Synthesis, structural and spectroscopic study of aromatic thioester compounds

Article abstract of DOI:10.1016/j.molstruc.2006.04.013

Eight novel thioesters were obtained and characterized by elemental analysis, mass spectrometry, infrared and NMR spectroscopy. The data were analyzed on the basis of the relevant resonance structures. The X-ray single crystal structure of o-methoxythiophenyl 4-methylbenzoate is reported.

Full text of DOI:10.1016/j.molstruc.2006.04.013

Journal of Molecular Structure 825 (2006) 53–59
www.elsevier.com/locate/molstruc
Synthesis, structural and spectroscopic study of aromatic
thioester compounds
a,
b
c
b
Jorge L. Jios *, Srecko I. Kirin , Thomas Weyhermuller , Nils Metzler-Nolte ,
¨
a,d,
*
´
Carlos O. Della Vedova
a
´
Laboratorio de Servicios a la Industria y al Sistema Cientıfico (UNLP-CIC-CONICET), Departamento de Quı´mica,
Facultad de Ciencias Exactas, Universidad Nacional de La Plata, 47 esq. 115, (1900) La Plata, Argentina
Institute of Pharmacy and Molecular Biotechnology, University of Heidelberg, Im Neuenheimer Feld 364, D-69120 Heidelberg, Germany
b
c
Max Planck Institute for Bioinorganic Chemistry, Stiftstraße 34–36, D-45470 Mu¨lheim an der Ruhr, Germany
CEQUINOR (UNLP-CONICET), Departamento de Quı´mica, Facultad de Ciencias Exactas, Universidad Nacional de La Plata,
47 esq. 115, (1900) La Plata, Argentina
d
Received 9 February 2006; received in revised form 31 March 2006; accepted 3 April 2006
Available online 24 May 2006
Abstract
Eight novel thioesters were obtained and characterized by elemental analysis, mass spectrometry, infrared and NMR spectroscopy.
The data were analyzed on the basis of the relevant resonance structures. The X-ray single crystal structure of o-methoxythiophenyl 4-
methylbenzoate is reported.
Ó 2006 Elsevier B.V. All rights reserved.
Keywords: Resonance structures; Thioesters; Conformational analysis; X-ray diffraction
1. Introduction
Whether the discussed structural difference supported by
thermodynamical data may explain huge differences of
about 2000-fold more reactivity of a thioester compound
compared with its equivalent oxoester in the reaction with
an alquil cyanoacetate [4] represents an open question.
Moreover, the general reactivity tendency is just opposite
when thioesters and oxoesters hydrolyze in basic solutions
[5].
Our previously reported structural study on CF3C(O)-
SOC(O)CF3, a molecule specially suited to compare thioes-
ter and oxoester properties, shows that the deviation from
0° of the O@CASAO and the O@CAOAS torsional angles
would indicate a more extended double bond character of
the CAO than of the CAS single bond. The opposite trend
is obtained by analyzing the rotational energy barriers
around the CAS and CAO single bonds. Values of 13.2
and 10.6 kcal molꢀ1 were calculated for the height of the
barrier around the CAS and CAO single bonds, respective-
ly, with B3LYP/6-31G* approximation [6].
Structural and conformational properties of thioesters
of the type RC(O)SR’ are of great interest because of their
close relation to many biomolecules such as coenzyme A.
The different reactivity of thioesters as compared to oxoest-
ers has attracted considerable interest in the past years.
Many organic chemistry and biochemistry books dealing
with enzymatic reactions of coenzyme A attribute its acetyl
transfer ability to the lower degree of electron delocaliza-
tion or resonance of a lone electron pair of the bridging sul-
fur with the C@O group of a thioester in relation to the
similar interaction in an oxoester [1–3]. The hydrolysis of
a thioester is also more favored than that of the corre-
sponding ester: DG° at pH 7 amounts to ꢀ7.5 kcal molꢀ1
(1 cal = 4.18 J) for the hydrolysis of coenzyme A.
*
Corresponding author. Tel./fax: +0054 221 4714527.
E-mail addresses: jljios@quimica.unlp.edu.ar (J.L. Jios), nils.
A plausible way to explain these rather controversial
results has been recently reported by Drueckhammer
metzler-nolte@urz.uni-heidelberg.de (N. Metzler-Nolte), carlosdv@
´
quimica.unlp.edu.ar (C.O. Della Vedova).
0022-2860/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2006.04.013

54
J.L. Jios et al. / Journal of Molecular Structure 825 (2006) 53–59
et al. [7]. Using theoretical tools the authors explain the
compared reactivity of oxoesters and thioesters towards
nucleophiles evaluating losses of delocalization energies
for the oxoester and thioester in going from the reactants
to the transition state.
quency data are included in Table 3. All EI (70 eV) mass
spectra of compounds 1–8 show the M+ peak. The two
ferrocenoyl compounds 1 and 2 also show a peak at m/
z = 213, corresponding to the [CpFeC5H4CO]+ cation.
1
Attempts to obtain low-temperature H NMR spectra
For the study of the molecular forms, several effects like
hydrogen bonding, lone pair – lone pair repulsion, dipole–
dipole interactions, aromaticity, anomeric effect, steric
interactions and mesomeric structures can be suggested to
rationalize these experimental observations, which can be
reproduced satisfactorily by computational chemistry
methods [8].
Moreover a number of studies on the bonding and sub-
stituent effect were reported employing vibrational spec-
troscopy [9–15], mass spectrometry [16–20], NMR
spectroscopy [9,10,21–23] and theoretical methods
[13,15,24,25]. The results of these studies were interpreted
in terms of the different resonance forms that can take
place mainly between the carbonyl and sulfur in the thioes-
ter moiety. In compounds with an aryl group adjacent to
the carbonyl group (R = Ar), resonance forms such as i–v
can be drawn:
for the title compounds have been performed in order to
assess populations and free-energy differences for E and Z
conformations around to the single C(O)AS bond. The
compounds were dissolved in acetone-d6 and spectra were
obtained in the range from 293 to 190 K. Down to this tem-
perature, no coalescence of any signals could be observed.
In the crystallographic determination of the structure of
the o-methoxythiophenyl 4-methylbenzoate 4, the number-
ing scheme used is shown in Fig. 1 and relevant crystallo-
graphic data in Table 4.
3. Discussion
3.1. NMR spectra
For the following discussion the title compounds
were classified in two series. Series I are the compounds
-
-
+
-
O
C
O
C
O
C
O
C
O
C
1
R
1
R
+
1
R
1
R
1
R
+
S
-
S
+
S
S
S
2
R
2
R
2
R
2
R
2
R
(iv)
(i)
(ii)
(iii)
(v)
The weight of resonance structures such as iii–iv can
1, 3, 5 and 7 (R1 = isobutyl) and Series II includes com-
pounds 2, 4, 6 and 8 (R1 = o-methoxyphenyl) (see
Scheme 1).
explain the high deshielding in thioesters, although the
weight of structures such as iii is not expected to be high
since the interaction between sulfur 3p orbitals and carbon
2p orbitals precludes strong resonance stabilization [26].
On the other hand, the existence of resonance form such
as iv was postulated as a back donation of electron density
from the carbonyl group to the sulfur atom according to
studies that showed a low basicity of the carbonyl group
in thioesters as measured by the change in the i.r. frequency
of external hydrogen bonding agents [26]. In contrast, 13C
NMR studies showed that thioesters were capable of
strong hydrogen bonding [9].
1
Low-temperature H NMR spectroscopy is the tool of
choice for the conformational analysis of compounds in
solution and some thioesters were successfully studied by
this method [23].
Surprisingly, only three solid state crystal structures of
organic thioesters were determined [27,28]. However, ‘‘in-
organic’’ thioesters have been fully described [4].
τO(8)C(8)C(9)C(14)=21.2º
=72.8º
τC(8)S(7)C(6)C(5)
2. Results
All H and 13C NMR spectral data are summarised in
1
Fig. 1. Crystal structure for the compound 4. Ellipsoids are drawn at the
Tables 2 and 3, respectively. The carbonyl infrared fre-
30% probability level.

J.L. Jios et al. / Journal of Molecular Structure 825 (2006) 53–59
55
Table 1
Melting points, yields and results of elemental analysis of the compounds 1–8a
Compound
Molecular formula
Yield (%)
m.p. (°C)
Calcd
C
Found
C
H
N
H
N
1
2
3
4
5
6
7
8
a
C
15H18OSFe
78
74
92
87
91
91
70
75
red oil
125.0–126.0
oil
85.0–86.5
oil
73.0–74.0
31.0–32.0
120.0–121.0
59.60
61.37
69.17
69.73
57.75
60.31
51.74
57.93
6.01
4.54
7.76
5.47
5.74
3.98
5.14
3.79
–
–
–
–
–
–
5.49
4.83
59.27
61.37
69.44
65.68
57.96
60.27
55.23
58.00
6.21
4.62
7.76
5.02
5.47
3.89
5.58
4.05
–
–
–
–
–
–
5.82
4.72
C18H16O2SFe
C12H16OS
C
C
C14H11O2SCl
C11H13O3SN
C
15H14O2S
11H13OSCl
14H11O4SN
Further molecular weight determination was performed by high resolution mass spectroscopy for the following compounds (compound; mass calcd;
mass found): 1, 302.04279, 302.04270; 4, 258.07144, 258.07140; 7, 239.06161, 239.06150.
1
3.2. H NMR spectra
As in the proton NMR spectra, ortho phenyl ring C-2 and
CAB positions show a similar behaviour with a deshielding
of 0.3–0.5 ppm when series II is compared to series I.
Protons H-2 and H-3 in para substituted phenyl rings
(compounds 3–8) show a typical AA0XX0 spin system pat-
tern due to magnetic inequivalence. The corresponding val-
ues of d and coupling constants (J) given in Table 2 were
calculated from the experimental spectra [29]. Looking at
the d values of the ortho phenyl or cyclopentadienyl ring
protons (H-2 and H–B), a change from the isobutyl (Series
I) to the o-methoxyphenyl group (series II) produces a
deshielding of 0.07 ppm in all cases. The possibility of char-
ge delocalization of the sulfur atom in series II such as the
form vi partially precludes resonance structures such as iii
and iv and reinforces structures such as v. A greater partic-
ipation of the latter form is responsible for the deshielding
observed for H-2 and H–B. Surprisingly, a long range desh-
ielding effect of the nitro group is observed even for the
rather distant CH2 protons in series I (see Table 2).
3.4. Carbonyl infrared spectroscopy
Compounds of series I show lower frequencies than
those of series II. The change from an alkyl group adjacent
to sulfur to an aryl group promotes resonance structures
such as vi that partially replace iii and thus lead to an
enhanced carbonyl double bond character and larger mCO
.
Therefore, the carbonyl frequency decreases with increases
of the electron donating power of substituents at the benzo-
yl moiety (R2) due to a greater weight of resonance forms
such as vii.
O
C
O
C
S+
-
S
R1
3.3. 13C NMR spectra
2
R2
R+
MeO
Our 13C NMR assignments fit well to calculations using
well-established increment rules [30].
(vi)
(vii)
The carbonyl carbon is most significant to evaluate the
substituent effect since it is especially sensitive to the weight
of the different structure contributions. The value of this
signal ranges from 187.9 to 194.0 ppm and experiences a
shielding of 2.6–3.3 ppm when going from series I
(d = 190.5–194.0) to series II (d = 187.9–190.7). In both
series, the ferrocene moiety causes a higher deshielding
due to its greater electronegativity compared to phenyl
rings, whereas withdrawing groups at the phenyl ring orig-
inate an upfield shift of this signal. The deshielding of the
C@O carbon in series I could be a consequence of a rela-
tively higher contribution of the resonance forms such as
iii and iv in which the double bonded sulfur in the thioke-
tone group (C@S) strongly deshields the carbonyl carbon
atom [31]. In series II these resonance forms are partially
Compounds of series I (with the exception of 3) show a
doublet structure of the C@O stretching band (see Table 3),
that could be owed to the existence of a syn-anti conforma-
tional equilibrium in the C(O)ASAR1 moiety (C@O double
bond with respect to the SAR1 single bond). The double
bond character (structures iii–iv) allows the existence of
conformational equilibrium. Whether one or two forms
can be present (rotational equilibrium) depends on the bal-
ance of several interactions as mentioned in the introduc-
tion of this paper.
3.5. Mass spectra
1
A remarkable difference is observed between series I and
II. In the series I, a peak at m/z = M-56 was observed cor-
responding to a Mc Lafferty rearrangement with loss of iso-
propene. This reaction is only possible in a spatial
precluded as was already discussed (see H NMR spectra).
Moreover, the ipso carbon atom bonded to sulfur in series
II causes a greater destabilization of structure iii since it is
sp2-hybridized while in series I it belongs to an sp3-hybrid.

Table 2
1H NMR dataa,b,c
H-2
H-3
–
H-30
–
H-40
–
H-50
–
H-60
–
H–B
H–C
C5H5
CH2
(CH3)2
CH
1
–
4.87, t, 1.9 Hz
4.46, t, 1.9 Hz
4.21, s
2.92, d, 6.4 Hz
1.02, d,
6.7 Hz
–
1.88, sep.,
6.7 Hz
–
2d
–
–
7.00, br.d,
8.1 Hz
7.43, ddd,
8.1, 7.9,
1.7 Hz
–
7.02, td, 7.7,
1.1 Hz
7.47, dd,
7.6, 1.6 Hz
4.94, t, 1.9 Hz
4.50, t, 1.9 Hz
4.32, s
–
3
4
7.88, 8.6,
1.8, 0.2 Hz
7.95, 8.6, 1.8,
0.2 Hz
7.22, 8.6,
1.8, 0.2 Hz
7.26, 8.6,
1.8, 0.2 Hz
–
–
–
–
–
–
–
–
–
2.98, d, 6.6 Hz
–
1.04, d,
6.7 Hz
–
1.90, sep.,
6.7 Hz
–
´
7.01, br.d,
8.1 Hz
7.44, ddd,
8.1, 7.5,
1.6 Hz
–
7.03, td, 7.4,
1.2 Hz
7.48, dd a,
7.4, 1.2 Hz
5
7.92, 8.8, 2.1,
0.2 Hz
7.99, 8.8, 2.0,
0.2 Hz
7.42, 8.9,
2.1, 0.2 Hz
7.45, 8.8,
2.0, 0.2 Hz
–
–
–
–
–
–
–
–
–
3.00, d, 6.7 Hz
–
1.03, d,
6.7 Hz
–
1.91, sep.,
6.7 Hz
–
6e
7.01, br.d,
8.6 Hz
7.46, ddd,
8.1, 7.3,
1.7 Hz
–
7.03, ddd,
7.7, 7.4, 1.0 Hz
7.47, dd, 7.7,
1.6 Hz
7
8
8.13, 9.0, 2.0,
0.2 Hz
8.20, 9.2, 2.1,
0.2 Hz
8.30, 9.0,
2.0, 0.2 Hz
8.32, 9.1,
2.1, 0.2 Hz
–
–
–
–
–
–
–
–
–
3.05, d, 6.7 Hz
–
1.05, d,
6.7 Hz
–
1.95, sep.,
6.7 Hz
–
7.04, br.d,
8.0 Hz
7.50, ddd,
8.1, 7.4,
1.6 Hz
7.06, ddd,
7.7, 7.4, 1.0 Hz
7.48, dd, 7.7,
1.4 Hz
a
d(1H) in ppm relative to TMS; CDCl3 as solvent, coupling constants [J(1H,1H)] in Hz; multiplicities, s: singlet; d: doublet; dd: double doublet; ddd: double double doublet; t: triplet; td: triple doublet;
sep: septuplet; br.d: broad doublet.
b
1H chemical shift of the substituents: 2 d = 3.88, s (OMe); 3 d = 2.40, s (Me); 4 d = 3.85, s (OMe), 2.42, s (Me); 6 d = 3.85, s (OMe); 8 d = 3.87, s (OMe).
c
H-2 and H-3 in compounds 3–8 showed an AA0XX0 spin system and coupling constant values were calculated from spectra (see text) [29].
d
Signals of H-50 partially superimposed with H-30.
Signals of H-40 and H-60 partially superimposed with H-3.
e

J.L. Jios et al. / Journal of Molecular Structure 825 (2006) 53–59
57
Table 3
13C NMR dataa,b and carbonyl infrared frequenciesc,d
mCO
CO
1
2
3
4
10
20
–
30
–
40
–
50
–
60
–
A
B
C
C5H5 CH2 (CH3)2 CH
1
2
3
4
5
6
7
8
1653 194.0
1670 190.7
–
–
–
–
–
–
–
–
–
79.7 71.5 68.8 70.4
36.7 21.8
29.0
–
115.8 159.7 111.4 131.5 121.0 137.4 78.9 71.8 69.1 70.7
–
–
1662 191.6 134.8 127.2
1670 188.8 134.2 127.7
129.0
129.3
144.0
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
37.2 21.9
28.8
–
28.7
–
28.6
–
144.3 115.6 159.7 111.6 131.7 121.1 137.3
–
–
1665 190.8 135.6 128.5e 128.8e 139.5
–
–
–
–
–
–
37.4 21.8
1671 188.1 135.1 128.9
1654 190.5 141.8 128.2
1677 187.9 141.4 128.5
128.9
128.2
128.5
139.8 114.9 159.6 111.6 131.9 121.2 137.1
150.4
150.5 114.1 159.5 111.7 132.3 121.3 136.9
–
–
–
–
–
–
–
37.8 21.8
–
–
a
b
c
d(13C) in ppm relative to TMS; CDCl3 as solvent.
13C chemical shift of the substituents: 2 d = 55.8 (OMe); 3 d = 21.6 (Me); 4 d = 56.0 (OMe); 21.7 (Me); 6 d = 56.0 (OMe); 8 d = 56.0 (OMe).
Frequencies in cmꢀ1
.
d
e
A doublet stretching band was observed in the following compounds (second peak in parentheses): 1 (1637); 5 (1648); 7 (1640).
Interchangeables.
Table 4
Crystallographic data for 4
the gas phase can be explained on the basis of (i) the lower
acidity of the aromatic proton and (ii) the high energy of
the species that must be formed (the methoxybenzyne
molecule).
Chemical formula
Fw
Space group
C15H14O2S
258.32
P21/c, No. 14
14.911(2)
7.3172(8)
12.3327(12)
90
104.95(2)
90
1300.0(3)
4
˚
a (A)
˚
b (A)
˚
O
c (A)
O
H
H
a (deg)
b (deg)
c (deg)
S+
S+
˚
V (A)
R2
R2
Z
iiib
iiia
T (K)
q calcd (g cmꢀ3
100(2)
1.320
)
Refl. collected/2Hmax
Unique refl./I>2r(I)
No. of params./ꢀre1str.
6019/124.96
1938/1797
163/0
1.54178/21.34
0.0691/1.117
0.1727
˚
3.6. Crystal structure of o-methoxythiophenyl 4-
methylbenzoate 4
k (A)/l(Ka), vl
R1a/goodness of fitb
wR2c (I>2r(I))
residual density (eA
ꢀ3
˚
)
+0.84/ꢀ0.93
The crystal structure of 4 can be analyzed to consist of
three planes A–B–C (Fig. 1). The planes A and C are con-
stituted by the 4-methylphenyl and 2-methoxyphenyl rings,
respectively, and plane B is formed by the thiocarbonyl
(AC(O)ASA) moiety. The torsion angles between the
atoms O(8)C(8)C(9)C(14) – connecting planes A and B –
and the atoms C(8)S(7)C(6)C(5) – connecting planes B
and C – are 21.2° and 72.8°, respectively, showing that
the aryl rings are almost orthogonal to each other.
P
P
a
Observation criterion: I > 2r(I). R1 = iFojꢀjFci/ jFoj.
P
2
1=2
b
c
GooF = ½ ½wðF 2o ꢀ F 2c Þ ꢁ=ðn ꢀ pÞꢁ
.
P
P
2
2
1=2
2
wR2 = ½ ½wðF 2o ꢀ F c2Þ ꢁ= ½wðF o2Þ ꢁꢁ where w ¼ 1=r2ðF 2oÞ þ ðaPÞ þ
bP; P ¼ ðF 2o þ 2F 2c Þ=3.
disposition as iiia. A similar behaviour for the other series
should give a peak at M-106 with loss of o-meth-
oxybenzyne iiib. The lack of such a peak for this series in
O
B
O
A
R1
C
2
R1
S
1
3
S
Fe2+
4
2
R
4: R1= C6H4(o-OMe); R2= Me
6: R1= C6H4(o-OMe); R2= Cl
8: R1= C6H4(o-OMe); R2= NO2
3: R1= iso-Bu; R2= Me
1: R1= iso-Bu
5: R1= iso-Bu; R2= Cl
7: R1= iso-Bu; R2= NO2
2: R1= C6H4(o-OMe)
Scheme 1. Structures and numbering of the thioesters 1–8.

58
J.L. Jios et al. / Journal of Molecular Structure 825 (2006) 53–59
The distance between S(7) and C(8) of the carbonyl
spectra were measured on a Mat 8200 instrument, EI
(70 eV), CI and FAB (glycerol matrix). Infrared spectra
were recorded on a Bruker Equinox55 FT-IR spectrometer
as KBr discs.
The ferrocenoyl chloride necessary to obtain com-
pounds 1 and 2 was prepared by refluxing a solution of fer-
rocene carboxylic acid in dichloromethane and oxalyl
chloride [32]. After removal of the solvent and excess oxalyl
chloride, the residue was dried in vacuum (dark red oil) fol-
lowed by washing with warm hexane to give red crystals in
quantitative yield (m.p. 50–51 °C).
The title compounds (Scheme 1) were synthesized as fol-
lows: the respective aroyl chloride (11.0 mmol) was dis-
solved in dry pyridine (2.0 ml). The solution was stirred
mechanically at 20 °C and then isobutanethiol (1.27 ml;
10.0 mmol) (compounds 1, 3, 5 and 7) or 2-methoxybenze-
nethiol (1.22 ml; 10.0 mmol) (compounds 2, 4, 6 and 8) was
added in one step. The reaction mixture became immedi-
ately warmer and was kept at 20–22 °C under Ar with stir-
ring for 2 h. After isolation, the crude product was purified
by silica gel column chromatography followed by crystalli-
zation or distillation.
˚
group (1.789(3) lA) is longer than that between S(7) and
˚
C(6) of the phenyl ring (1.772(3) A) and is in agreement
with the data found in the crystal structure of S,S-[sulfo-
nylbis(1,4-phenylene)] di(thiobenzoate) [27]. In contrast,
the crystal structure data of 4-n-pentylphenyl 40-cya-
nothiobenzoate have shown a bond distance S(7) –
C(8) = 1.747(4) shorter than S(7) – C(6) = 1.759(4) [28].
˚
A rather short intermolecular distance of 2.656 A is
found between the carbonyl oxygen atom and the proton
bonded to carbon C(3) showing a weak hydrogen bonding
interaction in solid state.
Different studies were performed about the syn-anti con-
formation reported for thioesters [13–15,23,25]. In this con-
text, the crystal structure of 4 reveals that this compound
adopts a syn conformation in solid state. This finding is
in agreement with the general preference of esters and relat-
ed compounds for this conformation [23]. This general
preference was also supported by ab initio quantum chem-
ical methods [25]. The conformation of 4 is readily under-
stood in terms of steric interactions between the 2-
methoxyphenyl moiety and 4-methylphenyl moiety, which
destabilizes the anti conformation.
5.1. X-ray Crystallographic data collection and refinement
4. Conclusion
Colourless needle-shaped crystals were obtained by slow
evaporation from a methanolic solution. A single crystal of
4 was coated with perfluoropolyether and mounted in the
nitrogen cold stream of a Siemens SMART-CCD diffrac-
tometer equipped with a sealed tube X-ray source and a
Title compounds were successfully obtained by reactions
of an aroyl chloride with isobutanethiol (compounds 1, 3, 5
and 7) or 2-methoxybenzenethiol (compounds 2, 4, 6 and
8) in dry pyridine and subsequently characterized by usual
spectroscopies and chemical analyses. From the IR spectra
it can be concluded that the compounds exist as one pre-
dominant syn form at room temperature. The most striking
feature which could be observed from the crystallographic
data of the compound 4 is the torsional angle around the
CAS bond (d(O@CASAC)) and the orientation of the
substituted phenyl group attached to the C(8) with respect
to the plane formed by the O@CASAC skeleton. The syn
orientation (O@C double with respect to the SAC single
bond) remains unaltered as compared with relevant ante-
cedents reported in the literature and the phenyl group is
rotated roughly 90° with respect to the plane formed by
the O@CASAC atoms.
˚
graphite monochromator (Cu-Ka, k = 1.54178 A). Final
cell constants were obtained from a least squares fit of
5286 reflections. Intensity data were corrected for absorp-
tion using the program SADABS [33]. The Siemens Shel-
XTL [34] software package was used for solution and
artwork of the structure, ShelXL97 [35] was used for the
refinement. The structure was readily solved by direct
methods and subsequent difference Fourier techniques.
All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms attached to carbon atoms were placed
at calculated positions and refined as riding atoms with iso-
tropic displacement parameters. Crystallographic data are
compiled in Table 1. The data were deposited at the
CCDC, deposit number 269665, and can be downloaded
free of charge.
5. Experimental
Melting points (uncorrected), yields and results of ele-
mental analyses of the title compounds are compiled in
Table 1. Reactions were carried out in ordinary glassware
and chemicals used without further purification. NMR
spectra were determined in CDCl3 solutions (15–40 mg in
0.7 ml) on a Bruker AM 360 spectrometer, 1H at
360.13 MHz and 13C at 95.56 MHz. Chemical shifts, d/
ppm, indicate a downfield shift from tetramethylsilane,
TMS, the internal standard. Coupling constants, J, are giv-
en in Hz (absolute values). Individual peaks are marked as:
singlet (s), doublet (d), triplet (t) or multiplet (m). Mass
Acknowledgments
Financial support by the Volkswagen Stiftung is grate-
fully acknowledged. The Argentinean authors acknowledge
´
the Fundacion Antorchas, Alexander von Humboldt
Foundation, DAAD (Deutscher Akademischer Aust-
auschdienst, Germany), Agencia Nacional de Promocion
´
´
´
Cientıfica y Tecnica (ANPCYT), Consejo Nacional de
´
´
Investigaciones Cientıficas y Tecnicas (CONICET), Comi-
´
sion de Investigaciones de la Provincia de Buenos Aires
(CIC) and Facultad de Ciencias Exactas (UNLP). CODV

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