Term: endo, exo, syn, anti
https://doi.org/10.1351/goldbook.E02094

Definition:
Descriptors of the relative orientation of groups attached to non-bridgehead atoms in a bicyclo[x.y.z]alkane (x ≥ y > z > 0). E02094-1.png If the group is orientated towards the highest numbered bridge (z bridge, e.g. C-7 in example below) it is given the description exo; if it is orientated away from the highest numbered bridge it is given the description endo. If the group is attached to the highest numbered bridge and is orientated towards the lowest numbered bridge (x bridge, e.g. C-2 in example below) it is given the description syn; if the group is orientated away from the lowest numbered bridge it is given the description anti. E02094-2.png

Related Terms:
1) syn (http://doi.org/10.1351/goldbook.S06216).
2) anti (http://doi.org/10.1351/goldbook.A00381).
3) bridgehead (http://doi.org/10.1351/goldbook.B00736-1).
4) bridge (http://doi.org/10.1351/goldbook.B00736-2).

Images:
1) Not defined (https://goldbook.iupac.org/img/inline/E02094-1.png) 
2) Not defined (https://goldbook.iupac.org/img/inline/E02094-2.png) 

Source: PAC, 1996, 68, 2193. 'Basic terminology of stereochemistry (IUPAC Recommendations 1996)' on page 2207 (https://doi.org/10.1351/pac199668122193)

Citation: '<i>endo</i>, <i>exo</i>, <i>syn</i>, <i>anti</i>' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.E02094

License: The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms.

Disclaimer: The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using.