transannular

Transannular is an adjective used in chemistry to describe interactions or reactions that occur across a ring system. The term combines trans- (across) and annular (ring), reflecting events that take place between atoms or functional groups that lie on opposite sides of a cyclic framework and are brought into proximity by the ring’s three-dimensional shape.

In macrocyclic and medium-sized rings, transannular phenomena arise when reactive sites on opposite faces of the ring encounter each other, enabling processes that connect non-adjacent positions. Such transannular processes can include cyclizations, additions, and rearrangements that form bridges or new bonds across the ring. Transannular effects can influence the conformation, reactivity, and overall topology of cyclic molecules, contributing to ring strain (transannular strain) in some cases and facilitating the construction of more complex, bridged, or polycyclic structures in natural products and synthetic targets.

The concept helps distinguish across-ring chemistry from traditional intramolecular reactions that occur between neighboring atoms along the same side of the ring. Transannular interactions are particularly discussed for eight- to fourteen-membered rings and larger macrocycles, where ring geometry often brings distant sites into reactive proximity. Studying transannular phenomena aids in understanding conformational preferences, reaction pathways, and strategies for assembling intricate cyclic architectures in organic synthesis.