A convenient method for conversion of the Z-isomer to the E-isomer from a mixture containing both isomers of fulgides

Article abstract of DOI:10.1016/S0040-4039(02)01514-9

A new method for the conversion of the Z-isomer of unsymmetrical substituted bis-methylenesuccinic anhydrides, 'fulgides', to their geometrical E-isomers was achieved by heating the Z-isomer under reflux or a mixture of both isomers in pyridine to give the E-isomer in near quantitative yield.

Full text of DOI:10.1016/S0040-4039(02)01514-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 6815–6817
A convenient method for conversion of the Z-isomer to the
E-isomer from a mixture containing both isomers of fulgides
Abdullah Mohamed Asiri
Chemistry Department, Faculty of Science, King Abdul Aziz University, Jeddah 21413, PO Box 80203, Saudi Arabia
Received 9 April 2002; revised 9 July 2002; accepted 19 July 2002
Abstract—A new method for the conversion of the Z-isomer of unsymmetrical substituted bis-methylenesuccinic anhydrides,
‘fulgides’, to their geometrical E-isomers was achieved by heating the Z-isomer under reflux or a mixture of both isomers in
pyridine to give the E-isomer in near quantitative yield. © 2002 Elsevier Science Ltd. All rights reserved.
Fulgides are potential candidates for many advanced
technology applications.1–4 The development of a
reversible optical information storage medium based on
photochromic organic compounds, as suggested by Hir-
shberg,5 became a potential commercial reality with the
discovery of the first thermally-stable fatigue-resistant
near colorless photochromic fulgides, which undergo
near quantitative conversion into their colored forms
on exposure to UV light.6 It is known that if the ketone
used in a Stobbe condensation, which is the normal
procedure for the synthesis of fulgides, is unsymmetri-
cal such as 3-acetyl-2,5-dimethylfuran, the condensa-
tion products, and hence the fulgides formed, will be
obtained as a mixture of both possible geometrical
isomers 1-E and 1-Z, respectively. The formation of
both isomers obviously leads to a lower yield of the
desired E-isomer, which is the only isomer that under-
goes ring closure to give the 7,7a-dihydrobenzofuran
derivative 1-C (Scheme 1). In this communication we
report a new method for the conversion of a mixture
containing both E and Z isomers to the desired E-iso-
mers in near quantitative yield (Table 1).
Thus, when a 1:1 mixture of both isomers, 1-E and 1-Z,
of a fulgide was heated in pyridine, a green coloration
was observed. After treating the solution with hydro-
chloric acid the green color faded and pale yellow
crystals of the E-isomer were isolated. The mechanism
which we suggest for this conversion is believed to be
through the formation of the green zwitterion 2
Scheme 1.
Keywords: fulgides; photochromic; photochromism; succinic anhydride; photochemistry; UV light.
E-mail: a asiri@hotmail.com
–
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01514-9

6816
A. M. Asiri / Tetrahedron Letters 43 (2002) 6815–6817
Table 1. 1H NMR data of the E- and the Z-isomers of
fulgide 1a
Table 2. 1H NMR data of the E- and the Z-isomers of
fulgide 1b
Assignment
E-Isomer
Z-Isomer
Assignment
E-Isomer
Z-Isomer
2-Me
5-Me
2.59
2.08
2.44
2.18
2-Me
5-Me
2.21
2.40
2.19
2.42
Ethylidene-Me
2.22
2.17
Ethylidene-Me
2.63
2.35
Furyl-4-H
5.90
5.97
Thionyl-4-H
5.50
6.53
Cyclopropyl-bridgehead-H (1H, m)
Cyclopropyl-H (9H, m)
3.01
0.40–1.15
3.17
0.4–1.40
Cyclopropyl-bridgehead-H (1H, m)
Cyclopropyl-H (9H, m)
3.04
1.21–0.30
3.11
1.20–0.50
(Scheme 2), which permits the rotation of the furyl ring
around the single bond. Intermediate 2 in the presence
of hydrochloric acid decomposes to give only the E-iso-
1
mer in very high purity. The H NMR spectrum of the
E-isomer showed a singlet at l 2.59 ppm for the methyl
group at C-2, similarly the Z-isomer showed the same
methyl singlet at l 2.44 ppm. This transformation of
the Z to the E isomer was also observed for some other
heterocyclic fulgides, for example 1b derived from thio-
1
phene. Tables 1 and 2 summarize the H NMR data of
both geometric isomers of fulgides 1a and 1b. This
transformation was also found to take place in the case
of simple aromatic fulgides such as fulgides 3a–c
1
(Scheme 3). Table 3 summarizes the H NMR data of
both E and Z isomers of fulgides 3a–c.
Scheme 3.
Experimental
organic layer was washed with water, dried (MgSO4)
and the solvent evaporated. The residue was recrystal-
lized from 3:7 chloroform–petroleum ether (40:60) to
give the pure E-isomer). Yield ( 0.95 g), mp 129°C, H
NMR (see Table 1).
A solution of a 1:1 mixture of fulgide 1a (1.0 g) and
anhydrous pyridine (20 ml) was refluxed for 4 h, cooled
and poured into dilute hydrochloric acid (5 M). The
aqueous solution was extracted with diethyl ether, the
1
Scheme 2.
Table 3. 1H NMR data of the E- and Z-isomers of fulgides 3a–c
Compound no.
R1
Me-2
Me-3
Aromatic-H
Other
3a-Z
3a-E
3b-Z
3b-E
3c-Z
3c-E
7.52
7.77
2.38
2.64
3.37
2.61
2.34
1.22
1.99
1.01
1.98
1.11
2.62
2.37
2.18
2.12
2.14
2.14
6.92
8.84
7.2–7.4
7.2–7.4
7.15–7.37
6.82–7.2
2.16, 2.39 3×Me
2.07,2.23 3×Me
3.73 MeO
3.76 MeO

A. M. Asiri / Tetrahedron Letters 43 (2002) 6815–6817
6817
Durr, H.; Bouas-Laurent, H., Eds.; Elsevier: Amsterdam,
1990; p. 314.
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