Use of chiral B(III) complexes in the cycloaddition of C,N-diphenylnitrone to tert-butyl vinyl ether

Article abstract of DOI:10.1016/S0957-4166(00)00405-5

The 1,3-dipolar cycloaddition of C,N-diphenylnitrone to tert-butyl vinyl ether in the presence of several chiral B(III) complexes which incorporate different bidentate ligands has been investigated. The use of these B(III) species reverses the endo/exo diastereoselectivity in relation to the uncatalysed reaction, giving trans cycloadducts as major products. Some of the catalysts gave very fast and high yielding reactions, but the enantioselectivities were only low to moderate. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0957-4166(00)00405-5

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 11 (2000) 4269–4278
Use of chiral B(III) complexes in the cycloaddition of
C,N-diphenylnitrone to tert-butyl vinyl ether
Pau Bayo´n, Pedro de March, Marta Figueredo,* Josep Font and Jordi Medrano
Departament de Qu´ımica, Universitat Auto`noma de Barcelona, 08193 Bellaterra, Spain
Received 24 July 2000; accepted 9 October 2000
Abstract
The 1,3-dipolar cycloaddition of C,N-diphenylnitrone to tert-butyl vinyl ether in the presence of several
chiral B(III) complexes which incorporate different bidentate ligands has been investigated. The use of
these B(III) species reverses the endo/exo diastereoselectivity in relation to the uncatalysed reaction, giving
trans cycloadducts as major products. Some of the catalysts gave very fast and high yielding reactions, but
the enantioselectivities were only low to moderate. © 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction
The 1,3-dipolar cycloaddition of nitrones to olefins is a widely spread methodology for the
preparation of isoxazolidines, which are valuable intermediates for the synthesis of bioactive
molecules.1 One of the main advantages of this reaction is that it enables the formation of up
to three new stereogenic centres in a single step. The relative configuration of the cycloadducts
depends on the exo/endo selectivity of the process. This diastereoselectivity is controlled by a
combination of electronic and steric factors. According to Sustmann’s classification,2 the
1,3-dipolar cycloadditions of nitrones to olefins are type II processes. Therefore, for a given
nitrone, the dominating frontier molecular orbital (FMO) interaction in the transition state (TS)
depends on the olefin nature. It is experimentally observed that electron deficient olefins are
more reactive towards nitrones than electron rich ones, while unactivated olefins are the less
reactive. With the two latter olefin categories, it is usually necessary to work at high tempera-
tures or pressures, or with large excesses of one of the reactants, in order to obtain reasonable
conversions. An attractive way to increase the rate of these cycloadditions is the use of Lewis
acids as promoters of the process. These species can interact with the nitrone and also with the
dipolarophile if it contains functional groups able to coordinate to the acidic centre. The effect
of this complexation will be a lowering of the FMO energy levels of the nitrone or the olefin
* Corresponding author. Tel: 34 935811853; fax: 34 935811265; e-mail: iqormafi@cc.uab.es
0957-4166/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(00)00405-5

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compared with those of the free reactant. In most successful examples of Lewis acid catalysed
cycloadditions of nitrones, the dipolarophile is an olefin conjugated to a carbonyl group,3–10
therefore the dominating TS interaction is HOMOnitrone–LUMOolefin and the catalytic effect can
be explained through the complexation of the acid with the olefin. Although less investigated,
several examples have also been described of Lewis acid catalysed cycloadditions of nitrones to
electron rich olefins.11–16 In these reactions the predominant TS interaction is HOMOolefin
–
LUMOnitrone and the catalytic effect has to be attributed to the complexation of the Lewis acid
with the nitrone. Two kinds of electron rich olefins have been used for these studies: ketene
acetals and vinyl ethers, but there is experimental evidence that the Lewis acid promoted
addition of ketene acetals to nitrones may happen, at least in some cases, through a stepwise
mechanism with the intermediacy of cationic species.17 Recently, Jørgensen et al. have achieved
excellent diastereo- and enantioselectivities in the cycloaddition of C,N-diarylnitrones to vinyl
ethers catalysed by chiral aluminium complexes.15a In these reactions the major product is the
cis-cycloadduct, which is also the predominant isomer of the uncatalysed cycloaddition. We13
and the group of Bosnich16 have found that this kind of reaction can also be accelerated with
several Ti(IV) species, Meske12 and the group of Scheeren11 have investigated the use of chiral
oxazaborolidines, and Furukawa and co-workers have described the catalysis by Pd(II) com-
plexes.14 All these catalysts gave variable diastereoselectivities, in some cases tuned by the metal
ligands, and, when chiral auxiliaries have been incorporated into the complex, the enantioselec-
tivities found were low to moderate.
We have now investigated the reaction of C,N-diphenylnitrone, 1, with tert-butyl vinyl ether,
2, in the presence of B(III) complexes (Scheme 1). The results are described herein.
Scheme 1.
2. Results and discussion
We had previously studied the uncatalysed reaction between 1 and 2 at 50°C in toluene (Table
1, entry 1).13a This cycloaddition proceeds very slowly to yield a mixture of the two diastereoiso-
meric isoxazolidines cis- and trans-3 in 70% yield with a strong predominance of the cis isomer,
presumably formed through an exo-TS from the more stable Z configuration of nitrone 1.18 The
B(III) promoted reactions were performed in CH2Cl2 with a fourfold excess of olefin and 20%
molar of catalyst (Fig. 1) referred to nitrone 1. Prior to the introduction of chiral boranes into
the reaction medium, we examined the influence of the presence of catecholborane, 4 (entry 2).
This complex caused a remarkable acceleration of the cycloaddition and it also induced a
complete inversion of the diastereoselectivity, yielding trans-3 with 97% de in 86% yield. In a
first set of experiments with chiral ligands, several boranes which incorporate bidentate
C2-symmetrical diolates, 5–8, were assayed. It was expected that those complexes bearing
aromatic residues, 5–7, may benefit from a p-stacking interaction with the phenyl groups of
nitrone 1, shielding one of its faces to the approach of the olefin in the TS. On the other hand,

P. Bayo´n et al. / Tetrahedron: Asymmetry 11 (2000) 4269–4278
4271
Table 1
Cycloaddition reaction of nitrone 1 to olefin 2 in the presence of B(III) complexes. [1]0=0.1 M, [2]0=0.4 M,
[catalyst]0=0.02 M
Entry
Catalyst
Solvent
Temp.
Time
Yielda
(%)
cis-3/trans-3b
Ee cis-3c
(%)
Ee trans-3c
(%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
None
4
5
6
7
8
9
9
9
Toluene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Toluene
Toluene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
THF
50°C
Rt
Rt
Rt
Rt
Rt
Rt
−20°C
Rt
−20°C
Rt
Rt
Rt
Rt
Rt
−40°C
Rt
−40°C
−20°C
−20°C
Rt
14 days
12 h
40 h
40 h
40 h
70
86
44
31
66
54
89
67
89
70
46
69
82
54
61
78
77
78
78
91
74
96
44
61
67
32
33:1
1:74
1:2
1:2
1:4
1:4
1:6
1:9
1:9
1:8
1:6
1:7
1:7
1:6
1:7
1:8
1:6
1:8
1:3
1:6
1:4
1:4
1:8
1:8
1:9
2:1
14
–
–
–
12
9
9
28
26
6
40 h
B2 min
45 min
B2 min
45 min
B2 min
B2 min
40 min
40 min
B2 min
15 min
B2 min
15 min
15 min
1 h
30 min
45 min
B2 min
30 min
30 min
40 h
8
–
40
8
9
13
14
–
29
16
–
10
11
12
13
14
14
15
15
15
15
16
17
18
19
20
8
–
16
29
24
28
33
28
31
32
26
24
–
11
13
13
13
16
16
10
11
57
–
DMF
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
Rt
Rt
Rt
Rt
8
–
–
19
20
21+ZnEt2 CH2Cl2
Rt
a Yield of isolated 3 by column chromatography.
1
b cis-3/trans-3 ratio determined by H NMR on the crude reaction mixtures.
c Ee determined by HPLC (Daicel Chiracel OD), only values over 5% are indicated.
the dicyclohexyl derivative 8 may introduce more steric hindrance in the proximity of the boron
atom. We found that the glycolate ligands (entries 3–6) were less effective than cathecolate,
giving lower reaction rates, yields and diastereoselectivities. The best ee within this series was
obtained with complex 7, which contains an electron-releasing p-methoxy substituent in the
aromatic rings, supporting the ‘p-stacking hypothesis’, but yet the enantioselectivity was quite
low. Crucial improvements of the asymmetric induction are often produced by diminishing the
reaction temperature, but the cycloaddition of 1 to 2 catalysed by the borane 7 was still too slow
to work at lower temperatures. Therefore, we decided to increase the acidity of the boron
complex, and hence its catalytic activity, by introduction of a triflate as the monodentate ligand.
The reaction in the presence of the triflate complex 9 at room temperature (entry 7) was indeed
very fast, and at −20°C (entry 8) both the diastereo- and the enantioselectivity of the process
improved substantially (80% de and 40% ee for the major cycloadduct trans-3).
In an attempt to gain some insight into the geometry of the reacting complex, a solution
containing equimolar amounts of nitrone 1 and the boron complex 9 in CD2Cl2 was analysed by

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Figure 1.
400 MHz 1H NMR. In the presence of 9, the signals of all protons of the nitrone resolved clearly
and could be completely assigned. The iminic proton experienced a considerable downfield
shifting (from l 7.96 to 8.54), while the ortho protons of the C-phenyl and N-phenyl groups (l
8.35 and 7.77, respectively) showed chemical shifts very similar to those observed for the free
nitrone.19 Presaturation of the signal corresponding to the iminic proton induced positive NOE
on both the C-phenyl and the N-phenyl Hortho, indicating the Z configuration of the nitrone in
the reactive complex. This observation strongly supports the hypothesis that the major trans
cycloadduct formed in the catalysed reaction is produced through an endo TS from the
complexed Z-nitrone.
The use of toluene as solvent (entries 9 and 10) did not produce beneficial changes. The
introduction of a second methoxy substituent in the aromatic residues of the diolate (entry 11)
and the increase in the size and rigidity of the diolate (entry 12) were ineffective. Complexes 12
and 13 having a C2-symmetrical cyclohexyldisulfonamide as the bidentate ligand were also tested
(entries 13 and 14). This kind of ligand showed the best enantioselectivity in the previous studies
with chiral Ti(IV) catalysts,13b but the performance of their boron complexes was not superior
to that of the diolate derivatives.
Comparison of the results of entries 5 and 7 (where all the other variables remained
unchanged) seemed to indicate that the inclusion of hydride as the monodentate ligand of the
catalyst was beneficial to the enantioselectivity in relation to the triflate. In an attempt to
understand the reasons for that, theoretical calculations of the reactive intermediates derived

P. Bayo´n et al. / Tetrahedron: Asymmetry 11 (2000) 4269–4278
4273
from nitrone 1 and complexes 7 or 9 were performed.† Molecular mechanics (MMFF94) were
used to obtain the energy global minimum and then the geometry of the intermediate complexes
were optimised applying semiempirical (AM-1) calculations. Depicted in Fig. 2 are the two
possible approaches, exo or endo, of the vinyl ether 2 to each face of nitrone 1 coordinated to
catalyst 7 with the optimised geometry; the re and si descriptors refer to the carbon atom of the
nitrone and the hydride ligand is shown in yellow. According to this model, the re-face of the
nitrone seems more available to the dipolarophile, and the endo approach to this face looks less
sterically encumbered than the exo approach. For the complex nitrone 1–catalyst 9, the
calculations gave an optimised trigonal geometry around the boron atom, as expected, with
similar accessibility to both faces of the complexed nitrone.
Figure 2.
Considering the above results, it was thought that B(III) complexes containing a fluoride
anion as the monodentate ligand could have higher catalytic activity than the hydride complexes
while preserving the tetragonal geometry, leading to a better asymmetric induction. This idea
was tested with complexes 14 and 15, prepared in situ by reaction of the corresponding triflates
9 and 10 with n-Bu4NF. In the presence of these fluoride complexes (entries 15–20) the
cycloaddition was indeed very fast, the yields were good and the enantioselectivities were similar
to that observed for the corresponding hydride complex, with higher diastereoselectivity
† The calculations were performed using the PC SPARTAN plus program of Wavefunction, Inc.

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P. Bayo´n et al. / Tetrahedron: Asymmetry 11 (2000) 4269–4278
(compare entries 5, 15 and 17). Unfortunately, lowering the reaction temperature or changing
the solvent did not produce significant improvements in the asymmetric induction.
It has been described that a nucleophilic oxygen atom may interact with two acidic metal
centres simultaneously, with the consequent loss of flexibility of the reacting system.20 To
propitiate this kind of interaction with nitrone 1, the bicentric complexes 16–20 were prepared,
and their influence in the studied cycloaddition was examined (entries 21–25). All these
complexes were efficient promoters of the reaction. Interestingly, the addition of 16 and 17, with
reversed proportion of the diolate ligands, gave almost identical results, while 18–20, which
include amino-alcohol moieties, gave better diastereoselectivities, but the enantiodifferentiation
was poor in all the cases. A last experiment was performed in the presence of complex 21, in
combination with ZnEt2 (entry 26). The purpose was to bring about the simultaneous coordina-
tion of the zinc nucleus with a sulphur atom of the diolate residue and the oxygen atom of the
nitrone, which at the same time should also be coordinated to the boron nucleus. This catalytic
system was not so effective in terms of reactivity and yield as all the former B(III) complexes,
giving poor asymmetric induction and with opposite diastereoselectivity.
In summary, the cycloaddition of nitrone 1 to vinyl ether 2 is efficiently promoted by B(III)
complexes giving fast reactions in high yields and with good diastereoselectivity in favour of the
trans-cycloadduct, contrary to the uncatalysed reaction, where the cis-cycloadduct predomi-
nates. Although the enantioselectivities are still quite low, the only efficient enantioselective
version previously described for this reaction, catalysed by (BINOL)–AlMe complexes,15a yields
almost exclusively cis-cycloadducts. Therefore, the improvement of the use of chiral B(III)
complexes emerges as a promising complement to the Al(III) promoted reactions.
3. Experimental
Previously described methods were used to prepare nitrone 1,21 (1R,2R)-hydroben-
zoin,22 (1S,2S)-1,2-dibenzylethyleneglycol,23 (1S,2S)-1,2-bis(p-methoxybenzyl)ethyleneglycol,24
(1R,2R)-dicyclohexylethyleneglycol,24 [(1R,2R)-2-hydroxy-1,2-diphenylethyl]pyridine-2,6-dicar-
boxylate,13b N-{(1R,2R)-2-[(methylsulfonyl)amino]cyclohexyl}metanesulfonamide,25 N-{(1R,2R)-
2 - [(p - toluenesulfonyl)amino]cyclohexyl} - p - toluenesulfonamide,25 N - {(1R,2R) - 2 - [(n - butyl-
sulfonyl)amino]cyclohexyl}butanesulfonamide,26 1,1-dimethyl-(S)-phenylglycinol,27 and
L
-tert-
leucinol.28 Commercially available vinyl ether 2 was distilled from sodium. Nitrone 1 was dried
by melting it under vacuum and refilling with nitrogen (three consecutive times). Reaction
mixtures were stirred magnetically. A commercial 1.0 M solution of cathecolborane in CH2Cl2
was used. TLC was performed using 0.25 mm Alugram Sil plates, Machery–Nagel. HPLC was
performed with a Daicel Chiracel OD 4.6×250 mm column connected to a Waters 600 pump,
under the following conditions: samples of 20 mL (8 mg/mL), hexane/2-propanol (98/2) as the
mobil phase, 350–300 psi pressure and 1 mL/min flow. Infrared spectra were recorded on a
1
Nicolet 5 ZDX spectrophotometer. H and 13C NMR spectra were recorded in the Servei de
Ressona`ncia Magne`tica Nuclear de la Universitat Auto`noma de Barcelona on a Bruker AC-250-
WB or AM-400-WB instrument. Mass spectra were obtained on a Hewlett–Packard 5985B
instrument. HRMS was performed in the Departamento de Qu´ımica Orga´nica de la Universidad
de Zaragoza.

P. Bayo´n et al. / Tetrahedron: Asymmetry 11 (2000) 4269–4278
4275
3.1. Preparation of (2S,3S)-1,4-bis(2,4-dimethoxyphenyl)-2,3-butanediol
To a stirred suspension of CuI (190 mg, 1.0 mmol) in anhydrous THF (16 mL) at −40°C
under a nitrogen atmosphere, a solution of 10.8 mmol of 2,4-dimethoxyphenylmagnesium
bromide (prepared from 16 mL, 10.8 mmol of 1-bromo-2,4-dimethoxybenzene and 294 mg, 12.1
mmol of Mg in 5 mL of anhydrous THF) was added dropwise. The mixture was stirred for 5
min and a solution of (2S,3S)-1,2:3,4-diepoxybutane29 (prepared from 3.3 mmol of 2,3-O-iso-
propylidene- -threitol 1,4-bismethanesulphonate) in ether (3 mL) was added. The mixture was
L
allowed to warm up to 0°C and stirred for 2 h. Cold water was added, the organic phase was
separated and the aqueous layer extracted with ether (2×10 mL). The combined organic extracts
were dried over anhydrous MgSO4 and the solvent evaporated under vacuum. The crude
material was purified by silica gel flash chromatography (hexane/EtOAc, 5/2), followed by
recrystallisation in toluene to give 775 mg (2.1 mmol, 64%) of the diol: mp 82–83°C (toluene);
1
IR (KBr): 3459, 3352 (br), 2935, 2838, 1616, 1509, 1457, 1260, 1211, 1157, 1122, 1041 cm−1; H
NMR (250 MHz, CDCl3): l 7.05 (d, J=8.4 Hz, 1H), 6.43 (s, 1H), 6.41 (m, 1H), 3.76 (s, 3H),
3.75 (s, 3H), 3.65 (m, 1H), 2.90–2.70 (complex absorption, 2H), 2.64 (br s, 1H); 13C NMR (62.5
MHz, CDCl3): l 159.5, 158.3, 131.5, 119.1, 104.2, 98.5, 73.3, 55.3, 34.1; MS (m/z): 362 (M+, 2),
181 (10), 152 (31), 151 (100), 121 (29); HRMS (EI) (M+) calcd for C20H26O6 362.1729, found
362.1719. [h]2D0 +7.2 (c 1.9, CHCl3).
3.2. Preparation of (2S,3S)-1,4-bis(4-methoxybenzylthio)-2,3-butanediol
Triethylamine (1.6 mL, 11.5 mmol) and LiOH·H2O (510 mg, 12.1 mmol) were added to a
solution of 4-methoxybenzylmercaptane (1.8 mL, 12.9 mmol) in 4 mL of CHCl3 placed in a
pressure resisting flask. The mixture was stirred at 50°C for 30 min, then it was cooled to room
temperature and a solution of (2S,3S)-1,2:3,4-diepoxybutane28 (prepared from 3.6 mmol of
2,3-O-isopropylidene- -threitol 1,4-bismethanesulphonate) in ether (3 mL) was added. The
L
mixture was heated at 80°C for 20 h, then cooled, acidified with 3% HCl (pH 3) and extracted
with CH2Cl2 (2×5 mL). The combined organic extracts were dried over anhydrous MgSO4 and
the solvent evaporated under vacuum. The crude material was purified by silica gel flash
chromatography (CH2Cl2), followed by recrystallisation in EtOAc/pentane to give 1.22 g (3.1
mmol, 86%) of the diol: mp 90–91°C (EtOAc/pentane); IR (KBr): 3423 (br), 2917, 2847, 1616,
1511, 1244, 1180, 1110, 1026 cm−1; 1H NMR (250 MHz, CDCl3): l 7.20 (d, J=8.7 Hz, 2H), 6.82
(d, J=8.7 Hz, 2H), 3.77 (s, 3H), 3.66 (s, 2H), 3.57 (m, 1H), 2.65–2.45 (complex absorption, 3H);
13C NMR (62.5 MHz, CDCl3): l 158.6, 129.9, 129.8, 113.9, 70.6, 55.1, 35.6, 35.0; MS (CI, NH3):
412 (M++18, 2), 395 (M++1, 26), 121 (100). Anal. calcd for C20H26O4S: C, 60.89; H, 6.64; S,
16.25. Found: C, 60.68; H, 6.38; S, 16.06. [h]2D0 −62.7 (c 3.0, CHCl3).
3.3. Preparation of the chiral complexes 5–13
A Schlenk flask containing the bidentate ligand (0.05 mmol) under nitrogen was connected to
a vacuum pump (0.1 Torr) at room temperature for 1 h. Anhydrous CH2Cl2 (1 mL) was added,
followed by 1 M solution of BH3·THF or (nBu)2B(OTf) (50 mL, 0.05 mmol) in CH2Cl2. The
mixture was stirred at room temperature for 1 h. With the only exception of 11, which
precipitated, all complexes gave yellow or pink solutions, depending on the ligands.

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P. Bayo´n et al. / Tetrahedron: Asymmetry 11 (2000) 4269–4278
3.4. Preparation of the chiral complexes 14 and 15
A Schlenk flask containing the diol (0.05 mmol) under nitrogen was connected to a vacuum
pump (0.1 Torr) at room temperature for 1 h. Anhydrous CH2Cl2 (1 mL) and a 1 M solution
of (nBu)2B(OTf) (50 mL, 0.05 mmol) in CH2Cl2 were consecutively added, and the mixture stirred
at room temperature for 30 min. Then a 1 M solution of Bu4NF in THF (50 mL, 0.05 mmol)
was added and the mixture was stirred at room temperature for 14 h. After the addition of
Bu4NF, the colour of the solutions of complexes 9 and 10 shifted from pale-yellow and pink,
respectively, to strong violet.
3.5. Cycloaddition of nitrone 1 to vinyl ether 2 promoted by complexes 4–15. General procedure
A solution of nitrone 1 (50 mg, 0.25 mmol) in CH2Cl2 (1.5 mL) was placed in a Schlenk flask
under nitrogen. While stirring at the reaction temperature (see Table 1), a freshly prepared
solution of the catalyst (0.05 mmol) was added dropwise through a syringe. After stirring for 1
h at the reaction temperature, the vinyl ether 2 (130 mL, 1.0 mmol) was added and the evolution
1
of the reaction mixture was monitored by TLC or H NMR analyses of aliquot samples.
The reaction mixture was filtered through a pad of silica gel, which was then washed with
MeOH/CHCl3, 1/9. The overall solution was concentrated and the residue was purified by flash
chromatography on silica gel (hexane/ether, 3/1) to yield a mixture of cis- and trans-3. The
diastereoisomers were separated by a second flash chromatography on silica gel (hexane/ether,
29/1) and their ee’s were determined by HPLC.
3.6. Preparation of 16
A Schlenk flask containing (1S,2S)-1,2-dibenzylethyleneglycol (13 mg, 0.05 mmol) under
nitrogen was connected to a vacuum pump (0.1 Torr) at room temperature for 1 h. Anhydrous
CH2Cl2 (0.9 mL) was added, followed by a 1 M solution of cathecolborane (100 mL, 0.10 mmol)
in CH2Cl2. The solution was stirred at room temperature for 1 h.
3.7. Preparation of 17
A Schlenk flask containing cathecolborane (5 mg, 0.05 mmol) under nitrogen was connected
to a vacuum pump (0.1 Torr) at room temperature for 1 h. Another Schlenk flask containing
2,3-butanediol (24 mg, 0.10 mmol) under nitrogen was also connected to a vacuum pump (0.1
Torr) at room temperature for 1 h. Anhydrous CH2Cl2 (0.5 and 0.4 mL) was added, respec-
tively, to each flask. Then, 1 M BH3·THF (100 mL, 0.1 mmol) was added to the suspension of
(1S,2S)-1,2-dibenzylethyleneglycol, which dissolved completely. This solution was stirred for 1 h
and then added dropwise via a syringe to the first flask containing the cathecolborane.
Anhydrous CH2Cl2 (0.9 mL) was added, followed by a 1 M solution of cathecolborane (100
mL, 0.10 mmol) in CH2Cl2. The solution was stirred at room temperature for 1 h.
Acknowledgements
We gratefully acknowledge financial support of CIRIT (1997SGR-00003) and DGES
(PB97-0215) and the Ministerio de Educacio´n y Ciencia and the Generalitat de Catalunya

P. Bayo´n et al. / Tetrahedron: Asymmetry 11 (2000) 4269–4278
4277
for grants (to P.B. and J.M., respectively). We thank Dr. J. Gar´ın for performing the
HRMS.
References
1. Tufariello, J. J. 1,3-Dipolar Cycloaddition Chemistry; John Wiley & Sons: New York, 1984; Vol. 2, Chapter 9; (b)
Torssell, K. B. G. Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis; VCH Verlagsgesellschaft:
Weinheim, 1988.
2. Sustmann, R. Pure Appl. Chem. 1974, 40, 569–593.
3. (a) Kanemasa, S.; Uemura, T.; Wada, E. Tetrahedron Lett. 1992, 33, 7889–7892; (b) Kanemasa, S.; Oderaotoshi,
Y.; Tanaka, J.; Wada, E. J. Am. Chem. Soc. 1998, 120, 12355–12356.
4. Murahashi, S.; Imada, Y.; Kohno, M.; Kawakami, T. Synlett 1993, 395–396.
5. (a) Gothelf, K. V.; Jørgensen, K. A. J. Org. Chem. 1994, 59, 5687–5691; (b) Gothelf, K. V.; Thomsen, I.;
Jørgensen, K. A. J. Am. Chem. Soc. 1996, 118, 59–64; (c) Gothelf, K. V.; Hazell, R. G.; Jørgensen, K. A. J. Org.
Chem. 1996, 61, 346–355; (d) Gothelf, K. V.; Jørgensen, K. A. J. Chem. Soc., Perkin Trans. 2 1997, 111–115; (e)
Jensen, K. B.; Hazell, R. G.; Gothelf, K. V.; Jørgensen, K. A. J. Org. Chem. 1997, 62, 2471–2477; (f) Jensen, K.
B.; Gothelf, K. V.; Jørgensen, K. A. Helv. Chim. Acta 1997, 80, 2039–2045; (g) Sanchez-Blanco, A. I.; Gothelf,
K. V.; Jørgensen, K. A. Tetrahedron Lett. 1997, 38, 7923–7926.
6. (a) Hori, K.; Kodama, H.; Ohta, T.; Furukawa, I. Tetrahedron Lett. 1996, 37, 5947–5950; (b) Hori, K.; Kodama,
H.; Ohta, T.; Furukawa, I. J. Org. Chem. 1999, 64, 5017–5023.
7. (a) Kobayashi, S.; Akiyama, R.; Kawamura, M.; Ishitani, H. Chem. Lett. 1997, 1039–1040; (b) Kobayashi, S.;
Kawamura, M. J. Am. Chem. Soc. 1998, 120, 5840–5841; (c) Kobayashi, S.; Kawamura, M. Tetrahedron Lett.
1999, 40, 3213–3216.
8. Gilberstone, S. R.; Lopez, O. D. J. Am. Chem. Soc. 1997, 119, 3399–3400.
9. Minakata, S.; Ezoe, T.; Nakamura, K.; Ryu, I.; Komatsu, M. Tetrahedron Lett. 1998, 39, 5205–5208.
10. (a) Desimoni, G.; Faita, G.; Mortoni, A.; Righetti, P. Tetrahedron Lett. 1999, 40, 2001–2004; (b) Desimoni, G.;
Faita, G.; Mella, M.; Righetti, P.; Zema, M. Tetrahedron 1999, 55, 8509–8524.
11. (a) Seerden, J.-P. G.; Scholte op Reimer, A. W. A.; Scheeren, H. W. Tetrahedron Lett. 1994, 35, 4419–4422; (b)
Seerden, J.-P. G.; Kuypers, M. M. M.; Scheeren, H. W. Tetrahedron: Asymmetry 1995, 6, 1441–1450; (c) Seerden,
J.-P. G.; Boeren, M. M. M.; Scheeren, H. W. Tetrahedron 1997, 53, 11843–11852.
12. Meske, M. J. Prakt. Chem. 1997, 426–433.
13. (a) Bayo´n, P.; de March, P.; Figueredo, M.; Font, J. Tetrahedron 1998, 54, 15691–15700; (b) Bayo´n, P.; de
March, P.; Espinosa, M.; Figueredo, M.; Font, J. Tetrahedron: Asymmetry 2000, 11, 1757–1765.
14. Hori, K.; Ito, J.; Ohta, T.; Furukawa, I. Tetrahedron 1998, 54, 12737–12744.
15. (a) Simonsen, K. B.; Bayo´n, P.; Hazell, R. H.; Gothelf, K. V.; Jørgensen, K. A. J. Am. Chem. Soc. 1999, 121,
3845–3853; (b) Simonsen, K. B.; Jørgensen, K. A.; Hu, Q.-S.; Pu, L. Chem. Commun. 1999, 811–812; (c) Jensen,
K. B.; Hazell, R. G.; Jørgensen, K. A. J. Org. Chem. 1999, 64, 2353–2360.
16. Ellis, W. W.; Gavrilova, A.; Liable-Sands, L.; Rheingold, A. L.; Bosnich, B. Organometallics 1999, 18, 332–338.
17. (a) Camiletti, C.; Dhavale, D. D.; Gentilucci, L.; Trombini, C. J. Chem. Soc., Perkin Trans. 1 1993, 3157–3165;
(b) Ohtake, H.; Imada, Y.; Murahashi, S.-I. J. Org. Chem. 1999, 64, 3790–3791.
18. Gothelf, K. V.; Hazell, R. G.; Jørgensen, K. A. Acta Chem. Scand. 1997, 50, 1234–1235.
19. (a) Koyano, K.; Suzuki, H. Tetrahedron Lett. 1968, 15, 1859–1864; (b) Koyano, K.; Suzuki, H. Bull. Chem. Soc.
Jpn. 1969, 42, 3306–3309.
20. Such interaction has been postulated for an alkoxide in: Denmark, S. E.; O’Connor, S. P. J. Org. Chem. 1997,
62, 584–594; and for a carbonyl oxygen atom with two boron centres in: Iseki, K. Tetrahedron 1998, 54,
13887–13914.
21. Wheeler, O. H.; Gore, P. H. J. Am. Chem. Soc. 1956, 78, 3363–3366.
22. Wang, Z.-M.; Sharpless, K. B. J. Org. Chem. 1994, 59, 8302–8303.
23. Devine, P. N.; Oh, T. Tetrahedron Lett. 1991, 32, 883–886.
24. De March, P.; Figueredo, M.; Font, J.; Medrano, J. Tetrahedron 1999, 55, 7907–7914.

4278
P. Bayo´n et al. / Tetrahedron: Asymmetry 11 (2000) 4269–4278
25. Takahashi, H.; Kawakita, T.; Ohno, M.; Yoshioka, M.; Kobayashi, S. Tetrahedron 1992, 48, 5691–5700.
26. Denmark, S. E.; O’Connor, S. P.; Wilson, S. R. Angew. Chem., Int. Ed. 1998, 37, 1149–1151.
27. Corey, E. J.; Ishihara, K. Tetrahedron Lett. 1992, 33, 6807–6810.
28. Nishiyama, H.; Kondo, M.; Nakamura, T.; Itho, K. Organometallics 1991, 10, 500–508.
29. Feit, P. W. J. Med. Chem. 1964, 7, 14–17.
.
.