P-35 末端シロキシビニル基を有するホモポリプレニルアレン類似体のジアステレオ選択的環化反応(ポスター発表の部)

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Biomimetic polyene cyclization is an important key step in the concise total synthesis of polycyclic natural products. In particular, the Lewis acid-promoted diastereoselective cyclization of polyenic aldehyde acetals to 4β(axial)-alkoxypolycycles has been established by Johnson et al. However, excess SnCl_4 is often required as Lewis acid and there are no methods available for the synthesis of 4α(equatorial)-alkoxypolycycles. We present the SnCl_4 (10 mol%)-catalyzed polycyclization of homo(polyprenyl)arene analogues bearing terminal siloxyvinyl groups, which are much more reactive than other initiators such as acetals, aldehydes, and ketones. The diastreoselectivities were independent of (E)/(Z)-stereochemistry of the terminal trialkylsiloxyvinyl group of starting materials. The α(equatorial)/β(axial) selectivity of 4-siloxy group at polycycles could be controlled by the nucleophilicity of pro-C(9) and the steric effect of a silyl group. The cyclization of tert-butyldiphenylsilyl polyenol ethers gave 4α(equatorial)-siloxypolycycles as a major isomer in high yield. On the other hand, the cyclization of triisopropylsilyl polyenol ethers gave 4β(axial)-siloxypolycycles as a major isomer in high yield. A strong nucleophilicity of pro-C(9), a (6E)-geometry and a bulky silyl group effectively favored the 4α-preference (up to 95% ds), while a weak nucleophilicity of pro-C(9), a (6Z)-geometry and less steric hindrance of a silyl group favored the 4β-preference (up to >99% ds). Therefore, 4α-selective cyclization would proceed concertedly or stepwise through an antiperiplanar transition state, while 4β-selective cyclization would proceed stepwise through a synclinal transition state. With this approach, two natural diterpenoids, 18-norabieta-8,11,13-trien-4-ol and its 4-epimer, were distereoselectively synthesized from the corresponding silyl (6E)-dienol ethers. Although it was difficult to directly generate silyloxocarbenium ion intermediates from aldehydes and ketones with silyl Lewis acids, we succeeded in their catalytic generation with SnCl_4 from silyl enol ethers instead of carbonyl compounds. The present results demonstrate the synthetic advantages of using polyprenoid analogues bearing a terminal siloxyvinyl group as substrates of polyene cyclization with respect to both the reactivity and 4α/4β-diastereocontrol.
2006-09-15

P-35 末端シロキシビニル基を有するホモポリプレニルアレン類似体のジアステレオ選択的環化反応(ポスター発表の部)

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