Department of Chemistry, Center for Chemical Methodology and Library Development at Boston University (CMLD-BU), Life Science and Engineering Building, Boston University, 24 Cummington Street, Boston, MA 02215, USA.
Angew Chem Int Ed Engl. 2010 Sep 17;49(39):7096-100. doi: 10.1002/anie.201003469.
Chiral α,β-dihydroxy carboxylic acids catalyze the enantioselective addition of alkenyl- and aryl boronates to chromene acetals. The optimal carboxylic acid is a tartaric acid amide, easily synthesized via a 3-step procedure. The reaction is enhanced by the addition of Lanthanide triflate salts such as cerium(IV)-and ytterbium(III) triflate. The chiral Brønsted acid and metal Lewis acid may be used in as low as 5 mol % relative to acetal substrate. Optimization of the reaction conditions can lead to yields >70% and enantiomeric ratios as high as 99:1. Spectroscopic and kinetic mechanistic studies demonstrate an exchange process leading to a reactive dioxoborolane intermediate leading to enantioselective addition to the pyrylium generated from the chromene acetal.
手性 α,β-二羟基羧酸催化烯烃基和芳基硼酸酯对色烯缩醛的对映选择性加成。最佳的羧酸是酒石酸酰胺，可通过三步法轻松合成。添加镧系三氟甲磺酸盐（如四价铈和三价镱）可增强反应。手性 Brønsted 酸和金属路易斯酸的用量可低至相对于缩醛底物的 5mol%。优化反应条件可以得到大于 70%的产率和高达 99:1 的对映体过量值。光谱和动力学机理研究表明，该反应经历了交换过程，生成了反应性二氧代硼烷中间体，从而对色烯缩醛生成的吡喃翁进行对映选择性加成。
