Substrate-imposed steric constraints can be overriden by the pronounced preference of strong peracids for epoxidation on the π face, which has the highest electron density. For example, the syn:anti ratio for reaction (1) with CF3CO3H in CH2Cl2 is 82:18, that with CH3CO3H in toluene is 3:97.
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Charles Fehr (1998) studied this question.