Chan Wing-Kei, Yu Wing-Yiu, Che Chi-Ming, Wong Man-Kin
Department of Chemistry and Open Laboratory of Chemical Biology of the Institute of Molecular Technology for Drug Discovery and Synthesis, The University of Hong Kong, Pokfulam Road, Hong Kong, China.
J Org Chem. 2003 Aug 22;68(17):6576-82. doi: 10.1021/jo034296d.
A beta-cyclodextrin-modified ketoester 2 was prepared by covalent attachment of a reactive ketone moiety to beta-cyclodextrin. Treatment of 2 with Oxone as terminal oxidant would produce CD-substituted dioxirane, which can effect stereoselective alkene epoxidation. The 2-mediated (S)-alpha-terpineol epoxidations proceeded to give terpineol oxides in high yields, and the stereoselectivities (i.e., cis-/trans-epoxide ratio) decreased from 2.5:1 to 1:1.2 with increasing steric bulkiness of the terpenes. This steric-dependent stereoselectivity can be understood based on different binding geometries of the 2/terpene inclusion complexes according to the (1)H NMR titration and 2D ROESY experiments. Enantioselective epoxidation of styrenes has also been achieved with 2 as catalyst (20-50 mol %) in aqueous acetonitrile solution, and up to 40% ee was obtained in 4-chlorostyrene epoxidation at 0 degrees C. Similar enantioselectivities were also obtained for the 2-mediated epoxidation of 1,2-dihydronaphthalene (37% ee), 4-chlorostyrene (36% ee), and trans-stilbene (31% ee).
通过将反应性酮部分共价连接到β-环糊精上制备了β-环糊精修饰的酮酯2。用Oxone作为终端氧化剂处理2会生成CD取代的二氧杂环丙烷，其可实现立体选择性烯烃环氧化。2介导的（S）-α-萜品醇环氧化反应以高产率生成萜品醇氧化物，并且随着萜类化合物空间位阻的增加，立体选择性（即顺式/反式环氧化物比率）从2.5:1降至1:1.2。根据1H NMR滴定和二维ROESY实验，基于2/萜类化合物包合物的不同结合几何结构，可以理解这种空间依赖性立体选择性。在乙腈水溶液中，以2作为催化剂（20-50 mol%）也实现了苯乙烯的对映选择性环氧化，在0℃下4-氯苯乙烯环氧化反应中获得了高达40%的对映体过量值。对于2介导的1,2-二氢萘（37% ee）、4-氯苯乙烯（36% ee）和反式二苯乙烯（31% ee）的环氧化反应，也获得了类似的对映选择性。
