Stereocontrolled asymmetric synthesis of syn-E-1,4-diol-2-enes using allyl boronates and its application in the total synthesis of solandelactone F
作者:Anna Robinson、Varinder K. Aggarwal
DOI:10.1039/c2ob06975j
日期:——
The solandelactones A–H comprise a novel class of oxygenated fatty acids bearing an eight-membered lactone, transcyclopropane, and a 2-ene-1,4-diol subunit. The relative stereochemistry of the 1,4-diol subunit is anti in solandelactones A, C, E & G, and syn in solandelactones B, D, F & H. Having prepared one member of the solandelactones bearing anti stereochemistry (solandelactone E), we have targeted the syn series and developed methodology for the synthesis of enantioenriched syn-2-ene-1,4-diols. The methodology comprises asymmetric deprotonation of an alkyl 2,4,6-triisopropylbenzoate using sBuLi/sparteine, followed by addition of the α-lithiobenzoate to β-silyl vinyl boronic acid ethylene glycol ester. The boron-ate complex generated undergoes a 1,2-metallate rearrangement furnishing an intermediate allyl boronic ester which is trapped by an aldehyde in the presence of MgBr2 to furnish anti-β-hydroxyE-allylsilanes in good yields, high diastereoselectivity and high enantioselectivity. These sensitive products were oxidized using mCPBA to the corresponding epoxides and subsequently treated with acid to furnish syn-E-2-ene-1,4-diols (∼4 : 1 d.r.). Application of the methodology to appropriately functionalized aldehyde and ω-alkenyl 2,4,6-triisopropylbenzoate coupling partners, led to a short, highly selective route to solandelactone F (bearing a syn-E-2-ene-1,4-diol).
Solandelactones A-H是一类新颖的含氧脂肪酸,具有八元内酯、反式环丙烷和2-烯-1,4-二醇子单元。在solandelactones A、C、E和G中,1,4-二醇子单元的相对立体化学为反式,而在solandelactones B、D、F和H中为顺式。我们已合成了具有反式立体化学的solandelactone E,并针对顺式系列开展研究,开发了合成富含对映体的顺式2-烯-1,4-二醇的方法。这一方法包括使用sBuLi/sparteine进行烷基2,4,6-三异丙基苯甲酸酯的非对称去质子化,随后将α-锂苯甲酸酯加入到β-硅基乙烯基硼酸乙二醇酯中。生成的硼酸盐复合物经历1,2-金属化重排,生成中间体烯丙基硼酸酯,在MgBr2存在下与醛反应,产生高产率、高立体选择性和高对映选择性的反式β-羟基E-烯丙基硅烷。这些敏感产物使用mCPBA被氧化为相应的环氧化物,并随后用酸处理生成顺式E-2-烯-1,4-二醇(约4:1的双对映体比)。将该方法应用于适当官能化的醛和ω-烯烃-2,4,6-三异丙基苯甲酸酯的偶联反应,得到了通向solandelactone F(具有顺式E-2-烯-1,4-二醇)的一条简短且高选择性路线。