Epimers, and the Nomenclature of Bicyclic Compounds

The term epimer originated in sugar chemistry and can be illustrated with the examples of D-lyxose (24) and D-xylose (25). There are three stereogenic centres in each of these molecules the configurations at C(3) and C(4) are the same in both, whereas the configuration of C(2) in 24 differs from that in 25. 

Two diastereoisomers that differ in configuration at only one stereogenic carbon are called epimers. The term is quite general, though it is 

An exo epimer has a substituent directed toward the smallest bridge substituents directed toward the largest bridge are endo. 

There are a couple of further points, one of which is exemplified by camphor (21) and the borneols. These molecules have the functional groups on one of two two-carbon bridges. Accordingly, the numbering is arranged to be as low as possible, and so in camphor, for example, the carbonyl carbon is C(2). 

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