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Markush structures (-R) are chemical symbols used to indicate a collection of chemicals with similar structures. They are commonly used in chemistry texts, and also in patent claims.
Markush structures are depicted with R groups, in which the side chain can be a structure type, e.g. 'cyclohexyl'. This more general depiction of the molecule, versus detailing every atom in the molecule, is used to protect intellectual property. The company which files the patent makes a general claim for the usage of the molecule without revealing to their competitors the exact molecule for which they are declaring a useful application.
In Patents
They are named after Dr Eugene A. Markush, founder of the Pharma Chemical Corporation in New Jersey. He was involved in a legal case that set a precedent for generic chemical structure patent filing. The patent filing was US Application 611,637, filed January 9, 1923. Markush was awarded a patent from the US Patent Office for “Pyrazolone Dye and Process of Making the Same” on August 26, 1924.
In describing a chemical, a Markush structure allows the patent-holder to list several active/effective structural formulas. Patent applicants are required to reveal their best known embodiment implementing their invention and patent claims can be rejected or invalidated as indefinite if they are too vague.