Markush structures are named after Dr. Eugene Markush, who was involved in the legal case that set the precedent for such generalized chemical structures to be used in chemical patents. Read more on Dr. Markush at http://www.colorantshistory.org/MarkushBiography.html .
Chemical patents are an important tool for many chemists, especially for synthetic chemists. Up to 77% of new compounds are reported in the patent literature prior to disclosure in other types of publications (e.g., journal articles), and many are never disclosed in other publications. (Reference: Scifinder Patent Important Info) For this reason, it is essential for chemists to be familiar with searching for chemical information in the patent literature.
Chemical patent searching has many similarities to general patent searching, and can be achieved to some degree through the use of keywords and patent classification searching. However, many chemical compounds are increasingly more difficult to describe with keywords. For this reason, many chemical patent searches are conducted through the use of CAS Registry numbers or Markush structures.
Markush structures are generalized structures that may include all of the compounds from a specific chemical family. Through the use of Markush structures, an inventor can simplify a patented compound to a basic skeletal stricture. In this way, broader protections are provided for an invention, since simple modifications are covered that do not impact the overall properties of the compound.
Markush Example: 
For searching chemical patent literature:
Out of the four free patent databases on the opening page of this guide, only the WIPO website allows for any degree of chemical structure searching or patents. However, WIPO does not allow for Markush structure searching. More information on the WIPO’s chemical structure searching can be found here: https://patentscope.wipo.int/search/help/en/chemsearch_help.pdf
Markush structure searching is, however, offered through the use of commercial structure searching databases including Scifinder and Reaxys.
If you are not familiar with Scifinder and Reaxys, here is a link to the UC guide that addresses these chemical structure searching databases: http://guides.libraries.uc.edu/c.php?g=222777&p=1473454
Here are some additional links for help when searching Markush structures in Scifinder and Reaxys:
Scifinder:
Help: https://scifinder.cas.org/help/scifinder/R42/index.htm
Video: https://scifinder.cas.org/help/scifinder/R42/index.htm
Reaxys:
Details of Markush
