Influence of a 2-fluoro substituent on diastereoselectivity in the 1,3-dipolar cycloadditions of nitrones
作者:Masataka Ihara、Yuko Tanaka、Nobuyuki Takahashi、Yuji Tokunaga、Keiichiro Fukumoto
DOI:10.1039/a702810e
日期:——
It is clear that the role of 1,2-asymmetric induction on the 1,3-dipolar cycloaddition of nitrones is influenced by the presence of a fluorine atom at the C-2 position. 2-Fluoro nitrones, synthesized by three different methods, have been subjected to the intermolecular 1,3-dipolar cycloaddition with ethyl vinyl ether. The stereostructures of isoxazolidines formed were determined by their conversion into 2,7-dioxa-6-azabicyclo[3.2.1]octanes. The diastereoselectivity of 2-fluoro nitrones was the reverse of that of the corresponding 2-hydro nitrones. This fact supports that the conformation with relief from the dipole repulsion between the fluorine atom and the oxygen atom of the nitrone is a preferred one for 2-fluoro nitrones, while the corresponding 2-hydro nitrones adopt the conformation with the least 1,3-allylic strain.
很明显,在氮酮的1,3-偶极环加成反应中,1,2-不对称诱导作用受到C-2位置氟原子存在的影响。通过三种不同方法合成的2-氟氮酮,与乙烯基乙醚进行了分子间的1,3-偶极环加成反应。所得异恶唑烷的立体结构通过其转化为2,7-二氧-6-氮杂双环[3.2.1]辛烷来确定。2-氟氮酮的非对映选择性与相应的2-羟氮酮相反。这一事实支持以下观点:氟原子与氮酮氧原子之间偶极排斥得到缓解的构象是2-氟氮酮的优选构象,而相应的2-羟氮酮则倾向于采取1,3-同系物应变最小的构象。