EXPERIMENTS DIRECTED TOWARDS THE SYNTHESIS OF ANTHRACYCLINONES .28. TITANIUM(IV)-MEDIATED AND TIN(IV)-MEDIATED CYCLIZATIONS OF ORTHO-METHALLYL-SUBSTITUTED HOMOCHIRAL DIOXANS

Metal chloride cyclizations of the 4-demethoxy anthraquinonyl 1',3'-di oxan (6) are highly diastereoselective, giving 9-chloro-9-methylanthra cyclinones in high yield. The selectivity is inversely proportional to the strength of the Lewis acid, with the mild Lewis acid tin(IV) chlo ride/dimethylformamide affording an 82% yield of the diastereomer (21) . The predominant products have a (7S) configuration and a trans relat ionship between the C7 side chain and the 9-chloro substituent. Cycliz ation of the analogous dimethoxy 1',3'-dioxan (4) with tin(IV) chlorid e/dimethylformamide is considerably less selective, but the major prod uct also has a (7S)-trans stereochemistry. Titanium(IV) chloride effec ts stereorandom cyclization of (4).