endo - exo isomerism

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In organic chemistry, endo - exo isomerism is a special form of isomerism that only occurs in substituted, bridged bicyclic hydrocarbons . The terms endo and exo are relative stereodescriptors and are now only used in systematic names for achiral pseudo-asymmetric stereogenic centers on the framework of bicycloalkanes and bicycloalkenes and their heteroanalogues.

The isomer in which the substituent R is on the opposite side of the shortest bridge ( marked in green ) is called endo ( endo , Greek, inside) . The term exo ( exo , Greek, outside) denotes the isomer in which the substituent R is on the side of the shortest bridge:

The endo - exo isomerism can be easily recognized using structural formulas:

  • endo

  • exo

  • endo

  • exo

This bicycle has three bridges: Two each with 2 carbon - atoms , and one with a carbon atom (peak top). The residues R can therefore occupy two different positions on the one "two-carbon bridge" :

  • if they are on the same side as the other “two-carbon bridge” marked in blue , this corresponds to an endo position of the R radicals in the bicycle.
  • if they are on the opposite side to the other ( blue marked) “two-carbon bridge”, this corresponds to an exo position of the radicals R in the bicycle.

Bicycles that are simply substituted on the shortest bridge cannot be described with the endo-exo definition. These isomers are classified using the syn - anti notation .

Web links

Individual evidence

  1. ^ Bernhard Testa: Fundamentals of Organic Stereochemistry , Verlag Chemie, Weinheim, 1983, pp. 128–130, ISBN 3-527-25935-X .