Stereochemical Characterization of Polyketide Stereotriads Synthesized via Hydrogen-Mediated Asymmetric syn-Crotylation

The stereoselective access to stereotriads as important polyketide building blocks is reported on the basis of the Krische-type hydrogen-mediated syn-crotylation. The products were obtained with an extremely high diastereoselectivity (dr >99:1), and the newly formed syn stereocenters were controlled solely by the chiral catalyst. The stereochemistry was assigned by crystallography and HPLC for both product manifolds. This extension of the burgeoning transfer hydrogen methodology gives divergent asymmetric access to anti,syn and syn,syn polyketide stereotriads from the same α-chiral starting material and avoids potentially epimerizable aldehyde intermediates.本研究报道了基于克里施型（Krische-type）氢介导顺式巴豆基化反应的立体选择性合成方法，可制备作为重要聚酮砌块的立体三单元体（stereotriad）。所得产物具有极高的非对映选择性，非对映异构体比例（dr）>99:1，且新生成的顺式立体中心仅由手性催化剂调控。两类产物系列的立体化学构型均通过晶体学与高效液相色谱法（HPLC）得以确定。该研究拓展了新兴的转移氢化方法学，可从同一α-手性起始原料出发，通过发散式不对称合成途径获得反式,顺式（anti,syn）与顺式,顺式（syn,syn）两类聚酮立体三单元体，同时规避了潜在的易发生差向异构化的醛类中间体。