Stereoparent descriptors

Two other nomenclature terms that have been used to store stereochemical information in connection tables are the labels aor b indicating whether a substituent points below or above a parent (ring) system, respectively. This method is frequently used for steroids. The actual structure can differ from the parent not only by additional substitution but also by, for example, ring contraction, which is coded as modifier to the parent. Stereoparent descriptors lack the generality of the CIP rules as a separate reference model has to be defined for each ring system. The prime benefit of this system is the ease with which the descriptor can be assigned by humans given a known reference template. 

Stereochemistry is expressed in C A index names by three methods stereoparents, systematic stereodescriptors, and descriptors for co-ordination compounds. We have developed a separate procedure for converting each of these three types of stereodescriptors to atom/bond specific descriptors in the Registry connection table. Table 1 gives statistics for the Registry File as of February, 1990. At that time, the total file contained over 10 million registered substances. 

The procedure for converting substances described as modified stereoparents is based on the use of stereospecific templates, one for each stereoparent. The template will have nomenclature locant numbers and parity descriptors for all stereocentres. The procedure for adding atom/bond specific stereodescriptors to the connection table, in simplified form, consists of the following steps ... 

There remain a small number of structures with stereochemical descriptors that have yet to be converted to atom/bond-specific representation. As noted above, these are a special group of stereoparents and coordination compounds, scheduled for future conversion. r... 

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