1979 年 27 巻 2 号 p. 331-340
The total syntheses of (±)-isopetasol (24), (±)-3-epiisopetasol (17), (±)-warburgiadione (23), (±)-petasitol (27a) are described. A key intermediate (11) was synthesized by the Robinson annulation of 2, 3-dimethylcyclohexane-1, 4-dione (6), which was prepared starting from 2, 3-dimethylphenol (2) and methyl vinyl ketone via intermediates 8 and 9. Reduction of 11 with NaBH4 gave epimeric alcohols (13a and 14a) in the 7 : 3 ratio. Stereochemistry of 13a and 14a and the coresponding acetates (13b and 14b) were confirmed by NMR spectrometry. Introduction of the C-7 side chain in 13a was made with CH3Li to give 16 which was treated with HCl-MeOH to afford (±)-17. While, the same procedure applied to 14a was unsucessful. Ketal (19) derived from 11 was converted to isopropyl alcohol (21) by two routes. Treatment of 21 with HCL-MeOH gave (±)-isopetasone (22). (±)-23 and (±)-24 were obtained from 22 by dehydrogenation and reduction, respectively. Dehydrogenation of 21 by 2, 3-dichloro-5, 6-dicyano-1, 4-benzoquinone (DDQ) gave a cross dienone (25). Deketalization of 25 followed by reduction afforded (±)-27a and (±)-3-epipetasitol (28).
