A concise, flexible, and efficient total synthesis of the cytotoxic resin glycosides ipomoeassin B (1) and ipomoeassin E (2) is reported which features the advantages of a novel protecting group strategy employing (Z)-3-dimethyl(phenyl)silyl-2-propenoic acid as cinnamic acid surrogate. The use of this readily available compound allowed the macrocycle of the glycolipids to be formed by ring closing olefin metathesis (RCM) with the aid of the second generation Grubbs carbene complex 12. The resulting E/Z mixture could be selectively hydrogenated using Wilkinson's catalyst [RhCl(PPh3)(3)] without affecting the unsaturated esters in the periphery of the compound, before the C-silyl group was cleaved off with TASF [tris(dimethylamino)sulfonium difluorotrimethylsilicate] under notably mild conditions to release the required cinnamate moiety. Other key steps of the synthesis route comprise the formation of the disaccharide linkage by the trichloroacetimidate method, the formation of the chiral acid segment 19 via a VO(acac)(2)-catalyzed, tert-BuOOH-induced oxidative rearrangement of the optically pure furyl alcohol (-)-15 (Achmatowicz-type reaction), and a reductive cleavage of the 4,6-O-p-methoxybenzylidene acetal in 5 with NaBH3CN and Me3SiCl (TMSCl), the regiochemical course of which was found to be opposite to that previously reported in the literature for sterically less encumbered substrates.
Copper(II)-Catalyzed Silylation of Activated Alkynes in Water: Diastereodivergent Access to<i>E</i>- or<i>Z</i>-β-Silyl-α,β-Unsaturated Carbonyl and Carboxyl Compounds
作者:Joseph A. Calderone、Webster L. Santos
DOI:10.1002/anie.201310695
日期:2014.4.14
Copper(II)‐catalyzed silylation of substituted alkynylcarbonyl compounds was investigated. Through the activation of Me2PhSiBpin in water at room temperature and open atmosphere, vinylsilanes conjugated to carbonyl groups are synthesized in high yield. A surprising diastereodivergent access to olefin geometry was discovered using a silyl conjugate addition strategy: aldehydes and ketones were Z selective
研究了铜(II)催化的取代炔基羰基化合物的硅烷化反应。通过在室温和开放气氛下在水中活化 Me 2 PhSiBpin,可以高产率合成与羰基共轭的乙烯基硅烷。使用甲硅烷基共轭加成策略发现了对烯烃几何形状的惊人非对映发散途径:醛和酮具有 Z 选择性,而酯和酰胺仅转化为E 产物。