Extracyclic stereocontrolled alkylation of (1R,5S)-4-ethyl-6,6-dimethyl-3-(phenylsulfonyl)bicyclo[3.1.1]hept-3-en-2-one. A highly stereocontrolled synthesis of (-)-kanshone A
作者:Michiharu Kato、Masataka Watanabe、Bahlul Z. Awen
DOI:10.1021/jo00071a025
日期:1993.9
(1R,5S)-4-Ethyl-6,6-dimethyl-3-(phenylsulfonyl)bicyclo[3.1.1]hept-3-en-2-one (7) was prepared from (+)-nopinone (1) in six steps and 70% overall yield via (1R,5R)-6,6-dimethyl-3-(phenylthio)bicyclo-[3.1.l]hept-3-en-2-one (2). Alkylation reactions of 7 with alkyl bromides 16 (a, allyl; b, 3-methyl-2-butenyl; c, propargyl; d, benzyl bromide) in the presence of K2CO3 in MeCN proceeded in regio- and extracyclic stereocontrolled fashion to give, as the major product, mixtures of gamma-alkylated products 17a-d possessing a new chiral center of R configuration adjacent to a ring and 18a-d possessing that of S configuration, whose ratios are 17a-18a, 10:1, 17b-18b, 7:1; 17c-18c, 13:1; and 17d-18d, 18:1, along with alpha-alkylated products 19a-d and 0-alkylated products 20a,b on reactions with 16a,b. In addition, reaction of 7 with methyl bromoacetate (16e) provided 17e as the sole product. In the presence of a combined reagent, K2CO3-Cs2CO3 (9:1), in MeCN, considerably high diastereoselection was detected, i.e., reactions of 7 with 16a,b produced mixtures of 17a and 18a, and 17b and 18b, in 20:1 and 12:1 ratios, respectively. Reaction products were separated by chromatography on silica gel, while the major diastereomers 17a,c-e, highly crystalline themselves, were readily obtained as pure crystals by recrystallization. Mechanism of diastereoselection and the scope and limitations of the extracyclic stereocontrolled alkylation are briefly discussed. In the application of 17 as the synthetic intermediate for the asymmetric synthesis, starting with (1R,5S)-6,6-dimethyl-4-[(1R)-1-methyl-3-butenyl]-3-(phenylsulfonyl)bicyclo[3.l.l]hept-3-en-2-one (17a), (-)-kanshone A (8), a nardosinane sesquiterpene, was synthesized in a highly stereoselective fashion in 12 steps via (1R,4R,5R)-4,6,6-trimethyl-4-[(1R)-l-methyl-3-butenyl]bicyclo[3.1.1]heptan-2-one (30) and its cyclobutane-ring opening product, (4S,4aR,5R)-1-acetoxy-4-isopropenyl-4a,5-dimethyl-3,4,4a,5,6,7-hexahydronaphthalene (34).
(1R,5S)-4-乙基-6,6-二甲基-3-(苯基亚砜基)双环[3.1.1]庚-3-烯-2-酮 (7) 由 (+)-诺蒎酮 (1) 经过六步反应和 70% 总收率制得,其中关键中间体为 (1R,5R)-6,6-二甲基-3-(苯硫基)双环[3.1.1]庚-3-烯-2-酮 (2)。在甲基乙腈 (MeCN) 中以 K₂CO₃ 为碱的条件下,化合物 7 与不同烷基溴 (16,a: allyl;b: 3-甲基-2-丁烯基;c: 丙炔基;d: 苯甲基溴;e: 甲基溴乙酸酯) 进行烷基化反应,分别以区域和全环立体受控的方式进行,主要生成含有 R 型新手性中心(邻近环)的 γ-烷基化产物 17a-d 和含有 S 型手性中心的 18a-d,它们的比例分别为:17a-18a,10:1;17b-18b,7:1;17c-18c,13:1;17d-18d,18:1。此外,反应还生成少量的 α-烷基化产物 19a-d 和 β-烷基化产物 20a,b。当使用 16a 和 16b 作为反应物时,烷基化反应主要生成 γ-烷基化产物。值得注意的是,当使用甲基溴乙酸酯 (16e) 进行反应时,仅生成 17e 作为唯一产物。在 MeCN 中使用 K₂CO₃ 和 Cs₂CO₃ (9:1) 的混合碱时,观察到较高的对映体选择性,即化合物 7 与 16a 和 16b 的反应分别以 20:1 和 12:1 的比例生成对映体混合物 17a-18a 和 17b-18b。
反应产物通过硅胶柱色谱分离,其中主要的对映异构体 17a、17c 和 17e,由于其高度结晶性,可通过重结晶直接获得纯晶体。文中简要讨论了对映体选择性的机制以及全环立体受控烷基化的范围和限制。
在将 17 作为不对称合成的合成中间体应用于天然产物合成时,以 (1R,5S)-6,6-二甲基-4-[(1R)-1-甲基-3-丁烯基]-3-(苯基亚砜基)双环[3.1.1]庚-3-烯-2-酮 (17a) 为起始原料,通过 12 步反应,以高立体选择性合成了 (-)-kanshone A (8),一种nardosinane型倍半萜。其合成过程中依次通过 (1R,4R,5R)-4,6,6-三甲基-4-[(1R)-1-甲基-3-丁烯基]双环[3.1.1]庚烷-2-酮 (30) 和其环丁烷开环产物 (4S,4aR,5R)-1-乙酰氧基-4-异丙烯基-4a,5-二甲基-3,4,4a,5,6,7-六氢萘 (34) 完成。