This is defined by analogy with enantiomer excess, as D1 - D2 [and the percent diastereoisomer excess as 100(D1 - D2)], where the mole fractions of two diastereoisomers in a mixture or the fractional yields of two diastereoisomers formed in a reaction are D1 and D2 (D1 + D2 = 1). The term is not applicable if more than two diastereoisomers are present. Frequently this term is abbreviated to d.e. See stereoselectivity; diastereoisomerism.
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