Electron deficient, bivalent sulfur atoms have two areas of positive electrostatic potential, a consequence of the low-lying σ* orbitals of the C-S bond that are available for interaction with electron donors including oxygen and nitrogen atoms and, possibly, π-systems. Intramolecular interactions are by far the most common manifestation of this effect, which offers a means of modulating the conformational preferences of a molecule. Although a well-documented phenomenon, a priori applications in drug design are relatively sparse and this interaction, which is often isosteric with an intramolecular hydrogen-bonding interaction, appears to be underappreciated by the medicinal chemistry community. In this Perspective, we discuss the theoretical basis for sulfur σ* orbital interactions and illustrate their importance in the context of drug design and organic synthesis. The role of sulfur interactions in protein structure and function is discussed and although relatively rare, intermolecular interactions between ligand C-S σ* orbitals and proteins are illustrated.

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Catalytic antibodies that control the reaction pathways of the Diels-Alder cycloaddition have been generated. One antibody catalyzes the favored endo and the other the disfavored exo pathway to yield the respective cis and trans adducts in enantiomerically pure form. A comparison of the x-ray structure of the hapten with the calculated geometry of the transition structure showed that [2.2.2] bicyclic compounds are excellent mimics of the transition state of the Diels-Alder reaction. To achieve catalysis and the high degree of stereoselectivity shown here, the antibody must simultaneously control the conformation of the individual reactants and their relation to each other. In the case of the disfavored process, binding energy must be used to reroute the reaction along a higher energy pathway. The rerouting of reaction pathways has become a major focus of antibody catalysis and other disfavored reactions can be expected to be catalyzed so long as the energy barrier is not extreme. The energy requirements needed for absolute control of all of the stereoisomers of many Diels-Alder reactions fall in the energy range (approximately 20 kilocalories per mole) deliverable by antibody binding.