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Communication | Special issue | Vol 64, No. 1, 2004, pp.57-60
Published online, 2nd November, 2004
DOI: 10.3987/COM-04-S(P)36
Mode Switching during the Cycloaddition of 5-Fluoro-1,3-dimethyluracil with Naphthalene from 1,4- to 1,2-Addition

Kazue Ohkura,* Tatsuyuki Sugaoi, Tetsuya Ishikara, and Koh-ichi Seki*

*Faculty of Pharmaceutical Sciences, Health Sciences University of Hokkaido, Ishikari-Tobetsu, Hokkaido 061-0293, Japan

Abstract

UV-irradiation of a solution of 5-fluoro-1,3-dimethyluracil (5-FDMU) and naphthalene (1) in acetonitrile effected both 1,2- and 1,4-cycloaddition. The 1,2-cycloaddition proceeds more smoothly than the alternative 1,4-cycloaddition. The 1,2-cycloadduct (2), however, is labile to the UV-light used for the reaction, and is rapidly converted to 5-FDMU and naphthalene, while the 1,4-adduct (3) is insensitive to UV-light. Thus, 2 is obtained as the major product by short-period irradiation, while 3 is produced predominantly when the irradiation duration is prolonged. Mode-selective 1,2-cycloaddition is achieved when UV-irradiation is performed in the presence of piperylene.