[bmu123]

Having a bit of trouble with which diastereomer forms and why. I've drawn what I think the diastereomers are (see images attached) but I'm not sure which will predominate and why

[Babcock_Hall]

Have you drawn two mirror-image isomers?  What is the definition of diastereomers?

[Dan]

bmu123: Forget about the mirror plane. You have tried to take a shortcut without understanding the fundamental concept and it has bitten you on the backside. Think about the reaction by drawing out and considering the mechanism of the reaction.

[bmu123]

Not sure about it 'biting me on the backside' but thanks for the advice.

[Babcock_Hall]

Did you intend to draw two mirror-image isomers or something else?

[orgopete]

This problem "seems like" two problems put together. As I recall, the actual examples that Dan was referring to were epoxidation of cyclic alkenes. You should find these examples in your textbook or lecture notes. You must use these principles. Then you must choose a conformation in which these principles will apply to the epoxide.

Hint, hydrogen bonding.

This is just my opinion, but I'd be surprised that the selectivity would be very large, but I could be wrong. If this is from an actual experiment, it would be interesting to see actual values.