Photochromism of dihydroindolizines. Part III [1]. Synthesis and photochromic behavior of novel photochromic dihydroindolizines incorporating a cholesteryl moiety

Article abstract of DOI:10.1007/s00706-004-0175-9

Ten novel photochromic dihydroindolizines (DHIs) based on 1,5-electrocyclization and bearing a cholesteryl moiety at 7′-position of the DHI skeleton were synthesized. 1D, 2D, NOESY ¹H NMR spectra, mass spectrometry, and elemental analysis were used for their characterization. Irradiation of the DHIs in CH₂Cl₂ solution with polychromatic light leads to the formation of red to red-violet colored betaines. Most colored betaine forms are notable in CH₂Cl₂ solution at room temperature because of their slow 1,5-electrocyclization except in one case, where the colored betaine could be observed only after cooling with liquid nitrogen due to the fast electrocyclization back reaction. The kinetics of the reverse 1,5-electrocyclization of the colored betaines into the corresponding DHIs were detected using both UV/VIS and flash photolysis measurements. The presence of three isosbestic points in the fading spectrum of the betaines proved that the thermal back reaction to the DHIs follows a first order mechanism. Tuning of the photophysical properties of DHIs and their colored betaines was achieved by change of substituents in the ester and fluorene regions. A notable increase of the t₃₀-value of some betaines by a factor ranging between 1.01 and 1.57 compared with the betaine form of dicyanopridazine DHI standard (t₃₀ = 243 min) was observed. The high photo-fatigue resistance of these betaines will help to find their applications. Springer-Verlag 2004.

Full text of DOI:10.1007/s00706-004-0175-9

Monatshefte f€ur Chemie 135, 1173–1188 (2004)
DOI 10.1007/s00706-004-0175-9
Photochromism of Dihydroindolizines.
Part III [1]. Synthesis and Photochromic
Behavior of Novel Photochromic
Dihydroindolizines Incorporating
a Cholesteryl Moiety
ꢀ;y
Saleh A. Ahmed
Chemistry Department, Faculty of Science, Assiut University, 71516 Assiut, Egypt
Received December 17, 2003; accepted (revised) January 15, 2004
Published online August 23, 2004 # Springer-Verlag 2004
Summary. Ten novel photochromic dihydroindolizines (DHIs) based on 1,5-electrocyclization and
bearing a cholesteryl moiety at 70-position of the DHI skeleton were synthesized. 1D, 2D, NOESY 1H
NMR spectra, mass spectrometry, and elemental analysis were used for their characterization. Irradia-
tion of the DHIs in CH2Cl2 solution with polychromatic light leads to the formation of red to red-violet
colored betaines. Most colored betaine forms are notable in CH2Cl2 solution at room temperature
because of their slow 1,5-electrocyclization except in one case, where the colored betaine could be
observed only after cooling with liquid nitrogen due to the fast electrocyclization back reaction. The
kinetics of the reverse 1,5-electrocyclization of the colored betaines into the corresponding DHIs were
detected using both UV=VIS and flash photolysis measurements.
The presence of three isosbestic points in the fading spectrum of the betaines proved that the
thermal back reaction to the DHIs follows a first order mechanism. Tuning of the photophysical
properties of DHIs and their colored betaines was achieved by change of substituents in the ester
and fluorene regions. A notable increase of the t30-value of some betaines by a factor ranging between
1.01 and 1.57 compared with the betaine form of dicyanopridazine DHI standard (t30 ¼ 243 min) was
observed. The high photo-fatigue resistance of these betaines will help to find their applications.
Keywords. Photochromism; Dihydroindolizines; 1,5-Electrocylization; Half-lives; Photo-fatigue.
Introduction
Photochromism, as a branch of photochemistry, is currently attracting a great deal
of interest. Its reversibility is the inherent characteristic of this phenomenon, which
ꢀ
E-mail: saleh_63@hotmail.com
y
Present address: Special Division for Human Life Technology, National Research Institute of
Advanced Industrial Science and Technology, 1-8-31, Midorigaoka, Ikeda, Osaka 563-8577, Japan

1174
S. A. Ahmed
distinguishes it from other photoinduced reactions [2–4]. The applications config-
ure the fascinating ability of modulating given physical properties through an
external stimulation, namely, upon light irradiation. Variable transmission materi-
als, optical switches and memories [5–10], and biological sensors are among the
most interesting. In the field of these variable-transmission optical materials, the
main characteristics strived for, are i) the ability to be activated by sunlight (helio-
chromism) and bleached through a thermal process, ii) extended absorption in the
visible region to provide a neutral color upon irradiation, iii) a suitable fading rate
(is related to the stability of the colored state), iv) high efficiency of coloration
upon irradiation (colorability), and v) a high resistance to photodegradation (fati-
gue resistance) [11–13].
Since the discovery of the photochromic dihydroindolizines (DHIs) and tetra-
hydroindolizines (THIs) by D€urr in 1979 [14, 15], they became a very interesting
class of photochromic molecules and have been extensively studied and received
particular attention owing to their remarkable photofatigue-resistance and wide
broad photochromic properties [16–31]. Due to these important and interesting
properties, they have already found their applications in optical technology [32],
ophthalmic lenses [33], data storage [34], photoswitches [1, 16, 35, 36], dental
filling materials [21, 26, 37], IR-sensitive photoswitchable materials [1, 16, 21],
and DNA markers [38].
These interesting molecules undergo a photo-induced change of color in solu-
tions and in polymer matrices when exposed to UV radiation or direct sunlight.
They return to the initial state when the illumination ceases, normally via a thermal
pathway. The photochromic behavior of DHIs is based on a reversible pyrroline
ring opening, induced by light, that converts a colorless form (usually named the
‘closed form’) to the colored form (betaine form). The thermal back reaction – the
1,5-electrocyclization – from the ring-open betaine to DHI shows rates extending
from milliseconds to several weeks [19, 21, 31, 39–42] depending on the substi-
tuents and structure of the involved molecule. This interesting wide range in the
lifetime of the colored form leads these molecules to find many applications
[19, 21, 32–38].
In continuation of our pervious work dealing with the synthesis and photochro-
mic properties of dihydroindolizines (DHIs) [1, 16–21, 34, 39] we wish to report
the synthesis, and photochromism of novel photochromic dihydroindolizines bear-
ing a cholesterol moiety in the region B (Scheme 1) as the first entry to supramo-
lecular photoresponsive self-assembling organogelators based on the photochromic
DHI skeleton.
Scheme 1

Photochromism of Dihydroindolizines
1175
Results and Discussion
Preparation of Cholesterol Pyridazine Precursor 7
Cholesterol pyridazine precursor 7 was prepared by five steps as shown in Scheme 2.
Treatment of citraconic anhydride (1) with hydrazine hydrate in refluxing ethanol
led to ring expansion and 3,6-dihydroxy-4-methylpyridazine (2) was formed in
93% yield [44–46]. Chlorination of diol 2 with phosphoryl chloride afforded
3,6-dichloro-4-methyl-pyridazine (3) in good yield (73%). Oxidation of the methyl
group in 3 by KMnO4=H2SO4 led to the formation of 3,6-dihydroxypyridazine-4-
carboxylic acid (4) in moderate yield (44%). Dechlorination of 4 with hydrogen
and 10% Pd=C in an ammonia=methanol mixture gave the corresponding pyrid-
azine-4-carboxylic acid 5 in low yield (10%) [47–49].
Esterification of 5 with cholesterol (6) in the presence of DMAP and DCC at
low temperature under anhydrous conditions gave the corresponding pyridazine-4-
carboxylic acid cholesteryl ester 7 in 18% yield. The constitutions of 1–5 as well as
the cholesterol pyridazine precursor 7 were established by elemental analysis and
spectral data.
Preparation of Dialkyl 70-Chloresterylcarbonyl-10H-spiro[substituted
fluorene-9,10-pyrrolo[1,2-b]pyridazine]-20,30-dicarboxylates,
DHIs (11a–11j)
Electrophilic addition of spirocyclopropenes 8a–8j to cholesteric pyridazine pre-
cursor 7 using the cyclopropene route (Scheme 2) in dry ethereal solution at room
Scheme 2

1176
S. A. Ahmed
temperature under dry nitrogen in the absence of light for 24–40 h (TLC-controlled
using CH2Cl2 as eluent) led to the formation of the photochromic dialkyl 70-chlo-
resterylcarbonyl-10H-spiro[substituted fluorene-9,10-pyrrolo[1,2-b]pyridazine]-20,30-
dicarboxylates 11a–11j (Scheme 3).
The formation of DHIs 11a–11j proceeded by a cyclopropyl-allylanion rear-
rangement [16, 49] of 9a–9j. The dark colored betaines 10a–10j underwent
Scheme 3

Photochromism of Dihydroindolizines
1177
Fig. 1. Representation of the optimized (MM2) structure of DHI 11a
1,5-electrocyclization to give the corresponding DHIs 11a–11j. Irradiation of DHIs
11a–11j with UV-light gave the dark colored betaine forms 10a–10j with high
reaction quantum yield (ꢀ ffi 0.4) [21, 51]. The high colorability as well as high
photostability of betaines 10a–10j renders wide applications to this class of photo-
chromic compounds.
Characterizations of the new photochromic DHIs 11a–11j were done using
spectral as well as analytical tools. In addition, the chemical structure of 11h
1
was assigned by 2D-NMR spectroscopy. The H-1H-correlation of 11h was used
1
to assign the protons of the pyridazine ring (region C) and in the H,1H-COSY
spectrum both 20- and 30- methyl ester groups showed no coupling with other
protons and appeared as two singlets at ꢁ ¼ 3.39 and 4.02 ppm. The 80-CH signal
was shifted to high field and appeared as a doublet of triplets at ꢁ ¼ 5.00 ppm
3
showing two coupling systems. The first is due to J-coupling with the 80a-CH,
4
which appears as doublet at ꢁ ¼ 5.70 ppm, the second is due to J-coupling with
the 60-CH, which appears as a double doublet at ꢁ ¼ 6.92 ppm. Further assign-
ments of 60-CH, 80-CH, and 80a-CH were done by the aid of NOESY spectrum
of 11h. Here we observed that 80a-CH at ꢁ ¼ 5.27 ppm is close in space to both
80-CH at ꢁ ¼ 5.04 ppm and 1-CH of the fluorene moiety at ꢁ ¼ 7.67 ppm. This
system proves that 80a-CH is in 80a-position and not at 60-position. Additionally,
the connectivity between 80-CH and 8-CH of the fluorene part at ꢁ ¼ 7.55 ppm
was observed. This vicinity of 80-CH with 8-CH proved that the pyridazine
moiety is perpendicular to the fluorene skeleton as proved by a molecular mod-
eling calculation [1, 19–21] of 11h. It showed that the distance between 0both
0
0
˚
8 a-CH, 8 -CH and 1-CH, 8-CH of the fluorene moiety is <3 A. Probably 6 -CH
and 30-CH3 of the ester group at ꢁ ¼ 4.01 ppm are also near to each other
(Fig. 1).
Photophysical Properties of New Photochromic DHIs 11a–11j
and their Corresponding Betaines 10a–10j in Solution
The UV-Vis spectra of 11a–11j and their corresponding betaines 10a–10j were
measured in dichloromethane. All DHIs showed a yellow color in solution as well
as in the solid state. The intensities (log ") of these bands were found to be between

1178
S. A. Ahmed
Fig. 2. Kinetic UV-Vis spectrum of the thermal fading of betaine 10a to DHI 11a (cycle time ¼ 20 s,
run time ¼ 600 s) in CH2Cl2 (c ¼ 2 Â 10À5 mol dmÀ3) at 296 K
Fig. 3. Kinetic UV-Vis spectrum of the thermal fading of betaine 10h to DHI 11h (cycle
time ¼ 1.5 s, run time ¼ 80 s) in CH2Cl2 (c ¼ 2 Â 10À5 mol dmÀ3) at 296 K
3.90 and 4.22. The absorption maxima of DHIs 11a–11j lie between 379–391 nm
(Figs. 2, 3). As established previously, these absorption bands can be assigned to
ꢀ
phore [1–3] of the DHIs system.
the locally excited ꢂ–ꢂ -transition located in the butadienyl-vinyl-amine chromo-
Irradiation of DHIs 11a–11j in CH2Cl2 solution with polychromatic light led to
the formation of red to red-violet colored betaines 10a–10j. The colored betaine
forms 10a–h and 10j (Figs. 2, 3) are obvious in CH2Cl2 solution at room tempera-
ture because of their slower 1,5-electrocyclization. Only for betaine 10i the color is
observed after cooling with liquid nitrogen due to the fast electrocyclic back reac-
tion. The betaines 10a–10j revealed absorption spectra in the visible region
between 507–537 nm (Table 1). Millisecond flash photolysis (Fig. 4) was used
to determine in addition to the ꢃmax (525 Æ 12 nm) of betaine 10i, the kinetics of
the thermal back reaction (10i ! 11i).

Photochromism of Dihydroindolizines
1179
Table 1. UV-Vis absorption data of DHIs 11a–11j and their corresponding betaines 10a–10j in
CH2Cl2 solution (23ꢁC, c ¼ 2 Â 10À4 mol dmÀ3
)
10=11
ꢃmax DHI=nm
log " DHI
ꢃmax betaine=nm
color of betaine
a
b
c
d
e
f
392
389
388
386
389
388
385
389
379
388
4.00
3.98
3.97
3.94
3.92
3.90
3.90
3.93
4.22
3.94
345, 512
346, 511
346, 509
348, 509
346, 507
344, 512
531
red
red
red
red
red
red
g
h
i
red-violet
red-violet
red
537
525 Æ 12
344, 510
j
red
Fig. 4. Millisecond flash photolysis spectrum of 10i for determination of the absorption maxium of
the betaine form at different wavelengths (325–850 nm every 25 nm) in CH2Cl2 at 296 K
Effect of Substituents on the ꢃmax of Photochromic DHIs 11a–11j
Effect of Substituents in the Fluorene Part (Region A)
A hypsochromic shift in the absorption maxima from 3 to 7 nm was observed in
2,7-disubstituted fluorene DHIs 11h and 11g compared with the unsubstituted DHI
11a. A more significant hypsochromic shift by 11 nm in 2,7-dinitrofluorene DHI
11i compared with the unsubstituted DHI 11a was recorded (Table 1). These results
were in good agreement with former studies on pyridazine DHIs [18, 21, 42].
Effect of Substituents in the Double Bond Part (Region B)
Generally, changing the alkyl ester group has resulted in very small influence or no
effect on the ꢃmax of DHIs [3, 19]. A small hypsochromic shift of 6 nm was
observed by changing the alkyl ester from a methyl to a tert-butyl group.

1180
S. A. Ahmed
Effect of Substituents on ꢃmax and Colors of Betaine 10a–10j
Substitution in the Fluorene Part (Region A)
Interestingly, the absorption maxima of 2,7-dichloro- and 2,7-dibromosubstituted
fluorene betaines 10g and 10h were shifted bathochromically by 19–25 nm com-
pared with betaine 10a, so that the color changes from red to red-violet. In addi-
tion, a pronounced bathochromic shift was observed in the absorption maxima of
the betaine form of 2,7-dinitrosubstituted fluorene DHI 10i by 13 nm compared
with betaine 10a. This large shift due to the delocalization of betaine charges (CT)
was affected by the electron attracting nitro group [1, 3, 14, 15, 19, 39]. A notable
bathochromic shift by 6 nm from 2,7-dichlorosubstituted betaine 10g to 2,7-dibro-
mosubstituted betaine 10h was also recorded.
Substitution in the Double Bond Part (Region B)
The change of the alkyl ester group in a fixed betaine system generally has no
pronounced influence on the ꢃmax of the betaine form as well as on the color [1, 3,
19, 21]. In betaines 10a–10f, a small bathochromic shift was observed upon chang-
ing the alkyl ester group from methyl to cyclohexyl by 5 nm while the norbornyl
betaine 10f showed no absorption change compared with betaine 10a.
Kinetics of the 1,5-Electrocyclization of Betaines
10a–10j to DHIs 11a–11j
Irradiation of DHIs 11a–11h and 11j with long-wavelength UV or visible light
(ꢃmax ¼ 400–450 nm) at room temperature afforded the colored betaines 10a–10h
and 10i. The kinetics of the reverse 1,5-electrocyclization of the colored betaines
10a–10j into the corresponding DHIs 11a–11j were detected using both UV-Vis
(in the case of 10a–10h and 10j) and flash photolysis measurements (in the case
10i). The presence of three isosbestic points in the fading spectrum of betaines 10
(e.g. Fig. 2) proved that the thermal back reaction to DHIs 11 follows a first order
Table 2. Kinetic data of the thermal 1,5-electrocyclization of betaines 10a–10j to their corresponding
DHIs 11a–11j in CH2Cl2 solution (23ꢁC, c ¼ 2 Â 10À4 mol dmÀ3
)
10=11
k (1=s) Â 10À3
t1=2=s
a
b
c
d
e
f
9.9
13.3
16.9
21.7
11.0
12.8
49.5
99.0
3872
10.0
70
52
41
32
63
54
14
7
g
h
i
0.179
68
j

Photochromism of Dihydroindolizines
1181
mechanism [1, 3, 4, 21]. The decrease in absorption of the colored betaine with
time was measured at six different wavelengths at the same time. The cyclization
rate constant (k) and the half-lives (t1=2) are listed in Table 2.
Effect of Substituents on the Half-lives of the Colored Betaines 10a–10j
Effect of Substitution in the Fluorene Region (Region A)
It should be noted that substitution in the fluorene part leads to a decrease in the
half-lives of the corresponding betaines by a factor of 106 [1, 21]. To obtain a
stable betaine, the negative charge on C-10-atom should be delocalized by
mesomerism [3]. In our case (Table 2) the half-life of the 2,7-disubstituted fluorene
betaines 10g–10i showed a decrease in the half-lives by a factor of 5–10 in the case
of 2,7-dihalosubstituted fluorene 10g and 10h and by a factor of 391 in case of the
dinitrosubstituted fluorene 10i [27, 28] compared with the unsubstituted betaine
10a. This is mainly attributed to the highly electron attracting nitro groups. The
2,7-dichlorosubstituted betaine 10g showed a higher t1=2 than 2,7-dibromofluorene
betaine 10h by a factor of 2 while betaine 10g showed a higher half-life than the
dinitrosubstituted fluorene 10i by a factor of 78. These interesting results reflect
well the tuning of the photophysical properties by changing the substitutions in the
DHI system, which may lead to tunable systems for applications.
Effect of Substitution in the Double Bond Part (Region B)
Changing of the alkyl group of the ester from methyl to ethyl and isopropyl or tert-
butyl leads to a decrease in the half-life time by a factor of 0.7–2.2. This may be
due to the strong electron donation of the tert-butyl group compared with the
methyl group. The norbornyl ester betaine 10f showed a smaller t1=2 by a factor
of 1.3 than the betaine 10a, in which the bulky norbornyl groups might attribute
with an electron donating effect.
Photo-Fatigue Resistance of Photochromic DHIs 11a–11h
and 11j and their Corresponding Betaines 10a–10h and 10j
In studying the quality of a photochromic system or in other terms, the thermal full
reversibility of a specific photochromic molecule, the problem of carrying out a
large number of colorization–decolorization cycles arises frequently. The gradual
loss of the ability to change color by exposure to visible or ultraviolet light in this
context has been termed fatigue [2]. Gautron [52] has advanced a quantitative
approach to measure the fatigue in photochromic systems.
Irradiation of spirodihydroindolizines 11a–11h and 11j with polychromatic light
(ꢃ ¼ 200–400 nm) produced the colored betaines 10a–10h and 10j. Upon continued
irradiation they decomposed after some time. Betaine 10i was not measured under
these conditions due to its fast thermal bleaching to DHI 11i. However, if oxygen is
excluded, these systems are notably more stable. It is possible that in the presence of
oxygen, the betaines 10a–10h and 10j act as sensitizers towards singlet oxygen [3].
From the photodegradation data represented in Table 3 and Fig. 5, it is clear that
the betaine form of the dimethyl ester pyridazine DHI 11a (t30 ¼ 276 min) is more

1182
S. A. Ahmed
Table 3. Photodegradation of betaines 10a–10h and 10j in dichloromethane (c ¼ 2 Â 104 mol dmÀ3
)
at 23ꢁC
t1=2=s
t30-betaine-DHI=min
F
a
70
52
41
32
63
54
14
7
276
264
259
246
213
195
462
381
278
243
1.14
1.09
1.07
1.01
0.87
0.80
1.93
1.57
1.14
1.0
b
c
d
e
f
g
h
j
68
56.2
Standard
Fig. 5. Time-relative absorbance relationship for the photodegradation experiment for determination
of the t30-value of betaines 10a–10h and 10j
stable than the standard dicyanopyridazine DHI ((t30 ¼ 243 min) by a factor of 1.14.
This is due to the high electron attraction of cyano groups in the reference com-
pound. The betaine forms with tert-butyl, cyclohexyl, and norbornyl ester groups
10d–10f showed a lower stability than dimethyl substituted betaine 10a probably
due to the steric hindrance of the bulky ester groups [28]. Interestingly, the 2,7-
dichloro betaine 10g showed a highly pronounced increase in photostability
(t30 ¼ 462 min) by a factor of 1.7 compared with unsubstituted betaine 10a and by
a factor of 1.9 compared with the standard betaine. On the other hand, the 2,7-
dibromo betaine 10h showed a lower t30-value (t30 ¼ 381 min) (Fig. 5) than the
2,7-dichloro betaine 10g, which may be due to the larger size of the bromine atom.
Betaine 10j showed no photostability difference from the unsubstituted betaine 10a.
In conclusion, the main motivation behind this work was to synthesize,
develop, and tune the photophysical properties of novel photochromic DHIs based
on 1,5-electrocylization bearing a cholesteryl moiety at the 70-position of the DHI
skeleton. The red to red-violet colored betaines produced by UV irradiation

Photochromism of Dihydroindolizines
1183
returned back through 1,5-electrocyclization to the corresponding DHIs 11a–11j
with different rate constants depending on the subsituents in both the fluorene and
ester regions. Substituent effects on the absorption maxima of DHIs 11 and
betaines 10 as well as on the half-lives of betaines 10 were investigated. Tuning
of the photophysical properties of DHIs 11a–11j and their colored betaines
10a–10j by change of substituents in the ester and fluorene regions was clearly
achieved. A notable increase of the t30-values of betaines 10a–10h and 10j by a
factor ranging between 1.01 and 1.57 compared with the betaine form of dicya-
nopridazine DHI used in former studies as standard (t30 ¼ 243 min) was observed.
The high photo-fatigue resistance of these betaines will help to find applications.
This study can be considered as the key step for the preparation of a photorespon-
sive self-assembling network based on the DHI skeleton, which will be discussed
in details in the forthcoming paper.
Experimental
All reactions were carried out under N2 with the use of standard Schlenk techniques, but no special
precautions were taken to exclude O2 during work up. Solvents were pre-dried and distilled from
˚
appropriate drying agents and stored over molecular sieve (5A) in brown bottles under a N2 atmo-
sphere. All chemicals were obtained from commercial sources, unless otherwise stated, and used as
received. Spirocyclopropene derivatives 8a–8i were obtained via photolysis of the corresponding
pyrazoles prepared according to reported procedures [13, 14]. Photolysis was carried out in the
photochemical reactor of Schenck [54] made from Pyrex (ꢃ>290 nm). The source of irradiation
was a high-pressure mercury lamp Philips HPK 125 W. Solutions to be photolyzed were flushed with
dry N2 for 30 min before switching on the UV lamp.
The progress of the reaction and the purity of the products isolated were monitored using TLC.
Separation and purification of all synthesized photochromic materials were carried out using column
chromatography (1m  2 cm) on silica gel and CH2Cl2 as eluent. Melting points were measured on a
Gallenkamp or a B€uchi (Smp-20) melting point apparatus. 1H NMR spectra were measured in CDCl3
with a JOEL EX270 400 MHz FT-NMR spectrometer, with chemical shift (ꢁ) relative to TMS in ppm. IR
spectra were measured on a JASCO FT=IR-230 infrared spectrometer FTS 3000. Fast atom bombard-
ment (FAB) mass spectra were recorded with a Finnigan MAT SSQ710 mass spectrometer. UV-spectra
were recorded on a FT-UV-Vis HP 6543 computer-spectrometer. Millisecond flash photolysis was
carried out with a 12V (50 W) halogen lamp, Photoflash (METZ 32 Z-1). Molecular modeling calcula-
tions (MM2) were carried out using version 5 of Hyperchem program.
For photodegradation experiments (photo-fatigue), the initial absorbance of pyridazine DHIs 11a–
)
11h and 11j was measured using a 3 cm3 quartz cell in degassed CH2Cl2 (c ¼ 2 Â 10À4 mol dmÀ3
solution. Before the beginning of the degradation experiment, the degassed DHIs 11a–11h and 11j
solutions were irradiated for two minutes to achieve equilibrium between DHIs and betaines. The
extinction was set as 100% at the beginning. The analysis was carried out from absolute destruction
time (every 30 min). At the fixed point recommended, the absorbance of the betaine form decreased
through irradiation to 30% of the beginning extinction (Fig. 5). This time is called in our work the t30-
value and the stability factor (F) can be calculated from the ratio of each t30-value and the t30-value of
dicyanopyridazine betaine, which has been used as standard [21, 53].
Pyridazine-4-carboxylic acid cholesteryl ester (7, C32H48N2O2)
To a solution of 0.039 g of 5 (0.32 mmol) and 0.124 g of cholesterol (6) (0.32 mmol) in 20 cm3 of dry
CH2Cl2 at 5ꢁC under N2 0.11g of DCC (0.56 mmol) and 0.03g of DMAP (0.22mol) were added. The

1184
S. A. Ahmed
reaction mixture was stirred at this temperature and monitored by TLC until completion of the reaction
(9h). The eliminated dicyclohexyl urea was filtered off, the filtrate was evaporated under reduced
pressure, and the ester was purified by column chromatography on silica gel and CH2Cl2:CH3OH (9:1)
1
as eluent yielding 28mg (18%) as white solid. H NMR (CDCl3, 400 MHz): ꢁ ¼ 9.86 (s, 3-CH-
pyridazine), 9.47 (d, J ¼ 8.4 Hz, 6-CH pyridazine), 8.15 (d, J ¼ 8.4 Hz, 5-CH-pyridazine), 5.47 (d,
J ¼ 7.90Hz, CH¼cholesteryl), 4.56 (m, CH-cholesteryl), 0.68–2.51 (m, 43H, cholesteryl) ppm; IR
(KBr): ꢄꢀ¼ 3132 (C–H, pyridazine), 2836–2989 (C–H, cholesteryl), 1727 (40-C¼O), 1608 (C¼N),
1532 (C¼C), 1262, 1210, 1027, 946, 850, 733 cmÀ1; MS: m=z ¼ 493 [Mþ].
General Procedure for the Preparation of Dialkyl 70-Chloresterylcarbonyl-10H-
spiro[substituted fluorene-9,10-pyrrolo[1,2-b]pyridazine]-20,30-
dicarboxylate (11a–j)
A mixture of 1 mmol of spirocyclopropenes 8a–8j in dry ether (50 cm3) and 0.387g of cholesteryl
pyridazine 7 (1mmol) was stirred at room temperature under dry N2 with exclusion of light for
24–40 h (TLC-controlled). Ether was evaporated under reduced pressure and the pure products were
separated by at least twice column chromatography on silica gel using CH2Cl2 as eluent. The pure
product was recrystallized from a suitable solvent to give white to yellow needles.
Dimethyl 70-chloresterylcarbonyl-10H-spiro[fluorene-9,10-pyrrolo[1,2-b]pyridazine]-20,30-
dicarboxylate (11a, C51H62N2O6)
Yield 68% as white needles; mp 152ꢁC (ether:pentane ¼ 8:2); 1H NMR (CDCl3, 400 MHz): ꢁ ¼ 7.73–
7.74 (dd, J ¼ 7.50 Hz, CH-arom), 7.53–7.55 (d, J ¼ 7.50Hz, CH-arom), 7.26–7.33 (m, CH-arom), 7.18–
7.20 (dd, J ¼ 7.50Hz, CH-arom), 5.80–5.83 (t, J ¼ 2.2 Hz, 60-CH), 5.34–5.36 (d, J ¼ 7.50Hz, CH¼
cholesteryl), 5.27–5.28 (t, J ¼ 2.2 Hz, 80a-CH), 4.92–4.94 (dt, J ¼ 2.2 Hz, 80-CH), 4.42–4.45 (m, CH-
cholesteryl), 4.03 (s, 20-CH3), 3.30 (s, 30-CH3), 0.68–2.51 (m, CH2-cholesteryl) ppm; IR (KBr): ꢄꢀ¼ 3032
(C–H, arom), 2855–2979 (C–H, aliph), 1752 (30-C¼O), 1728 (70-C¼O), 1706 (20-C¼O), 1604 (C¼N),
1543 (C¼C), 1445, 1322, 1254, 1232, 1123, 1039, 930, 849, 741 cmÀ1; MS: m=z ¼ 799 [Mþ].
Diethyl 70-chloresterylcarbonyl-10H-spiro[fluorene-9,10-pyrrolo[1,2-b]pyridazine]-
20,30-dicarboxylate (11b, C53H66N2O6)
Yield 55% as white needles; mp 134ꢁC (ether:pentane ¼ 7:3); 1H NMR (CDCl3, 400 MHz): ꢁ ¼ 7.76–
7.79 (dd, J ¼ 7.50Hz, CH-arom), 7.52–7.55 (d, J ¼ 7.50 Hz, CH-arom), 7.25–7.34 (m, CH-arom),
7.15–7.19 (dd, J ¼ 7.50Hz, CH-arom) 5.82–5.85 (t, J ¼ 2.2 Hz, 60-CH), 5.37–5.39 (d, J ¼ 7.50 Hz,
CH¼cholesteryl), 5.29–5.31 (t, J ¼ 2.2 Hz, 80a-CH), 4.90–4.94 (dt, J ¼ 2.2 Hz, 80-CH), 4.49–4.53 (m,
20-CH2), 4.40–4.43 (m, CH-cholesteryl), 3.70–3.73 (m, 30-CH2), 0.66–2.46 (m, 20,30CH3 þ CH2-cho-
lesteryl) ppm; IR (KBr): ꢄꢀ¼ 3059 (C–H, arom), 2846–2992 (C–H, aliph), 1749 (30-C¼O), 1724 (70-
C¼O), 1704 (20-C¼O), 1611 (C¼N), 1539 (C¼C), 1443, 1321, 1262, 1227, 1127, 1037, 935, 841,
747 cmÀ1; MS: m=z ¼ 827 [Mþ].
Diisopropyl 70-chloresterylcarbonyl-10H-spiro[fluorene-9,10-pyrrolo[1,2-b]pyridazine]-
20,30-dicarboxylate (11c, C55H70N2O6)
Yield 53% as white needles; mp 127ꢁC (ether:pentane ¼ 6:4); 1H NMR (CDCl3, 400 MHz): ꢁ ¼ 7.70–
7.73 (dd, J ¼ 7.50Hz, CH-arom), 7.55–7.57 (d, J ¼ 7.50 Hz, CH-arom), 7.22–7.33 (m, CH-arom),
7.24–7.27 (dd, J ¼ 7.50Hz, CH-arom), 5.80–5.83 (t, J ¼ 2.2 Hz, 60-CH), 5.37–5.39 (d, J ¼ 7.50 Hz,
CH¼cholesteryl), 5.29–5.35 (m, 30-CH þ 80a-CH), 4.92–4.95 (dt, J ¼ 2.2 Hz, 80-CH), 4.48–4.54 (m,
CH-cholesteryl), 4.40–4.43 (m, 20-CH), 0.52–2.53 (m, 20,30(CH3)2 þ CH2-cholesteryl) ppm; IR

Photochromism of Dihydroindolizines
1185
(KBr): ꢄꢀ¼ 3057 (C–H, arom), 2835–2997 (C–H, aliph), 1746 (30-C¼O), 1722 (70-C¼O), 1703 (20-
C¼O), 1609 (C¼N), 1540 (C¼C), 1447, 1320, 1264, 1229, 1125, 1031, 937, 839, 749 cmÀ1; MS:
m=z ¼ 855 [Mþ].
Di-tert-butyl 70-chloresterylcarbonyl-10H-spiro[fluorene-9,10-pyrrolo[1,2-b]pyridazine]-
20,30-dicarboxylate (11d, C57H74N2O6)
Yield 47% as white needles; mp 107ꢁC (ether:pentane ¼ 5:5); 1H NMR (CDCl3, 400 MHz): ꢁ ¼ 7.78–
7.83 (dd, J ¼ 7.50Hz, CH-arom), 7.52–7.55 (d, J ¼ 7.50 Hz, CH-arom), 7.25–7.32 (m, CH-arom),
7.23–7.28 (dd, J ¼ 7.50Hz, CH-arom), 5.82–5.85 (t, J ¼ 2.2 Hz, 60-CH), 5.35–5.37 (d, J ¼ 7.50 Hz,
CH¼cholesteryl), 5.30–5.33 (m, 80a-CH), 4.94–4.96 (dt, J ¼ 2.2 Hz, 80-CH), 4.50–4.54 (m, CH-
cholesteryl), 0.56–2.69 (m, 20,30(CH3)3 þ CH2-cholesteryl) ppm; IR (KBr): ꢄꢀ¼ 3054 (C–H, arom),
2846–2990 (C–H, aliph), 1748 (30-C¼O), 1721 (70-C¼O), 1707 (20-C¼O), 1612 (C¼N), 1547 (C¼C),
1442, 1318, 1264, 1227, 1129, 1037, 932, 842, 745 cmÀ1; MS: m=z ¼ 883 [Mþ].
Dicyclohexyl 70-chloresterylcarbonyl-10H-spiro[fluorene-9,10-pyrrolo[1,2-b]pyridazine]-
20,30-dicarboxylate (11e, C61H78N2O6)
Yield 49% as white needles; mp 118ꢁC (ether:pentane ¼ 6:4); 1H NMR (CDCl3, 400 MHz): ꢁ ¼ 7.76–
7.80 (dd, J ¼ 7.50Hz, CH-arom), 7.54–7.59 (d, J ¼ 7.50 Hz, CH-arom), 7.24–7.36 (m, CH-arom),
7.22–7.29 (dd, J ¼ 7.50Hz, CH-arom) 5.80–5.84 (t, J ¼ 2.2 Hz, 60-CH), 5.34–5.36 (d, J ¼ 7.50 Hz,
CH¼cholesteryl), 5.35–5.39 (m, 80a-CH), 5.11–5.15 (m, 30-OCH), 4.92–4.95 (dt, J ¼ 2.2 Hz, 80-CH),
4.49–4.55 (m, CH-cholesteryl), 4.40–5.48 (m, 20-OCH), 0.58–2.78 (m, 20,30-cyclohexyl þ CH2-cho-
lesteryl) ppm; IR (KBr): ꢄꢀ¼ 3052 (C–H, arom), 2844–2998 (C–H, aliph), 1747 (30-C¼O), 1720 (70-
C¼O), 1701 (20-C¼O), 1613 (C¼N), 1542 (C¼C), 1445, 1315, 1264, 1229, 1128, 1038, 932, 843,
746 cmÀ1; MS: m=z ¼ 935 [Mþ].
Di-exo-norbonyl 70-chloresterylcarbonyl-10H-spiro[fluorene-9,10-pyrrolo[1,2-b]pyridazine]-
20,30-dicarboxylate (11f, C63H78N2O6)
Yield 42% as white needles; mp 127ꢁC (ether:pentane ¼ 6:4); 1H NMR (CDCl3, 400 MHz): ꢁ ¼ 7.77–
7.85 (dd, J ¼ 7.50Hz, 2H, CH-arom), 7.57–7.61 (d, J ¼ 7.50 Hz, CH-arom), 7.24–7.35 (m, CH-arom),
7.27–7.32 (dd, J ¼ 7.50Hz, CH-arom), 5.81–5.84 (t, J ¼ 2.2 Hz, 60-CH), 5.32–5.36 (d, J ¼ 7.50 Hz,
CH¼cholesteryl), 5.29–5.33 (m, 80a-CH), 5.00–5.06 (m, 30-OCH), 4.92–4.95 (dt, J ¼ 2.2 Hz, 80-CH),
4.56–4.59 (m, CH-cholesteryl), 4.34–5.39 (m, 20-OCH), 0.62–2.72 (m, 20,30-exo-norbonylþ CH2-
cholesteryl) ppm; IR (KBr): ꢄꢀ¼ 3050 (C–H, arom), 2849–2995 (C–H, aliph), 1746 (30-C¼O), 1721
(70-C¼O), 1703 (20-C¼O), 1617 (C¼N), 1541 (C¼C), 1447, 1312, 1267, 1230, 1129, 1037, 934, 847,
744 cmÀ1; MS: m=z ¼ 959 [Mþ].
Dimethyl 70-chloresterylcarbonyl-10H-spiro[2,7-dichlorofluorene-9,10-
pyrrolo[1,2-b]pyridazine]-20,30-dicarboxylate (11g, C51H60Cl2N2O6)
1
Yield 56% as pale yellow needles; mp 189ꢁC (ether:methanol ¼ 9:1); H NMR (CDCl3, 400MHz):
ꢁ ¼ 7.64–7.66 (d, J ¼ 3.56 Hz, CH-arom), 7.61–7.63 (d, J ¼ 4.40Hz, CH-arom), 7.54–7.55 (d,
J ¼ 1.76Hz, CH-arom), 7.43–7.44 (d, J ¼ 1.76, CH-arom), 7.37–7.39 (m, CH-arom), 5.86–5.89 (t,
J ¼ 2.2 Hz, 60-CH), 5.35–5.37 (d, J ¼ 7.50Hz, CH¼cholesteryl), 5.24–5.26 (t, J ¼ 2.2 Hz, 80a-CH),
4.93–4.95 (dt, J ¼ 2.2 Hz, 80-CH), 4.44–4.48 (m, CH-cholesteryl), 4.09 (s, 20-CH3), 3.33 (s, 30-CH3),
0.61–2.70 (m, CH2-cholesteryl) ppm; IR (KBr): ꢄꢀ¼ 3062 (C–H, arom), 2854–2992 (C–H, aliph),
1745 (30-C¼O), 1720 (70-C¼O), 1708 (20-C¼O), 1614 (C¼N), 1540 (C¼C), 1448, 1315, 1269, 1230,
1132, 1034, 938, 849, 743 cmÀ1; MS: m=z ¼ 868 [Mþ].

1186
S. A. Ahmed
Dimethyl 70-chloresterylcarbonyl-10H-spiro[2,7-dibromofluorene-9,10-
pyrrolo[1,2-b]pyridazine]-20,30-dicarboxylate (11h, C51H60Br2N2O6)
Yield 51% as yellow needles; mp 176ꢁC (ether:methanol¼ 8:2); 1H NMR (CDCl3, 400 MHz):
ꢁ ¼ 7.33–7.35 (d, J ¼ 2.2 Hz, CH-arom), 7.20–7.26 (m, CH-arom), 6.98–6.99 (d, J ¼ 0.88 Hz, CH-
arom), 5.87–5.89 (t, J ¼ 2.2 Hz, 60-CH), 5.34–5.36 (d, J ¼ 7.50 Hz, CH¼cholesteryl), 5.26–5.28 (t,
J ¼ 2.2 Hz, 80a-CH), 4.94–4.95 (dt, J ¼ 2.2 Hz, 80-CH), 4.42–4.46 (m, CH-cholesteryl), 4.07 (s, 20-
CH3), 3.31 (s, 30-CH3), 0.64–2.74 (m, CH2-cholesteryl) ppm; IR (KBr): ꢄꢀ¼ 3068 (C–H, arom), 2850–
2998 (C–H, aliph), 1744 (30-C¼O), 1721 (70-C¼O), 1707 (20-C¼O), 1617 (C¼N), 1545 (C¼C), 1443,
1309, 1272, 1229, 1135, 1031, 937, 852, 747 cmÀ1; MS: m=z ¼ 959 [Mþ2].
Dimethyl 70-chloresterylcarbonyl-10H-spiro[2,7-dinitrofluorene-9,10-
pyrrolo[1,2-b]pyridazine]-20,30-dicarboxylate (11i, C51H60N4O10)
Yield 42% as yellow needles; mp 199ꢁC (ether:methanol¼ 5:5); 1H NMR (CDCl3, 400 MHz):
ꢁ ¼ 8.54–8.56 (d, J ¼ 2.2 Hz, CH-arom), 8.33–8.38 (m, CH-arom), 8.29–8.30 (d, J ¼ 2.24 Hz, CH-
arom), 7.91–7.93 (q, J ¼ 3.96 Hz, CH-arom), 6.78–6.79 (d, J ¼ 2.20Hz, 60-CH), 5.59–5.62 (d,
J ¼ 7.50Hz, CH¼cholesteryl), 5.30–5.32 (t, J ¼ 2.2 Hz, 80a-CH), 4.99–5.01 (dt, J ¼ 2.2 Hz, 80-CH),
4.58–4.60 (m, CH-cholesteryl), 4.11 (s, 20-CH3), 3.37 (s, 30-CH3), 0.60–2.71 (m, CH2-cholesteryl)
ppm; IR (KBr): ꢄꢀ¼ 3070 (C–H, arom), 2851–2997 (C–H, aliph), 1744 (30-C¼O), 1720 (70-C¼O),
1709 (20-C¼O), 1617 (C¼N), 1547 (C¼C), 1440, 1303, 1272, 1231, 1137, 1039, 938, 853, 749 cmÀ1
MS: m=z 889 [Mþ].
;
Trimethyl 70-chloresterylcarbonyl-10H-spiro[fluorene-9,10-pyrrolo[1,2-b]pyridazine]-
4,20,30-tricarboxylate (11j, C53H64N2O8)
Yield 44% as white needles; mp 141ꢁC (ether:pentane ¼ 5:5); 1H NMR (CDCl3, 400 MHz): ꢁ ¼ 7.70–
7.72 (d, J ¼ 7.50 Hz, CH-arom), 7.51–7.53 (d, J ¼ 7.50Hz, CH-arom), 7.24–7.30 (m, CH-arom), 7.16–
7.18 (dd, J ¼ 7.50Hz, CH-arom), 5.79–5.81 (t, J ¼ 2.2 Hz, 60-CH), 5.32–5.34 (d, J ¼ 7.50 Hz,
CH¼cholesteryl), 5.23–5.27 (t, J ¼ 2.2 Hz, 80a-CH), 4.90–4.92 (dt, J ¼ 2.2 Hz, 80-CH), 4.44–4.46
(m, CH-cholesteryl), 4.01 (s, 20-CH3), 3.56 (s, 4-CH3), 3.31 (s, 30-CH3), 0.63–2.70 (m, CH2-choles-
teryl) ppm; IR (KBr): ꢄꢀ¼ 3070 (C–H, arom), 2843–2987 (C–H, aliph), 1745 (30-C¼O), 1720 (70-
C¼O), 1713 (4-C¼O), 1705 (20-C¼O), 1611 (C¼N), 1547 (C¼C), 1440, 1311, 1269, 1220, 1137,
1028, 938, 856, 747 cmÀ1; MS: m=z ¼ 857 [Mþ].
Acknowledgements
€
The author is very indebted to Prof. Dr. H. D€urr at the University of Saarland, Saarbrucken, Germany,
for his continuous encouragement and help. Many thanks are also given to Dr. M. Tanaka in AIST,
Osaka, for his helpful assistance during the progress of this manuscript.
References
[1] Part II: Ahmed SA (2002) J Phys Org Chem 15: 392
[2] Brown GH (1971) Photochromism. Wiley-Interscience, New York
€
[3] Durr H, Bouas-Laurent H (eds) (1990) Photochromism: Molecules and Systems. Elsevier,
Amsterdam
[4] Crano JC, Guglielmetti RJ (eds) (1999) Photochromic and Thermochromic Compounds, vol. I
and II. Kluwer Academic Publishers, Plenum Press, New York
[5] Pimienta V, Froute C, Deniel MH, Lavabre D, Guglielmetti RJ, Micheau JC (1995) J Photochem
& Photobiol 122: 199

Photochromism of Dihydroindolizines
1187
[6] Irie M (2000) Chem Rev 100: 1685
[7] Berkovic G, Krongauz V (2000) Chem Rev 100: 1741
[8] Kawata S, Kawata Y (2000) Chem Rev 100: 1777
[9] Tsivgoulis GM, Lehn J-M (1995) Angew Chem Int Ed Engl 10: 1119
[10] Gilat SL, Kawai SH, Lehn J-M (1995) Chem Eur J 5: 285
[11] Meyer JL, Levoir P, Dubest R (1995) Analyst 120: 447
[12] Cerqueira NMFSA, Oliverira-Campos AMF, Coelho PJ, Melo de Carvalho LH, Samat A,
Guglielmetti RJ (2002) Helv Chem Acta 85: 442
[13] Ortica F, Levi Brun P, Gugielmetti RJ, Mazzucato U, Favaro G (2001) J Photochem Photobiol A
139: 133, and the references therein
€
[14] Hauck G, Durr H (1979) Angew Chem 91: 1010; (1979) Angew Chem Int Ed 18: 945
[15] Durr H, Hauck G (1979) D Offenl 29 06 193
€
€
[16] Ahmed SA, Hartmann T, Huch V, Durr H, Abdel-Wahab AA (2000) J Phys Org Chem 13: 539
[17] Tan YS, Ahmed SA, Durr H, Huch V, Abdel-Wahab AA (2001) Chem Commun 14: 1246
€
[18] Fromm R, Ahmed SA, Hartmann T, Huch V, Abdel-Wahab AA, D€urr H (2001) Eur J Org Chem
21: 4077
[19] Ahmed SA, Abdel-Wahab AA, D€urr H (2003) J Photochem Photobiol A 154: 131
€
[20] Ahmed SA, Weber C, Hozien ZA, Hassan KhM, Abdel-Wahab AA, Durr H (unpublished results)
[21] Ahmed SA (2000) PhD Thesis, Saarland-Assiut Universities
€
[22] Durr H, Spang P (1984) Angew Chem 96: 277
[23] Bouas-Laurent H, Durr H (2001) Pure Appl Chem 73: 639
€
€
€ €
[24] Dorweiler C, Munzmay T, Spang P, Holderbaum M, Durr H, Raabe E, Kurger K (1988) Chem
Ber 121: 843
[25] D€urr H, Schommer C, M€unzamy T (1986) Angew Chem 98: 565
[26] Burtscher P, D€urr H, Rheinberger V, Salz U (1995) German Pat DE 195200160
€
[27] Durr H, Gross H, Zils KD (1983) D Offenl Pat 3220275A1
€
€
[28] Durr H, Jonsson HP, Scheidhauer P, Munzmay T, Spang P (1985) D Offenl 35214325
[29] Durr H, Janzen KP, Thome A, Braun B (1988) D Offenl 35214325
€
€
€
[30] Durr H, Gross H, Zils KD, Hauck G, Hermann H (1983) Chem Ber 116: 3915
[31] Tan YS, Hartmann Th, Huch V, Durr H, Kossanyi K (2001) J Org Chem 66: 1130
€
[32] Weber C, Rustemeyer F, D€urr H (1998) Adv Mater 10: 1348
[33] D€urr H, Amlung M, Rustemeyer F, Tan YS (1998) D Offenl 198 349 408
[34] Hartmann H, Ahmed SA, Tan YS, D€urr H, Renn A, Wild U (2004) (in press)
€
[35] Durr H, Thome A, Kilburg H, Bossmann S, Blasius E, Janzen K, Kranz C (1992) J Phys Prg
Chem 5: 689–698
€
[36] Durr H (1994) Chimia 48: 514–515
[37] Burtscher P, Durr H, Rheinberger V, Salz U (1995) German Pat 195200160
€
€
[38] Gogritchiani E, Hartmann Th, Palm B, Samsoniya Sh, Durr H (2002) J Photochem Photobiol B
67: 18
[39] Ahmed SA, Abdel-Wahab AA, D€urr H (2003) In: Horspool WM, Lenci F (eds) CRC Handbook
of Org Photochem Photobiol, 2nd ed. CRC Press, New York
€
[40] Durr H, Ma Y (1994) D Offenl 4444 244.09
[41] Andreis C, Durr H, Wintgens V, Valat P, Kossanyi J (1997) Chem Eur J 3: 509
€
€
[42] Bleisinger H, Scheidhauer P, Durr H, Wintgens V, Valat P, Kossanyi J (1998) J Org Chem 63:
990
€
€
[43] Ahmed SA, Sallenave X, Fages F, Mieden-Gundert G, Muller WM, Muller U, Vogtle F,
€
Pozzo J-L (2002) Langmuir 18(19): 7096
[44] Heinisch G (1973) Monatsh Chem 104: 953
[45] Heinisch G (1973) Monatsh Chem 104: 1372
[46] Heinisch G (1976) Monatsh Chem 107: 799

1188
S. A. Ahmed: Photochromism of Dihydroindolizines
[47] Neises B, Steglich W (1978) Angew Chem 90: 556
[48] Alvaro M, Garcia H, Iborra I, Miranda MA, Primo J (1987) Tetrahedron 43: 143
[49] Heinisch G, Jentzsch A, Pailer M (1974) Monatsh Chem 105: 648
[50] Staley W (1977) In: Lehr RE, Marchand AP (eds). Pericyclic Reactions, Academic Press,
New York
€
[51] Fromm R (1996) PhD Thesis, Universitat des Saarlands, Saarbrucken
[52] Gautron R (1968) Bull Soc Chim France 3190
€
€
[53] Bach V (1987) PhD Thesis, Universitat des Saarlands, Saarbrucken
[54] Schonberger A (1958) Praparative Organische Photochemie, chapt 1. Springer, Berlin
€
€
€