Whereas the cycloisomerization of phenyl-substituted α-hydroxyallene 1 to 2,5-dihydrofuran 2 catalyzed by gold(I) or gold(III) chloride takes place with considerable epimerization, high levels of axis-to-center chirality transfer can be reached in the presence of additives (e.g. 2,2′-bipyridine), by using a moderately coordinating solvent (e.g. THF), or by lowering the reaction temperature. The increased stability and selectivity of the catalysts thus formed allows their application to the cycloisomerization of various functionalized α- and β-hydroxyallenes, as well as a decrease of the catalyst loading to 0.1%.
在
金(I)或
金(III)
氯化物催化下,苯基取代的β-羟基烯丙基1到
2,5-二氢呋喃2的环化异构化会发生相当大的外消旋化,但在添加物(如2,2′-联
吡啶)存在下,使用适度配位溶剂(如
四氢呋喃)或降低反应温度,可以实现高
水平的轴对中心手性转移。因此,催化剂的稳定性和选择性提高,使其可用于各种官能化的β-和β-羟基烯丙基的环化异构化,同时可将催化剂用量降至0.1%。