The Photoreactions Of Thiazoles With O-quinones

Due to the importance of thiazole derivatives and their potential biological and pharmaceutical applications, we studied photochemical reactions of thiazoles with o- quinones to develop green method to synthesize various thiazole derivatives. Based on the photoreactions of o-quinone with a series of substituted thiazoles, we obtained structurally diversified compounds containing thiazole units. The main work described in this thesis includes:1. The photoreactions of alkyl or aryl substituted thiazoles with o- quinones including phenanthrenequinone (PQ) and N-acetyl isatin (IS) were studied. We found that the the photoreactivity of the alkyl-substituted thiazoles in photochemical cycloaddition is generally lower than their oxazole analogues. The reactivity of thiazoles in the photochemical cycloaddition can be improved through the changes of the different substituents in the thiazole ring. For example, in the reaction of 2-methyl thiazole with PQ, only hydrogen abstraction product was isolated. However, in the reaction of 2,4-diphenyl thiazole with PQ the main product was [2+2] photochemical cycloaddition product. We also found that the position of the substitution groups in the thiazole had influence to the stereoselectivity of the photocycloaddition. The substituent at C4 caused only exo- conformation but no endo-conformation in the reaction of thiazoles with IS.2. We discussed the electronic effect of substituents and found that electron-donating substituents can increase the reactivity of thiazoles, while the electron-withdrawing substituents usually decrease the photocycloaddition reactivity of thiazoles. We chose five kinds of thiazoles with 5-methoxyl substituent which was relatively strong electron-donor substituent. These thiazoles showed high activity and regioselectivity in the reaction with PQ to give [4+2] cycloaddition products with high yield. Then the difference between the reaction of 5-methoxythiazoles and that of 5-methoxyoxazoles with PQ was studied. We found that in the photoreactions of 5-methoxyoxazoles with PQ, not noly [4+2] cycloadducts but also the [2+2] photocycloadducts were present in the final products.