10a
11
12
13
14
15
16
17
b
-
34
-
-
16
-
-
-
15
a N-oxide was generated with NEt3, in other cases on the Amberlyst A-21 column;b Dipolarophile was added after dipole.
Scheme 3
- O
CO2Me
O
O
N
MeO2C
+ N
N
+
6
C
CO2Me
ee 66%
(-)- Sparteine 0.5 eq
+
Yb(OTf)3 0.5 eq
CH2Cl2
72%
CF3
CF3
ee 8%
1 : 2.2
CF3
7
8
mmol) was added to the clear solution and stirring was
continued for 30 minutes. A solution of dipole in the same
solvent generated by passing a hydroximinoyl chloride (1
mmole) solution through a column of Amberlyst 21 was added
dropwise over 30 minutes. The obtained solution was stirred for
ca. 19 h, water was added to quench the reaction and the
reaction mixture was diluted with dichloromethane. The organic
phase was washed with water, dilute hydrochloric acid and
water, dried (magnesium sulfate) and the solvent was evaporated
in vacuo. The crude product was purified by flash column
chromatography on silica gel and the enantiomeric excess of
separated regioisomers was determined by HPLC analysis
(Astec Cyclobond I 2000 RN).
General procedure for the cycloaddition reactions with
application of sparteine and lithium alkoxides. N-butyl
lithium (2.5 M solution in hexanes) was added to a solution of
(-)-sparteine in dry diethyl ether at -78 °C. Z 2-penten-1-ol (1
mmol) was added dropwise and the mixture was stirred at 0 °C
for 30 minutes. A solution of nitrile oxide was added dropwise
over 30 minutes and a milky reaction mixture was stirred at 0 °C
for 5 hours and overnight at room temperature. A clear reaction
mixture was neutralized with ammonium chloride solution and
worked-up as above.
unexpectedly good ee of 66% for regioisomer 4 (Scheme
3). This result broadens the synthetic utility of sparteine-
lanthanide catalyst. Further studies are in progress to
improve efficiency of the catalyst and to explore it in
other asymmetric reactions.
EXPERIMENTAL
Reagent grade chemicals were used without further purifycation
unless otherwise noted. Elemental analyses were performed at
Microanalysis Laboratory of Institute of Organic Chemistry,
Polish Academy of Sciences, Warsaw. Spectra were obtained as
1
follows: IR spectra on JASCO FTIR-420 spectrometer, H NMR
spectra on Varian 500 UNITY plus-500 and Varian 200 UNITY
plus 200 spectrometers in deuterated chloroform using TMS as
internal standard. Flash-chromatography was carried out using
silica gel S 230-400 mesh (Merck).
Hydroximinoyl acid chlorides were prepared from the
corresponding aryl aldehyde oximes and NCS in DMF [35]. The
corresponding nitrile oxides were generated in situ by
dehydrohalogenation with triethylamine or on Amberlyst A-21
column [9].
General procedure for the cycloaddition reactions with
application of sparteine-lanthanide complexes: A mixture of
(-)-sparteine and ytterbium triflate in dry dichloromethane was
stirred at room temperature for 30 minutes. Z 2-penten-1-ol (1
[4-Ethyl-3(4-trifluoromethylphenyl)-4,5-dihydroisoxazol-
4-yl]methanol (4a) and [5-ethyl-3(4-trifluoromethylphenyl)-
4,5-dihydroisoxazol-4-yl]methanol (5a). These compounds
were obtained as colorless oils. Regioisomer-5 (4a): ir (neat)
1
OH 3330, 3140, 1619, 1520 (Ph), 1324 (CF3) cm-1; H nmr

244
M. Gucma and W. M. Go ꢀbiewski
Vol 45
(CDCl3, 200 MHz) ꢀ 7.77 (d, J = 8.5 Hz, 2H, 5’-H, 3’-H), 7.66
(d, J = 8.5 Hz, 2H, 6’-H, 2’-H), 4.81 (m, 1H, 5-H), 4.01 (m,
2H, CH2OH), 3.64 (m, 1H, 4-H), 2.18 (s, 1H, OH) 1.69 (m,
2H, CH2CH3), 0.94 (t, J = 7.6 Hz, 3H). Anal. Calcd. for
C13H14NO2: C, 57.14; H, 5.16. Found: C, 56.81; H, 5.01.
Regioisomer 4 (5a): ir (neat) OH 3328, phenyl 1619, 1530,
6.3 Hz, 3H, CH3CH). Anal. Calcd. for C13H12F3NO3: C, 54.36;
H, 4.21. Found: C, 54.59; H, 4.17.
Acknowledgements. This work was supported in part by the
Polish Ministry of Science and Higher Education Research
(Grant 429E-142/S/06), what is gratefully acknowledged.
1
CF3 1326 cm-1; H nmr (CDCl3, 200 MHz) ꢀ 7.96 (d, J = 8.4
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(m, 2H, 4-H, 5-H), 4.24 (m, 2H, CH2OH), 2.08 (m, 2H,
CH2CH3), 0.98 (t, J = 7,6 Hz, 3H). Anal. Calcd. for C13H14NO2:
C, 57.14; H, 5.16. Found: C, 56.95; H, 4.98.
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[4-Ethyl-3-(4-isopropylphenyl)-4,5-dihydroisoxazol-5-yl]-
methanol (4b) and [5-ethyl-3-(4-isopropylphenyl)-4,5-dihydro-
isoxazol-4-yl]methanol (5b). These compounds were obtained
as colorless oils; regioisomer-5 (4b): ir (neat) OH 3400,
1
phenyl 1610, 839 cm-1; H nmr (CDCl3, 200 MHz) ꢀ 7.67 (d, J
= 8.2 Hz, 2H, 2’-H, 6’-H), 7.27 (d, J = 8.2 Hz, 2H, 3’-H, 5’-
H), 7.20 (s, 1H, OH), 4.51 (dt, J = 8.3; 6.4 Hz, 1H, 5-H), 3.85
(d, J = 6.4 Hz, 1H, CH-CH2-OH), 3.61 (dt, J = 8.3; 6.7 Hz, 1H,
4-H), 2.93 (septuplet, J = 6.8 Hz, 1H, CH(CH3)2), 1.99 (qd, J =
6.7 Hz, 2H, CH2CH3), 1.26 (d, J = 6.8 Hz, 6H, (CH3)2CH-),
1.16 (t, J = 6.7 Hz, 3H, CH3CH2). Anal. Calcd. for C15H21NO2:
C, 72.84; H, 8.56. Found: C, 72.53; H, 8.41. Regioisomer 4
(5b): ir (neat) OH 3309, phenyl 1620, 841 cm-1; 1H nmr
(CDCl3 , 200 MHz) ꢀ 7.87 (s, 1H, OH), 7.56 (d, J = 8.0 Hz,
2H, 2’-H, 6’-H), 7.25 (d, J = 8.0 Hz, 2H, H–3’, H–5’), 5.63 (m,
2H, 4-H, 5-H), 4.75 (d, J = 5.5 Hz, 2H, CH-CH2OH), 2.93
(septuplet, J = 6.8 Hz, 1H, CH(CH3)2), 1.97 (dq, J = 6.9; 3.0
Hz, 2H, CH2CH3), 1.25 (d, J = 6.5 Hz, 6H, (CH3)2CH), 0.91 (t,
J = 7.8 Hz, 3H, CH3CH2). Anal. Calcd. for C15H21NO2: C,
72.84; H, 8.56. Found: C, 72.46; H, 8.39.
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1
OH 3380, phenyl 1636, 840 cm-1; H nmr (CDCl3 , 200 MHz) ꢀ
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H, 5’-H), 5.65 (m, 2H, 4-H, 5-H), 4.86 (d, J = 6 Hz, 2H,
CH2OH), 2.71 (q, J = 7.5 Hz, 2H, CH2Ar), 2.19 (m, 2H, CH-
CH2CH3), 1.25 (t, J = 7.5 Hz, 2H, Ar-CH2CH3), 1.02 (t, J = 7.5
Hz, 3 H, CH3CH2CH-). Anal. Calcd. for C14H19NO2: C, 72.07; H,
8.21. Found: C, 72.36; H, 8.59.
[5-Ethyl-3-(4-methoxyphenyl)-4,5-dihydroisoxazol-4-yl]-
methanol (5d). This compound was obtained as a colorless oil;
1
ir (neat) OH 3429, phenyl 1608, 832 cm-1; H nmr (CDCl3, 200
MHz) ꢀ 6.93 (d, J = 8.5 Hz, 2H, 5’-H, 3’-H), 6.84 (d, J = 8.5 Hz,
2H, 6’-H, 2’-H), 4.05 (m, 2H, 4-H, 5-H), 3.91 (m, 2H, CH2OH),
3.78 (s, 3H, CH3O), 1.65 (m, 2H, CH3CH2CH), 0.88 (m, 3H,
CHCH2CH3). Anal. Calcd. for C13H17NO3: C, 66.36; H, 7.28.
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zole-5-carboxylate (7) and methyl 3-(4-trifluoro-methyl-
phenyl)-5-methyl-4,5-dihydroisoxazole-4-carboxylate (8).
These compounds were obtained as colorless oils. Regioisomer-
5: ir (neat) CO 1741, phenyl 1619, 1482, 847, CF3 1324 cm-1; 1H
nmr (CDCl3, 200 MHz) ꢀ 7.97 (d, J = 8.1 Hz, 2H, 5’-H, 3’-H),
7.68 (d, J = 8.1 Hz, 2H, 6’-H, 2’-H), 4.84 (d, J = 4.2 Hz, 1H, 5-
H), 4.02 (dq, J = 7.3; 4.2 Hz, 4-H), 3.81 (s, 3H, CH3O), 1.43 (d,
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54.36; H, 4.21. Found: C, 54.48; H, 4.01. Regioisomer-4: ir
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1
(neat) CO 1742, phenyl 1620, 849, 1325 (CF3) cm-1; H NMR
(CDCl3 , 200 MHz) ꢀ 7.82 (d, J = 8.6 Hz, 2H, 5’-H, 3’-H), 7.65
(d, J = 8.6 Hz, 2H, 6’-H, 2’-H), 5.15 (quintuplet, J = 6.3 Hz, 1H,
5-H), 4.11 (d, J = 6.3 Hz, 4-H), 3.73 (s, 3H, CH3O), 1.49 (d, J =
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Jan-Feb 2008
Enantioselective 1,3-Dipolar Cycloaddition Reactions Using Complexes of (-)-Sparteine
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