Photochemistry of nonconjugated bichromophoric systems. Cyclomerization of 7,7'-polymethylenedioxycoumarins and polymethylenedicarboxylic acid 7-coumarinyl diesters

Abstract

Upon irradiation of dicoumarins with ultraviolet light, intramolecular cycloadducts with the syn configuration are formed. The structure of the isomers was proved unequivocally by nmr spectral study and dipole moment measurements. Upon irradiation of 7,7'-polymethylenedioxycoumarins with a chain length exceeding four units, the syn head-to-tail isomer is the most abundant regioisomer. On irradiation of the polymethylenedicarboxylic acid (7-coumarinyl) diesters, the syn head-to-head isomer is formed predominantly. In the former class neither the chain length nor the solvent polarity seem to exert an important influence on the ratio between the two regioisomers. In the latter this ratio is more affected by length of the link between the two chromophores. Upon increase of the concentration of the dioxycoumarins, the amount of intermolecular photoproducts increases. Upon sensitization, intermolecular reaction prevails. Steric factors such as methyl substitution on the photoreactive double bond were investigated. The photocycloadditions are reversible upon irradiation at ~300 nm.During recent years some examples of intramolecular photopolymerization between suitable chromophores, which are linked by a flexible chain, have been reported.5 In all cases mentioned except the biscinnamates,5h the 1,7 dimer50•9 ****and the A^-ZV'-alkylene bismaleimides,6r the intramolecular reaction is limited to systems in which the two functions are separated by three methylene units.In view of the hypothesis put forward in the photocyclomerization of jV",2V'-alkylene bismaleimides51 and in view of the assumption that an excimer could be an intermediate in the coumarin dimerization,6 the photochemistry of some symmetric and asymmetric di-coumarins7 was studied. We wish here to report some synthetic aspects of this study.Although the dimerization of coumarins is a longknown reaction,8 only a limited number of photoreac-(1) Abstracted in part from the Ph.D. thesis of L. H. Leenders. Presented in part at the 6th International Photochemistry Conference, Bordeaux, France, Sept 1971. (2) Predoctoral Fellow, N. F. W. 0. (aspirant, Belgisch National Fonds voor Wetenachappelijk Onderzoek).(3) Predoctoral Fellow, I. W. O. N. L. (Instituut tot Aanmoediging van het Wetenschappelijk Onderzoek in Nijverheid en Landbouw).(4) All inquiries should be addressed to this author.(5) (a) F. D.

Coumarin–nucleotide base pairs. Ultraviolet absorption, fluorescence, and photochemical study

“…The following compounds had been described previously: 9-(3-bromopropy l)adenine, 9-(5-bromopentyl)adenine (5), I -methylthymine (22), 9-methyladenine (23), 7-methoxycoumarin (24). All these materials have been synthesized in this work by alkylation of a heterocycle with an appropriate 1,w-dibromoalkane or methyl iodide in DMF solution in the presence of K2C03.…”

Topochemical photodimerization of 7-acetoxycoumarin: the acetoxy group as a steering agent

“…The results of our study on 7-acetoxycoumarin reveal that electronic and steric aspects play, if any, only a minor role in the solid state reactions. It is significant that 7-acetoxycoumarin does not dimerize in solution (7), probably owing to the deactivation of the active double bond by the electron withdrawing acetoxy group. Therefore, it appears that adoption of a crystal packing arrangement favourable for bimolecular reaction can overcome the disadvantageous inherent electronic factors.…”

Supramolecular Control of the Template‐Induced Selective Photodimerization of 4‐Methyl‐7‐O‐hexylcoumarin

“…The yellow oil was purified by chromatography (SiO 2 ) with 70 % ethyl acetate/30 % hexane and the product was isolated as a white solid (44 mg, 13 % N-(2,7-Di-tert-butyl-9,9-dimethyl-9H-xanthen-4-ylmethylene)-N'-pyridin-2-yl-hydrazine (4): Compound 8 (20 mg, 0.06 mmol) was dissolved chloroform (10 mL) to which was added (15 mg, 0.14 mmol) 2-hydrazinopyridine; the mixture was then stirred at room temperature for 24 h. The solvent was removed and then subjected to chromatography (SiO 2 ) with a gradient of 5 % ethyl acetate/95 % hexane to 50 % ethyl acetate/50 % hexane. The product was isolated as a white solid (17 (5): N,N-Dimethyl hydrazine (50 mL, 0.65 mL) was added to a solution of dialdehyde 7 (23 mg, 0.06 mmol) in ethanol (10 mL), solubilized by heating; the mixture was then refluxed for 8 h. The solvent was removed under reduced pressure and the white solid was further dried under vacuum to quantatively afford compound 5. N-Methyl-N-pyridin-2-ylhydrazine (9): N-methylhydrazine (20 mL, 360 mmol) was added to 2-bromopyridine (5 mL, 5 mmol) and the mixture was refluxed under an argon atmosphere for 2 h. The residual solvent was distilled off and the oil fully dried under vacuum.…”

“…Under typical experimental conditions, however, the two syn (2 A and 2 B) photoproducts are formed predominately, while only trace amounts of the anti dimers 2 C and 2 D are observed. [7] The ratio of coumarin photoproducts can significantly be altered through control of the reaction conditions such as solvent choice, [9,11,12] Lewis acid addition, [13,14] micellar environment, [15,16] solid-state irradiation, [13,14] modification of the coumarin precursor, [17,18] use of inclusion complexes [15] and tethered precursors, [17, 19±21] or recently with pseudo-rotaxanes. [22] Much effort has been focused on crystal engineering and packing within the solid state to influence the product distribution.…”