P-21 二ヨウ化サマリウムによる環化反応を駆使した生理活性ジテルペン類の全合成研究(ポスター発表の部)

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Highly oxygenated diterpenoids such as grayanotoxin (1), kalmanol (2), vinigrol (3), taxol (4), ingenol ester (5) can be found among some of the best known biologically active natural products. The total synthesis of these diterpenes has stood for over 20 years as a major challenge for organic chemists. We started the program directed toward the total synthesis of these diterpenes with the aim of exploring a general and flexible synthetic method for the stereoselective formation of highly functionalized carbocycles via the reductive coupling reactions mediated by samarium(II) iodide (SmI_2). Samarium(II) iodide (SmI_2) has evolved as a unique, single-electron reducing reagent for promoting reductive couplings, such as ketone-olefin and pinacol couplings, to effectively accomplish carbon-carbon bond formation under exceedingly mild conditions. Few areas of synthetic chemistry have witnessed growth as explosive as that brought about by the application of SmI_2 to organic synthesis. However, a limited number of highly stereocontrolled carbon-carbon bond formation reactions has been reported. In conjunction with our synthetic study on vinigrol (3), unique tricyclic diterpenoid isolated from Virgaria nigra as the antihypertensive and platete aggregation inhibiting substance, stereoselective formation of the bicyclo[5.3.1]undecane skeleton was attempted. The 8-membered ring of the bicyclo[5.3.1]undecane system of 3 was successfully cyclized through the intramolecular Barbier type reductive coupling induced by SmI_2 of the aldehyde 9 prepared from (+)-dihydrocarvone. A reliable synthetic scheme for stereoselective construction of the decalin moiety of 3 was also developed by using the reductive coupling mediated by SmI_2. Treatment of the anti-β-hydroxy ketone 11 and 13 with SmI_2 resulted in the exclusive formation of the trans-decalin diols 12 and 14. Obviously, the stereochemical course of the SmI_2-mediated annulation reactions is completely stereocontrolled by chelation of the Sm^<3+> cations generated in the initial reductive processes, with the hydroxyl groups incorporated within the starting materials. On the other hand, when the syn-β-hydroxy ketones 17 and 20 were reacted with SmI_2, the hydroxyl group directed cyclization also took place in a stereoselective manner to give the cis-decalin diols 18 and 19 (1:3) and cis-decalin diols 21 and 22 (2:1), respectively. In each case, diastereoselectivity was little affected by the addition of HMPA in the reaction medium. Furthermore, the hydroxyl-directed pinacol coupling of the keto aldehyde 31 promoted by SmI_2 exclusively afforded the trans-decalin cis-triol 32. Interestingly, the pinacol cyclization reaction of 31 performed in the presence of HMPA provided the trans-decalin trans-triol 33 as the sole product.
天然有機化合物討論会の論文
1995-09-01

P-21 二ヨウ化サマリウムによる環化反応を駆使した生理活性ジテルペン類の全合成研究(ポスター発表の部)

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