In general, we've learned that alkoxide groups are awful leaving groups, unless we're talking about epoxide rings whose ring strain allow the alkoxide to act as a leaving group in a ring-opening reaction. However, if a structure contained both an epoxide and a relatively good leaving group like Chloride, what would predominate if a strong base/nucleophile was introduced: Epoxide opening or the substitution reaction? An example is provided below of what I'm referring to:
If I perform, the epoxide ring opening, I get something unusual:
I left the stereochemistry ambigious because I honestly did not know how to assign it since a new stereocenter is formed.
If I perform the Sn2 substitution, the reaction is completed quickly:
Notice that both of my proposed mechanism reform the epoxide ring, but do generate entirely different structures.