32 Total Synthesis of (±)-Gibberelline A_1 by Using SmI_2-induced Sequential Reaction

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  • 32 SmI_2の誘起する連続環化を用いる(±)-ジベレリンA_1の全合成(口頭発表の部)
  • 三仏忌考--成道会の儀礼について(3)
  • サンブツキコウ ジョウドウエ ノ ギレイ ニ ツイテ 3

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Samarium(II) iodide (SmI_2) has been effective in carbon-carbon bond forming reactions and considerable attention has been focused on various intramolecular processes. During the studies on SmI_2-induced cyclization, we found a novel sequential cyclization, reductive cyclization-Dieckmann condensation-lactonization, of keto diesters I, to produce bicyclo[4.n+2.0]alkanones II (n=1, 2) bearing γ-lactone. In order to demonstrate the synthetic utility of this reaction, we choose gibberellin A_1 as a target, which is one of representative gibberellins and has a unique structural feature with eight asymmetric centers. Here we wish to describe a new route for the total synthesis of (±)-gibberellin A_1. Our synthetic strategy involves SmI_2-induced sequential reaction, IV to III, as a crucial step as shown in Scheme 2. Intermediate 17 corresponding to VI was prepared by 7 steps from enone 13, involving stereoselective aldol reaction of the lithium enolate of 13 with aldehyde 12, 1,2-addition of a propagyl group with enone 14 and introduction of methoxycarbonylmethyl group into β-position in α,β-unsaturated ketone 15. The key intermediate 26 was synthesized from 17 as shown in Scheme 3, which involves the construction of D ring using SmI_2-HPMA-induced reductive coupling, stereoselective introduction of hydroxymethyl group into α-position of ester carbonyl in 21, and formation of Z-α,β-unsaturated ester group using Still's method. Treatment of 26 with 3 equivalent of SmI_2 in THE for 10 min at room temperature gave gibbane skeleton 27 with desire stereochemistry in 67% yield (Scheme 4). 27 was easily transformed to (±)-gibberellin A_1 by 4 steps.

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