If the ring closure is an #S_N2# reaction then what should be the arrangement of the #-O-C-C-X# system?
The O-C and C-X bonds must be anti to each other.
One way to prepare an epoxide is to react a halohydrin with a base.
An example is the conversion of 3-bromo-2,3-dimethylbutan-2-ol to 2,3-epoxy-2,3-dimethylbutane.
The reaction is really an intramolecular Williamson ether synthesis.
Step 1 is an acid-base reaction. The OH⁻ removes a proton from the alcohol. This converts it to an alkoxide ion and enhances the nucleophilicity of the oxygen atom.
Step 2 is an intramolecular
The C-O⁻ bond and the C-Br bond must therefore be anti to each other.