Summary.

• Overall transformation :  C=C to epoxide
• Reagent : a peracid, RCO₃H
• Regioselectivity : not relevant since both new bonds are to the same O
• Stereoselectivity : syn since the two new s bonds form at the same time from the peracid.

MECHANISM FOR REACTION OF ALKENES WITH PERACID
A single step reaction involving several changes. Start at the C=C as the nucleophile, make a bond to the slightly electrophilic O, break the weak O-O bond and form a C=O, break the original C=O to make a new O-H bond, break the original O-H to form the new C-O bond ! (phew !)