Isomeric cis-cyclohexano-13-crown-4 ethers: A low-temperature 1H and 13C NMR investigation

Article abstract of DOI:10.1002/(SICI)1097-458X(199810)36:10<687::AID-OMR352>3.0.CO;2-S

The two positionally isomeric cyclohexanotetraoxacyclotridecanes (13-crown-4 ethers) were synthesized and studied via low-temperature NMR methods. For the 1,4,8,11-tetraoxa isomer, the cyclohexane ring inversion is a degenerate process, whereas for the 1,4,7,11-tetraoxa isomer, a preference of 1.4 kJ mol^-1 for the form in which the propyleneoxy group is equatorial was determined. Molecular mechanics calculations using MM⁺ indicated a preference of 0.6 kJ mol^-1 for this conformer. Resonance assignment was facilitated by the synthesis of a selectively deuterated derivative and by COSY, HMQC and HMBC experiments. The results were compared with those for the related 10-crown-3 system and ¹³C chemical shift trends are discussed in terms of MM⁺ calculated geometries. John Wiley & Sons Ltd.

Full text of DOI:10.1002/(SICI)1097-458X(199810)36:10<687::AID-OMR352>3.0.CO;2-S

MAGNETIC RESONANCE IN CHEMISTRY
Magn. Reson. Chem. 36, 687È692 (1998)
Isomeric cis-cyclohexano-13-crown-4 ethers: a
low-temperature 1H and 13C NMR investigation¤
G. W. Buchanan,* M. Gerzain and R. C. Laister
Ottawa–Carleton Chemistry Institute, Department of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa, K1S 5B6, Canada
Received 30 December 1997; revised 1 April 1998; accepted 2 April 1998
ABSTRACT: The two positionally isomeric cyclohexanotetraoxacyclotridecanes (13-crown-4 ethers) were synthe-
sized and studied via low-temperature NMR methods. For the 1,4,8,11-tetraoxa isomer, the cyclohexane ring inver-
sion is a degenerate process, whereas for the 1,4,7,11-tetraoxa isomer, a preference of 1.4 kJ mol~1 for the form in
which the propyleneoxy group is equatorial was determined. Molecular mechanics calculations using MM` indi-
cated a preference of 0.6 kJ mol~1 for this conformer. Resonance assignment was facilitated by the synthesis of a
selectively deuterated derivative and by COSY, HMQC and HMBC experiments. The results were compared with
those for the related 10-crown-3 system and 13C chemical shift trends are discussed in terms of MM` calculated
geometries. ( John Wiley & Sons Ltd.
KEYWORDS: stereochemistry; 13-crown-4 ethers; tetraoxacyclotridecanes
INTRODUCTION
not expected to be equivalent. In this study we exam-
ined the conformational dependence of the chemical
shifts in 1 and the position of the equilibrium between
2A and 2B via one- and two-dimensional NMR
methods at temperatures where ring inversion is slow
on the NMR time-scale.
Recent reports from this laboratory have been con-
cerned with low-temperature 1H and 13C NMR chemi-
cal shifts and conformational analysis of a variety of
cis-1,2-cyclohexano crown ethers with ring sizes of 9, 10
and 15.1h3 The only unsymmetrical case was the 10-
crown-3 system2 in which a preference of 2.8 kJ mol~1
was determined for the conformation having the propyl-
eneoxy group in the equatorial disposition.
For the case of cis-1,2-cyclohexano-13-crown-4, there
are two positional isomers, 1 and 2. Cyclohexane ring
inversion in 1 interconverts the degenerate forms 1A
and 1B, whereas for 2 and populations of 2A and 2B are
RESULTS AND DISCUSSION
Chemical shift assignments
For 1 at 300 K, the assignments (Table 1) of the 13C
resonances for the carbons of the 13-membered ring
were initiated at the C-10 site using a combination of
COSY and 1HÈ13C HMQC experiments. The high-Ðeld
resonance position for this carbon, its relative intensity
and the correlation of its bonded protons with aliphatic
protons on oxygenated carbon all indicated that the
line at 31.02 ppm must be due to C-10. It then followed
directly that C-9,11 must resonate at 69.82 ppm. The
remaining distinction between C-8,12 and C-7,13 was
made by comparing the spectrum of 1 with that for 1-7,
13-d . For the cyclohexyl carbons, assignments were
4
made by analogy with related materials studied in these
laboratories.1h3
At 210 K, where ring inversion is slow on the NMR
time-scale, the pattern of shift di†erences for the cyclo-
hexyl carbons of 1 is similar to that found for
cyclohexyl-9-crown-31 and -15-crown-53 and other
simpler derivatives4 in that the 13C chemical shift for
the methine carbon bearing equatorial oxygen is
deshielded substantially (by 5.5 ppm) relative to its
counterpart bearing axial oxygen. The pattern of 1H
shifts is also normal with the axial methine proton on
C-1 being shielded relative to its equatorial counterpart
¤ Taken in part from the PhD Thesis of M. Gerzain, Carleton Uni-
versity, February 1997.
* Correspondence to: G. W. Buchanan, OttawaÈCarleton Chemistry
Institute, Department of Chemistry, Carleton University, 1125
Colonel By Drive, Ottawa, K1S 5B6, Canada.
( 1998 John Wiley & Sons, Ltd.
CCC 0749-1581/98/100687È06 $17.50

688
G. W. BUCHANAN, M. GERZAIN AND R. C. LAISTER
Table 1. 13C and 1H NMR chemical shifts for 1 (d from
TMS ^ 0.01)a
ment in which a correlation was observed between the
axial C-1 methine proton at 3.19 ppm and the 13C reso-
nance at 63.60 ppm, thereby establishing that the C-7
site gives rise to this resonance and the C-13 site is sub-
stantially deshielded (i.e. by 5.42 ppm). From low-
temperature COSY and HMQC experiments, it was
subsequently deduced that the C-8 resonance was at
63.76 ppm whereas that for C-12 appeared at 66.74
ppm. With respect to the large shift di†erence observed
between C-7 and C-13, it is useful to examine the pre-
ferred geometry of 1 as calculated by MM` with
respect to the torsion angles of the macrocyclic ring (see
Table 3). The most signiÐcant di†erence between the
environments for these two carbons involves the
C-7ÈO-1ÈC-1ÈC-2 angle of 85.0¡ vs. the C-13ÈO-
4ÈC-2ÈC-1 angle of [ 163.6¡. Based on the known
dependence of the gamma steric shift4 on dihedral
angle, it is expected that the C-7 site should indeed be
substantially shielded relative to C-13, in agreement
with experiment.
1 (300 K)
1A (210 K)
Site
13C
1H
3.50
13C
1H
3.19
3.81
1.50
1
2
3
4
5
6
7
8
9
10
11
12
13
76.68
76.68
27.81
22.41
22.41
27.81
66.40
68.01
69.82
31.02
69.82
68.01
66.40
77.45
71.95
27.28
18.73
23.97
25.72
63.60
63.76
(68.18)b
31.25
(68.41)b
66.74
69.02
3.50
1.88, 1.38
1.60, 1.30
1.60, 1.30
1.88, 1.38
3.60, 3.62
3.75, 3.52
3.56
1.68
3.56
3.75, 3.52
3.60, 3.62
1.27
1.60, 1.18
1.95, 1.02
3.70, 3.68
3.50, 3.58
3.40, 3.62
1.58
3.40, 3.60
3.42, 3.80
3.42, 3.50
a 0.1 M solutions in CD Cl .
2
2
b Parentheses indicate possible assignment interchange.
The 13C NMR spectra of 2 at 300 and 210 K are
depicted in Figs 1 and 2, respectively. At 300 K, again
the starting point for the macrocyclic carbon chemical
shift assignment (Table 2) was in the propyleneoxy unit
(i.e. C-11ÈC-13). Using COSY and HMQC methods, it
was determined that the 13C resonance at 31.1 ppm has
directly bonded protons resonating at 1.64 ppm and
hence was assigned to C-12, since there were COSY
connections for the 1.64 ppm resonance to two sets of
aliphatic protons on carbons bearing oxygen. Distinc-
on C-2. With respect to the carbons of the macrocyclic
ring, the C-10 assignment is straightforward. From
COSY and HMQC spectra the C-9 and C-11 reso-
nances were identiÐed as being at 68.2 and 68.4 ppm,
but no distinction between these resonances for assign-
ment purposes was possible. Using 1-7,13-d , it was
4
possible to ascertain that resonances at 69.02 and 63.60
ppm arose from these sites. Distinction between C-7
and C-13 was made on the basis of a 3J HMBC experi-
Figure 1. 100 MHz 13C NMR spectrum of 2 at 300 K.
MAGNETIC RESONANCE IN CHEMISTRY, VOL. 36, 687È692 (1998)
( 1998 John Wiley & Sons, Ltd.

NMR OF 13-CROWN-4 ETHERS
689
(a)
(b)
Figure 2. 100 MHz 13C NMR spectra of 2 (a) oxygenated carbons and (b) non-oxygenated carbons, at 210 K.
tion between the C-11 and C-13 sites was made possible
only at 210 K by the observation of a 3J HMBC corre-
lation between the axial proton on C-2 in the major
conformer 2B and the 13C line at 60.57 ppm. From
coalescence results it was clear that C-13 of 2A reson-
ates at 66.85 ppm. Hence the assignment of the room
temperature shift of 63.72 ppm to the C-13 position and
that at 71.00 ppm to C-11 was made. It is interesting
that the highly shielded 13C resonance for the C-13 site
of 2B relative to 2A is consistent with the result found
recently for the related 10-crown-3 derivative.2
resonance at 64.17 in 2B. This permitted the assignment
of the 64.17 ppm line to C-7 and hence the line at 64.69
ppm to C-8 of 2B. No distinction was possible between
resonances for C-9 and C-10 of 2 at either 300 or 210 K.
The assignments of the cyclohexyl resonances at both
temperatures were carried out by methods analogous to
those used earlier.1,2
Conformational free energies
From the relative intensities of the 13C resonances for
2A and 2B, one can obtain an estimate of the equi-
librium constant K for their conformational intercon-
version at 210 K. This determination carries the implicit
In a similar fashion, the assignments for C-7 vs. C-8
were made, the key observation being the 3J HMBC
correlation observed at 210 K between the equatorial
proton resonance for C-1 (at 3.85 ppm) and the 13C
assumption that the spinÈlattice relaxation times (T )
1
( 1998 John Wiley & Sons, Ltd.
MAGNETIC RESONANCE IN CHEMISTRY, VOL. 36, 687È692 (1998)

690
G. W. BUCHANAN, M. GERZAIN AND R. C. LAISTER
Table 2. 13C and 1H NMR chemical shifts for
TMS ^ 0.01)a
2
(d from
2 (300 K)
1H
2B (210 K)
13C 1H
2A (210 K)
13C
Site
13C
1
2
3
4
5
6
7
8
9
10
11
12
13
76.75
76.42
27.64
22.91
22.10
28.41
66.36
68.54
(70.55)b
(70.00)b
71.00
31.12
63.72
3.42
3.58
78.51
69.77
26.78
18.83
23.99
27.07
68.29
68.29
71.86
68.82
68.66
29.76
66.85
72.01
77.50
25.68
23.99
18.59
27.25
64.17
64.69
(68.82)c
(68.97)c
69.97
29.86
60.57
3.85
3.20
2.00
1.63,1.20
1.30
1.58
3.27
3.62, 3.41
3.82
3.54
1.84, 1.37
1.58, 1.37
1.58, 1.28
1.84, 1.37
3.81, 3.40
3.56
3.60, 3.44
3.62, 3.55
3.82, 3.50
1.64
3.64, 3.44
1.62
3.10
3.68, 3.39
a 0.1 M solutions in CD Cl .
2
2
b,c Parentheses indicate possible interchange of assignments.
and the nuclear Overhauser enhancements (NOEs) for
isomeric pairs of carbons are the same within error
limits. The results of such measurements, based on eight
pairs of clearly resolved 13C resonances (Fig. 2) lead to
a value of K \ 2.2 ^ 0.4, which, using the equation
[*G¡ \ RT ln K, translates into a conformational free
energy di†erence ([*G¡) of 1.4 ^ 0.3 kJ mol~1 at 210
K between 2A and 2B. The observed preference for the
conformer with the equatorial propyleneoxy group is
consistent with that observed in the 10-crown-3
analog,2 although the energy di†erence between con-
formers 2A and 2B is only half that found for the corre-
sponding cis-cyclohexyl-10-crown-3 conformers. This
attenuated di†erence in the present case may be due to
reduced repulsive transannular interactions in the
macrocyclic ring involving the equatorial methine
proton of the less populated 2A form.
sesses four corners. The present conformation calcu-
lated for the 1,4,7,11-tetraoxacyclotridecane derivative
2B is exactly analogous to that found for the carbo-
cyclic analog.
From the standpoint of 13C NMR chemical shift dif-
ferences between the conformers, the largest di†erences
are the shielding of the C-1 and C-13 sites in conformer
2B by ca. 6 ppm relative to 2A [Fig. 2(a)]. From the
aforementioned calculated torsion angles, it is clear that
there is a gamma-gauche interaction between C-1 and
C-13 in conformer 2B which is lacking in 2A. Such
interactions are well known6 to be responsible for
upÐeld shifts in 13C NMR of 5È7 ppm, in agreement
with the present Ðndings.
Table 3. MM` calculated total energy and macrocyclic
torsion angles (¡) for 1, 2A and 2B
Results of molecular mechanics calculations (Table 3
and Fig. 3) support this observed preference for con-
former 2B. The calculated gas-phase energy di†erence
between 2A and 2B is 0.6 kJ mol~1. We regard this
level of agreement between theory and experiment as
being satisfactory. In general the angles are of compara-
ble magnitude for the two conformers with three excep-
tions. In the case of 2A, the C-2ÈC-1ÈO-1ÈC-7 angle
is [89.1¡ whereas in 2B it is [178.4¡. Conversely, the
C-13ÈO-4ÈC-2ÈC-1 angle is [179.5¡ in 2A and
68.9¡ in 2B. Third, the C-8ÈO-2ÈC-9ÈC-10 angle in
2A changes from an essentially trans geometry
([171.3¡) to a nearly perpendicular structure ( [ 87.2¡).
The fact that the only major structural di†erences calcu-
lated between 2A and 2B amount to transoidÈgauche
“trade-o†sÏ in the geometries of two CÈCÈOÈC units
and one trans to perpendicular change is consistent with
the small di†erence in their calculated total energies.
It is of interest that according to early calculations by
Dale4 the lowest energy conformation of cyclotridecane
has three “cornersÏ and comes close to the diamond
lattice conformation of cyclotetradecane, which pos-
Network
1
2A
2B
O-4ÈC-2ÈC-1ÈO-1
O-1ÈC-7ÈC-8ÈO-2
O-2ÈC-9ÈC-10ÈO-3
C-2ÈC-1ÈO-1ÈC-7
C-1ÈO-1ÈC-7ÈC-8
C-7ÈC-8ÈO-2ÈC-9
C-8ÈO-2ÈC-9ÈC-10
C-9ÈC-10ÈO-3ÈC-11
C-10ÈO-3ÈC-11ÈC-12
C-12ÈC-13ÈO-4ÈC-2
C-13ÈO-4ÈC-2ÈC-1
O-3ÈC-11ÈC-12ÈC-13
C-11ÈC-12ÈC-13ÈO-4
C-11ÈO-3ÈC-12ÈC-13
O-2ÈC-9ÈC-10ÈC-11
C-9ÈC-10ÈC-11ÈO-3
C-10ÈC-11ÈO-3ÈC-12
O-3ÈC-12ÈC-13ÈO-4
55.5
57.3
[54.7
[63.9
53.2
[89.1
159.3
48.5
[74.4
[51.2
[178.4
153.4
156.3
[87.2
175.0
85.0
[166.8
166.7
154.0
178.0
[171.3
[172.4
173.9
[170.1
[179.5
[61.4
79.6
[171.1
178.4
83.9
[163.6
68.9
55.5
54.6
169.7
46.9
[74.7
164.6
54.9
MM` energy (kJ mol~1)
108.85
108.01
107.38
( 1998 John Wiley & Sons, Ltd.
MAGNETIC RESONANCE IN CHEMISTRY, VOL. 36, 687È692 (1998)

NMR OF 13-CROWN-4 ETHERS
691
Finally, it is intuitively reasonable that there are only
small di†erences in total energy as calculated by MM`
between 1 and either 2A or 2B (Table 3). The results
show that 1 is the least stable form by 0.84 kJ mol~1
relative to 2A, which is in turn calculated to be 0.63 kJ
mol~1 less stable than 2B.
EXPERIMENTAL
Materials
The preparation of 1 was carried out via catalytic hydrogenation of
the known5 benzo-13-crown-4 ether precursor according to the fol-
lowing procedure. Benzo-13-crown-4 (310 mg, 1.30 mmol) was dis-
solved in absolute ethanol (10 ml) containing an Rh on alumina
catalyst (62 mg, 20% by weight). Hydrogenation was carried out for 8
h at 30 ¡C using an American Instrument high-pressure apparatus
operating at 600 psi. After removal of the catalyst by gravity Ðltration,
the solvent was removed via rotoevaporation. The oily product was
puriÐed in 70% overall yield using preparative thin-layer chromatog-
raphy with hexaneÈdiethyl ether (1 : 2) as eluent. Elemental analysis:
calculated for C
H
O , C 63.9, H 9.9; found, C 63.7, H 9.8%.
13 24
4
The synthesis of 1-7,13-d was accomplished via an analogous pro-
4
cedure. For the preparation of the benzo-13-crown-4-d precursor,
4
reduction of diethyl-3,7-dioxanonadioate was accomplished using
LiAlD to furnish the requisite 3,7-dioxa-1,9-nonanediol-d . 13C
4
4
NMR (CDCl ), 72.02, 68.36, 61.12 (quintet, 1J \ 21.5 Hz), 29.59;
3
CD
b.p. 100È102 ¡C/0.2 mmHg, reported5 105 ¡C/0.2 mmHg.
For the preparation of 2 the synthesis of the unsymmetrical benzo-
13-crown-4 ether 3 was necessary and this was carried out via the
sequence shown in Fig. 4. A (ethyl 6-chloro-3-oxahexanoate) was pre-
pared starting from 3-chloropropan-1-ol (Aldrich), 4.72 g (50 mmol)
and ethyl diazoacetate (Aldrich) 5.70 g (50 mmol), mixed in a pre-dried
Figure 3. MM` calculated geometries for 1, 2A and 2B.
Figure 4. Synthetic scheme for preparation of 3.
MAGNETIC RESONANCE IN CHEMISTRY, VOL. 36, 687È692 (1998)
( 1998 John Wiley & Sons, Ltd.

692
G. W. BUCHANAN, M. GERZAIN AND R. C. LAISTER
250 ml two-necked reaction vessel under argon with 50 ml of spectro-
each of the 512 values of the evolution time with a
grade CH Cl . The Ñask was cooled in an iceÈwater bath. Subse-
2
2
digital resolution of ca. 8 Hz per point in F and 4 Hz
quently, Ðve drops of the BF É Et O catalyst were added with a
1
3
2
syringe through a septum. A highly exothermic reaction occurred with
per point in F . The raw data were zero Ðlled in F
2
1
vigorous N release. After stirring for 3 h the reaction mixture was
prior to transformation using the qsine window func-
2
allowed slowly to reach ambient temperature. The solvent was then
tion for both F and F . The proton relaxation delays
removed via rotoevaporation to give a crude oily product. PuriÐ-
cation was accomplished via Ñash silica gel chromatography (grade
60, 230È400 mesh) using ethyl as solvent. Subsequent vacuum distilla-
tion yielded pure A as judged via NMR spectroscopy; b.p. 88È90 ¡C/4
1
2
were set to 1 s. Delays were chosen to emphasize J
values of 135È140 Hz. For the long-range HMBC
experiment, delays were chosen to emphasize a coupling
of 7.5 Hz.
mmHg). 13C NMR, d \ 170.0, 68.3, 68.0, 60.7, 41.6, 32.5 and 14.1
C
ppm.
For the reduction of A to B, all glassware was dried at 120 ¡C for 24
h. A 500 ml three-necked round-bottom Ñask, equipped with a con-
denser and dropping funnel, cooled in an iceÈsalt bath. A suspension
The 1HÈ1H COSY experiments were run using
N-type phase cycling with a 45¡ mixing pulse. The free
induction decays were acquired over 1024 data points
for each of the 256 values of the evolution time with a
digital resolution of 5 Hz per point. The raw data were
of LiAlH (800 mg, 21.1 mmol) in 250 ml of dry THF was introduced
4
into the Ñask and stirred for 15 min. Subsequently A (6.35 g, 35.1
mmol) was added dropwise and the reaction mixture was stirred over-
night. The resulting suspension of lithium alkoxide was decomposed
via addition of 1.5 ml of distilled water followed by 10 ml of 10%
H SO . Following suction Ðltration the solvent was removed by
zero Ðlled in F prior to transformation using the qsine
1
window function for both F and F . The data were
2
4
rotoevaporation. The crude product was dissolved in CH Cl and
1
2
2
2
symmetrized about the diagonal.
washed with water. After drying, the solvent was removed in vacuo
and the product was distilled under reduced pressure to give B, b.p.
78È82 ¡C/4 mmHg. Mass spectrum (CI), [M ] 1] 138.9. 13C NMR,
d \ 71.9, 61.2, 41.6, 32.2 and 67.2 ppm.
C
By analogous methods, C (9-chloro-3,6-dioxanonan-1-ol) was pro-
Molecular mechanics and geometry optimization
duced from B in 70% overall yield. C, b.p. 106È108 ¡C/9.5 mmHg.
Mass spectrum (CI), [M ] 1] 182.9. 13C NMR (CDCl ), d \ 41.9,
3
C
All calculations were performed on a Pentium 200 com-
puter using the Hyperchem (release 5.02) implementa-
tion of the MM` empirical force Ðeld.7 The isomeric
cyclohexyl-13-crown-4 ethers 1, 2A and 2B were con-
structed through the graphical interface. The torsion
angles O-1ÈC-7ÈC-8ÈO-2 and O-2ÈC-9ÈC-10ÈO-
3 were restrained to either ] or [ gauche conforma-
tions. The PolakÈRibiere conjugate gradient algorithm
was employed to minimize the resulting structures to a
gradient termination parameter of 0.001 kcal mol~1
Ó~1 (1 kcal \ 4.184 kJ). The eight minimized structures
were then subjected to a molecular dynamics simulation
at 300 K for 10 ps, followed by cooling to 0 K over 20
ps using a 1 fs step size. During the simulation, the two
O-CÈC-O units were restrained using the default force
constant of 16 kcal mol~1 K~2. Unrestrained mini-
mizations were then performed to obtain the Ðnal struc-
tures.
67.6, 32.6, 70.3 ppm (intensity 2), 72.6 and 61.6. The bischloroether D
was produced from C in 85% yield via conventional treatment with
SOCl .
2
Condensation of D with catechol yielded 3 in 24.3% yield using the
following method. To
a 4 l three-necked round-bottomed Ñask
equipped with a condenser and a dropping funnel was introduced
catechol (2.93 g, 27.0 mmol), 2.0 l of distilled water and LiOH É H O
2
(2.24 g, 54.0 mmol). This mixture was stirred for 1 h at 40 ¡C. Subse-
quently D (5.47 g, 27.0 mmol) was added dropwise with stirring and
the mixture was reÑuxed for 5 days. After cooling, the mixture was
acidiÐed to pH 1È2 via addition of 6 M HCl. Following extraction
with CH Cl (3 ] 100 ml), the organic layer was washed with 2%
2
2
aqueous KOH and dried over Na SO . The solvent was removed by
2
4
rotoevaporation and the resulting oil puriÐed by column chromatog-
raphy (silica gel, grade 60, 230È400 mesh) using hexaneÈacetone (5 : 1)
as eluent to give 3, an oil, C
H
O . High-resolution mass spectrum
13 18
4
calculated 238.120 51, found 238.119 33. 13C NMR (CDCl ), d \
3
C
149.6, 150.6, 120.0, 121.8, 123.2, 115.3, 68.9, 30.1, 66.6 68.5, 68.6, 69.0
and 70.8.
Catalytic hydrogenation of 3 to 2 was accomplished by methods
analogous to those for the preparation of 1. Yield of 2 was 75%. Ele-
mental analysis: CalcÏd for C
H10.0.
H
O ; C, 63.9; 9.9. Found C, 64.1;
13 24
4
Spectra
REFERENCES
All spectra were recorded using a Bruker AMX-400
NMR spectrometer equipped with a 5 mm inverse
probe and an Aspect X32 computer. An Aspect 3000
process controller was employed and all standard
microprograms used are in the Bruker Software
Library. Chemical shifts are relative to an internal TMS
standard. The normal sweep width was 13 150 Hz with
16K data points and an acquisition time of 0.625 s.
For the 1HÈ13C HMQC experiments, the free induc-
tion decays were acquired over 1024 data points for
1. G. W. Buchanan, A. B. Driega, A. Moghimi, C. Bensimon and K.
Bourque, Can. J. Chem. 71, 951 (1993).
2. G. W. Buchanan, M. Gerzain and K. Bourque, Magn. Reson.
Chem. 35, 283 (1997).
3. G. W. Buchanan, K. Bourque, G. K. Diedrich and M. Z. Khan,
Magn. Reson. Chem. 25, 65 (1987).
4. J. Dale, Acta Chem. Scand. 27, 1115 (1973).
5. U. Olsher and J. Jagur-Grodzinski, J. Chem. Soc., Dalton T rans.
501 (1981).
6. N. K. Wilson and J. B. Stothers, T op Stereochem. 8, 1 (1973).
7. N. L. Allinger, Y. H. Yuh and J.-H. Lii, J. Am. Chem. Soc. 111, 8551
(1989).
( 1998 John Wiley & Sons, Ltd.
MAGNETIC RESONANCE IN CHEMISTRY, VOL. 36, 687È692 (1998)