Synthesis, crystal structure and molecular modeling (AM1) of two 5-arylidene derivatives of Meldrum's acid

Article abstract of DOI:10.1023/A:1009535215383

The synthesis and structural characterization of two 5-Arylidene derivatives of Meldrum's acid (2,2-dimethyl-1,3-dioxane-4,6-dione) are described: 5-(4-Nitrobenciliden)-2,2-dimethyl-1,3-dioxane-4,6-dione (3a), and 5-(4-Methoxybenciliden)-2,2-dimethyl-1,3-

Full text of DOI:10.1023/A:1009535215383

Journal of Chemical Crystallography, Vol. 30, No. 3, 2000
Synthesis, crystal structure and molecular modeling (AM1)
of two 5-arylidene derivatives of Meldrum’s acid
Hector Novoa de Armas,(1)* Norbert M. Blaton,(1) Oswald M. Peeters,(1)
´
Camiel J. De Ranter,(1) Margarita Suarez,(2) Estael Ochoa,(2)
´
Yamila Verdecia,(2) and Esperanza Salfran(2)
´
Received March 7, 2000
The synthesis and structural characterization of two 5-Arylidene derivatives of Meldrum’s
acid (2,2-dimethyl-1,3-dioxane-4,6-dione) are described: 5-(4-Nitrobenciliden)-2,2-dimethyl-
1,3-dioxane-4,6-dione (3a), and 5-(4-Methoxybenciliden)-2,2-dimethyl-1,3-dioxane-4,6-dione
(3b). The structure of 3a was refined to R1 ϭ 0.0421 for 2148 reflections (with I Ͼ 2␴ (I)).
Crystal data for 3a: C13H11NO6, orthorhombic, space group Pbca, a ϭ 16.008(3), b ϭ 6.137(1),
3
˚
˚
c ϭ 25.281(5) A, V ϭ 2483.6(8) A , Z ϭ 8. The structure of 3b was refined to R1 ϭ 0.0496
for 4681 reflections (with I Ͼ 2␴(I)). Crystal data for 3b: C14H14O5, triclinic, space group P1,
˚
a ϭ 9.131(2), b ϭ 9.922(2), c ϭ 14.490(3)A, Ͱ ϭ 85.076(6), ͱ ϭ 84.80(3), Ͳ ϭ 89.37(2)Њ,
V ϭ 1302.4(5) A , Z ϭ 4. The results of crystallographic and molecular modeling (AM1)
3
˚
were compared. The molecules in the crystal are held together, in both compounds, by van
der Waals forces and CUHиииO hydrogen bond interactions.
KEY WORDS: Crystal structure; AM1; X-ray diffraction; Meldrum’s acid.
8
Introduction
thesis of pyrido[2,3-d]pyrimidines.7, Recently, we
have reported the structural study of 2,5-dioxo-
Isopropylidene alkylidenemalonates1 obtained
1,2,3,4,5,6,7,8-octahydroquinolines9 as potential cal-
cium channel modulators, synthesized from the ap-
propriate 5-arylidene substituted Meldrum’s acid (3),
dimedone and ammonium acetate.
from Meldrum’s acid, have achieved considerable in-
terest as highly reactive Michael acceptors,2 electron-
deficient dienophiles,3 strongly polarized hetero-
dynes,4 and precursors of the corresponding saturated
compounds5 as well as of methylene ketenes.6 Our
own interest in these structures stems from their use
as a Knovenagel product in the synthesis of different
2(1H)pyridones derivatives in a Hantzsch-like reac-
tion as Michael acceptor. In several papers we have
reported the use of 5-arylidene-2,2-dimethyl-1,3-di-
oxan-4,6-diones (3) as a key intermediate in the syn-
Experimental
Synthesis of 5-Arylidene-2,2-dimethyl-1,3-dioxan-
4,6-diones (3)
A mixture of Meldrum’s acid (20 mmol) and
the corresponding aromatic aldehyde (20 mmol) was
dissolved in acetonitrile 20 mL, and piperidine (0.1
mL) was added. The resulting solution was stirred at
room temperature for 10 h. Afterwards, it was poured
into ice-water, and the precipitate was collected by
filtration. Further purification was accomplished by
recrystallization from ethanol.
(1) Laboratorium voor Analytische Chemie en Medicinale Fysico-
chemie, Faculteit Farmaceutische Wetenschappen, Katholieke
Universiteit Leuven, Van Evenstraat 4, B-3000 Leuven,
Belgium.
(2) Laboratorio de Sı´ntesis Orga´nica, Facultad de Quı´mica. Uni-
versidad de La Habana, 10400 La Habana, Cuba.
* To whom correspondence should be addressed.
189
1074-1542/00/0300-0189$18.00/0  2000 Plenum Publishing Corporation

190
Novoa de Armas et al.
Table 1. Crystal Data and Structure Refinement
Compound
CCDC deposit no.
Color/shape
Chemical formula
Formula weight
Temperature
C13H11NO6
CCDC-1003/5758
Colorless/plate
C13H11NO6
277.23
C14H14O5
CCDC-1003/5759
Yellow/prism
C14H14O5
265.25
293(2)
293(2)
Crystal system
Space group
Orthorhombic
Pbca
Triclinic
Pළ1
Unit cell dimensions
˚
˚
˚
a ϭ 16.008(3) A
a ϭ 9.131(2) A
Ͱ ϭ 85.076(6)Њ
ͱ ϭ 84.80(3)Њ
Ͳ ϭ 89.38(2)Њ
˚
b ϭ 6.137(1) A
b ϭ 9.922(2) A
˚
˚
c ϭ 25.281(5) A,
(From 27 reflections, 10.2 Ͻ ␪ Ͻ 53.1)
Volume
c ϭ 14.490(3) A
(From 38 reflections, 12.56 Ͻ ␪ Ͻ 36.90)
3
3
˚
˚
2483.6(8) A
1302.4(5) A
Z
8
4
Density (calculated)
Absorption coefficient
Diffractometer/scan
Radiation/wave length
F(000)
1.483 mg mϪ3
1.020 mmϪ1
Siemens P4/Ͷ/2␪
1.337 mg mϪ3
0.860 mmϪ1
Siemens P4/Ͷ/2␪
˚
˚
1.54184 A
1152
1.54184 A
552
Crystal size
␪ range for data collection, deg
Index ranges
0.40 ϫ 0.25 ϫ 0.06 mm
1 to 69.21
Ϫ1 Յ h Յ 19, Ϫ1 Յ k Յ 6, Ϫ30 Յ l Յ 1 Ϫ10 Յ h Յ 1, Ϫ11 Յ k Յ 11, Ϫ17 Յ l Յ 17
0.76 ϫ 0.68 ϫ 0.38 mm
1 to 69.04
Reflections measured
Independent/observed
reflections
3367
5656
2148 (Rint ϭ 0.031)/1755[I Ͼ 2␴(I)]
4681 (Rint ϭ 0.039)/4401[I Ͼ 2␴(I)]
Absorption correction
Range of rel. transm. factors
Data/restraints/parameters
Goodness-of-fit on F2
SHELXL97 weight parameters
Final R indices [I Ͼ 2␴(I)]
R indices (all data)
Largest diff. Peak/hole
Semi-empirical from ␺-scans23
0.547/0.941
2148/0/184
1.055
0.0615, 0.9104
R1 ϭ 0.0421, wR2 ϭ 0.1220
R1 ϭ 0.0526, wR2 ϭ 0.1220
0.20/Ϫ0.20
Semi-empirical from ␺-scans23
0.499/0.721
4681/0/350
1.092
0.0627, 0.2666
R1 ϭ 0.0476, wR2 ϭ 0.1295
R1 ϭ 0.0496, wR2 ϭ 0.1314
0.22/Ϫ0.25
Spectroscopy
(BDH and Fluka) and were used without further
purification. Aromatic aldehydes were distilled be-
fore use.
Melting points were determined in a capillary
tube in an Electrothermal C14500 apparatus and are
uncorrected. The NMR spectra were recorded on a
Bruker DPX300 spectrometer (300 MHz-1H and
75.47 MHz-13C). Chemical shifts are given as ͳ values
against tetramethylsilane as the internal standard,
and J values are given in Hz. The IR spectra were
measured with a Shimadzu FTIR 8300 instrument as
potassium bromide pellets. Mass spectra were ob-
tained with a Hewlett Packard 5989 A machine. Mi-
croanalyses were performed in a Perkin Elmer 2400
CHN by the Servicio de Microana´lisis of Universidad
Complutense de Madrid. The reactions were moni-
tored by TLC and performed on silica-gel plates
(Merck 60F250) using hexane:ethyl acetate (8:2) as the
eluent. Commercially available starting materials and
reagents were purchased from commercial sources
5-(4-Nitrobenciliden)-2,2-dimethyl-1,3-dioxane-4,6-
dione (3a)
Following the general procedure gave 3a, 88%
yield, m.p. 142-143ЊC, ␯max/cmϪ1 1759 (CuO), 1730
(CuO), 1629 (CuC), 1527 and 1350 (NO2); 1H NMR
(DMSO-d6) ͳ 8.49 (1H, s, CH-7), 8.28 (2H, d, H3Ј,
H5Ј, J ϭ 8.7 Hz), 8.05 (2H, d, H2Ј, H6Ј, J ϭ 8.7 Hz),
1.77 (6H, s, CH3); 13C NMR (DMSO-d6) ͳ 164.1 (CO),
163.2 (CO), 153.6 (C7), 147.8 (C1Ј), 138.2 (C4Ј), 132.2
(C2Ј, C6Ј), 122.8 (C3Ј, C5Ј), 118.8 (C5), 104.9 (C2),
26.9 (CH3); m/z: 277 (Mϩ, 5%), 219 (100), 202 (98),
172 (46) and 101 (52). Anal Calc. for

5-Arylidene derivatives of Meldrum’s acid
191
Table 2. Atomic Coordinates (ϫ104) and Equivalent Isotropic
Table 3. Atomic Coordinates (ϫ104) and Equivalent Isotropic
2
3
2
3
˚
˚
Displacement Parameters (A ϫ 10 ) for Compound 3b
Displacement Parameters (A ϫ 10 ) for Compound 3a
x
y
z
U(eq)a
x
y
z
U(eq)a
O(1)
O(3)
O(4)
O(6)
O(11)
O(12)
N(1)
C(2)
C(4)
C(5)
C(6)
C(7)
C(21)
C(22)
C(1Ј)
C(2Ј)
C(3Ј)
C(4Ј)
C(5Ј)
C(6Ј)
4138(1)
2995(1)
2518(1)
4764(1)
3632(1)
3060(1)
3412(1)
3717(1)
3081(1)
3858(1)
4297(1)
4159(1)
4295(2)
3395(2)
3923(1)
3562(1)
3391(1)
3588(1)
3948(1)
4122(1)
3974(2)
1613(2)
772(2)
6877(1)
6764(1)
5975(1)
6194(1)
3319(1)
3822(1)
3753(1)
7084(1)
6234(1)
6017(1)
6355(1)
5525(1)
7096(1)
7618(1)
5090(1)
5151(1)
4716(1)
4218(1)
4141(1)
4580(1)
45(1)
40(1)
41(1)
60(1)
72(1)
69(1)
48(1)
40(1)
32(1)
33(1)
39(1)
36(1)
59(1)
60(1)
35(1)
41(1)
43(1)
38(1)
42(1)
39(1)
O(1A)
O(3A)
O(4A)
O(6A)
O(11A)
C(2A)
C(4A)
C(5A)
C(6A)
C(7A)
C(8A)
C(21A)
C(22A)
C(1Ј A)
C(2Ј A)
C(3Ј A)
C(4Ј A)
C(5Ј A)
C(6Ј A)
O(1B)
O(3B)
O(4B)
O(6B)
O(11B)
C(2B)
6809(1)
8868(2)
6095(1)
10350(2)
8058(2)
7984(2)
7088(2)
8555(2)
9355(2)
9184(2)
7099(3)
7251(3)
8902(3)
8809(2)
7815(2)
7540(2)
8266(2)
9285(2)
9562(2)
1501(1)
3378(2)
775(1)
4586(2)
3591(2)
2709(2)
1721(2)
3064(2)
3748(2)
3710(2)
2799(2)
2058(3)
3828(3)
3546(2)
2722(2)
2702(2)
3512(2)
4331(2)
4354(2)
Ϫ208(1)
715(1)
541(2)
1908(1)
997(1)
66(1)
71(1)
73(1)
85(1)
70(1)
63(1)
56(1)
54(1)
62(1)
56(1)
78(1)
97(1)
87(1)
51(1)
57(1)
57(1)
53(1)
60(1)
58(1)
72(1)
70(1)
71(1)
93(1)
80(1)
71(1)
58(1)
57(1)
65(1)
58(1)
90(1)
108(1)
91(1)
55(1)
67(1)
68(1)
61(1)
69(1)
64(1)
3251(1)
1523(1)
7237(1)
1218(1)
2652(1)
2632(1)
1702(1)
3366(1)
7740(1)
353(2)
1559(2)
4362(1)
4849(1)
5799(1)
6301(1)
5839(1)
4890(1)
1591(1)
760(1)
5469(3)
Ϫ2501(3)
Ϫ4913(3)
Ϫ3177(3)
2082(3)
1498(3)
2471(3)
4090(3)
2188(3)
150(4)
2753(4)
741(3)
Ϫ1314(3)
Ϫ2595(4)
Ϫ1810(3)
201(4)
2137(2)
1287(1)
Ϫ472(2)
441(2)
1050(2)
1355(2)
1481(2)
307(2)
Ϫ699(3)
Ϫ1640(2)
1357(2)
440(2)
385(2)
1263(2)
2172(2)
2202(2)
5325(2)
4187(1)
4683(2)
2495(2)
4064(2)
5480(2)
4639(2)
3806(2)
3424(2)
3309(2)
5148(3)
5908(3)
6451(2)
3542(2)
4582(2)
4770(2)
3929(2)
2884(2)
2708(2)
1457(3)
a U(eq) is defined as one third of the trace of the orthogonalized
Uij tensor.
3044(1)
1411(1)
6989(1)
882(1)
C(4B)
C(5B)
C(6B)
C(7B)
2411(1)
2430(1)
1521(1)
3196(1)
7388(2)
Ϫ10(2)
1137(2)
4173(1)
4566(1)
5496(1)
6069(1)
5702(1)
4771(1)
C13H11NO6(277.23) C, 56.32; H, 4.00; N, 5.05. Found:
C, 56.47; H, 4.27; N, 5.34.
C(8B)
C(21B)
C(22B)
C(1Ј B)
C(2Ј B)
C(3Ј B)
C(4Ј B)
C(5Ј B)
C(6Ј B)
5-(4-Methoxybenciliden)-2,2-dimethyl-1,3-dioxane-
4,6-dione (3b)
Following the general procedure gave 3b, 80%
yield, m.p. 127-128ЊC, ␯max/cmϪ1 3210 (NH), 1690
(CuO), 1660 (CuO), 1600 (CuC); 1H NMR
(DMSO-d6) ͳ 8.30 (1H, s, CH-7), 8.22 (2H, d, H3Ј,
H5Ј, J ϭ 8.9 Hz), 7.07 (2H, d, H2Ј, H6Ј, J ϭ 8.9
Hz), 3.89 (3H, s, OCH3), 1.72 (6H, s, CH3); 13C
NMR (DMSO-d6) ͳ 164.1 (CO), 163.2 (CO), 160.2
(C4Ј), 156.6 (C7), 137.1 (C2Ј, C6Ј), 124.5 (C1Ј), 114.3
(C3Ј, C5Ј), 111.5 (C5), 104.2 (C2), 55.8 (OCH3), 27.0
(CH3); m/z: 262 (Mϩ, 32%), 204 (72), 176 (48), 160
(100), 145 (63), 117 (52), 89 (56). Anal Calc. for
C14H14O5 (262.26) C, 64.12; H, 5.38. Found: C, 64.43;
H, 5.61.
a U(eq) is defined as one third of the trace of the orthogonalized
Uij tensor.
using the program SHELXS9710 and refined on F2
with SHELXTL9711 with scattering factors from In-
ternational Tables.12 Other programs used include
XSCANS13 for computing data collection, cell re-
finement and data reduction; DIAMOND14 for com-
puting molecular graphics; PLATON15 for computing
publication material. Non-H atoms were refined an-
isotropically by full-matrix least-squares techniques.
H atoms were calculated geometrically and included
in the refinement, but were restrained to ride on their
parent atoms. The isotropic displacement parameters
Crystallography
Suitable crystals were obtained from slow evapo-
ration in ethanol at room temperature. The structures
were solved by direct methods and Fourier synthesis

192
Novoa de Armas et al.
˚
Table 4. More Relevant Bond Distances [A], Valence Angles [Њ] and Torsion Angles [Њ] for the More
Stable Conformation of Compounds 3a and 3b
3a
3a
3b
3b X-ray
3b X-ray
AM1
X-ray
AM1
Molecule A
Molecule B
Bond distances
O1UC2
C2UO3
O3UC4
C4UO4
C4UC5
1.430
1.440(2)
1.429
1.437(2)
1.431(2)
1.358(2)
1.206(2)
1.483(2)
1.472(2)
1.206(2)
1.347(2)
1.356(2)
1.448(2)
1.437(2)
1.433(2)
1.350(2)
1.204(2)
1.483(2)
1.473(2)
1.204(2)
1.349(2)
1.356(3)
1.448(2)
1.428
1.377
1.229
1.470
1.479
1.229
1.373
1.349
1.452
1.438(2)
1.350(2)
1.199(2)
1.485(2)
1.487(3)
1.201(2)
1.346(2)
1.345(3)
1.464(3)
1.426
1.380
1.230
1.465
1.476
1.230
1.375
1.353
1.443
C5UC6
C6UO6
C6UO1
C5UC7
C7UC1Ј
Bond angles
O1UC2UO3
C2UO3UC4
O3UC4UC5
C4UC5UC6
C5UC6UO1
C6UO1UC2
C5UC7UC1Ј
C7UC1ЈUC2Ј
C7UC1ЈUC6Ј
107.5
109.48(15)
119.13(14)
115.51(15)
116.86(16)
116.03(16)
119.12(15)
132.93(18)
124.87(17)
116.34(18)
107.5
109.64(14)
118.87(13)
116.11(15)
116.50(14)
116.80(14)
119.90(13)
134.14(16)
125.95(15)
116.95(15)
109.27(15)
118.34(14)
116.38(15)
116.66(16)
116.18(14)
118.90(14)
135.17(16)
125.78(16)
117.27(15)
118.4
118.1
113.8
118.6
117.6
129.4
122.8
117.8
118.4
118.1
113.6
118.6
117.4
130.9
123.6
117.6
Torsion angles
O1UC6UC5UC7
O3UC4UC5UC7
O4UC4UC5UC6
O6UC6UC5UC4
C4UC5UC7UC1Ј
C6UC5UC7UC1Ј
C5UC7UC1ЈUC2Ј
Ϫ166.3
166.3
157.5
Ϫ159.7
Ϫ4.3
180.0
Ϫ163.68(17)
166.10(18)
154.22(18)
Ϫ156.68(19)
Ϫ8.9(4)
Ϫ164.8
164.1
154.7
Ϫ158.4
Ϫ6.7
177.9
Ϫ164.56(16)
168.41(15)
159.80(18)
Ϫ153.07(18)
9.6(3)
162.31(17)
Ϫ162.55(16)
Ϫ159.54(19)
154.53(18)
Ϫ11.9(3)
177.89(19)
Ϫ29.3(3)
Ϫ177.43(17)
16.6(3)
170.7(3)
Ϫ10.2(3)
Ϫ34.5
Ϫ24.4
of the H atoms were fixed to 1.3 times Ueq of their
parent atoms. Final R and weighted R values and
data collection parameters for both compounds are
given in Table 1, and the final geometrical parameters
are given in Table 2 (compound 3a) and Table 3
(compound 3b).
Discussion
Compounds 3a and 3b were obtained as crystal-
line solids from Meldrum’s acid (1) and the corre-
sponding aromatic aldehyde 2 in the presence of cata-
lytical amounts of piperidine in 88 and 80% yield
respectively (see Scheme 1).
Molecular modeling
Full energy minimization in vacuo was carried
out using semiempirical AM1 calculations16 with
the aid of the MOPAC17 molecular orbitals set.
Previously, the molecular geometry was optimized
using Allinger’s Molecular Mechanics18 with PCMO-
DEL program.19 The structures with lowest energies
following the AM1 optimization were used for
comparison with the X-ray structures. Calculations
were performed on an IBM-RS6000 workstation.
Scheme 1
The semiempirical (AM1) optimized geometries
for lowest energy conformers for compounds 3a
(⌬H ϭ Ϫ110.1 kcal/mol) and 3b (⌬H ϭ Ϫ154.7 kcal/
mol) are presented in Figs. 1 and 2, respectively.
The crystal structures within the atomic numbering

5-Arylidene derivatives of Meldrum’s acid
193
Fig. 1. Plot showing the lowest energy conformer for compound
3a obtained by semiempirical calculations (AM1), ⌬H ϭ Ϫ110.1
kcal/mol.
Fig. 3. A view of 3a with the atomic numbering scheme. Displace-
ment ellipsoids are drawn at 50% probability level for non-H atoms.
scheme for 3a and the two crystallographic indepen-
dent molecules of 3b are shown in Figs. 3 and 4,
respectively. Most relevant structural features are
given in Table 4, along with those predicted by AM1
calculations for comparison purposes. Theoretical
calculations are in agreement with the structural pa-
rameters derived from X-ray crystallography. Quan-
tum chemical calculations showed that the six-
member alkyl moiety adopts a flattened boat confor-
mation with the C2 and C5 atoms out of the main
boat plane.
cyclohexane chair (16% chair with O3A pointing
down 17% twist boat with axis through O1A and
C6A pointing down 67% boat with bowsprit at C2A
pointing up). Similarly, molecule B (puckering pa-
˚
rameters: Q ϭ 0.485(2) A, ␪ ϭ 74.4(2)Њ, ␸ ϭ 59.2(2)Њ)
represents 82% of puckering in an ideal cyclohexane
chair (17% chair with O3B pointing up 2% twist boat
with axis through O1B and C6B pointing up 81%
boat with bowsprit at C2B pointing down).
The bond lengths and angles are in good
agreement with values retrieved from the Cambridge
Structural Database.22 There appears to be a more
single-bond character in the C5UC7 bonds of 3b
X-ray crystallography data shows that the diox-
ane-dione ring system in both compounds can be
described as being mainly in a boat conformation
˚
molecules [mean value: 1.356(2) A] than in 3a
20
˚
with puckering parameters Q ϭ 0.485(2) A, ␪ ϭ
˚
[1.345(3) A]. In 3a the dioxane-dione has local mirror
104.1(2)Њ and ␸ ϭ 117.8(2)Њ. This ring in compound 3a
represents 81% of puckering in an ideal cyclohexane
chair (16% chair with O3 pointing down, 6% twist
boat with axis through O1 and C6 pointing down
78% boat with bowsprit at C2 pointing up).21 In com-
pound 3b, molecule A (puckering parameters: Q ϭ
symmetry to within experimental error, but in 3b
˚
molecules, C4UC5 is longer [1.483(2) A] than
˚
C5UC6 [1.472(2) A]. (Further geometrical calcula-
tions with PLATON14 revealed no higher symmetry
˚
0.466(2) A, ␪ ϭ 75.4(2)Њ, ␸ ϭ 66.1(2)Њ), the dioxane–
dione ring presents 78% of puckering in an ideal
Fig. 2. Plot showing the lowest energy conformer for compound
3b obtained by semiempirical calculations (AM1), ⌬H ϭ Ϫ154.7
kcal/mol.
Fig. 4. A view of the two independent molecules of 3b and their
atomic numbering scheme. Displacement ellipsoids are drawn at
50% probability level for non-H-atoms.

194
Novoa de Armas et al.
6. McNab, H. J. Org. Chem. 1981, 46, 2809.
and no solvent accessible areas.) The molecules in
the crystal, in both compounds, are packed at normal
van der Waals distances. There is only one C-HиииO
intermolecular interaction in 3a [C6иииO6: 3.250(3),
H6иииO6: 2.53 and C6UH6иииO6: 134 (Ϫx ϩ 1, Ϫy
ϩ 1, Ϫz ϩ 1)], which stabilizes the molecules in the
crystal. In compound 3b [C8BиииO6B: 3.378(3),
H14CиииO6B: 2.45 and C8B-H14CиииO6B: 162 (Ϫx,
Ϫy ϩ 1, Ϫz ϩ 1)].
7. Rodr´ıguez, R.; Suarez, M.; Ochoa, E.; Morales, A.; Gonza´lez,
L.; Martı´n, N.; Quinteiro, M.; Seoane, C.; Soto. J.L. J. Hetero-
cycl. Chem. 1996, 33, 45.
8. Rodr´ıguez, R.; Sua´rez, M.; Ochoa, E.; Pita, B.; Espinosa, R.;
Martin, N.; Quinteiro, M.; Seoane, C.; Soto. J.L. J. Heterocycl.
Chem. 1997, 34, 957.
9. Sua´rez, M.; Ochoa, E.; Verdecia, Y.; Pita, B.; Mora´n, L.;
Mart´ın, N; Quinteiro, M.; Seoane C.; Soto, J.L.; Novoa, H.;
Blaton, N.; Peters, O. Tetrahedron 1999, 55, 875.
10. Sheldrick, G.M. SHELXS97, Program for the Solution of
Crystal Structures; University of Go¨ttingen: Germany, 1997.
11. International Tables for X-Ray Crystallography; Dodrecht:
Kluwer Academic Publishers, 1992; Vol. C.
12. Sheldrick, G.M. SHELXL97, Program for the Refinement of
Crystal Structures; University of Go¨ttingen: Germany, 1997.
13. XSCANS: X-ray Single Crystal Analysis System, version 2.2;
Siemens Analytical X-ray Instrument: Madison, WI, 1996.
14. Bergerhoff, G. DIAMOND-Visual Crystal Structure Informa-
tion System; Bonn, Germany, 1996.
15. Spek, A.L. PLATON, An Integrated Tool for the Analysis of
the Results of a Single Crystal Structure Determination. Acta
Cryst. 1990, C-34.
Acknowledgment
This work was supported in part by the K.U.
Leuven (Belgium) through an IRO scholarship for
H. Novoa and Proyectos Alma Mater (Universidad
de La Habana, Cuba).
16. Dewar, M.J.S.; Zoebisch, E.G.; Hearly, E.F.; Stewart, J.J.P.
J. Am. Chem. Soc. 1985, 107, 3902.
17. Stewart, J.J.P. QCPE Program No. 455.
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