Abstract
(+)-Vibsanin A (1) was isolated from Viburnum odoratissimum by Kawazu in 1980. This natural product showed piscicidal activity. The structure of 1 is characterized by an 11-membered ring skeleton containing an all-carbon asymmetric quaternary stereocenter. Interested in the unique structure, we have involved in synthetic study of 1. We first synthesized the upper-half segment 4. 4-Pentyn-l-y1 acetate (7) was converted to aldehyde 12, which was subjected to olefination using the Ando reagent to provide (Z)-α,β-unsaturated ester 13. The reduction of 13, followed by the Sharpless asymmetric epoxidation of the resultant allylic alcohol 6, provided epoxy-alcohol 14 in 97% ee. The alcohol 14 was oxidized to aldehyde 4. The coupling reaction of 4 and the model lower-half segment 15 afforded 16-A and 16-B, which were converted to iodoalkene-aldehyde 19. The intramolecular Nozaki-Hiyama-Kishi reaction of 19 proceeded smoothly, giving the cyclized product 20. The isomerization of the exo-olefin in 20 occurred under the Mitsunobu condition to result in the formation of an undecadiene skeleton. The selective removal of the p-nitrobenzoyl group in 21 furnished 22, the norprenyl analogue of 1. We next developed a stereoselective synthesis of the all-carbon quaternary stereocenter through zinc-mediated Barbier-type allylation of sugar-derived aldehyde 23 with allylic chloride 9. The major isomer 24 was converted to alcohol 26 via hydroboration-oxidation. Further synthetic endeavor toward the total synthesis of 1 is in progress.
