Dr. Gaurav Jhaa Blogs

Isomers hosting three epoxides prefer axial orientation by a huge margin (>40kcal/mol) despite having severe steric problems.  Among the two axial isomers, the ‘all-cis’ structure 1B is less stable than the cis-trans hy-brid 1A though the C-C bond alternation is moderate in 1B compared to 1A .  This reduced bond alternation in 1B along with the fact that it also has a planar cyclohexane ring unlike 1A suggests the possibility of cyclic conjuga-tion à la aromatic delocalization in 1B ; however, it is not dominating the energetics compared to the repulsive O…..O interactions. The splaying of cis oxygen atoms that adversely affect C-C bonding is less in 1B (0.600Å) than 1A (0.624Å), facilitating cyclic delocalization in ‘all-cis’ form.  Overlap Control of Conformations Bonding in epoxides is usually explained by two classical models, though both have their own drawbacks.  The popular bent ‘banana bond’ model of Coulson is borrowed from cyclopropane that uses isovalent hybridiz