Scheme 2.

The “acylal mechanism” for Ψ generation modified to account for epimerization at C2′ via the boxed reaction manifold^a

^aThe stereospecific hydration across C=N could occur at the active site or in solution with a directing hydrogen bond between the fluoro group and incoming water molecule. The reaction manifold made accessible by the longer lifetime of the acylal/oxocarbenium intermediates is boxed.