Smallest Bicycles in Medicinal Chemistry: Where Are We Now?

The replacement of aromatic rings with saturated molecular frameworks is a recent development encapsulated by the motto "escape from flatland" and conceptualized through the use of saturated benzene bioisosteres. This Review summarizes the application of the smallest bicyclic and spirocyclic ring systems as saturated scaffolds, focusing on applications in constructing bioactive molecules. We discuss considerations of their molecular strains, their potential to serve as saturated benzene isosteres in terms of both volume and geometry, and structural data derived from small-molecule and protein crystallography. Additionally, we present general approaches to synthesis, examine the current commercial availability of functional building blocks, and present existing examples of applications of bicyclic systems in drug discovery programs. At least eight structures based on the smallest skeletons have advanced to clinical trials, with one, vanzacaftor, recently receiving U.S. FDA approval. Our analysis indicates that small bicyclic fragments are represented exceptionally unevenly. While bicyclo[1.1.1]pentane and spiro[3.3]heptane have become routine and indispensable in medicinal chemistry over the past decade, ladderane and housane remain exotic and unexplored. We highlight knowledge gaps, aiming to stimulate interest in small saturated skeletons for innovative molecular engineering.