Rate enhancement of biomimetic polyene cyclizations by a cation-stabilizing auxiliary
Rate enhancement of biomimetic polyene cyclizations by a cation-stabilizing auxiliary
复制标题
通过阳离子稳定助剂提高仿生多烯环化的速率
DOI：
10.1021/ja00253a048
复制
发表时间：
1987
影响因子：
15
通讯作者：
J. Steele
中科院分区：
文献类型：
--
作者：
W. S. Johnson;S. Lindell;J. Steele
（“CS”）辅助。因此，多烯Ib进行立体选择性闭环，以77%的产率得到D-高甾体产物，而Ia的产率为30%。[1]我们现在公开了一种巨大的速率加速，伴随着在由这种相同的（异丁烯基）助剂介导的涉及形成三个环的环化中的产率增加四倍。这一发现不仅对皮质激素的合成具有实用价值，而且可能具有生物学意义，像2a这样在前Cl 1位带有羟基的底物可能是重要的皮质激素前体。2不幸的是，这种官能化底物的环化速率衰减了几个数量级，3推测是吸电子烯丙基杂原子诱导的前-C-8，9烯键的低亲核性的结果。由于竞争过程的参与，这些缓慢的环化导致产率低下，推测是底物的二聚化211和酸对双键的破坏，特别是在终止子中。本研究的目的是看看是否可以消除这一困难
(“CS") auxiliary. Thus the polyene lb underwent stereoselective ring closure to give D-homosteroidal products in 77% yield as compared with 30% for la. 1 We now disclose an enormous rate acceleration attended by a fourfold increase in yield in a cyclization involving formation of three rings mediated by this same(isobutenyl) auxiliary. This discovery not only has practical potential for the synthesis of corticoids butmay also have biological sig-nificance.Substrates like 2a with a hydroxyl group at pro-Cl 1 are po-tentially important corticoid precursors. 2 Unfortunately the rate of cyclization of such functionalized substrates is attenuated by several orders of magnitude, 3 presumably a result of the low nucleophilicity of the pro-C-8, 9 olefinic bond induced by the electron-withdrawing allylic heteroatom. These slow cyclizations result in poor yield due to the involvement of competing processes, presumably dimerization of the substrate211 and destruction of the double bonds by acid, particularly in the terminator. The aim of the present study was to see if this difficulty could be obviated