Cis and trans descriptors of stereochemistry

The stereochemical outcome is known for many epoxide-opening reactions. Examination of the effects of substituents and conditions on stereochemistry can provide insight into the mechanisms of the rearrangements which are the focus of this section. The words syn and anti are used here to describe mechanistic features, with the terms cis and trans retained as structural descriptors. The vast majority of epoxide-opening reactions occur with anti stereospecificity, i.e. backside displacement of the cleaved C—O bond. Anti preference dominates sufficiently to warrant calling this the normal stereochemical outcome. 

The unique identification of stereoisomers in the Registry System was originally accomplished via a controlled vocabulary field called the stereochemical descriptor which was introduced with the CA Ninth Collective Period in 1972. This descriptor was used in both the Registry structural record and in the substance index heading in printed CA indexes, with slight differences in punctuation. The CAS stereochemical descriptors used standard stereochemical terms such as alpha, beta, cis, trans, endo, exo, syn, anti, R, S, E, and Z R and S being used for relative stereochemistry. The... 

Data). It allows the specification of tetrahedral stereocenters, cis/trans double bonds, and allene type stereochemistry by specifying parities associated with the ligands of a stereocenter (tetrahedral) or stereopair (cis/trans or allene). Each of these descriptors is assigned to a stereogroup which can be absolute ( 0 ), relative (uppercase letter), or racemic (lowercase letter). Figure 10 shows a stereocenter block describing atom 3 of molecule 21. 

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