Conversion of 2,4-dihydroxybenzaldehyde to 2-benzoyloxy-4-hydroxybenzaldehyde and structural characterization of both precursor and product

Article abstract of DOI:10.1023/A:1009570027191

2,4-Dihydroxybenzaldehyde can be acylated at the 2 position if the more reactive 4 position is first protected as the methylmethoxy ether. Deprotection permits isolation of 2-benzoyloxy-4-hydroxybenzaldehyde. 2,4-Dihydroxybenzaldehyde, C₇H

Full text of DOI:10.1023/A:1009570027191

Journal of Chemical Crystallography, Vol. 29, No. 2, 1999
Conversion of 2,4-dihydroxybenzaldehyde to 2-benzoyloxy-
4-hydroxybenzaldehyde and structural characterization of
both precursor and product
Zhiyong Hu,(1) Michaele J. Hardie,(2) Pannee Burckel,(2) A. Alan Pinkerton,(2)*
and Paul W. Erhardt(1)*
Received January 4, 1999
2,4-Dihydroxybenzaldehyde can be acylated at the 2 position if the more reactive 4 position
is first protected as the methylmethoxy ether. Deprotection permits isolation of 2-benzoyloxy-
4-hydroxybenzaldehyde. 2,4-Dihydroxybenzaldehyde, C7H6O3, crystallizes in P21/c, with a ϭ
˚
7.3762(6), b ϭ 11.7455(9), c ϭ 7.3149(6) A, ͱ ϭ 90.533(2)Њ, Z ϭ 4, and DX ϭ 1.448 g
cmϪ3. The individual molecules are characterized by an intramolecular hydrogen bond. The
molecules form sheets with additional intermolecular hydrogen bonds. 2-Benzoyloxy-4-hy-
droxybenzaldehyde, C14H10O2, crystallizes in Pca21, with a ϭ 24.2629(14), b ϭ 3.8445(2),
Ϫ3
˚
c ϭ 12.3727(8) A, Z ϭ 4, and DX ϭ 1.394 g cm . The molecules consist of two planar halves
twisted by 51.63(5)Њ about the ester linkage. Intermolecular hydrogen bonding joins the
molecules into ribbons approximately parallel to c.
KEY WORDS: 2-Benzoyloxy-4-hydroxybenzaldehyde; 2,4-dihydroxybenzaldehyde; hydrogen bonding;
x-ray structure.
Introduction
Experimental
As part of a medicinal chemistry program to
Potassium carbonate, 1.38 g (10 mmol), was
added to a solution of 1.38 g (10 mmol) of 2,4-dihy-
droxybenzaldehyde in 70 ml of anhydrous acetone.
The mixture was stirred at room temperature for 15
min and then cooled to 5–10ЊC. A solution of 0.81 g
(10 mmole) of chloromethyl methyl ether in 2 ml of
ethyl acetate was added dropwise and the mixture
stirred for 2 h at 5–10ЊC followed by an additional
15 h at room temperature. Water, 50 ml, was added to
the reaction medium and the new mixture extracted
three times with 70-ml portions of ethyl ether. The
organic phases were combined, washed once with 70
ml of saturated aqueous sodium chloride, once with
70 ml of water, and then dried for several hours over
anhydrous magnesium sulfate at 2ЊC. Filtration of
the combined organic phase followed by evaporation
provided a light yellow oil which displayed a 2 spot
prepare novel anticancer agents, we required 2-ben-
zoyloxy-4-hydroxybenzaldehyde, 1, as a key synthetic
intermediate. Toward this end, the commercial avail-
ability of 2,4-dihydroxybenzaldehyde, 2, affords an
appropriately substituted benzene ring. However, ‘in-
tramolecular hydrogen bonding of the 2-hydroxy
group to the adjacent aldehyde function minimizes
reaction at this site’1 compared to the more reactive
4-position hydroxy group such that 1 is difficult to
obtain by direct acylation of 2. 2-Hydroxy-4-
(methoxymethoxy)benzaldehyde, 3.
(1) Center for Drug, Design and Development, University of To-
ledo, Toledo, Ohio 43066.
(2) Department of Chemistry, University of Toledo, Toledo,
Ohio 43606.
* To whom correspondences should be addressed.
185
1074-1542/99/0200-0185$16.00/0 1999 Plenum Publishing Corporation

186
Hu, Hardie, Burckel, Pinkerton, and Erhardt
thin-layer chromatogram when assessed on silica gel
using hexane : ethyl acetate (3 : 1) as developer. A
spot at Rf ϭ 0.17 was identical to that of the starting
material and a spot at 0.31 was presumed to be the
product. This oil was further purified by silica gel
column chromatography employing hexane : ethyl ac-
etate (3 : 1) as the eluent. Thin-layer chromatography
(same system) was used to evaluate and combine
column fractions having Rf values near 0.30. Evapora-
tion of the combined fractions provided 0.93 g (51%)
of a white solid: mp 53–54ЊC [Literature 52–53ЊC1
and 51–52ЊC2]; 1H-NMR (CDCl3) ͳ 11.37 (s, 1H, OH),
9.74 (s, 1H, CHO), 7.44–7.46 (d, J ϭ 8 Hz, 1H, ArH),
Scheme 1.
Table 1. Crystal Data and Structure Refinement for 1 and 2
1
2
Formula
C14H10O4
C7H6O3
CCDC deposit no.
Color/shape
Formula weight
Temperature
CCDC-1003/5512
Colorless/needle
242.22
CCDC-1003/5513
Colorless/columnar
138.12
293(1) K
180(1) K
Crystal system
Space group
Unit cell dimensions
Orthorhombic
Pca21
a ϭ 24.2629(14) A
b ϭ 3.8445(2) A
c ϭ 12.3727(8) A
Monoclinic
P21/c
a ϭ 7.3762(6) A
b ϭ 11.7455(9) A
c ϭ 7.3149(6) A
˚
˚
˚
˚
˚
˚
ͱ ϭ 90.533(2)Њ
633.71(9) A
3
3
˚
˚
Volume
1154.11(12) A
Z
4
4
Density (calculated)
Diffractometer/scan
Radiation/wavelength
1.394 g cmϪ3
1.448 g cmϪ3
Siemens SMART/Ͷ scans
Mo-KͰ (graphite monochromated)/
0.71073 A
504
Siemens SMART/Ͷ scans
Mo-KͰ (graphite monochromated)/0.71073 A
˚
˚
F(000)
288
Crystal size
0.28 ϫ 0.07 ϫ 0.05 mm
0.3 ϫ 0.3 ϫ 0.1 mm
␪ range for data collection
Index range
2.35 to 29.85Њ
h ϭ Ϫ33 Ǟ 33, k ϭ Ϫ4 Ǟ 5, l ϭ Ϫ16 Ǟ
2.76 to 29.79Њ
h ϭ Ϫ9 Ǟ 10, k ϭ Ϫ15 Ǟ 9, l ϭ Ϫ9 Ǟ 10
13
Reflections collected
Independent/observed
reflections
7284
4454
2644/1653 [I Ͼ 2␴(I)]
1642/1358 [I Ͼ 2␴(I)]
R(int)
0.0468
0.0268
Absorption coefficient
Absorption correction
Rel. Transm. factor range
Refinement method
Computing
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I Ͼ 2␴(I)]
R indices (all data)
0.103 mmϪ1
0.115 mmϪ1
SADABS
0.706–0.978
SADABS
0.700–0.992
Full-matrix least-squares on F2
SHELXTL, v. 5
2644/1/204
Full-matrix least-squares on F2
SHELXTL, v. 5
1642/1/115
1.135
1.062
R1 ϭ 0.0590, wR2 ϭ 0.0848
R1 ϭ 0.1122, wR2 ϭ 0.1006
Ϫ0.1(15)
0.019(2)
0.167, Ϫ0.119 e A
R1 ϭ 0.0461, wR2 ϭ 0.1270
R1 ϭ 0.0568, wR2 ϭ 0.1393
N/A
Absolute structure parameter
Extinction coefficient
Max., min. peak
N/A
Ϫ3
Ϫ3
˚
˚
0.331, Ϫ0.246 e A

2-Benzoyloxy-4-hydroxybenzaldehyde
187
Table 2. Atomic Coordinates and Equivalent Isotropic Displacement
2
˚
Parameters (A ) for 1
x
y
z
U(eq)/U(iso)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
C(8)
0.53867(11)
0.59332(11)
0.61412(10)
0.58330(10)
0.52786(10)
0.50583(12)
0.60699(12)
0.71158(11)
0.76471(10)
0.76934(12)
0.82071(14)
0.86709(13)
0.86272(13)
0.81190(12)
0.51905(11)
0.66814(7)
0.58401(9)
0.70527(8)
0.4856(14)
0.6149(12)
0.5067(10)
0.4661(12)
0.6466(12)
0.7368(13)
0.8232(13)
0.9032(14)
0.8975(14)
0.8070(11)
0.8151(8)
0.8955(8)
0.8039(8)
0.6310(8)
0.5586(8)
0.6459(8)
0.5306(9)
0.7928(8)
0.9123(7)
1.0728(9)
1.1699(9)
1.1014(10)
0.9456(9)
0.8515(9)
0.9139(7)
0.8982(5)
0.3577(7)
0.6215(6)
0.827(8)
1.015(7)
0.425(6)
0.592(7)
0.601(8)
1.113(8)
1.283(9)
1.163(9)
0.888(7)
0.740(8)
0.7600(2)
0.7830(3)
0.8810(2)
0.9599(2)
0.9342(2)
0.8361(2)
1.0622(3)
0.8468(2)
0.8918(2)
0.9922(3)
1.0306(3)
0.9701(3)
0.8703(3)
0.8311(3)
0.6617(2)
0.9108(2)
1.1322(2)
0.7679(2)
0.658(3)
0.728(3)
0.988(2)
0.823(2)
1.073(2)
1.031(3)
1.103(3)
0.998(3)
0.825(3)
0.768(3)
0.0464(7)
0.0480(7)
0.0432(7)
0.0415(7)
0.0453(8)
0.0452(7)
0.0536(9)
0.0453(7)
0.0390(6)
0.0469(8)
0.0551(9)
0.0598(9)
0.0585(9)
0.0521(8)
0.0645(7)
0.0492(5)
0.0661(7)
0.0658(7)
0.060(10)
0.056(9)
C(9)
C(10)
C(11)
C(12)
C(13)
C(14)
O(1)
O(2)
O(3)
O(4)
H(1)
H(2)
H(5)
H(6)
H(7)
H(10)
H(11)
H(12)
H(13)
H(14)
0.031(7)
0.056(8)
0.058(9)
0.057(9)
0.070(11)
0.075(10)
0.070(9)
0.056(9)
6.60–6.66 (m, 2H, ArH), 5.22 (s, 2H, CH2) and 3.48
(s, 3H, OCH3).
(2 : 1) as developer. This oil was further purified by
silica gel column chromatography employing hex-
ane : ethyl acetate (3 : 1) as the eluent. Thin layer
chromatography (same system) was used to evaluate
and combine column fractions having Rf values near
0.35. Evaporation of the combined fractions provided
1.20 g (84%) of a white solid: mp 66–67ЊC; 1H-NMR
(CDCl3) ͳ 10.09 (s, 1H, CHO), 8.24–8.26 (m, 2H,
ArH), 7.91–7.93 (d, J ϭ 8 Hz, 1H, ArH), 7.55–7.70
(m, 3H, ArH), 7.00–7.10 (m, 2H, ArH), 5.28 (s, 2H,
CH2) and 3.52 (s, 3H, OCH3); 13C-NMR (CDCl3) ͳ
187.1, 164.8, 162.8, 153.9, 134.0, 131.9, 130.3 (double
intensity), 128.8 (double intensity), 128.7, 122.7,
114.3, 110.7, 94.3 and 56.4; Elemental Analysis The-
ory C 67.13% and H 4.89%, Found C 67.03% and
H 4.97%.
2-Benzoyloxy-4-(methoxymethoxy)benzaldehyde, 4
Benzoyl chloride, 0.7 g (5 mmol), was added
dropwise to a solution of 0.91 g (5 mmol) of 3 in 20
ml methylene chloride also having 0.76 g (7.5 mmol)
of triethylamine at 0ЊC. The reaction medium was
stirred at 0ЊC for 30 min and then at room tempera-
ture for 2 h. The mixture was diluted with 80 ml of
methylene chloride and then washed consecutively
with 10 ml 10% aqueous HCl, 10 ml water, 10 ml
saturated aqueous sodium bicarbonate, 10 ml satu-
rated aqueous sodium chloride, and 10 ml water. The
organic phase was dried for several hours over anhy-
drous magnesium sulfate at 2ЊC and then evaporated
to provide a light yellow oil whose thin layer chroma-
togram displayed a single spot at Rf ϭ 0.35 when
assessed on silica gel using hexane : ethyl acetate
2-Benzoyloxy-4-hydroxybenzaldehyde, 1
Trimethylsilylbromide, 1.53 g (10 mmol), was
added dropwise to a solution of 0.572 g (2 mmol) of

188
Hu, Hardie, Burckel, Pinkerton, and Erhardt
˚
Table 3. Bond Lengths (A) and Angles (Њ) for 1
C(1)UO(1)
C(1)UC(2)
C(1)UC(6)
C(2)UC(3)
C(3)UC(4)
C(3)UO(2)
C(4)UC(5)
C(4)UC(7)
C(5)UC(6)
C(7)UO(3)
1.359(3)
1.391(4)
1.394(4)
1.360(4)
1.398(4)
1.409(3)
1.410(3)
1.444(4)
1.368(4)
1.225(4)
C(8)UO(4)
C(8)UO(2)
C(8)UC(9)
C(9)UC(14)
C(9)UC(10)
C(10)UC(11)
C(11)UC(12)
C(12)UC(13)
C(13)UC(14)
1.188(3)
1.379(3)
1.477(4)
1.389(4)
1.391(4)
1.385(4)
1.377(5)
1.376(5)
1.373(5)
O(1)UC(1)UC(2)
C(14)UC(9)UC(10)
O(1)UC(1)UC(6)
C(2)UC(1)UC(6)
C(3)UC(2)UC(1)
C(2)UC(3)UC(4)
C(2)UC(3)UO(2)
C(4)UC(3)UO(2)
C(3)UC(4)UC(5)
C(3)UC(4)UC(7)
C(5)UC(4)UC(7)
C(6)UC(5)UC(4)
C(5)UC(6)UC(1)
117.0(3)
O(3)UC(7)UC(4)
O(4)UC(8)UO(2)
O(4)UC(8)UC(9)
O(2)UC(8)UC(9)
C(14)UC(9)UC(8)
C(10)UC(9)UC(8)
C(11)UC(10)UC(9)
C(12)UC(11)UC(10)
C(13)UC(12)UC(11)
C(14)UC(13)UC(12)
C(13)UC(14)UC(9)
C(8)UO(2)UC(3)
125.7(3)
119.4(3)
122.3(2)
120.7(3)
118.6(3)
123.2(2)
120.8(3)
115.9(2)
116.5(3)
121.8(2)
121.7(3)
121.6(3)
119.4(2)
122.4(3)
126.5(3)
111.0(2)
117.6(3)
123.0(3)
119.9(3)
119.8(3)
120.5(3)
120.0(3)
120.3(3)
119.0(2)
4 in 30 ml of methylene chloride also having 1.0 g of
4 A molecular sieves at Ϫ30ЊC. The reaction medium
glass capillary with epoxy resin. A colorless crystal
of 2 of approximate dimensions 0.3 ϫ 0.3 ϫ 0.1 mm
was similarly mounted. Preliminary examination and
data collection were performed at 293(1) K for 1
and 180(1) K for 2 on a Siemens SMART Platform
diffractometer equipped with a CCD detector. 0.3Њ
omega scans were carried out at three different phi
settings corresponding to a nominal hemisphere of
data. The frame time was set to 60 sec for 1 and 8
sec for 2. The final unit cells were obtained from the
refinement of up to 8192 reflections after integration
(SAINT3). The intensity data were corrected for de-
cay and absorption (SADABS4). The structures were
solved by direct methods (SHELXS-865). All nonhy-
drogen atoms were refined with anisotropic displace-
ment parameters by full matrix least-squares on F2
(SHELXL-936). Hydrogen atoms were located from
the difference maps and refined with isotropic dis-
placement parameters.
˚
was stirred at Ϫ20 to Ϫ30ЊC for 2 h and then at room
temperature for 15 h. The mixture was poured into
200 ml of saturated aqueous sodium bicarbonate and
quickly extracted three times with 150-ml portions
of ethyl ether. The combined organic phase was dried
for several hours over anhydrous magnesium sulfate
at 2ЊC and then evaporated to produce a white solid
whose thin layer chromatogram displayed a single
spot at Rf 0.24 on silica gel when developed with
hexane : ethyl acetate (2: 1). Recrystallization was ef-
fected from chloroform : cyclohexane (1: 1) to pro-
duce 0.45 g (93%) of colorless crystals suitable for X-
ray analysis: mp 139–140ЊC; 1H-NMR (CDCl3) ͳ 9.99
(s, 1H, CHO), 8.20–8.23 (m, 2H, ArH), 7.52–7.81 (m,
4H, ArH), 7.05 (s, 1H, OH), 6.70–6.79 (m, 2H, ArH);
13C-NMR (CDCl3) ͳ 187.3, 165.2, 162.2, 153.9, 134.2,
132.7, 130.4 (double intensity), 128.8 (double inten-
sity), 128.5, 121.8, 114.1, and 110.5; Elemental Analy-
sis Theory C 69.42% and H 4.13%, Found C 69.50%
and H 4.12%.
Details of data collection and structure refine-
˚
Table 4. Hydrogen Bonds (A/Њ) for 1
DUHииииA
DUH
HиииA
ϽDUHиииA
X-ray crystallography
O(1)UH(1)ииииO(3)a
0.88(3)
1.86(3)
173(3)
A colorless needle of 1 of approximate dimen-
sions 0.28 ϫ 0.07 ϫ 0.05 mm was mounted on a fine
a Ϫx ϩ 1, Ϫy ϩ 1, z Ϫ1/2.

2-Benzoyloxy-4-hydroxybenzaldehyde
189
Table 5. Atomic Coordinates and Equivalent Isotropic Displacement
2
˚
Parameters (A ) for 1
x
y
z
U(eq)
O(1)
O(2)
O(3)
C(1)
C(2)
C(3)
C(4)
C(5)
C(6)
C(7)
H(2Ј)
H(3)
H(3Ј)
H(5)
H(6)
H(7)
Ϫ0.10153(14)
0.08015(15)
0.65849(13)
0.18840(17)
0.21091(17)
0.36902(18)
0.50697(16)
0.48764(18)
0.32990(18)
0.02843(18)
Ϫ0.010(4)
0.384(2)
0.25000(10)
0.43684(9)
0.38716(9)
0.24456(11)
0.36380(11)
0.40959(11)
0.33779(11)
0.21883(11)
0.17408(11)
0.19492(12)
0.395(3)
0.4907(16)
0.334(2)
0.1684(16)
0.0919(16)
0.1092(17)
0.43556(15)
0.34233(17)
0.09910(15)
0.30042(17)
0.28887(17)
0.22011(18)
0.16301(17)
0.17191(18)
0.23967(18)
0.37488(19)
0.391(4)
0.0361(3)
0.0387(3)
0.0322(3)
0.0238(3)
0.0251(3)
0.0262(3)
0.0235(3)
0.0253(3)
0.0257(3)
0.0290(3)
0.084(9)
0.033(4)
0.064(7)
0.037(4)
0.034(4)
0.039(5)
0.207(2)
0.052(3)
0.127(2)
0.249(2)
0.729(3)
0.581(2)
0.313(2)
0.023(3)
0.377(2)
ments are given in Table 1. Fractional atomic coordi-
nates and selected bond lengths and angles are given
in Tables 2 to 7.
reactive 2 position. Cleavage of the MOM ether in
the resulting penultimate intermediate, 4, can then
be readily accomplished with trimethylsilyl bromide8
to afford 1. Since this overall strategy had not been
previously deployed to redirect an acylation type re-
action which also produces an ester intermediate that
has a high degree of susceptibility toward migration,9
the gentle conditions afforded by the trimethylsilyl
bromide cleavage of the MOM ether were specifically
selected to minimize potential acyl transfer reactions
that might result in formation of the 4 position ester,
5. Such an unwanted intermolecular acyl transfer re-
action might be additionally driven by the ability of
Results and discussion
Following a strategy utilized by others,1,2 the un-
desirable regioselectivity exhibited by 2 may be
masked by first protecting the 4 position hydroxy
functionality with a methoxymethyl group (MOM)
so as to form compound 3. This can subsequently be
forced to undergo the requisite acylation7 at the less
˚
Table 6. Bond Lengths (A) and Angles (Њ) for 2
O(1)UC(7)
O(2)UC(2)
C(3)UC(4)
C(1)UC(2)
C(1)UC(6)
1.2420(18)
1.3514(15)
1.3467(15)
1.4129(18)
1.4073(18)
C(1)UC(7)
C(2)UC(3)
C(3)UC(4)
C(4)UC(5)
C(5)UC(6)
1.4286(17)
1.3832(17)
1.3888(17)
1.4061(18)
1.3735(18)
C(2)UC(1)UC(6)
C(2)UC(1)UC(7)
C(6)UC(1)UC(7)
O(2)UC(2)UC(1)
O(2)UC(2)UC(3)
C(1)UC(2)UC(3)
C(2)UC(3)UC(4)
118.44(11)
121.68(12)
119.87(12)
121.83(12)
117.70(12)
120.47(12)
119.73(12)
O(3)UC(4)UC(3)
O(3)UC(4)UC(5)
C(3)UC(4)UC(5)
C(4)UC(5)UC(6)
C(1)UC(6)UC(5)
O(1)UC(7)UC(1)
117.11(12)
121.92(11)
120.97(11)
118.92(12)
121.47(12)
124.52(13)
C(2)UO(2)UH(2Ј)
C(4)UO(3)UH(3Ј)
C(2)UC(3)UH(3)
C(4)UC(3)UH(3)
C(4)UC(5)UH(5)
107.4(18)
108.7(15)
121.4(10)
118.9(10)
121.2(11)
C(6)UC(5)UH(5)
C(1)UC(6)UH(6)
C(5)UC(6)UH(6)
O(1)UC(7)UH(7)
C(1)UC(7)UH(7)
119.8(11)
117.8(10)
120.7(10)
119.1(11)
116.4(11)

190
Hu, Hardie, Burckel, Pinkerton, and Erhardt
˚
Table 7. Hydrogen Bonds (A/Њ) for 2
DUHииииA
DUH
HииииA
ϽDUHииииA
O(2)UH(2Ј)ииииO(1)
O(3)UH(3Ј)ииииO(1)a
0.90(3)
0.88(3)
1.87(3)
1.82(3)
147(2)
168(2)
a
x ϩ 1, Ϫy ϩ 3/2, z Ϫ 1/2.
5 to reestablish the energetically favorable intramo-
lecular hydrogen bond as initially present in 2.
Unambiguous confirmation of structure 1 is pro-
vided by X-ray analysis of our final product (Fig.
1). Clearly the synthetic strategy was successful and
acylation has taken place at the 2 position. The two
halves of the molecule are essentially planar, however
they are twisted with respect to each other by 51.63Њ
about the ester linkage (C(8)UO(2)). In the solid
state, the molecules are linked into zig-zag ribbons by
hydrogen bonds between the hydroxyl and aldehyde
functionalities (Fig. 2). The ribbons are approxi-
mately parallel to the c axis and interleave to form
sheets perpendicular to the b axis.
Fig. 1. ORTEP plot of molecule 1—50%
probability ellipsoids.
sponsible for the distinct regiochemical behavior that
it exhibits during reactions conducted in non-aqueous
solvents. In addition, parallel sheets of planar mole-
cules are formed by intermolecular hydrogen bonds
(Fig. 4) similar to those observed for 1. The layers
The x-ray structure determination (Fig. 3) of
the starting material 2 confirms the presence of the
intramolecular hydrogen bond postulated1 to be re-
˚
are close together (ca. 3.35 A) and manifest a slipped
ȏ stacking.
Fig. 2. Packing of molecules in 1 showing hydrogen bonds.

2-Benzoyloxy-4-hydroxybenzaldehyde
191
Fig. 3. ORTEP plot of molecule 2—50% probability
ellipsoids.
Fig. 4. Packing of molecules in 2 showing hydrogen bonds.
3. SAINT, Siemens Energy and Automation; Madison, WI,
1994–1996.
Acknowledgment
4. Sheldrick, G.M. SADABS; University of Go¨ttingen: Ger-
many, 1996.
5. Sheldrick, G.M. Acta Crystallogr. 1990, A46, 467.
6. Sheldrick, G.M. SHELXL93; University of Go¨ttingen: Ger-
many, 1993.
We thank the College of Arts and Sciences of
the University of Toledo for generous financial sup-
port of the x-ray diffraction facility.
7. Vogel, A. Vogel’s Textbook of Practical Organic Chemistry,
5th ed., revised by Furnis, B.S. et al.; Wiley: New York, 1989;
pp 695–705.
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