Promising test results on the biological activity of our previously described naphtho1,2‐doxazoles and heterobenz1,2‐doxazoles, obtained by the photochemical cyclization of 5‐phenylethenyl‐ and ...5‐heteroarylethenyloxazoles, prompted us to continue with the photochemical synthesis of substituted naphtho1,2‐doxazoles, and to extend the photochemical cyclization to the synthesis of naphtho/heterobenz2,1‐doxazoles from 4‐(aryl/heteroarylethenyl)oxazoles. The required p‐ and o‐phenyl‐substituted 5‐arylethenyloxazoles were prepared from the corresponding α,β‐unsaturated aldehydes and the TosMIC reagent (tosylmethyl isocyanide) by the Van Leusen reaction. The substituted 4‐(aryl/heteroarylethenyl)oxazoles were prepared by the Wittig reaction starting from various phosphonium salts and 2‐H/methyl‐4‐oxazolecarbaldehydes.
The biological activity of naphtho1,2‐doxazoles and heterobenz1,2‐doxazoles prompted us to investigate the photochemical synthesis of substituted naphtho1,2‐d/2,1‐doxazoles. The required p‐ and o‐phenyl‐substituted 5‐arylethenyloxazoles were prepared by the Van Leusen reaction. The substituted 4‐(aryl/heteroarylethenyl)oxazoles were prepared by the Wittig reaction.
To study the effects of the position of the hexatrienyl moiety on the oxazole ring, novel substituted cis/trans‐2/4/5‐(2‐vinylstyryl)oxazoles have been synthesized. These novel compounds were ...prepared by Wittig reaction from the diphosphonium salt of α,α′‐o‐xylenedibromide, formaldehyde and the corresponding 2‐methyl‐4‐, 4‐methyl‐2‐, 2‐pheny‐5‐ and 4‐methyl‐5‐oxazolecarbaldehyde, respectively. Aldehydes were synthesized by using several synthetic approaches. By applying intramolecular photocycloaddition, 2‐methyl‐4‐(2‐vinylstyryl)oxazole afforded, as major product, fused oxazoline‐benzobicyclo3.2.1octene with small quantities of 4‐(1,2‐dihydronaphthalen‐2‐yl)‐2‐methyloxazole, as electrocyclization product. Upon irradiation of 4‐methyl‐5‐(2‐vinylstyryl)oxazole, endo‐ and exo‐benzobicyclo2.1.1hexene products were formed by 2+2 cycloaddition; this was the first instance of the 1,4‐closure to the bicyclo2.1.1hexene skeleton in the 2‐, 4‐, and 5‐oxazole‐stilbene derivatives studied so far. Derivatives 2‐phenyl‐5‐ and 4‐methyl‐2‐(2‐vinylstyryl)oxazole did not react and gave only high‐weight molecular products but were crucial as a comparison in the overall mechanistic study. We have found that, depending on the position of the hexatrienyl moiety in the oxazole ring, as well as on the position of the methyl/phenyl substituents, these new vinylstyryl‐2/4/5‐oxazole derivatives show diverse photochemical behavior.
To study the effects of the position of the hexatrienyl moiety on the oxazole ring, targeted substituted cis/trans‐2/4/5‐(2‐vinylstyryl)oxazoles have been synthesized and irradiated. These new vinylstyryl‐2/4/5‐oxazole derivatives showed diverse photochemical behavior and gave new functionalized oxazole and oxazoline polycyclic products.
A new approach to benzofquinoline derivatives has been found by an effective formal 4 + 2 photocycloaddition process from novel aryl-substituted o-vinylstyryl-2-oxazoles. All of the ...o-vinylstyryl-2-oxazoles were synthesized by a multicomponent Wittig reaction from the diphosphonium salt of α,α′-o-xylene dibromide, formaldehyde, and 5-tolyl-, 4-phenyl-5-methyl-, and 4,5-diphenyloxazole-2-carbaldehydes. TD-DFT calculations revealed that the intramolecular photocyclization in 2-(2-vinylstyryl)oxazoles to form benzofquinoline derivatives proceeds on the S1 PES via a stepwise pathway, namely by 10π followed by 6π ring closure. On that path the existence of an S0/S1 conical intersection was indicated. The reactivity of the photocyclization steps depends on the substitution pattern at positions 4 and 5 of the oxazole ring, where the aryl group in position 5 deactivates the reaction.
A new synthetic approach is presented for the synthesis of naphthoxazoles and fused heterobenzoxazoles. The starting 5-(aryl/furyl/thienyl/pyridyl ethenyl)oxazoles are prepared from the corresponding ...α,β-unsaturated aldehydes using Van Leusen reagent in very good yields and are transformed into naphthoxazoles and fused heterobenzoxazoles on irradiation under aerobic conditions and in the presence of iodine.
The photochemical behaviour of 2,2′-(o-phenylenedivinylene)dithiophenes (7a–c), 3,3′-(o-phenylenedivinylene)dithiophene (8a) and 3,3′-(o-phenylenedivinylene)dibenzothiophene (8b) was studied under ...the low concentrations. An intramolecular reaction via the 2,3-dihydronaphthalene intermediate has been observed in all studied examples accompanied by dimerization and polymerization. The 1,2-dihydro-2,3-dithienylnaphthalenes (9a–c) were isolated (7–9%) from the 2-thiophene derivatives while the 3-thiophene derivatives gave polycyclic structures (13–44% yield).
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Four novel
trans,
trans-2,5-distyrylfuran derivatives (
E,
E-X-DStFs, X
=
Cl, OCH
3, N(CH
3)
2, NO
2) have been synthesized and characterized by
1H and
13C NMR and UV–vis spectroscopy. The ...photophysical properties of the excited states of these poorly photoreactive compounds have been studied in two solvents of different polarities by stationary and pulsed techniques and also by the help of semiempirical quantum-mechanical calculations. The high fluorescence efficiency of these systems was accompanied by a modest triplet production (that becomes substantial in the nitro-derivative) investigated by nanosecond laser flash photolysis. The solvent effect on the absorption and emission spectra and on the photobehaviour allowed to evidence the presence of charge transfer (CT) excited states in the dimethylamino- and, particularly, in the nitro-derivative.
Stilbeneylsydnone derivatives were synthesized by a sequence of reactions in good yields. Irradiation of 3-stilbeneyl-4-methylsydnone
4 gives 1
H-1,2-benzodiazepine derivative
7 as the main product ...along with 2-methylquinoline derivative
20. Irradiation of 3-stilbeneyl-4-phenylsydnone
5 afforded only 1
H-1,2-benzodiazepine derivative
8 whereas on irradiation of 4-unsubstituted 3-stilbeneylsydnone
3 no benzodiazepine derivative was detected. An efficient novel photochemical approach to 1
H-1,2-benzodiazepines has been found from the new 3-(
o-stilbeneyl)-4-substituted-sydnones via intramolecular 1,7-electrocyclization reaction of the photogenerated nitrile imines.
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Novel
cis- and
trans-(
o-H/Me/vinyl) substituted styryl furo-benzobicyclo3.2.1octadiene derivatives (
7a,
b,
8) were prepared and transformed to the novel naphthofuran derivatives of ...benzobicyclo3.2.1octadiene (
6a,
b) and novel phenanthrene-benzobicyclo3.2.1octadiene derivative (
11) by photochemical electrocyclic ring closure in the presence of iodine and by intramolecular photoinduced 4+2 cycloaddition, respectively. These novel annelated bicyclo3.2.1octadiene derivatives (
6a,
b,
11) are especially interesting for their rigid methano-bridged junction of two aromatic units at defined geometrical arrangement and thereby as potentials for molecular clips.
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Conformations, spectra and photophysics of a series of new organic fluorophores, 2,3-distyrylbenzofuran derivatives, have been studied by a combined theoretical and experimental approach. Ground ...electronic state geometries have been investigated by Hartree-Fock ab initio methods and Density Functional Theory. Electronic spectra have been calculated with the CS INDO S-CI and SDT-CI procedures. Spectral and photophysical behaviour has been investigated by stationary and time-resolved techniques. Solvatochromism of these compounds has been analyzed. The UV–vis absorption spectra of the substituted compounds are very similar, showing a red shift in the series H
<
Cl
<
OCH
3
<
NH
2
<
NO
2. The CS INDO CI analysis of the electronic spectra of all rotamers shows coherence with the prevalent presence of one non-planar conformer. These compounds are very stable and show an intense and structured fluorescence indicating that the emitting state is the same as that reached by absorption, i.e. the
|
π
H
π
L
∗
〉
singlet state. The nitro-derivative behaviour is exceptional if compared to the other compounds since it displays a strong fluorosolvatochromism, due to an intramolecular charge transfer state.
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