Four-membered rings 2-substituted oxetane-3-ones

Formation of the four-membered ring of fluorinated oxetanes as for their hydrocarbon congeners is normally accomplished by one of the two general approaches the cyclization of y-substituted alcohols or [2 + 2] cycloaddition reaction. 

The diol is made into an epoxide by an intramolecular substitution reaction that is Sn2 and so goes with inversion. There are two possible rings that could form, depending on which hydroxyl group attacks, but (as you will shortly see) three-membered rings form faster than four-membered ones, and the reaction gives none of the oxetane. The other hydroxyl group can now be protected as a benzyl ether. 

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