Experiments Directed Towards the Synthesis of Anthracyclinones. XXXVI Asymmetric Dihydroxylations of Alkylidene Anthracyclinones

The methylidene tetracycle (2) has been synthesized in 11 steps from quinizarin (5) in an overall yield of 38% by using a highly ecient selective dihydroxylation step and an intramolecular ene cyclization. Also prepared with the selective dihydroxylation methodology were the silyloxy alkene (3) and the 6-demethoxy alkene (4). A mixture (1 : 2) of the (E)- and (Z)-isomers of the ethylidene compound (6) has been prepared by similar methods. The products resulting from the reactions of AD-mix-α and AD-mix-β on the alkenes (1)–(3) and (6) have been investigated and their stereochemistries assigned by using 1 H n.m.r. and NOESY experiments, and molecular modelling of acetonide derivatives. An X-ray crystal structure of the acetate (64) has confirmed the relative stereochemical assignments.