13 ノピノン誘導体のシクロブタン環開環と天然物合成(口頭発表の部)

抄録

By the cyclobutane ring cleavage, nopinone (1) gives a cyclohexanone bearing an isopropyl unit at its C(4) position. This cleavage reaction is highly usefull in natural product synthesis employing derivatives of 1 as chiral starting materials, provided that the cleavage occurs without loss of optical purity. We first examined the cleavage reaction of cis-3-methylnopinone (2) employed as a model. It was found that the cyclobutane ring of 2 regioselectively underwent the cleavage reaction to provide iodoketone 6 with trimethyiodosilane and diacetate 7 with BF_3・Et_2O-Zn(OAc)_2-Ac_2O with almost no loss of optical purity, respectively. Synthetic approach to penlanfuran (12): 1 was converted to diacetate 15 by employing the above BF_3-OEt_2-Zn(OAc)_2-Ac_2O conditions, and the product 15 was then transformed into (R)-(-)-cryptone (14) through an efficient four-step sequence (Scheme 1). While the Michael addition of a 3-(furyl)methyl magnesium chloride to 14 was unsuccessful, 14 was converted to 24 via oxaspiropropane derivative 23 according to the Tanis's procedure (Scheme 2). Conversion of 24 to 12 is currently under investigation. Synthesis of (+)-β-elemenone (26) and an elemenolide 42 via the common intermediate 28: β,β-Disubstituted nopinone 27 was prepared from 1 through a seven-step sequence showen in Scheme 3 in 27% overall yield. The product 22 was then submitted to the cyclobutane cleavage with the same combination of reagents as above to give 28 directly. The lithium enolate derived from 28 was proved to be amenable to regioselective alkylation process, leading to alcohol 37 with acetone, while keto ester 38 with methyl bromoacetate. (+)-β-Elemenone (26) was obtained from 37 by dehydration, while synthesis of an elemanolide 42 from γ-lactone obtained from 38 is now in progress.