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■ A Facile Synthesis of 7-Phenylpyrido[3,4-d]pyridazine-1,4(2H,3H)-dione

Yoshiro Usui,* Yukio Hara, Norio Shimamoto, Shojiro Yurugi, and Toru Masuda

*Central Research Division, Takeda Chemical Industries, Ltd., 17-85 Jusohonmachi 2-chome, Yodogawa-ku, Osaka 532-8686, Japan

Abstract

1,4-Cycloaddition reactions of 4-phenyloxazole (IV) with a variety of maleic acid derivatives were undertaken. Maleimide derivatives were proved to be the most efficient dienophiles, affording 1,4-cycloadduct, 3-phenyl-7-oxa-2-aeabicyclo[2.2.1]hept-2-ene-5,6-dicarboximide derivatives (XII) in good yields.
Acid-catalyzed dehydration of XII, followed by the reaction of the resulting XIII with hydrazine, gave 7-phenylpyrido[3,4-d]pyridazine-l,4(2H,3H)-dione (II), which is the key compound for the synthesis of a potent diuretic, DS-511 (I). Structures of by-products formed in those reactions were elucidated.