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Recent Changes in the Scaffold Diversity of Organic Chemistry as Seen in the CAS Registry.

A large set of organic compounds extracted from the CAS Registry is analyzed to study recent changes in structural diversity. The diversity is characterized using the framework content of the compounds; the framework of a molecule is the scaffold consisting of all its ring systems and all the chain fragments connecting them. The compounds are partitioned based on their year of first report in the literature, which allows framework occurrence frequencies to be compared across a 10-year interval. The results are consistent with a process in which frameworks with the greatest frequency of use in the past are the most likely to be used again, but it is also found that the frequency ordering changes over time. These fluctuations in ordering are attributed to stochastic factors, scientific and economic, that can affect how chemical space is explored. Framework diversity is found to have increased over time despite the extensive reuse of a relatively small number of frameworks; this increase is due to the large number of new frameworks. The long tail of the framework distribution, composed of frameworks that occur in few compounds or only one compound, is found to be a large and growing part of framework space.