Macrocyclic rings containing sp centers display a conformational preference for the sp centers to avoid transannular nonbonded interactions by orienting perpendicular to the plan of the ring. Clark W. Still proposed that the ground state conformations of  macrocyclic rings, containing the energy minimized orientation of the sp center, display one face of an olefin outwards from the ring. Addition of reagents from the outside the olefin face and the ring (peripheral attack) is thus favored, while attack from across the ring on the inward diastereoface is disfavored. Ground state conformations dictate the exposed face of the reactive site of the macrocycle, thus both local and distant stereocontrol elements must be considered. The peripheral attack model holds well for several classes of macrocycles, though relies on the assumption that ground state geometries remain unperturbed in the corresponding transition state of the reaction.