Interlocked and Knotted Rings in Biology and Chemistry

Abstract

Catenanes are chemical entities consisting of two or more interlocked rings. Such systems, as well as knotted molecules, belong to a branch of chemistry which is related to mathematics, molecular biology and graphic art. Many endeavours have been made to build such topologicaly novel compounds. The statistical methods of preparation, although very simple to conceive and to attempt, are of little practical utility. The directed strategy, using the classical reactions of organic synthesis, offers an interesting alternative route to prepare interlocked ring systems. The most recent approaches, utilizing templated reactions either on transition metals or on organic electron donoracceptor pairs are certainly the most efficient ones for making catenanes. For instance, a gram-scale preparation of [3]-catenanes (two peripheral rings separately interlocked to a central cycle) has been achieved using the transition metal template approach. The metal is able to gather and pre-oriente the several fragments to be incorporated into the target molecule. Although knotted forms of DNA are relatively common, no synthetic molecular knot had been obtained until recently. Here again, the success of the synthetic approach relies on the use of transition metals as templating species. Multinuclear double-stranded helices are interesting intermediate compounds which should allow the preparation of molecular knots and catenanes of increasing topological complexity. The first molecular trefoil knot has only been synthesized and fully characterized recently, although theoretical discussions on chemical knots were published over thirty years ago.