In addition he included compounds where an oxygen atom provides the link between the two C 6 C 3 units. The inclusion of the lignans as a type of neolignan has not been generally accepted. Table 1 shows some of the ways these sub-classes of lignans have been described. Table 1 Comparison of Nomenclature Proposals Skeleton e.g. This nomenclature problem is further complicated by the retrieval problem as the Chemical Abstracts index name of lignans and neolignans use systematic nomenclature.
Primed numbers refer to the aryl group attached to the index heading parent.
An asterisk refers to a carboxy group. Existing trivial names for lignans and neolignans may still be used but it is recommended that the semisystematic name for the lignan or neolignan also be quoted in a paper when first encountered. It should be noted that several different numbering systems are in use so that it must be clear with a modified trivial name which system is being used. This group is also considered to include examples where the two units are joined by an ether oxygen atom which for nomenclature purposes is treated as a linking oxygen of an assembly. The locants of the bond linking the two units are cited in front of the name and with the second number primed.