P-15 官能基化されたアリルシランの分子内反応を用いる5員環の合成および環化の立体化学(ポスター発表の部)
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β-(Ethoxycarbonyl)allylsilane is a useful organic bifunctional unit which can react with both nucleophile and electrophile. We synthesized α-methylene-γ-lactones fused to various carbocycles utilizing intramolecular cyclization of this unit with aldehyde. Our next focus is to synthesize five-membered carbocycles, instead of γ-lactone, by this chemistry. Here we describe synthesis of bicyclo[4.3.0]nonane and spiro[4.n]alkane carbon frameworks through intramolecular cyclization of β-(ethoxycarbonyl)allylsilane and α-(trimethylsilylmethyl)divinyl ketone derivatives, respectively. (1) Synthesis of Bicyclo[4.3.0]nonane Ring System β-(Ethoxycarbonyl)allylsilane derivatives 1a-d were synthesized according to Schemes 2 and 3. Treatment of 1a-d with Lewis acid afforded cyclohexanones 9a,b. While bicyclic compounds 10a,b were also produced together with 9a,b by treatment with TBAF. It was found that the stereochemistry of the products depends on the geometry of the precursor and not on the reagent, Lewis acid or fluoride. Thus 1a and 1c afforded cis-isomer 9a (and 10a) predominantly, while trans-isomer 9b (and 10b) was obtained as the major product from 1b. The allylsilane 1d afforded both isomers with 1:1 ratio. The stereoselectivity of the cyclization reaction could be rationalized by interaction between two oxygen functionalities (eg. Scheme 4). Compounds 9a and 9b were converted to methylenated bicyclo[4.3.0]nonanes 12a and 12b, respectively, by aldol condensation of keto aldehyde 11a,b. (2) Synthesis of Spiro[4.5]decane and Spiro[4.4]nonane Ring Systems α-(Trimethylsilylmethyl)divinyl ketone derivatives 16a-c were synthesized from cyclohexanones 13a-c via β-(ethoxycarbonyl)allylsilanes 14a-c by the route shown in Scheme 6. Nazarov cyclization of 16a-c was performed by treatment with FeCl_3 in CH_2Cl_2 giving spiro[4.5]-decanes 17a-c and 18a,b (17a:18a=17b:18b=5:1). Similarly, methylenated spiro[4.4]nonane 20 was obtained by treatment of 19 with FeCl_3 at 0℃. However, when the reaction was carried out at room temperature, rearranged product 21 was afforded from 19 via 20.
- 天然有機化合物討論会の論文
- 1995-09-01
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