Ab initio molecular orbital calculations of 3,4‐dihydro‐1,2‐dioxin, 3,6‐dihydro‐1,2‐dioxin, 4 H ‐1,3‐dioxin (1,3‐diox‐4‐ene), and 2,3‐dihydro‐1,4‐dioxin (1,4‐dioxene)

Optimized geometries and total energies for 3,4‐dihydro‐1,2‐dioxin( 1 ), 3,6‐dihydro‐1,2‐dioxin ( 2 ), 4 H ‐1,3‐dioxin(1,3‐diox‐4‐ene, 3 ), and 2,3‐dihydro‐1,4‐dioxin (1,4‐dioxene, 4 ) were calculated using ab initio 3‐21G, 6‐31G*, andMP2/6‐31G*//6‐31G* methods. The half‐chair conformers of 1 ( C 1 ), 2 ( C 2 ), 3 ( C 1 ), and 4 ( C 2 ) are more stablethan their respective planar structures [ 1 ( C s ), 2 ( C 2 v ), 3 ( C s ), and 4 ( C 2 v )]. Among the four isomers 1 – 4 , the half‐chair conformer of 3 is the moststable. It is 53.1, 54.6, and 3.4 kcal mol −1 more stable than 1 , 2 , and 4 , respectively. Thelargest energy difference (19.0 kcal mol −1 ) isobserved between the half‐chair and planar conformers of 2 . The boatconformers of 2 and 4 are less stable than their respectivehalf‐chair conformers, but are more stable than their planar structures.Hyperconjugative orbital interactions (anomeric effects) contribute to thegreater stability of 3 ( n O(3) →σ* C(2)— O (1) , n O(3) →σ*   C(2)—H   ax, n O(3) →σ*   C(2)—H   ax) and of 4 ( n O(1) →σ*   C(2)—H   ax). The ab initio calculated structural features of the half‐chairconformations of the dihydrodioxins 1 – 4 are comparedwith the half‐chair conformations of cyclohexene and the chairconformations of cyclohexane, oxacyclohexane (tetrahydropyran),1,2‐dioxacyclohexane (1,2‐dioxane), 1,3‐dioxacyclohexane (1,3‐dioxane), and1,4‐dioxacyclohexane (1,4‐dioxane) © 1997 by John Wiley &Sons, Inc.  J Comput Chem 18 : 1392–1406, 1997