Studies on Terpenoids and Related Alicyclic Compounds. XVIII. Stereoselective Synthesis of (±)-Furanofukinol and (±)-Petasalbin

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A stereoselective synthesis of (±)-furanofukinol (1a) and (±)-petasalbin (2) starting from the diene adduct (6) is described. Reduction of the diketone (8) with NaBH₄ gave the 3β-ol (9) as a main product. Attempted synthesis of 1a from 9, by reduction of 9 with LiAlH₄ followed by catalytic reduction, was unsuccessful due to decomposition of 11. Reduction of the diketone (12) gave the 3β-ol (13). The conformations of 13 and its acetate (14) were shown to be non-steroidal (13b and 14b, Fig. 1) by nuclear magnetic resonance (NMR) and nuclear Overhauser effect (NOE) studies. Reduction of 13 with LiAlH₄ gave a mixture of 1a and the 6α-ol (15), whereas reduction of 13 with Na metal dissolved in refluxing EtOH gave 1a stereoselectively. On the basis of NMR and infrared (IR) spectral comparisons, 1a and its diacetate (1g) were identical with the natural compounds (1a and 1g, respectively). Reduction of (±)-ligularone (4) under thermodynamic conditions gave 2 stereoselectively. IR and NMR spectral comparisons showed (±)-2 to be identical with petasalbin.