A practical kinetic resolution of 4-acetyl[2.2]paracyclophane

Article abstract of DOI:10.1016/S0957-4166(01)00452-9

The kinetic resolution of 4-acetyl[2.2]paracyclophane has been realized by borane reduction in the presence of a CBS catalyst. The structure of the two diastereomeric alcohol products has been established. The ketone was recovered in the more than 99% e.e. at 69% conversion.

Full text of DOI:10.1016/S0957-4166(01)00452-9

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 12 (2001) 2625–2630
A practical kinetic resolution of 4-acetyl[2.2]paracyclophane
a
a
b
a
Philippe Dorizon, Catherine Martin, Jean-Claude Daran, Jean-Claude Fiaud and
a,
Henri B. Kagan *
a
Laboratoire de Catalyse Mol e´ culaire (ESA 8075) Institut de Chimie Mol e´ culaire d’Orsay, Universit e´ Paris-Sud,
9
1405 Orsay, France
b
Laboratoire de Chimie de Coordination du CNRS (UPR 8241), 205 route de Narbonne 31077 Toulouse cedex, France
Received 2 October 2001; accepted 15 October 2001
Abstract—The kinetic resolution of 4-acetyl[2.2]paracyclophane has been realized by borane reduction in the presence of a CBS
catalyst. The structure of the two diastereomeric alcohol products has been established. The ketone was recovered in more than
99% e.e. at 69% conversion. © 2001 Elsevier Science Ltd. All rights reserved.
1
. Introduction
enantiopure 4-ethene[2.2]paracyclophane, itself ob-
tained from 4-acetyl[2.2]paracyclophane 1. A recent
10
The [2.2]paracyclophane system can give rise to many
chiral derivatives. The first known enantiopure com-
pound was the [2.2]paracyclophane carboxylic acid,
report on the resolution of 1 by the SAMP-hydrazone
method prompted us to describe herein an efficient
kinetic resolution in the borane reduction of 4-ace-
tyl[2.2]paracyclophane 1.
1
prepared in 1955 by Cram et al. by resolution. The
2
,3
resolution was later improved by several groups. In
the last decade there has been a renewal of interest in
enantiopure [2.2]paracyclophane derivatives, since they
can be used for the preparation of chiral ligands for
Asymmetric reduction of (±)-1 generates the
diastereomeric alcohols 2 and 3 (Scheme 1). The partial
conversion of racemic 1 can give rise to partially enan-
tioenriched 1, 2 and 3. The relationships between the
enantiomeric excesses of these three compounds (e.e.1,
e.e.2 and e.e.3), the conversion (C) and the
4–7
asymmetric catalysis
or as chiral auxiliaries in stoi-
8
chiometric asymmetric synthesis. The [2.2]paracyclo-
phane skeleton may also be useful as a structural
element of complex molecules, such as helicenophanes
1
1
diastereomeric ratio (d.r.=2/3) have been calculated.
9
for example. In that case, a good starting material is
These relationships allow the self-consistency of experi-
OH
HO
O
O
(S,SP)-2
(R,RP)-2
OH
HO
(SP)-1
(RP)-1
(R,SP)-3
(S,RP)-3
Scheme 1. Reduction of racemic 4-acetylparacyclophane 1.
*
Corresponding author. E-mail: kagan@icmo.u-psud.fr
0
957-4166/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(01)00452-9

2
626
P. Dorizon et al. / Tetrahedron: Asymmetry 12 (2001) 2625–2630
mental data to be checked. The action of a chiral
catalyst or reagent on a racemic substrate with creation
of a new stereocenter may be highly diastereoselective
but not necessarily enantioselective. This has been dis-
3. Kinetic resolution by CBS-catalyzed reduction
Chiral oxazaborolidines derived from proline and ana-
logues are known to be good enantioselective catalysts
11,12
17
cussed by us in a general form.
It means that the
in many reductions by borane (CBS catalysts). For
formation of products such as 2 and 3 in high e.e. is not
automatically associated with good kinetic resolution of
the starting material 1.
example, acetophenone 6a (Scheme 2) has been trans-
formed by borane into (R)-7b (96% e.e.) in the presence
of catalytic amounts of (S)-5. Similarly, the dimethyl
derivative 6b provided (R)-7b (97% e.e.).
By using 15 mol% of (S)-5 at 0°C we investigated the
2
. Kinetic resolution mediated by aminoindanol 4
influence of the loading of borane (BH ·THF or
3
BH ·Me S) on the conversion, the product distribution
3
2
It is known that some oxazaborolidines can mediate the
13
and the enantiomeric excesses of the various products.
The results obtained in the reduction of (±)-1 at the 0.2
g scale are listed in Table 1. In these experiments
ketone 1 was added as a solid to a THF solution of
asymmetric reduction of ketones by borane. Aminoin-
danol 4 has been previously described to give excellent
results (95% e.e. in the presence of 1 mol% of 4) in the
14
reduction of acetophenone. Aminoindanol 4 has also
15,16
BH and catalyst.
3
been used catalytically.
The amino alcohol 4 gener-
ates an oxazaborolidine which mediates or catalyzes the
asymmetric reduction. Asymmetric reduction of (±)-4-
acetylparacyclophane 1 at 0°C in the presence of 1.25
mol% of 4 and an excess of borane (2.5 mol equiv.)
gave a total conversion into an almost equimolar
amount (53:47) of diastereomeric alcohols 2 and 3 in
very high enantiomeric excess (89 and >99% e.e.,
respectively). The enantiomeric excesses of ketone 1 and
alcohols 2 and 3 could be nicely analyzed by HPLC
Complete reduction of 1 provided a 2.4:1 mixture of
diastereomeric alcohols 2 and 3 (35.9 and 89.3% e.e.,
respectively). The relationships between d.r. and e.e.s
for 100% conversion is 2/3=e.e. /e.e. , established by
3
2
18
Guett e´ and Horeau. This relationship is well satisfied,
since e.e. /e.e. =2.48, against 2.4 measured. When the
3
2
reaction was stopped before completion, the enan-
tiomeric excess of the remaining ketone 1 increased
with conversion (compare entries 4 and 3), as is usual in
(
Daicel Chiralcel OD-H, see Section 6). In order to be
19
a kinetic resolution process. Simultaneously the enan-
synthetically useful, this process must be catalytic with
respect to the chiral auxiliary 4. Unfortunately, by
decreasing the amount of 4 (for example to 22 mol%)
the two alcohols were recovered in near racemic form,
presumably because of the competing uncatalyzed
borane reduction. Furthermore, the recovered 1 at par-
tial conversion is also close to racemic composition.
For that reason we investigated the borane reduction
catalyzed by other types of chiral oxazaborolidines.
tiomeric excess of the product 2 decreased.
In Table 2, the results for the addition of a THF
solution of 1 (instead of addition of 1 as a solid) are
shown. The optimal experimental conditions at a 0.2 g
scale (entry 1, Table 2) with (S)-5 as a catalyst are
described in Section 6. Acetylparacyclophane 1 was
recovered with 98.8% e.e. (S) and 33% isolated yield.
NH2
OH
H
R
R
Ph
Ph
O
BH3
OH
N
B O
R
Cat = (S)-5
R
Me
4
5
6
7
a: R = H
b: R = Me
a: R = H
b: R = Me
Scheme 2.
Table 1. CBS reduction of (9)-4-acetyl[2.2]paracyclophane 1 (addition of 1 as a solid)
a,b
c
Conversiond (%) Recovered 1
d.r.e,d 2/3
e.e. d (%)
e.e.3d (%)
Entry
Cat. 5 config. BH ·THF
Time (min)
3
2
(
mol equiv.)
e.e.1 (%)
1
2
3
4
(R)
(S)
(S)
(S)
0.90
0.60
0.60
0.30
80
100
50
100
85
78
–
2.4
2.1
2.3
35.9
64
64.6
90.1
89.3
\99
\99
\99
95.1 (Sp)
92.5 (Sp)
31.3 (Sp)
50
30
17.2
a
1
1
(0.2 g) added as a solid over 5 min to a mixture of catalyst 5 and BH ·THF at 0°C.
5 mol%.
3
b
c
Reaction time.
Measured by HPLC.
Diastereomeric ratio.
d
e

P. Dorizon et al. / Tetrahedron: Asymmetry 12 (2001) 2625–2630
2627
Table 2. (S)-CBS reduction of (9)-4-acetyl[2.2]paracyclophane 1 (addition of 1 dissolved in THF)
a,b
c
Conversiond (%) Recovered 1 e.e.1 d.r.e,d 2/3
sf
e.e.2d (%)
e.e.3d (%)
Entry
1
(g) Addition time
Time (min)
(min)
(%)
1
2
3
0.2
1
5
5
5
10
15
15
20
60
59
78.4
98.8 (Sp)
99.1 (Sp)
84 (Sp)
9.8
9.4
9.4
23
27
4
78.1
74
57.7
\99
\99
\99
a
A solution of 1 in THF was added to a mixture of (S)-CBS and BH ·Me S (0.6 mol equiv.) at 0°C.
3
2
b
c
d
e
f
1
5 mol%.
Reaction time.
Measured by HPLC.
Diastereomeric ratio.
Calculated by the formula: s=krel=[ln(1−c)(1−e.e. )]/[ln(1−c)(1+e.e. )].
19
1
1
The reaction has also been carried out successfully at
the 1 g scale (entry 2, Table 2), with an isolated yield of
product. Here e.e.p is the e.e. of ketone 1 obtained
through oxidation of the mixture of diastereomeric
alcohols 2 and 3. This second approach gives C=0.65
(65%) leading to s=15.6.
33% in 1 (99% e.e.). The mixture of diastereomeric
alcohols is strongly enriched into 2 (d.r.=9.4, 74% e.e.).
An attempt to scale up the reaction to 5 g failed, since
ketone 1 was recovered with only 84% e.e. for 78%
conversion. This reflects the complexity of the reduc-
tions using CBS catalysts, needing a careful optimiza-
4. Assignment of stereochemistry to alcohols
(−)-2 and (+)-3
2
0
tion for a given scale and a given substrate. For that
reason, we investigated the procedure described by
The kinetic resolution of (±)-1 provided recovered (S )-
p
17
Corey et al. with an inverse addition, e.g. the addition
of a THF solution of borane to the solution containing
the ketone and the catalyst. Some representative results
are indicated in Table 3. The procedure is very satisfac-
tory with 10 mol% of catalyst at the 0.2 g, as well as at
the 3 g scale (entry 3, Table 3). The synthetic usefulness
of the process is well demonstrated by the optimization
1 when (S)-CBS catalyst 5 has been used. The major
alcohol 2, which was simultaneously produced in large
excess (and in high e.e.) needs to be of (R)-configura-
tion at the newly created stereogenic center. This
hypothesis is based on the established rule in the CBS
reduction that an arylmethylketone was consistently
transformed into an excess of (R)-alcohol when (S)-5
17
at the 3 g scale, leading to 23% of recovered (+)-(S )-1
was the catalyst. It means that the major alcohol (−)-2
p
(
>99% e.e.) and to 73% of a mixture of diastereomeric
has the (R,R )-configuration. This was confirmed by
p
21
alcohols 2 and 3, which on reoxidation using PCC,
oxidation of the crude mixture of (−)-2 and (+)-3 (9.8:1)
gave (−)-(S )-1 (54% e.e.) in 53% total yield. The final
(entry 1, Table 2), which generated (−)-(R )-acetylpara-
p
p
mass balance of the process was 23% of (S )-1 (>98%
cyclophane 1 in 80% e.e. The minor alcohol (+)-3
necessarily has the alternate (S,R )- or (R,S )-configu-
p
2
2
e.e.) and 53% of (R )-1 (54% e.e.).
p
p
p
ration. The reoxidation of the minor diastereomer (+)-3
The intrinsic efficiency of the kinetic resolution of
racemic 1 can be appreciated by the value of the
(>99% e.e.) gave the ketone (+)-1 of known (S )-abso-
p
2
lute configuration. The relative stereochemistry of (+)-
stereoselectivity factor s=k , using the formula
3 has been subsequently confirmed by X-ray crystal
structural analysis. The molecular structure with (R,Sp)
configuration is represented in Fig. 1. Owing to the lack
of heavy atoms (Z>Si) the Flack parameter is unreliable
and the absolute configuration could not be deter-
mined. The paracyclophane skeleton compares well
rel
s=ln[(1−C)(1−e.e. )/(1−C)(1+e.e. )] established for
1
1
19
reactions which are first-order in substrate. The reac-
tion in entry 3 of Table 3 (69% conversion) gives s=12.
The main source of error is the accuracy on the mea-
surement of conversion. Conversion can be accurately
calculated by the formula C=e.e. /(e.e. +e.e. ), where
23
with the related structure of racemic 1. Alcohol (+)-3
was isolated after flash-chromatography of the products
s
s
p
e.e. =e.e. and e.e. is the enantiomeric excess of the
s
1
p
Table 3. (S)-CBS reduction of (9)-4-acetyl[2.2]paracyclophane 1 (addition of borane to the ketone)
a
Timeb (min)
c
d.r.d,e 2/3
e
e.e.2c (%)
e.e.3c (%)
Entry
Cat. 5 (%)
1
(g)
Addition
Conversion (%)
Recovered 1
s e
time (min)
e.e.1 (%)
1
2
3
15
10
10
0.2
0.2
3
5
5
10
25
45
45
53
71
69
98 (Sp)
\99 (Sp)
\99 (Sp)
6.3
3.4
2.6
13
11
12
88.8
80
60.8
\99
\99
76.8
a
BH ·Me S (0.6 mol equiv.) added in THF to a mixture of (S)-CBS and 1 at 0°C.
Reaction time.
Measured by HPLC.
3
2
b
c
d
e
Diastereomeric ratio.
Calculated by the formula: s=krel=[ln(1−c)(1−e.e. )]/[ln(1−c)(1+e.e. )].
19
1
1

2
628
P. Dorizon et al. / Tetrahedron: Asymmetry 12 (2001) 2625–2630
in a kinetic resolution of racemic 1 catalyzed by (S)-5.
Although being the minor product, the diastereomeric
alcohol 3 ([h] =+40 (CH Cl ), >99% e.e.) was easily
isolated. Its crystal structure shows a conformation
where the hydroxyl group is away from the paracy-
clophane skeleton.
Finally, the relative and absolute configuration of the
diastereomeric alcohols 2 and 3 have been firmly estab-
lished using chemical correlation and X-ray crystallog-
raphy.
D
2
2
6
. Experimental
The epimeric alcohol 2 of (R,R )-configuration has not
p
been isolated enantiomerically pure. A sample of 75.8%
6
.1. General
e.e. has a specific rotation of [h] =−110.4 (c 1.35,
D
CH Cl ). It was then calculated that the enantiopure
1
13
2
2
H and C NMR spectra were recorded at 250 and 63
MHz, respectively, with a Bruker AM 250 instrument.
Chemical shifts are denoted in ppm (l) relative to TMS
(
R,R )-2 must have a value of [h] =−145.6 (CH Cl ).
p
D
2
2
The absolute configuration of (+)-4-acetyl[2.2]para-
cyclophane 1 ([h] =+65 (c=0.8, CHCl )) has been
D
3
1
13
(
H and C). Coupling constants are reported in Hz.
established as (S ) by comparison with the literature
p
Optical rotations: Perkin–Elmer 241 polarimeter (589
nm, 20°C). Concentrations (c) are reported in g/100
mL. High resolution mass spectra (HRMS) were per-
formed with a GC/MS Finnigan-MAT-95-S. Analytical
HPLC were recorded on a HPLC machine equipped
with a Spectra Series P100 pump and a Spectra Series
UV100 detector. The chiral stationary phase was a
Daicel Chiralcel OD-H column. All reactions were car-
ried out under argon in oven-dried glassware using
standard vacuum-line techniques. All commercial
reagents were used as received. Borane dimethyl sulfide
and borane tetrahydrofuran complexes were purchased
from Acros. (S)-CBS was purchased from Fluka.
2
value. We use here the CIP nomenclature modified by
Prelog and Helmchen (see also Ref. 25).
2
4
5. Conclusion
We have developed a simple way to resolve 4-ace-
tyl[2.2]paracyclophane by a catalytic asymmetric reduc-
tion. This enantiopure compound is an interesting
intermediate since it can be transformed into a wide
variety of enantiopure paracyclophane derivatives such
1
as 4-[2.2]paracyclophane carboxylic acid, 4-ethene[2.2]-
1
0
26
paracyclophane
or 4-bromo[2.2]paracyclophane.
Figure 1. CAMERON view of the molecule 3 with atom labelling scheme. Ellipsoids are drawn at 50% probability.

P. Dorizon et al. / Tetrahedron: Asymmetry 12 (2001) 2625–2630
2629
6
.2. Asymmetric reduction of racemic 1 using (1R,2S)-
6.3.3. (R,S )-(+)-1-[4-[2.2]-Paracyclophanyl]-ethan-1-ol
p
1
(
+)-cis-1-amino-2-indanol 4
3. H NMR (CDCl , 250 MHz) l (ppm) 1.57 (3H, d,
3
J=7.6 Hz,+1H, s large); 2.87–3.15 (7H, m); 3.58–3.65
13
A 1 M solution of BH ·THF (1 mL, 1 mmol) was
added dropwise to a solution of 4 (80 mg, 0.5 mmol) at
(1H, m); 4.84 (1H, m); 6.31–6.51 (7H, m). C NMR
CDCl , 62 MHz) l (ppm): 25.7; 33.2; 34.4; 35.2; 35.3;
3
(
3
0
°C in THF (3 mL). After 2 h at 0°C, 4-acetylpara-
6
1
2
7.6; 128.2; 129.9; 131.6; 132.1; 133.0; 133.6; 134.9;
35.1; 139.3; 139.7; 140.4; 144.7. MS (EI) m/z: 252 (M ,
3); 130 (93), 129 (100), 104 (75). HRMS calcd for
cyclophane (0.1 g, 0.4 mmol) was added portionwise
over 5 min and the mixture was stirred for 5 min. Then
MeOH (0.5 mL) and a 2 M solution of HCl in EtOH
+
C H O 252.1514, found 252.1513. GC (200°C+10°C/
18
20
(
0.5 mL) were added. A white solid was filtered off and
min), Cpsil, 25 m, R (min): 2.56. HPLC (hex/iPrOH:
t
the solution was evaporated under reduced pressure.
The crude mixture was analyzed by HPLC (OD-H
Chiralcel, hexane/isopropanol: 98/2, 1 mL/min, 254
nm): see text. Purification by chromatography on silica
gel (CH Cl /pentane/AcOEt: 5/5/1) afforded a mixture
9
8/2, 1 mL/min, 254 nm) R (min): 43.89. >99% e.e.,
t
25
[h] +40 (c=1.35, CH Cl ). Mp=113°C (colorless
D 2 2
crystals).
2
2
of alcohols 2 and 3 (75 mg).
6.4. Oxidation of alcohol 3 into ketone 1
Alcohol 3 (2.2 g, 7.93 mmol) was dissolved in CH Cl
2
2
6.3. Asymmetric reduction of racemic 1 using (S)-CBS 5
(
12 mL). PCC (2.57 g, 11.9 mmol) was added and the
mixture was stirred 2 h at room temperature. The black
reaction mixture was diluted with five volumes of ether
and filtered through a pad of Celite. The solvent was
removed under reduced pressure and the residue was
purified by flash chromatography to give ketone 1 as a
white solid (1.6 g, 80%). Mp=110–112°C.
A solution of BH ·THF (1 M, 0.24 mL, 0.24 mmol)
was added to a solution of (S)-CBS (1 M, 0.06 mL,
3
0
4
.06 mmol) at 0°C in THF (1 mL). After 2 h at 0°C,
-acetylparacyclophane (0.1 g, 0.4 mmol) was added
portionwise over 5 min and the mixture was stirred for
0 min. MeOH (0.1 mL) and a solution of HCl in
5
EtOH (2 M, 60 mL) were added. A white solid was
filtered off and the solution was evaporated under
reduced pressure. The crude mixture was analyzed by
HPLC (OD-H Chiralcel, hexane/isopropanol: 49/1).
Results are collected in Table 1.
6
.5. X-Ray crystal structure of (R,S )-(+)-1-[4-[2.2]-
p
paracyclophanyl]-ethan-1-ol 3
The crystal data were collected using a Stoe IPDS
diffractometer operating at 180(2)° K. Intensities were
collected with graphite monochromatized MoKa radia-
tion (u=0.71073) using a 8 scan technique. Cell
parameters were refined using 8000 selected reflections.
BH ·SMe (0.11 mL, 1.2 mmol) was added to a solution
3
2
of (S)-CBS (1 M, 1.2 mL, 1.2 mmol) at 0°C in THF (40
mL). After 5 min at 0°C, 4-acetylparacyclophane (3 g,
12 mmol) was added and the mixture was stirred for 5
min. Then BH ·SMe (1.1 M, 0.57 mL, 6 mmol) in
3
2
2
7
THF (10 mL) was added over 10 min and the mixture
was stirred for 45 min. MeOH (3 mL) and a solution of
HCl in EtOH (2 M, 1 mL) were subsequently added. A
white solid was filtered off and the solution was evapo-
rated under reduced pressure. The crude mixture was
analyzed by HPLC (OD-H Chiralcel, hexane/iso-
propanol: 49/1). Purification by chromatography on
silica gel (CH Cl /AcOEt: 98/2) afforded 0.7 g of
The structure was solved by direct methods (SIR-97 )
and refined by least-squares procedures on F using the
28
CRYSTALS program. All H atoms attached to car-
bon were introduced in calculation in idealised posi-
tions [d(CH)=0.96 A] and treated as riding models. H
atom of the hydroxyl group was isotropically refined.
The drawing of the molecule was realized with the help
,
29
2
2
of CAMERON.
ketone 1 and 1.85 g of a mixture of alcohols 2 and 3.
Diastereoisomers 2 and 3 can be cleanly separated on
silica gel (CH Cl /pentane/AcOEt: 5/5/1) and pure
Crystal data for 3: [C H O]; M =252.35; orthorhom-
bic; space group P2 2 2 ; a=8.0079(6), b=13.0920(8),
, ,
c=26.739(2) A, V=2803.3(3) A , Z=8, zcalcd=1.196 g
cm , v=0.072 cm ; 2q =48.4°; reflections collected
18
20
r
2
2
alcohol 3 was recrystallized from ether. The results are
collected in Tables 2 and 3.
1 1 1
3
−
3
−1
max
unique used, 15853, 4413 (R =0.0276) 3901 (I>2|(I));
2
5
int
6
.3.1. 1-[4-[2.2]-Paracyclophanyl]-ethan-1-one 1. [h]D
parameters refined, 352; R/R =0.0311/0.0372; GOF=
w
+
65 (c=0.2, CHCl ) when e.e. >99%. HPLC (hex/
iPrOH: 98/2, 1 mL/min, 254 nm) R (min): 18.84
3
1
.016; D/| =0.042; [Dz] [Dz]max, −0.22, 0.22 e
max
min
t
−3
,
A ; Flack’s parameter=not reliable.
(
minor), 14.73 (major).
1
Crystallographic data (excluding structure factors) have
been deposited with the Cambridge Crystallographic
Data Center as supplementary publication N. CCDC
6
.3.2. 1-[4-[2.2]-Paracyclophanyl]-ethan-1-ol 2. H NMR
(
CDCl , 250 MHz) l (ppm) 1.27 (3H, d, J=6.4 Hz);
.73 (1H, s large); 2.80–3.33 (8H, m); 4.92 (1H, q,
J=6.4 Hz); 6.38–6.62 (7H, m). GC (200°C+10°C/min),
Cpsil, 25 m, R (min): 2.49. HPLC (hex/iPrOH: 98/2, 1
3
1
1
68418. Copies of the data can be obtained free of
charge on application to the Director, CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (fax: (+44)
1223-336-033; e-mail: deposit@ccdc.cam.ac.uk
t
mL/min, 254 nm) Rt (min): 20.93 (minor), 32.66
(
major).

2
630
P. Dorizon et al. / Tetrahedron: Asymmetry 12 (2001) 2625–2630
14. Didier, E.; Loubinoux, B.; RamosTombo, G. M.; Rihs,
Acknowledgements
G. Tetrahedron 1991, 47, 4941–4958.
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1
We thank CNRS, Universit e´ Paris-Sud and INCO-
Copernicus program (contract IC-CT960722) for their
financial support.
16. Yang, G. S.; Hu, J. B.; Zhao, G.; Ding, Y.; Tang, M. H.
Tetrahedron: Asymmetry 1991, 10, 4307–4311.
1
1
1
2
2
2
7. Corey, E. J.; Helal, C. J. Angew. Chem., Int. Ed. 1998, 37,
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