Subject aldol reaction, anti stereoselectivity

The degree of stereoselectivity of aldol reactions of simple cyclohexanone enolates has been a subject of some confusion. For cyclohexanone itself, it has been reported that reaction of the lithium enolate with benzaldehyde gives Ae two isomeric aldols (Scheme 1) in ratios of 52 48 in THF at -78 C and 50 50 in dimethoxyethane at -20 C. On the other hand, Seebach reports ratios of 79 21 at -78 C and 85 15 at -150 C. ° Hirama and coworkers reinvestigated the reaction of the lithium enolate of cyclohexanone with benzaldehyde (Scheme 1) and found anti.syn ratios of about 82 18 at -78 C. The ratio is... 

Cyclohepta-3,5-dienone)iron complexes can be stereoselectively methylated and hydroxylated. The electrophile adds exclusively anti to the tricarbonyliron fragment. Double methylation or hydroxylation of the a and a positions is accomplished in high overall yield (Scheme 4-146). Silyl enol ethers adjacent to tricarbonyl(Ti -diene)iron units can be subjected to Mukaiyama aldol reaction with aldehydes to provide aldol adducts with varying diastereoselectivity. This methodology has, for example, been applied to the enantioselective synthesis of the dienetriols streptenol C and D (Scheme 4-147). ... 

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