Detailed Studies of Perezone Rearrangements

The isomerization of perezone (1) into isoperezone (2) by means of 3,4,5,6-tetrahydro-2-pyrimidinethiol afforded 6-methoxyperezone (6), the sulfide 7, and the heterocycle 8 as by-products. Addition of a small amount of water to the reaction system increased the yield of 2 from 45 to 65%. The pathway for the formation of 6, 7, and 8 is discussed. The reaction of 2 with silica gel or with p-TsOH regioselectively yielded β-isopipitzol (3) or dihydroisoperezinone (4) in 72 and 82% yield, respectively.