39 π-アリルパラジウムへのアルケン、アレンの連続的分子内挿入反応を利用した新規骨格合成法と天然物合成への応用(口頭発表の部)

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Palladium(0)-catalyzed cyclization of 6-(2-alkenyl)-2,7-octadienyl acetates (7) was carried out. When the 6-substituent is an allyl group, tandem cyclization of 7a took place stereoselectively to give trans-bicyclo[3.3.0]octane 9a in 67% yield. The compounds 9b-d having (E)- or (Z)-2- alkenyl groups at 6-position underwent the cyclization leading to 12b-d, which resulted from β-elimination of the common 2-(vinylcyclopentyl)methylpalladium intermediates 8. There was found an unprecedented cis diastereoselection at C(3)-C(6) of 7a-d during the cyclization as well as trans diastereoselection at C(6)-C(7) in the particular formation of 9a. It is also worthy of mentioning that the trans-bicyclo-[3.3.0]octane 14 bearing a novel angular methyl group was formed in the Pd(0)-catalyzed tandem cyclization of 13 in 53% yield. The Pd(0)-catalyzed cyclization of 15 proceeded in acetic acid at 45℃ under a carbon monoxide atmosphere to give 16 accompanied with the meso products (yield 41%, selectivity 82%). This is resulted in the unprecedented carbopalladation of the π-allylpalladium intermediate (path a) forming the δ-alkenylpalladium A which underwent carbonylation leading to the product 16. This method was utilized to the synthesis of (±)-isoiridomyrmecin (5). The Pd(0)-catalyzed cylization of trienyl acetate 18, followed by carbonylation selectively gave the acrylic acid derivative 17, which was converted to (±)-5 in six steps. The Pd(0)-catalyzed cyclization of 24, in which the allylic acetate and the allenic moieties are tethered by two methylene carbons was examined. In this reaction, the initially formed π-allylpalladium intermediate 21 would undergo exclusively alkene insertion at the center of the allenic unit, resulting in a new π-allylpalladium complex 23, which in turn proceeded back to the product 25. In fact, shortening the tether by one carbon between the π-allylpalladium and the allenic moieties (cf. 15 and 24) has caused a drastic change of the regioselection in the cyclizations. This reaction was extended to the formation of tetracyclic diketone 6. The Pd(0)-catalyzed tandem cyclization of tetraenyl acetate 26 proceeded under carbon monoxide to give tetracyclic compound 6 in 22% yield. It is noteworthy that the six consecutive C-C bond formation involving four carbopalladation and two carbonylation insertions has taken place in one operation.
天然有機化合物討論会の論文
1996-09-02

39 π-アリルパラジウムへのアルケン、アレンの連続的分子内挿入反応を利用した新規骨格合成法と天然物合成への応用(口頭発表の部)

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