Titanium-catalyzed diastereoselective epoxidations of ene diols and allylic alcohols with beta-hydroperoxy alcohols as novel oxygen donors
Titanium-catalyzed diastereoselective epoxidations of ene diols and allylic alcohols with beta-hydroperoxy alcohols as novel oxygen donors
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DOI：
10.1021/jo970110x
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发表时间：
1997-05-16
影响因子：
3.6
通讯作者：
Renz, M
中科院分区：
文献类型：
--
作者：
Adam, W;Peters, K;Renz, M
β -氢过氧醇1-4作为有效的三齿氧供体，用于钛催化的高非对映选择性烯二醇5a-e的环氧化反应。因此，与双齿叔丁基过氧化氢相反，三齿β -羟基过氧醇有效地取代钛模板中的三齿环氧二醇产物6a-e，从而通过有效地补充氧转移所必需的负载络合物来维持催化循环。无论在烯二醇底物中双键的取代模式或二醇官能团的构型（红对三）如何，都观察到环氧二醇产品具有很高的非对映选择性。高立体化学控制是由于氧转移的刚性过渡态，这是由钛模板中的多个钛-氧键和配位所施加的。所观察到的烯二醇的红选择性源于其同丙烯基羟基与钛中心的附加键合，它以这样一种方式固定了底物构象，即要转移的氧原子从烯丙基氧官能团的一侧靠近（参见负载配合物a）。双齿烯二醇在钛模板中的额外结合也表现为烯二醇相对于单齿烯丙醇的反应性增强。然而，如果这些单齿底物具有1,2-烯丙基菌株，活性较低的烯丙醇也显示出高的红细胞选择性。第一次直接，非对映选择性和催化环氧化的烯二醇已可用于合成应用，而不诉诸保护基团的方法。
beta-Hydroperoxy alcohols 1-4 serve as effective tridentate oxygen donors for the highly diastereoselective, titanium-catalyzed epoxidation of ene diols 5a-e. Thus, in contrast to the bidentate tert-butyl hydroperoxide, the usual oxygen donor employed in Sharpless-type epoxidations and known to work poorly for polyhydroxy substrates, the tridentate beta-hydroperoxy alcohols efficiently replace the tridentate epoxy diol products 6a-e in the titanium template and thereby the catalytic cycle is sustained by replenishing with efficacy the loaded complex necessary for the oxygen transfer. Irrespective of the substitution pattern of the double bond or the configuration (erythro versus threo) of the diol functionalities in the ene diol substrate, high diastereoselectivities are observed for the epoxy diol products. The high stereochemical control is due to the rigid transition state for the oxygen transfer, which is imposed by the multiple titanium-oxygen bonding and coordination in the titanium template. The observed erythro selectivity for the ene diol derives from the additional bonding of its homoallylic hydroxy group to the titanium center, which fixes the substrate conformation in such a way that the oxygen atom to be transferred approaches from the side of the allylic oxygen functionality (cf. loaded complex A). This additional binding of the bidentate ene diol in the titanium template is also manifested in the enhanced reactivity of the ene diol versus the monodentate allylic alcohols. Nevertheless, the less reactive allylic alcohols also display a high erythro selectivity, provided these monodentate substrates possess 1,2-allylic strain. For the first time a direct, diastereoselective, and catalytic epoxidation of ene diols has been made available for synthetic applications, without recourse to protection group methodology.