Tin(IV) chloride and titanium(IV) chloride mediated cyclizations of the ortho-allyl-substituted homochiral hydroxyanthraquinone acetals (7)-(10), prepared by optimized redictive Claisen rearrangements, have afforded monochloro and dichloro tetracyclic products, the tereochemistry of which has been assigned by using n.m.r. techniques. An Sn2-like process in which the dioxolan ring is maintained as an ion pair intermediate is favoured when either tin(IV) chloride or titanium(IV) chloride is used at -78�. Thereafter the direction of addition of chloride at C9 is largely governed by the orientation of this ion pair. An alternative path which probably involves a free oxocarbenium ion predominates at higher temperatures. An adjacent methoxy group on the anthraquinone lowers the stereoselectivity at both C7 and C9, possibly by bidentate coordination of the Lewis acid involving the quinone carbonyl, the methoxy oxygen and the acetal oxygens.
由
氯化锡(IV)和
氯化
钛(IV)介导的正
烯丙基取代的同手性羟基
蒽醌缩醛(7)-(10)的环化反应是通过优化的重构克莱森重排法制备的,得到了单
氯和二
氯四环产物,其立体
化学结构已通过 n.m.r. 技术确定。在-78℃条件下使用
氯化锡(IV)或
氯化
钛(IV)时,二
氧戊环作为离子对
中间体保持的类似 Sn2 的过程更有利。此后,
氯化物在 C9 处的加成方向在很大程度上取决于该离子对的取向。在较高温度下,另一种可能涉及游离
氧羰基离子的路径占主导地位。
蒽醌上相邻的甲
氧基降低了 C7 和 C9 处的立体选择性,这可能是由于
路易斯酸与醌羰基、甲
氧基
氧和
乙缩醛氧的双齿配位。