抄録
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.
