Diastereoselective Nucleophilic and Electrophilic Attack on Double Bonds Free of Steric Effects

4 Diastereoselective Nucleophilic and Electrophilic Attack on Double Bonds Free of Steric Effects 

Suggest explanations for why aldehydes are less selective in the Felkin-Anh sense than ketones. 

The dioxan-5-one 5.159 is attacked by nucleophiles with a much higher degree of axial selectivity than cyclohexanones, whereas the dithia analogue 5.160 is attacked from the equatorial direction. Consider the arguments that might account for these two effects. 

The protonation of the steroidal enol ethers ( )-5.161 and (Z)-5.161 give diastereoisomeric products 5.162 and 5.163, respectively. Identify the features that explain why the geometry of the double bond appears to control the diastereoselectivity. 

Predict what stereochemistry you might expect for the borohydride reduction of 5-azaadamantanone 5.164. 

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