Conformational Studies of 1,3-Dithianes and their Oxo and Seleno Analogues Containing Organophosphorus Substituents. A Question of the Anomeric Effect

Abstract

This review summarizes the results of conformational studies on 2-organophosphorus substituted 1,3-dithianes, 1,3-dioxanes and 1,3-diselenanes. The synthetic approaches to the compounds investigated are briefly presented. The conformation of 2-phosphoryl-, 2-thiophosphoryl- and 2-selenophosphoryl-1,3-diheteroanes was determined by chemical, spectral and X-ray methods. The anomeric effect was found to operate in all compounds studied. Crystallographic, spectroscopic and thermodynamic data suggest the nx-[sgrave]∗c-p hyperconjugative interactions as one of the factors responsible for the anomeric effect. The axial preference of the phosphoryl group in 1,3-diheteroanes is also due to hydrogen bond formation between the phosphoryl oxygen and axial hydrogen(s) at C(4) and C(6). The role of repulsive interactions between the lone pairs on endocyclic heteroatoms and on the P=X heteroatoms are also discussed. The operation of the generalized anomeric effect was also found for 2-phosphonio-1,3-dithianes what is in a sharp contrast to the O-C-N⁺ system exhibiting the reverse anomeric effect.