Illustrated Glossary of Organic Chemistry

Diastereoselective: A process such as a chemical reaction or total synthesis in which a mixture of diastereomers is possible, but one of these diastereomers is the major product.
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Anti addition product
Only product formed
    
Syn addition product
Not formed
Addition of Br2 to 1,2-dimethylcyclohexene is diastereoselective because anti addition is the only reaction pathway, and only a single diastereomer of the product is produced.
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Mixture of diastereomers
 Addition of HBr to 1,2-dimethylcyclohexene is not diastereoselective because the product is a mixture of nearly equal amounts of two diastereomers.