(R)-(−)-enantiomer refers to the stereoisomer of a chemical compound that rotates polarized light to the left, or in a counterclockwise direction. It is also known as the “S” or “sinister” enantiomer, and is often abbreviated as (R).

The designation of (R) or (S) for a particular enantiomer is based on the sequence of atoms attached to the chiral center of the molecule, which determines its three-dimensional shape. In general, the (R) and (S) enantiomers of a molecule will have different physical and chemical properties, including different biological activities.

For example, the (R)-(−)-enantiomer of the drug modafinil is more active than its (S)-(+) enantiomer in promoting wakefulness, which is why it is used clinically as a treatment for sleep disorders.

It is important to note that the (R) and (S) designations do not necessarily indicate the relative safety or efficacy of a particular enantiomer, and that each enantiomer of a drug must be studied individually to determine its pharmacological properties and potential risks and benefits.

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