Selectivity
The chirality of the product of a Sharpless epoxidation can be predicted using the following mnemonic. Draw the double bond of interest lying flat. Draw a rectangle that around the double bond in the same plane as the carbons of the double bond. Towards the bottom right corner, draw the allylic alcohol. Place the other substituents in the appropriate corners. In this orientation, the (-) diester tartrate preferentially interacts with the top half of the molecule, and the (+) diester tartrate preferentially interacts with the bottom half of the molecule. This model seems to be valid despite substitution on the olefin. Selectivity decreases with larger R1, but increases with larger R2 and R3 (see introduction).
The product of allylic 1,2-diols is incorrectly predicted by this model.
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