Organic synthesis planning: Some hints from similarity

Despite of the present popularity of the new usages of similarity in chemistry, and of its counterpart dissimilarity, few approaches have used it in the design of organic synthesis. In addition mast of the known applications refer to the mere comparison between structures of the synthetic tree. We would like to discuss the power of the use of similarity in organic synthesis planning and to give some examples. The possibilities of analysis of a synthesis at different levels, from the single step to the entire tree, are presented and criticised. Special attention is dedicated to the role of similarity, whilst the identification of good descriptors is postponed to future developments.