A dipseudoacid, C16H18O6

Article abstract of DOI:10.1016/j.molstruc.2007.07.045

Pseudoacids, or cyclic oxocarboxylic acids commonly form non-planar, complementary and non-cooperative, dimeric hydrogen-bonds. A dehydration resistant arylpyran dipseudoacid, C₁₆H₁₈O₆, has been synthesized and characteriz

Full text of DOI:10.1016/j.molstruc.2007.07.045

Available online at www.sciencedirect.com
Journal of Molecular Structure 878 (2008) 149–154
www.elsevier.com/locate/molstruc
A dipseudoacid, C16H18O6
a
a,
*
Dmitriy V. Liskin , Edward J. Valente
a
Department of Chemistry & Biochemistry Mississippi College, Clinton, MS 39056, USA
Received 18 May 2007; received in revised form 30 July 2007; accepted 31 July 2007
Available online 15 August 2007
Abstract
Pseudoacids, or cyclic oxocarboxylic acids commonly form non-planar, complementary and non-cooperative, dimeric hydrogen-
bonds. A dehydration resistant arylpyran dipseudoacid, C16H18O6, has been synthesized and characterized. Crystals of trans-4,4,8,8-tet-
ramethyl-3,7-dihydroxy-1,2,3,4,5,6,7,8-octahydro-2,6-dioxaanthracen-1,5-dione occur in the monoclinic system and form linear
hydrogen-bonded chains; it has an elevated melting point without decomposition at 574.6 K.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Oxocarboxylic acids; Pseudoacids; Crystallography; Oxidation; Hydrogen-bonding
1. Introduction
Lattice stabilization by introduction of a supramolecu-
lar hydrogen-bonded network can be illustrated by the con-
trast between benzoic acid (mp 395 K) and terephthalic
acid (mp 673 K with sublimation). Benzoic acid forms local
dimeric hydrogen-bonds, while terephthalic acid forms infi-
nite linear chains [5,6]. At present, no example of a
dipseudoacid with a structure permitting linear hydrogen-
bonded chains has appeared. A simple system which may
be elaborated to show two pseudoacid functions is the
dehydration resistant arylpyran 3-hydroxy-4,4-dimethyl-
3,4-dihydroisobenzopyran-1-one (1), mp 331 K, 337 K [7].
Thus, the target molecule is 4,4,8,8-tetramethyl-3,7-dihy-
droxy-1,2,3,4,5,6,7,8-octahydro-2,6-dioxaanthracen-1,5-
dione (2) (Fig. 1).
In the usual scheme for forming oxocarboxylic acids
that can cyclize, the interacting functional groups are intro-
duced simultaneously by oxidative cleavage of a precursor
a-hydroxyketone [8]. In the present work, we pursued a
symmetric synthesis (Fig. 2) which begins with a double
alkylation of benzene with mesityl oxide to produce dike-
tone 3. After oxidation to bis(20-methyl-10-carboxyprop-
2-yl)benzene 4, double cyclization produces a mixture of
3,3,7,7-tetramethyl-1,2,3,5,6,7-hexahydro-s- and as-inda-
cen-1,5-diones (5, 6). Oxidation of 5 leads to the dipseudo-
acid system. The synthesis, structure, and spectral
properties of the trans-dipseudoacid 2 are described.
Hydrogen-bonding is one of the more important inter-
molecular forces of attraction affecting the properties of
polar organic substances. Systems such as purines and pyr-
imidines, carboxylic acids, oximes, and syn-amides engage
in multiple, complimentary hydrogen-bonding and pro-
duce local ring structures and in combination support
extended supramolecular structures [1,2]. Typically, these
paired donor–acceptor systems are approximately co-pla-
nar. Cyclic oxocarboxylic acids form an exception. Intra-
molecular cyclization of aldehyde or ketone group with a
carboxylic acid produces the lactol or pseudoacid function
in which the (chiral) hydroxyl and carbonyl groups are not
co-planar and lack an intervening p-system. It has been
found previously that pseudoacids commonly, but not
exclusively, form dimeric, complementary hydrogen-
bonded intermolecular structures with
a non-planar
2
R2 (12) ring [3,4]. These pair-wise hydrogen-bonds are
weaker than in carboxylic acid dimers since the linkage is
not cooperative. This is supported by the somewhat longer
˚
hydrogen-bonded OÆÆÆO distances in pseudoacids (2.8 A)
compared to those in carboxylic acids (2.6 A).
˚
*
Corresponding author. Tel.: +1 601 925 3424; fax: +1 601 925 3933.
E-mail address: valente@mc.edu (E.J. Valente).
0022-2860/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2007.07.045

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D.V. Liskin, E.J. Valente / Journal of Molecular Structure 878 (2008) 149–154
2.2. 1,4-Bis(10-carboxy-20-methylprop-20-yl)benzene (4)
In a 1 L flask fitted with a reflux condenser, 8.3 g
(30.3 mmol) diketone 3 was combined with 250 mL of 6%
commercial bleach. The two-phase mixture was stirred vig-
orously and heated to 70 oC for 18 h. After cooling, the
clear aqueous phase was separated from the yellow oily res-
idue by decantation through several layers of filter paper. A
sample of the aqueous phase showed absence of hypochlo-
rite on testing with potassium iodide. The aqueous mixture
was cooled to 2 oC, and 22 mL of 12 M HCl was added;
after vigorous agitation, a white precipitate separated.
The product was isolated by suction filtration, washed with
water, dried, and then recrystallized from a minimum of
95% ethanol producing 4 as a colorless solid; yield:
4.83 g, 60%. Analysis: 1H NMR (dH, CDCl3, 20 oC):
7.27, 4H, s, aryl-H’s; 6.27, 2H, broad s, hydroxyl-H; 2.64,
4H, s, CH2’s; 1.43, 12H, s, CH3’s; 13C{1H} (dC, CDCl3,
20 oC): 177.5, CO2; 145.2, CarACMe2; 125.4, CarAH;
48.1, CH2’s; 36.8, C(quaternary); 29.3 CH3’s.
Fig. 1. Arylpyran pseudoacid, 1, and dipseudoacid, 2.
2. Experimental
Reagents and solvents in the best commercial purities
were obtained from Aldrich Chemical Co. and used as
received. Nuclear magnetic resonance spectra were
obtained on a JEOL Eclipse 400 + (400 MHz) instrument;
(CH3)4Si reference. Melting points were determined on a
Shimadzu DSC-5 differential scanning calorimeter (under
dinitrogen, at a heating rate of 10 oC/min) and a melt-temp
capillary apparatus.
2.1. 1,4-Bis-(40-oxo-20-methylpent-20-yl)benzene (3)
Benzene (8.550 g, 109.6 mmol) was combined with
50 mL of petroleum ether and 33.758 g (253.2 mmol)
AlCl3. The mixture was stirred vigorously while 22.462 g
mesityl oxide (229.2 mmol) dissolved in 40 mL petroleum
ether was added dropwise over 30 min. During the addi-
tion, the mixture began to boil, and the reaction was con-
trolled by occasional cooling. After the addition, the
mixture was stirred for 1 h; stirring halts due to formation
of a solid product mass below a light-colored organic layer.
The contents of the reaction flask were transferred into a
2 L beaker half filled with ice and 150 mL of 12 M HCl;
the mixture was stirred until all of the solids were hydro-
lyzed. The organic phase was decanted, and the solvent
was removed, leaving an amber-colored oil, which crystal-
2.3. Cyclic diketones: 3,3,7,7-tetramethyl-1,2,3,5,6,7-hexahydro-
s-indacen-1,5-dione, ‘‘anti’’ isomer (5) and 3,3,7,7-tetramethyl-
1,2,3,5,6,7-hexahydro-as-indacen-1,5-dione, ‘‘syn’’ isomer (6)
About 30 mL of polyphosphoric acid (PPA) was placed
in a 100 mL beaker, and heated on a hot plate to 90 oC.
While stirring with a glass rod, 4.60 g (16.5 mmol) of 4
was added, and the mixture was continuously stirred while
maintaining 91–99 oC. After 20 min, 25 mL of PPA was
added, and heating and stirring continued for a total reac-
tion time of 1 h. The deep brown mixture was allowed to
cool to ambient temperature, and 100 g ice was added
and the mixture was stirred to discharge the dark color.
After extraction twice with 20 mL portions of ether, the
combined yellow organic phases were washed with satu-
rated NaHCO3, then with water. The monocyclized keto-
acid byproduct was isolated by acidifying the bicarbonate
wash solution; yield 2.205 g, 51%. After drying the ether
solution with anhydrous Na2SO4, ether was evaporated
leaving crude 5 and 6 as an orange-colored solid; yield
1
lized on standing. Yield 27.9 g, 93%. Analysis: H NMR
(dH, CDCl3, 20 oC): d 7.30, 4H, s, aryl-H; 2.70, 6H, s,
CH3CO; 1.77, 4H, s, CH2; 1.40, 12H, s, CH3’s; 13C{1H}
dC, CDCl3, 20 oC): 208.3, C@O; 145.8, CarACMe2; 125.5,
CarH; 57.1, CH2; 37.1, C quaternary); 32.0 CH3 terminal;
29.0 CH3’s.
Fig. 2. Symmetric synthesis of a dipseudoacid.

D.V. Liskin, E.J. Valente / Journal of Molecular Structure 878 (2008) 149–154
151
0.767 g, 19%. NMR analysis (see below) showed 78:22
ratio of anti to syn diketone isomers. They were separated
by column chromatography on silica gel (100 mesh; diam-
eter 1.5 cm, length 6 cm) eluting first with CHCl3 to remove
the less polar anti isomer 5; then switching to CHCl3:ethyl
acetate (3:1) to elute the minor syn isomer 6. Analysis for 5:
1H NMR (dH, CDCl3, 20 oC): 7.73, 2H, s, aryl-H; 2.63, 4H,
s, CH2; 1.41, 12H, s, CH3’s; 13C{1H} (dC, CDCl3, 20 oC):
202.7, C@O; 164.8, 132.4, CarAC(Me2); 129.7, CarH;
53.5, CH2; 38.64, C quaternary; 30.0, CH3. The crystal
structure of 5 was determined (see below). For 6: (dH,
CDCl3, 20 oC): 7.79, 2H, s, aryl-H; 2.66, 4H, s, CH2;
1.43, 12H, s, CH3’s; 13C{1H} (dC, CDCl3, 20 oC): 205.8,
C@O; 162.8, 140.6, CarACMe2; 118.6, CarAH; 53.7, CH2;
38.59, C(quaternary); 30.2, CH3’s.
CarACO;127.01, CarH; 102.87/102.85, OACAO; 37.10/
37.06, C(quaternary); 28.80/28.71, 21.87, CH3.
2.5. 4,4,8,8-Tetramethyl-3,7-dihydroxy-1,2,3,4,5,6,7,8-octahydro-
2,6-dioxaanthracen-1,5-dione, the dipseudoacid, (2)
This substance was found in the chloroform insoluble
component of the products of the nitric acid oxidation
(see above), and formed the most polar chromatographic
fraction. It is also derived from the Rf 0.2 material, which
was dissolved in 2 mL ethanol and 1 mL water, and treated
with a few drops of 0.5 N NaOH solution and allowed to
react for 5 min. The mixture was acidified with 0.5 N
HCl and became slightly cloudy. The alcohol was evapo-
rated under an air stream, and the noticeably cloudy aque-
ous layer was extracted with dichloromethane. On
evaporation dipseudoacid 2 was recovered as colorless
crystals; it was recrystallized from chloroform, acetone or
methanol. Melting point: 574.6 K (DSC peak; broad),
569–576 K (capillary, without decomposition). Analysis:
1H NMR (CDCl3, recrystallized from MeOH, 20 oC, most
resonances minutely doubled except hydroxyl H’s, dH):
8.06, 2H, s, aryl-H; 5.58, 2H, s, methine-H; 6.0, 2H, s
(br), OH; 1.46, 6H, s, CH3; 1.39, 6H, s, CH3; 13C{1H}
(dC, CDCl3, 20 oC, cis and trans isomers): 162.8, CO2;
145.2, CarACMe2; 128.8, CarACO; 126.2, CarH; 101.9,
OACAO; 38.4, C quaternary; 26.3, 21.4, CH3. The crystal
structure of 2 was determined (see below). In CDCl3, most
13C and 1H resonances are doubled from contributions for
each of the diastereomers. In acetone-d6, each resonance is
broadened into single features, indicating interconversion
between diastereomers is faster in this polar solvent. In
CDCl3 (70%): d4-methanol (30%) at 295 K, methyl protons
are a broad singlets, and two very broad carbon methyl res-
onances are seen suggesting an even faster rate of
interconversion.
2.4. Nitric acid oxidation of the ‘‘anti’’ diketone (5)
In a small glass vial, 0.050 g (0.21 mmol) of the ‘‘anti’’-
diketone 5 was dissolved in 1.0 mL concentrated sulfuric
acid and cooled to 0 oC. A cooled mixture of 1 mL concen-
trated (70%) nitric acid and 1 mL concentrated sulfuric
acid was added, and the resulting mixture was thoroughly
mixed at 0 oC. After 45 min at 0 oC, the sample was
allowed to warm to room temperature, and the mixture
was transferred at once into 50 mL water. The aqueous
solution formed was extracted twice with 10 mL portions
of CH2Cl2. On evaporation of the solvent, an oily pale-yel-
low solid was obtained. Thin layer chromatography (silica
gel, chloroform:ethyl acetate 10:1) showed essentially three
components with Rf 0.0 (2), 0.2 (7a), and 0.8 (7b). The most
polar component was nearly insoluble in CHCl3, which sol-
vent was used to transfer the mixture of the two less polar
components for separation by column chromatography on
silica gel (100 mesh; 1.2 cm diameter, 6 cm length). The col-
umn was eluted with CHCl3 at first, then combined with
increasing proportions of ethyl acetate. A small amount
of the most polar component (2) finally elutes with
CHCl3:ethyl acetate (1:1). Analyses: 7a, 1H NMR (d,
CDCl3, 20 oC, each resonance minutely doubled except
hydroxyl H): 8.15, 1H, s, aryl-H; 8.10, 1H, s, aryl-H;
6.42, 1H, s, HACONO2; 5.55, 1H, s, HACO2; 4.2, 1H, s
(br), OH; 1.502, 1.489, 6H, s, CH3; 1.420, 1.404, 6H, s,
CH3; 13C{1H} (d, CDCl3, 20 oC, cis and trans isomers):
163.05, 160.84/160.79, CO2; 145.72/145.68, 143.31,
CarACMe2; 128.98, 127.74, CarACO; 127.27/127.22,
126.41, CarH; 102.96, 102.09/102.00, OACAO; 38.567/
2.6. Crystallographic analysis
Crystal structures were obtained for compounds 3, 4, 5,
7a, and 2. Data were obtained on an Oxford Diffraction
Gemini S system with a charge coupled device operating
under the CrysalisPro system [9] using Mo or CuKa radia-
tion. Structures were solved with SHELXS-86 and refined
with SHELXL-97 [10]. Referenced CCDC numbers con-
tains the supplementary crystallographic data for these
structures. For open diketone 3, CCDC 647883, crystals
were obtained from the oil, monoclinic, space group P21/n,
˚
˚
˚
38.539, 37.007/36.965,
C
quaternary; 28.778/28.653,
a = 11.5966(15) A, b = 6.6798(7) A, c = 11.7635(13) A,
3
˚
27.330/27.056, 22.046/22.011, 21.919/21.891, CH3. This
component crystallizes as the cis isomer, and the crystal
structure of cis-7a was determined (see below). For 7b,
1H NMR (d, CDCl3, 20 oC, cis and trans isomers) 8.114/
8.111, 2H, s, aryl-H; 6.438/6.435, 2H, s, methine-H;
1.587/1.580, 6H, s, methyl-H; 1.539/1.527, 6H, s, methyl;
13C{1H} (d, CDCl3, 20 oC, cis and trans isomers):
160.49/160.44, CO2; 144.08, CarACMe2; 127.86/127.82,
b = 116.476(15)°, V = 815.7(2) A , with four molecules in
the cell, 293 K, 2550 data, R1 = 0.071, wR2 = 0.200
[I > 2r(I)], S = 1.01. For open diacid 4, CCDC 647881,
crystals from 95% ethanol, monoclinic, space group P21/n,
˚
˚
˚
a = 18.4117(18) A, b = 9.0649(10) A, c = 18.4506(19)A,
3
˚
b = 99.666(11)°, V = 3035.7(5) A , with eight molecules in
the cell, 293 K, 4968 data, R1 = 0.081, wR2 = 0.229
[I > 2r(I)], S = 1.08. For open diacid 4 at 102 K, CCDC

152
D.V. Liskin, E.J. Valente / Journal of Molecular Structure 878 (2008) 149–154
˚
647882, monoclinic, space group Cc, a = 21.8163(2) A,
˚
˚
b = 8.9653(1) A, c = 23.2716(2) A, b = 104.718(1)°, V =
3
˚
4402.33(7) A , with 12 molecules in the cell, 6914 data,
R1 = 0.036, wR2 = 0.096 [I > 2r(I)], S = 0.96. For cyclic
diketone 5, CCDC 647884, crystals from benzene, mono-
˚
˚
clinic, space group P21/n, a = 5.7765(2) A, b = 15.9490(5) A,
o
3
˚
˚
c = 7.5342(3) A, b = 110.015(5) , V = 652.20(7) A , with
four molecules in the cell, 293 K, 1910 data, R1 = 0.046,
wR2 = 0.133 [I > 2r(I)], S = 1.073. For the cis-heminitryl-
oxy pseudoacyl anhydride 7a, CCDC 647885, crystals
from chloroform, orthorhombic, space group Pbca, a =
˚
˚
˚
11.6085(11) A,
b = 3.6901(12) A,
c = 20.8707(19) A,
3
˚
V = 3316.8(5) A , with eight molecules in the cell, 294 K,
3344 data and R1 = 0.126, wR2 = 0.227 [I > 2r(I)],
S = 0.96. For the trans-dipseudoacid 2, CCDC 647886,
crystals from chloroform, monoclinic, space group C2/c,
Fig. 4. Ellipsoid plot of one of three inequivalent molecules of diacid 4 at
100 K.
˚
˚
˚
a = 14.2880(3) A, b = 11.2230(2) A, c = 10.1703(2) A,
o
3
˚
b = 101.898(2) , V = 1595.81(5) A , with four molecules
in the cell, 293 K, 1364 data, R1 = 0.038, wR2 = 0.116
[I > 2r(I)], S = 1.05. CCDC data can be obtained free of
charge from www.ccdc.cam.ac.uk/conts/retrieving.html
(or from the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033).
3. Results and discussion
In the system and synthetic strategy pursued, symmetry
and simplicity are repeatedly encountered. Dialkylation of
benzene proceeds in good yield to the diketone 3 which
shows four singlets for its 26 hydrogens in the PMR spec-
trum. In the crystal, molecules reside on inversion centers
of the monoclinic lattice (Fig. 3) and therefore show Ci
symmetry. The conformation adopted places one of the
side-chain methyl groups nearly in the ring plane. Diacid
4 is obtained by haloform oxidation of 3, and its PMR
Fig. 5. Ellipsoid plot of ‘‘anti’’-diketone 5.
spectrum also shows four singlets. In the solid state at
100 K, three diacid molecules comprise the asymmetric
unit, and each shows approximate Ci symmetry, though
not required by their general locations in the monoclinic
lattice (Fig. 4). Molecular conformations differ in the orien-
tation of the side-chain groups. In two of the three, the
conformations show methyl groups nearly in the approxi-
mate aryl ring plane. On warming to room temperature
(293 K), crystals of 4 show a transformation to a different
monoclinic cell without fracture. In this modification, two
molecules comprise the asymmetric unit, each with approx-
imate Ci symmetry, and with conformational variety as
before. Diacid molecules in each modification form infinite
hydrogen-bonded chains supported by the typical dimer
carboxylic acid motif.
Double cyclization of diacid 4 with polyphosphoric acid
produces a mixture of cyclic diketones 5 and 6 in relatively
poor yields. PMR spectra of the separated cyclic diketones
each show three singlets for their 18 hydrogens. Molecules
of the major isomer, 5, show Ci symmetry, occupying
inversion centers on the monoclinic lattice (Fig. 5). Oxida-
tion of 5 to form the diacetoxy diketone derivatives (8) pro-
ceeds in good yield, but subsequent saponification leads to
Fig. 3. Ellipsoid plot of diketone 3.

D.V. Liskin, E.J. Valente / Journal of Molecular Structure 878 (2008) 149–154
153
Fig. 6. Revised final oxidative steps; (a) Pb(OAc)4, AcOH, Ac2O; (b) MeOH, H2O, K2CO3, N2, 25 oC; (c) HNO3, H2SO4, 0 oC, then H2O; (d) NaOH(aq)
then H3O+.
,
unidentified polar products, without evidence for the for-
mation of di(a-hydroxy)diketones (9). The precursor for
periodate oxidative cleavage was therefore not accessible
by this route. Oxidation and oxidative cleavage of 5 was
accomplished with nitric acid (Fig. 6) following the discov-
ery of this reaction by Cooper et al. [7]. Diketone 5 seemed
less likely to undergo ring nitration due to steric con-
straints, so treatment with concentrated nitric and sulfuric
acid at 273 K and hydrolysis with water produced good
yields of a mixture of the diastereomeric heminitryloxy
pseudoanhydrides 7a, dinitryloxy pseudoanhydrides 7b,
and dipseudoacids 2. Nitryloxy derivatives were identified
by the characteristic magnetic resonance features of their
pseudoacyl H at about dH 6.5 ppm and pseudoacyl C at
about dC 103 ppm; in comparison, pseudoacyl H occurs
at about dH 5.5 ppm. For each of these reaction products,
the magnetic resonance spectra show solution mixtures of
diastereomers. Comparable carbon resonances varying in
the range <0.01–0.09 ppm. Aryl carbon assignments are
well fit by use of typical substitutent shieldings [11] and
supported by DEPT experiments. Compound 7a forms
the cis isomer on crystallization, and confirmed the nitryl-
oxy substitution (Fig. 7). Hydrolysis of the nitryloxy pseud-
oanhydrides with dilute aqueous base, then acidification
produces dipseudoacids 2, which crystallize as the trans
diastereomer.
In the solid state, molecules of the dipseudoacid, 2
(trans-4,4,8,8-tetramethyl-3,7-dihydroxy-1,2,3,4,5,6,7,8-octa-
hydro-2,6-dioxaanthracen-1,5-dione) show Ci symmetry,
lay on inversion centers of the monoclinic space group
C2/c, a half-molecule comprising the asymmetric unit
(Fig. 8). Hydroxy groups are disposed axially, as is typical
for this function and possibly indicating an operative ano-
meric effect. Molecules are hydrogen-bonded to neighbors
on each side forming infinite chains, with the links occur-
ring by complementary pairing (Fig. 9). Crystals of 2 melt
without decomposition or apparent sublimation at
574.6 K, over 230 K above that for either of the two mod-
ifications of 1 [5]. Clearly the formation of infinite chains
has lent considerable stability to the lattice of the dipseudo-
acid. Still, it is not as robust as terephthalic acid (mp
673 K), which also forms infinite hydrogen-bonded chains
[4]. Carboxylic acids form complementary and cooperative
dimeric hydrogen-bonds, in which p systems may be
thought to overlap throughout the process of proton
exchange between equivalent carboxyl groups. In pseudo-
acids, cooperativity is lost since proton exchange would
Fig. 8. Ellipsoid plot of the trans-dipseudoacid 2.
Fig. 7. Ellipsoid plot of cis-heminitryloxy pseudoanhydride 7a.

154
D.V. Liskin, E.J. Valente / Journal of Molecular Structure 878 (2008) 149–154
Fig. 9. Plot showing hydrogen-bonded chains formed by trans-dipseudoacid 2.
Table 1
with complementary but non-cooperative dimeric links,
and a remarkably high melting point.
Relevant metrics of complementary hydrogen-bonded pseudoacidsa
Angle OAHÆÆÆO (o)
Distance OÆÆÆO (A)
Reference
[7]
˚
Compound
Acknowledgements
1
172.2
159.4
169.1
168.9
169.8
2.759
2.833
2.809
2.767
2.789
1
[7]
Support for instrumentation is gratefully acknowledged
to the Camille and Henry Dreyfus Foundation, W.M.
Keck Foundation, and the National Science Foundation
(0618148).
2
This work
This work
7a
a
o
˚
OH bond lengths normalized to 0.96 A; angle esd’s 6 1.5 , distance
˚
esd’s 6 0.005 A.
References
involve inequivalent molecular forms (cyclic and open
forms) and rehybridization at the lactol carbon. Without
coplanarity of the interacting groups, effective p overlap
between the two forms does not occur. In solution, equili-
bration between the two diastereomeric of 2 occurs, with
the rate of interconversion apparently depending on sol-
vent polarity. An intermediate open form has not been
detected. Both 7a and 2 show complementary hydrogen-
bonding between pseudoacid groups. The hydrogen-bond
features (Table 1) are similar to those observed in the mod-
ifications of pseudoacid 1.
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4. Conclusions
[10] G.A. Sheldrick, Programs for Crystal Structure Analysis, Institut fur
¨
¨
A arylpyran dipseudoacid has been synthesized by oxi-
dation of the precursor anti-diketone with nitric and sulfu-
ric acids. The dipseudoacid shows chain hydrogen-bonding
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