Studies on Terpenoids and Related Alicyclic Compounds. X. Total Synthesis of Sesquiterpenoid, (±)-Ligularone

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The total synthesis of (±)-ligularone (3) via diene adduct (17) is described. 3, 5-Dimethylbenzofuran-4, 7-quinone (14) was prepared starting from resorcinol. A mixture of 14 and diene (16) was refluxed in benzene for 48 hr to afford the desired adduct (17) in 70% yield. Treatment of 17 with silica gel gave 10-epimer (20). 20 was reduced with NaBH₄ followed by treatment of aq-AcOH to give 3, 6-dioxo-9β-ol (21) and 9α-ol (22). Dehydration of 21 and 22 gave 23 which was epimerized to give 4β-methyl compound (24), in low yield. Catalytic reduction of 24 gave furanoeremophilan-3, 6-dione (28). 28 was also synthesized by catalytic reduction of 23 followed by acid-epimerization of the resulting diketone (30), in good yield. Stereochemistry of diketone (28) and (30) are discussed by nuclear magnetic resonance (NMR) spectrometry. Desulfurization of thioketal of 28 followed by catalytic reduction of the resulting product afforded (±)-ligularone (3). The infrared and NMR of (±)-(3) were identical with those of natural ligularone. Furanoeremophilan-3, 6, 9-trione (36) was synthesized from diene adduct (17) and (20) via (34) and (35) in good yield, respectively.