An effective method for the preparation of chiral polyoxy 8-membered ring enone corresponding to the B ring of taxol

An effective method for the preparation of 8-membered ring enone, (45,5R;7 R.8R)-4,8-bis(benzyloxy)-7-(t-butyldimethylsiloxy)-5-(4-methoxybenzyloxy)-2 ,6,6-trmethylcyclooct-2-enone (1) in sufficient quantities was developed. O ptically active trioxy aldehyde, (3R,4S)-4-benzyloxy-5-(t-butyldimethylsilo xy)-3-(4-methoxybenzoloxy)-2,2-dimethylpentanal (3), was prepared first by diastereoselective dihydroxylation of (2R,4S)-2-(4-methoxyphenyl)-5,5-dimet hyl-4-vinyl-1,3-dioxane derived from D-pantolactone. Next, 8-chloro-7-oxoal dehyde,(2R,3R,5R,6R)-2,6-bis(benzyloxy)-5-(t-butyldimethylsiloxy)-8-chloro- 3-(4-methoxybenzyloxy)-4,4-dimethyl-7-oxononanal (30), was newly synthesize d by the following reactions: i) MgBr2-OEt2-mediated diastereoselective ald ol reaction of the aldehyde 3 with (Z)-2-benzyloxy-1-methoxy-1-(trimethylsi loxy)ethene and ii) reaction of methyl (2R,3R,5R,6S)-2.6-bis(benzyloxy)-3.7 -bis(t-butyldimethylsiloxy)-5-(4-methoxybenzyloxy)-4,4-dimethylheptanoate w ith 1,1-dichloroethyllithium, followed by successive reductive dehalogenati on of thus formed 1,1-dichloroethyl ketone with 1.1-dichloroethyllithium. T hen, the chiral 8-chloro-7-oxoaldehyde 30 was converted to the 8-membered r ing enone 1 by SmI2-mediated aldol cyclization.