Stereoselective epoxidation
We started this section with a diastereoselective epoxidation of an alkene. The alkene was this one, and it has a substituent cis to the stereogenic centre. We can therefore expect it to have one important conformation, with H eclipsing the double bond. When a reagent—m-CPBA here—attacks this conformation, it will approach the less hindered face, and the outcome is shown.
Without the cis substituent, selectivity is much lower.
m-CPBA still attacks the less hindered face of the alkene, but with no cis substituent there are two low-energy conformations: one with H eclipsing the double bond, and one with Me eclipsing. Each gives a different stereochemical result, explaining the low stereoselectivity of the reaction.
You saw at the end of the last chapter that the reactions of m-CPBA can be directed by hydroxyl groups, and the same thing happens in the reactions of acyclic alkenes. This allylic alcohol epoxidizes to give a 95:5 ratio of diastereoisomers.
Drawing the reactive conformation explains the result. The thing that counts is the cis methyl group: the fact that there is a trans one too is irrelevant as it is just too far away from the stereogenic centre to have an effect on the conformation. The reaction uses a racemic mixture, but to explain the diastereoselectivity we just have to pick one enantiomer and show what happens to that.
