Stereoselective formation of dicondensed spiropyran product obtained from the reaction of excess Fischer base with salicylaldehydes: First full characterization by X-ray crystal structure analysis of a DC·acetone crystal

Article abstract of DOI:10.1016/j.tet.2005.05.005

The structure and stereochemistry of the dicondensed spiropyran product (DC-1, X=COOH) obtained from reaction of excess Fischer base with substituted salicylaldehydes has been fully assigned as C with (8R, 10R) configuration on the basis of single crystal X-ray diffraction analysis. The stereoselective formation of DC molecules indicates that the most plausible mechanism for DC formation involves dehydration of the cyclic carbinol intermediate with the aid of intramolecular H-bonding via transition structure TS₁ ^?.

Full text of DOI:10.1016/j.tet.2005.05.005

Tetrahedron 61 (2005) 6720–6725
Stereoselective formation of dicondensed spiropyran product
obtained from the reaction of excess Fischer base with
salicylaldehydes: first full characterization by X-ray crystal
structure analysis of a DC$acetone crystal
a
a
a
b,
Sam-Rok Keum,a, Byung-Soo Ku, Jin-Taek Shin, Jae Jung Ko and Erwin Buncel
*
*
aDepartment of New Material Chemistry, Korea University, Jochiwon 339-700, South Korea
bDepartment of Chemistry, Queen’s University, Kingston, Canada, K7L 3N6
Received 5 April 2005; revised 30 April 2005; accepted 2 May 2005
Abstract—The structure and stereochemistry of the dicondensed spiropyran product (DC-1, XZCOOH) obtained from reaction of excess
Fischer base with substituted salicylaldehydes has been fully assigned as C with (8R, 10R) configuration on the basis of single crystal X-ray
diffraction analysis. The stereoselective formation of DC molecules indicates that the most plausible mechanism for DC formation involves
dehydration of the cyclic carbinol intermediate with the aid of intramolecular H-bonding via transition structure TS‡1.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
photo and/or thermal relaxation to regenerate the spiro-
pyrans (Scheme 1).
The thermochromism and photochromism of spiropyrans
has received wide attention because of the potential
practical applications of these materials to a variety of
optoelectronic and molecular information devices.1 The
indolinobenzospiropyran structure, 3, that is 1,3,3-tri-
methyl-60-nitrospiro(indoline-2,20-benzopyran) derivatives,
typifies this class of compounds. These materials exemplify
some of the typical organic photochromic compounds
having high extinction coefficients in the near-infra region
and have thus been featured in a number of recent studies.1,2
Their photochromic behavior is normally based on the UV
irradiation-promoted opening of the spiro ring system (SP)
to produce the colored merocyanine species (MC) and their
In contrast to the many available reports1–4 on the synthesis
and structural and mechanistic aspects of spiropyran
species, little is known about the structural chemistry of
dicondensed (DC) products, although a few authors have
1
speculated on the most likely structure of DC through H
NMR spectroscopy. Interest in these dicondensed hetero-
cycles as additives in silver halide emulsions5,6 and as
components of thermal paper7,8 provides further motivation
for the unequivocal structure assignment of these com-
pounds. An ongoing research focus in our laboratories has
been the development of new optoelectronic materials. Thus
of special interest pertaining to these dicondensed adducts,
Scheme 1. Photochromism of indolinobenzospiropyrans.
Keywords: Dicondensed spiropyran; Single crystal X-ray diffraction analysis; Stereoselective formation; Cyclic carbinol intermediate; Intramolecular H-
bonding.
*
Corresponding authors. Tel.: C82 41 860 1332; fax: C82 41 865 5396 (S.-R.K.); tel.: C1 613 533 2653; fax: C1 613 533 6669 (E.B.); e-mail
addresses: keum@korea.ac.kr; buncele@chem.queensu.ca
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.05.005

S.-R. Keum et al. / Tetrahedron 61 (2005) 6720–6725
6721
representative compound, DC-1, which was obtained in
satisfactory crystalline form as a 1:1 complex with one mole
of solvent (acetone). We report here the first X-ray structural
determination of this class of compound.
2. Results
2.1. Synthesis
The reaction of Fischer base 1 and salicylaldehyde 2 in 1:1
molar ratio yields both the monocondensed product 3 and
the dicondensed product 4. The product ratios 3:4 are
w1:1–2:1 depending upon solvent used, the substituents
(X,Y) in the salicylaldehydes and reaction conditions.
Predominance of the dicondensed product could be
achieved by using a 2- to 3-fold excess of Fischer base
over the salicylaldehyde.
Figure 1. 1H NMR spectra of DC-1 in the range of 4–6 ppm at 140 8C, TZ
(a) 0; (b) 20; (c) 30; (d) 40; (e) 60 min.
From the reaction of 1 and 2 in 1:1 molar ratio, the
monocondensed spiropyran compounds (SP) are generally
formed as the major product when an electron-withdrawing
substituents is present in the salicylaldehyhyde.
DC materials melted around 170 8C but decomposed over
temperature ranges of 141–149 8C, with changes of colour
before melting. NMR monitoring showed that DC-1
survived heating up to 140 8C for 50 min in the solid
state, thereafter forming Fischer base and spiropyran.
DC/SP CFB
Resonances at 4.15 and 4.32 ppm are characteristic of DC
and 5.86 ppm for SP.3 In the presence of excess acid
decomposition to Fischer base and MCHC (protonated
Figure 2. Thermal transformation of DC-2 in diglyme, showing formation
of the corresponding SP; DC-2 (dotted line) and SP/MC photochromic
behavior (solid line) after irradiation in EtOH (2.19!10K5 M).
open-form spiropyrans) occurs in about 50 min.9
A
1H
NMR temperature study of the decomposition of DC-1 in
containing two indoline units, is the possibility that these
structures could function as optical switches.
DMF is shown in Figure 1.
In particular, the DC stereochemical aspects have not been
reported due to the lack of a convenient method for the
formation of DC crystals. This aspect is described in detail
in the following section, but the main conclusion can be
drawn here. Structure analysis was effected for one
The DC molecules are thus shown to be precursors of SP
molecules. The thermal transformation of DC molecules to
the corresponding SP molecules is confirmed further via
UV–vis spectral behavior of DC-2. SP–NO2, which exhibits
typical photochromic behavior shown in Figure 2.
Scheme 2. Synthetic scheme of SP and DC compounds.

6722
S.-R. Keum et al. / Tetrahedron 61 (2005) 6720–6725
Figure 3. ORTEP diagram with an atomic labeling system in the DC-1.
2.2. Structural analysis of DC-1 by X-ray diffraction
Selected bond lengths and bond angles are collected in
Table 1.
Structure analysis was effected for one representative
compound, DC-1 (XZCOOH), which was obtained in
satisfactory crystalline form, as an 1:1 complex with one
molecule of solvent (acetone). This is the first isolation of
‘dicondenced product’ from the reaction of Fischer base
with salicylaldehydes. Attempts to grow crystals of other
DC molecules such as DC-2 or DC-3 (Scheme 2) from
various solvents were unsuccessful for X-ray crystal
analysis. An ORTEP diagram with atomic labeling in the
DC-1 molecule is shown in Figure 3.
The main conclusion of the X-ray crystal structure study is
that the second molecule of Fischer base is attached to C(8),
1
as proposed previously from the H NMR study,15 that is
structure C in Scheme 3. The C(8)–C(9) and C(8)–C(21)
˚
distances are 1.534 and 1.510 A, respectively, typical of
C–C single bond. The enamine C(21)–C(22) bond length
˚
of the second Fischer base unit is 1.325 A which is typical of
the CZC bond. The O(1)–C(5) and O(1)–C(10) distances
˚
are 1.347 and 1.475 A, respectively. The most interesting
aspect is the conformational structure of0 the benzopyran
ring of DC-1. The four hydrogens (Ha, Ha Hb and Hc) are
located adjacent to each other, as depicted in Figure 4.
The crystal data, structure refinement, atomic coordinates
(!104) and equivalent isotropic displacement parameters
2
3
(A !10 ) for DC-1 are briefly given in the Section 9.
˚
Dihedral angles of DC-1 are 53.0, 170.4 and 172.168 for
Hb–C(8)–C(9)–Ha (q1), Hb–C(8)–C(9)–Ha0 (q2), and Hb–
C(8)–C(21)–Hc (q3), respectively. The double bond
C(21)]C(22) of the second Fischer base moiety has an E
configuration. From the 1H NMR vicinal coupling constant
values of DC-1, dihedral angles are calculated using the
modified Karplus equation.14 The dihedral angles (50.1,
170.4 and 172.18 for q1, q2 and q3, respectively) are in
complete agreement with calculated values.15
˚
Table 1. Selected geometrical parameters (bond lengths in A, angles in
degrees)
Bonds
Length
Bonds
Angle
O(1)–C(5)
1.347(4)
1.475(4)
1.444(4)
1.395(5)
1.446(5)
1.406(4)
1.378(4)
1.443(4)
1.521(5)
1.534(5)
1.510(5)
1.511(5)
1.325(5)
1.532(5)
1.515(5)
C(5)–O(1)–C(10)
O(1)–C(5)–C(6)
C(5)–C(6)–C(8)
C(21)–C(8)–C(6)
C(21)–C(8)–C(9)
C(6)–C(8)–C(9)
C(10)–C(9)–C(8)
C(22)–C(21)–C(8)
C(21)–C(22)–N(2)
C(21)–C(22)–C(23)
N(1)–C(10)–O(1)
N(1)–C(10)–C(9)
N(2)–C(22)–C(23)
O(1)–C(10)–C(9)
O(1)–C(10)–C(11)
120.9(3)
123.4(3)
120.0(3)
111.5(3)
111.6(3)
108.8(3)
113.8(3)
128.7(3)
123.2(3)
129.8(4)
104.9(3)
113.6(3)
107.0(3)
108.7(3)
108.0(3)
O(1)–C(10)
N(1)–C(10)
N(1)–C(17)
N(1)–C(20)
N(2)–C(22)
N(2)–C(29)
N(2)–C(32)
C(6)–C(8)
The stereochemical relationship of the two centers can now
be established. The absolute configuration of the product is
(8R, 10R). The geometry about the olefinic bond is also
determined. The enamine proton H(21) is located close to
the N-methyl group of the second Fischer base unit. The
stereoselective formation of (8R, 10R) isomer of DC
molecules from the reaction of Fischer base and salicyl-
aldehydes requires further consideration (see Section 3). 1H
C(8)–C(9)
C(8)–C(21)
C(9)–C(10)
C(21)–C(22)
C(22)–C(23)
C(23)–C(24)
Scheme 3. Proposed structures of DC molecules.

S.-R. Keum et al. / Tetrahedron 61 (2005) 6720–6725
6723
et al.13 later preferred C or D based on 1H NMR
considerations.
Unequivocal structure assignment of the dicondensed
product requires the following three segments. First, the
four isomeric structures A–D (Scheme 3) must be
considered. Second, since the dicondensed compound
contains two chiral centers, the stereochemical relationship
of these centers must be established. Finally, in the case of
structures B, C and D, the geometry about the olefinic bond
must be ascertained.
The unequivocal structure and stereochemistry of the DC
system is hereby established by X-ray single crystal analysis
as C in Scheme 3. Due to the lack of a convenient method
for the formation of DC crystals until now, no papers have
reported the detailed structure and the stereochemical
identification of the DC molecules. The X-ray single crystal
structure of the dicondensed product is important, not only
because of the molecular conformation including stereo-
chemistry at three stereogenic centers, C(8), C(10) and
C(21) (Fig. 3), but because this may give a clue to the
mechanism of DC formation in the reaction.
Figure 4. Structure of DC-1 showing the (8R, 10R, 21E) configuration.
NMR data of DC-2, before recrystallization, showed
formation of (8R/10S, 8S/10R) isomer in less than 1%.
The DC-1 molecules are stacked in linear chains in the
crystal. All molecules are juxtaposed alternatively. The
indole unit stays parallel to the plane of the second molecule
of Fischer base unit. The crystal DC-1 is monoclinic. All
molecules pack in the crystal with their long axes almost
parallel.
3.2. Mechanism of DC formation
Having established the structure of the dicondensed product
it is possible to rationalize the mechanism of its formation.
Dicondensed products can be formed from the reaction of
Fischer base and salicylaldehydes by either of two
variations of a reasonable pathway, as in Scheme 4. The
salicylaldehydes may condense with two molecules of
Fischer base via the intermediate carbinol (Path B) or the
Fischer base may undergo a Michael addition to the open
MC form of the spiropyran (Path A).
3. Discussion
3.1. Structure and stereochemistry of the DC molecules
Four isomeric structures (A–D) shown in Scheme 3 have
been proposed for the dicondensed product. Koelsch and
Workman10 assumed the structure of the product was A.
This conjecture was supported by the infrared studies of
Schiele and Arnold.11 Bertelson12 then pointed out that
structure B must also be considered as a possibility. Hinnen
The most plausible mechanism for its formation is Path B,
which involves dehydration of the carbinol intermediate.
Dehydration of this carbinol intermediate might occur via a
Scheme 4. Formation processes of DC molecules via Path A and Path B.

6724
S.-R. Keum et al. / Tetrahedron 61 (2005) 6720–6725
Scheme 5. Cyclic TS‡ in DC formation via Path A.
cyclic transition state, TS1‡, with the aid of intramolecular
H-bonding, as in Scheme 5. TS‡1 would lead to (8R, 10R) or
(8S, 10S) configuration, whereas TS‡2 leads to (8S, 10R) or
(8R, 10S) configuration. A steric effect may be involved
between the dimethyl groups of indoline and the forth-
coming phenolic oxygen moiety.
ethanol was refluxed for 8 h. The yellow precipitate was
filtered from the hot solution and washed thoroughly with
cold diethyl ether. Purification was carried out either by
recrystallization from acetone or by precipitation from
chloroform/diethyl ether. The product was identified by 1H
NMR and mass spectroscopy and gave satisfactory
elemental analysis.
This hypothesis is supported by the observation from the
X-ray data of DC-1, that the stereogenic centers C-8 and
C-10 have the RR or SS configuration. Thus the proton
locates at the same side of the pyranose ring oxygen in TS‡1.
Without this type of H-bonding, the epimeric proton on C-8
would not be steroselective. In addition, the fact that the
(8R, 10R) or (8S, 10S) DC isomer was formed stereo-
selectively may rule out the Path A formation mechanism
involving capture of the open merocyanine intermediate
prior to ring closure to the spiropyran, since stereo-
selectivity at C-10 could not be expected from the Michael
addition to the open merocyanine intermediate.
5.1.1. 4-(2-Methylene-1,3,3-trimethylindoline-20-yl)-6-
carboxylic-10,30,30-trimethyl-spiro[3,4-dihydro-2H-1-
benzopyran-2,20-indoline], DC-1. Yellow, yield 75%, mp
1
167 (dec 133) 8C, H NMR (400 MHz, CDCl3) d 1.25(s,
3H), 1.26 (s, 3H), 1.55 (s, 6H), 2.19 (dd, JZ14.2, 12.0 Hz,
1H), 2.43 (dd, JZ14.2, 4.20 Hz, 1H), 2.85 (s, 3H), 3.04 (s,
3H), 4.16 (d, JZ9.90 Hz, 1H), 4.27 (m, JZ12.4, 9.90,
4.20 Hz, 1H), 6.78 (t, 1H), 6.56 (d, 1H), 6.59 (d, JZ
7.50 Hz, 1H), 6.75 (d, JZ8.70 Hz, 1H), 6.86 (t, 1H), 7.10 (t,
1H), 7.18 (d, 1H), 7.15 (t, JZ7.50 Hz, 1H), 7.19 (d, 1H),
7.81 (d, JZ8.70 Hz, 1H), 8.06 (s, 1H); ES-Mass for
C32H34N2O3, Mw: 494; 105 (23.5), 158 (11.0), 174 (100),
494 (1.2) m/z (%); C, 77.7; H, 6.93; N, 5.66; O, 9.70
obtained C, 77.2; H, 7.10; N, 5.78, O, 9.92.
4. Conclusions
The results of X-ray structure determination for DC-1 as C
with (8R, 10R) or (8S, 10S) configuration are in complete
agreement with our earlier structural assignments based
solely on 1H NMR results for the series of DC compounds.15
The most plausible mechanism of DC formation involves
dehydration of the carbinol intermediate via the cyclic TS1‡
with the aid of intramolecular H-bonding (Path B in
Scheme 4), rather than capture of the open merocyanine
intermediate prior to ring closure to the spiropyran (Path A,
Scheme 4).
5.1.2. 4-(2-Methylene-1,3,3-trimethylindoline-20-yl)-6-
nitro-10,30,30-trimethylspiro[3,4-dihydro-2H-1-benzo-
pyran-2,20-indoline], DC-2. Yellow, yield 82%, mp 176
1
(dec 137) 8C, H NMR (400 MHz, CDCl3) d 1.31(s, 3H),
1.33 (s, 3H), 1.62 (s, 6H), 2.23 (dd, JZ14.3, 13.0 Hz, 1H),
2.85 (s, 3H), 3.00 (dd, JZ14.3, 4.87 Hz, 1H), 3.05 (s, 3H),
4.15 (d, JZ10.1 Hz, 1H), 4.32 (m, JZ13.0, 10.1, 4.87 Hz,
1H), 6.83 (t, 1H), 6.59 (d, 1H), 6.60 (d, JZ7.43 Hz, 1H),
6.75 (d, 1H), 6.86 (t, 1H), 7.06 (t, 1H), 7.07 (d, 1H), 7.09 (t,
JZ7.43 Hz, 1H), 7.11 (d, 1H), 7.96 (d, 1H), 8.23 (s, 1H);
ES-Mass for C31H33N3O3, Mw: 496; 118 (16.1), 132 (22.7),
174 (100) 323 (33.2), 496 (4.1) m/z (%); C, 75.13; H, 6.71;
N, 8.48; O, 9.68; found C, 74.9; H, 6.80; N, 8.57; O, 9.73.
5. Experimental
5.1. Materials
5.1.3. 4-(2-Methylene-1,3,3-trimethylindoline-20-yl)-6-
phenylazo-10,30,30-trimethyl spiro[3,4-dihydro-2HK1-
benzopyran-2,20-indoline], DC-3. Yellow, yield 73%, mp
Fischer base (2-ethylene-1,3,3-trimethylindoline) and
salicylaldehyde were available from Aldrich Chemical Co.
and were used without further purification.
1
170 (dec 142) 8C, H NMR (400 MHz, CDCl3) d 1.34 (s,
1H), 1.38 (s, 1H), 1.65 (s, 3H), 1.69 (s, 3H), 2.24 (dd, JZ
14.2, 12.0 Hz, 1H), 2.87 (s, 3H), 2.45 (dd, JZ14.2, 4.89 Hz,
1H), 3.06 (s, 3H), 4.26 (d, JZ10.0 Hz, 1H), 4.37 (m, JZ
12.0, 10.0, 4.89 Hz, 1H), 6.75 (t, 1H), 6.57 (d, 1H), 6.57 (d,
1H), 6.82 (d, JZ8.55 Hz, 1H), 6.85 (t, 1H), 7.08 (t, 1H),
7.19 (d, 1H), 7.09 (t, 1H), 7.22 (d, 1H), 7.79 (d, JZ8.55 Hz,
1H); ES-Mass for C37H38O, Mw: 554; 105 (35.4), 158
(16.4), 174 (100) 382 (22.9), 555 (0.5) m/z (%); C, 80.11; H,
The azoarylated salicylaldehydes were obtained from the
reaction of 1:1 molar ratio of commercially available
salicylaldehydes and the corresponding substituted benzene
diazonium salts, which were prepared from diazotization of
substituted anilines with nitrous acid.
For preparation of DC’s, a mixture of 5-substituted
salicylaldehyde and excess (2–3-fold) Fischer base in

S.-R. Keum et al. / Tetrahedron 61 (2005) 6720–6725
6725
Laurent, H., Eds.; Elsevier: London, 1992. (b) Pieroni, O.;
Fissi, A.; Angelini, N.; Lenci, F. Acc. Chem. Res. 2001, 34, 9.
(c) Berkovic, G.; Krongauz, V.; Weiss, V. Chem. Rev. 2000,
100, 1741.
6.90; N, 10.10; O, 2.88 obtained C, 80.6; H, 6.88; N, 10.2;
O, 2.32.
5.2. Measurements
2. Organic Photochromic and Thermochromic Compounds;
Crano, J. C., Gunglielmetti, R. J., Eds.; Topics in Applied
Chemistry; Kluwer Academic/Plenum: New York, 1999; Vols.
1 and 2.
Melting points were determined on a Fischer–Johns bloc
and are uncorrected. The 1H NMR spectra were taken with a
Bruker CXP-400 FT NMR spectrophotometer. Electrospray
(ES) mass spectra were recorded on a VG Quattro mass
spectrometer at Queen’s University.
3. (a) Keum, S. R.; Hur, M. S.; Kazmaier, P. M.; Buncel, E. Can.
J. Chem. 1991, 69, 1940. (b) Keum, S. R.; Lee, K. B.;
Kazmaier, P. M.; Buncel, E. Magn. Reson. Chem. 1992, 30,
1128. (c) Keum, S. R.; Lee, K. B.; Kazmaier, P. M.; Buncel, E.
Tetrahedron Lett. 1994, 35, 1015. (d) Keum, S. R.; Lee, M. J.;
Shin, S. T. Liq. Cryst. 2001, 28(11), 1587. (e) Keum, S. R.;
Shin, J. T.; Lee, S. H.; Shin, S. T. Mol. Cryst. Liq. Cryst. 2004,
411, 11. (f) Keum, S. R.; Ku, B. S.; Kim, S. E.; Choi, Y. K.;
Kim, S. H.; Koh, K. N. Bull. Korean Chem. Soc. 2004, 25(9),
1361.
5.3. X-ray crystallography
Crystal data for C35H40N2O4, MZ552.69, monoclinic, aZ
˚
˚
˚
˚
17.689(7) A, bZ11.770(4) A, cZ16.112(6) A, UZ
3
3150.3(19) A , TZ298(2) K, space group P2(1)/c, ZZ4,
m(Mo Ka)Z0.076 mmK1, 6487 reflections measured, 3926
unique (RintZ0.0423) which were used in all calculations.
The final wR(F2) was 0.1696 (all data). Intensity data for
DC-1 was collected using a Siemens SMART ccd area
detector mounted on a Siemen P4 diffractometer equipped
with graphite-monochromated Mo Ka radiation (lZ
4. (a) Irie, M.; Iwayanagi, T.; Taniguchi, Y. Macromolecules
1985, 18, 2418. (b) Malatesta, V.; Neri, C.; Wis, M. L.;
Montanari, L.; Millini, R. J. Am. Chem. Soc. 1997, 119, 3451.
(c) Hobley, J.; Malatesta, V.; Millini, R.; Mantanari, L.;
Parker, W. O. N., Jr. Phys. Chem. Chem. Phys. 1999, 1, 3259.
(d) Hirano, M.; Osakada, K.; Nohira, H.; Miyashita, A. J. Org.
Chem. 2002, 67, 533.
˚
0.71073 A) radiation source and a CCD detector. A total of
multi frames of two-dimensional diffraction images were
collected. The frames data were processed to give structure
factors using the program SAINT.16 The structure was
solved by direct methods and refined by full matrix least-
squares on F2 for all data using SHELXTL software.17
Hydrogen atom position were initially determined by geometry
and refined by a dreiding model. Non-hydrogen atoms were
refined using anisotropic displacement parameters.
5. Anon Res. Discl. 1982, 215, 72. CA, 97(18), 153836e.
6. Baralle, R. M.; Compere, M. A.; Praff, M. E.; Jordi F. X.;
Goumount, C. G. Fr. Demande, FR 2491638 Al, 1982; CA,
95(4), 33421c.
7. Ricoh Co., Ltd Jpn. Tokyo Koho, JP 55035277, 1980; CA,
95(4), 33421c.
8. Samat, A.; Guglielmetti, R.; Metzger, J. Fr. Demande, FR
2271938, 1975; CA, 85(16), 114776t.
Crystallographic data (excluding structure factors) for the
structures in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary
publication numbers CCDC 216486. Copies of the data can
be obtained, free of charge, on application to CCDC, 12
Union Road, Cambridge CB2 1EZ, UK [Fax: C44-1223-
336033 or e-mail: deposit@ccdc.cam.ac.uk].
9. Yoon, I. K. Master Thesis, Department of Chemistry, Graduate
School of Korea University, 1991.
10. Koelsch, C. F.; Workman, W. R. J. Am. Chem. Soc. 1952, 74,
6288.
11. Schiele, C.; Arnold, G. Tetrahedron Lett. 1968, 245.
12. Bertelson, R. C. Chapter 3 in Ref. 1(a).
13. Hinnen, A.; Audic, C.; Gautron, R. Bull. Soc. Chim. Fr. 1968,
2066.
Acknowledgements
14. Calculated using a modified Karplus equation. See: Garbisch,
E. W., Jr. J. Am. Chem. Soc. 1964, 86, 5561.
This work has been supported partly by the Brain Korea 21
project and in part by grant No. (R01-2003-000-10248-0)
from the Basic Research Program of the Korea Science and
Engineering Foundation (S.R.K.).
15. Keum, S. R.; Kazmaier, P. M.; Cheon, K. S.; R. Manderville,
R. A.; Buncel, E. Bull. Korean Chem. Soc. 1996, 17, 391.
16. SMART & SANIT software reference manuals, version 4.0,
Siemens Energy and Automation, Inc., Analytical Instrumen-
tation, Madison, Wisconsin, USA, 1996.
17. SHELXTL reference manual, version 5.03, Siemens Energy &
Automation, Inc., Analytical Instrumentation, Madison, Wis-
consin, USA, 1996.
References and notes
1. (a) Phorochromism: Molecules and Systems; Du¨rr, H., Bouas-