Borromean rings: a one-pot synthesis.

The aesthetics of beautiful molecules has been, and still is, a strong motivation for many chemists. Complex interwoven structures such as catenanes, rotaxanes, and knots are even nowadays appealing because of their interesting topological properties, although their functions as molecular machines have become more and more of the focus of interest. The ability of chemistry to realize the mathematical zoo of knots (Figure 1), at the molecular level still suffers from narrow limitations, and it is thus well-justified to consider the synthesis of trefoil knots as a major achievement. Borromean rings possess an even more complex topology than the trefoil knot, thus making them a challenging target for chemical synthesis. This is reflected in the fact that they have only so far been realized at the molecular level through the DNA nanoassembly methodology developed by Seeman and co-workers. Consequently, the first chemical synthesis of Borromean rings by Stoddart, Atwood, and their co-workers is certainly a highlight in supramolecular chemistry. Borromean rings consist of three rings which are entangled in a way which prevents separation of the rings just by changing their shape (Figure 2). Opening one of the rings, however, makes the whole assembly fall apart. It is this property which has made the