Alkenes neighboring group effects

Neighboring group effects are important in reactions of peroxyacids, as they were with reactions of hydroperoxides. Reexamination of the epoxidation of 171 using MCPBA rather than TBHP reveals that the peroxyacid gave a 95 5 mixture of 172 and 173. Using MCPBA rather than TBHP led to a slight increase in diastereoselectivity for the reaction, although this depends on the substitution pattern of the alkene. Sharpless... 

Neighboring group effects can compete with steric effects, as illustrated by treatment of oxetane 142 with diethylaluminum-At-methylaniline. This elimination reaction gave a 99% yield of (E)-alkene 143 in a synthesis of humulene. The aluminum coordinated to the oxetane oxygen on the less hindered p face in 144 and 145, the two key rotamers. Rotamer 144 is lower in energy due to the severe nonbonded interaction... 

The cyclopropanation is sensitive to steric effects, adding from the less-hindered face. Having a neighboring hydroxyl group will generally accelerate the reaction, and will direct the cyclopropanation syn to the hydroxyl group even into sterically congested alkenes. 

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