EXPERIMENTS DIRECTED TOWARDS THE SYNTHESIS OF ANTHRACYCLINONES .16. TIN(IV)-MEDIATED AND TITANIUM(IV)-MEDIATED CYCLIZATIONS OF ORTHO-ALLYL-SUBSTITUTED HOMOCHIRAL HYDROXYANTHRAQUINONE DIOXOLANS

Tin(IV) chloride and titanium(IV) chloride mediated cyclizations of the ortho-allyl-substituted homochiral hydroxyanthraquinone acetals (7)-(10), prepared by optimized reductive Claisen rearrangements, have afforded monochloro and dichloro tetracyclic products, the stereochemistry of which has been assigned by using n.m.r. techniques. An S(N)2-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-degrees. Thereafter the direction of addition of chloride at C 9 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 C 7 and C 9, possibly by bidentate coordination of the Lewis acid involving the quinone carbonyl, the methoxy oxygen and the acetal oxygens.