An encaged σ∗ orbital in 3-methoxy-2,4,10-trioxaadamantane. Preferred exo-anomeric effect as revealed by the crystal structure

Abstract

In the titled tricyclic orthocarbonate derivative, the three endocyclic C–O bonds are longer than the exocyclic C–O bond (∼1.40Å vs. ∼1.37Å). This indicates an anomeric-type interaction between the two electron lone pairs on the exocyclic oxygen atom and the antibonding orbitals of the two antiperiplanar endocyclic C–O bonds. The remaining endocyclic C–O bond – marginally shorter than the other two – apparently adds to this effect. Intriguingly, the antibonding orbital of the exocyclic C–O bond extends into the interior of the adamantyl cage, and is stereoelectronically prevented from overlapping with any of the six adjacent lone pairs. The results also seem to indicate a preference for interaction between a single donor oxygen atom and multiple acceptor antibonding orbitals rather than vice versa. The results add insightfully to the substantial body of evidence favouring the antiperiplanar lone pair hypothesis (ALPH).