The conformational analyses of 2,3-dihydro-5H-l,4-dioxepine (5), 5-methoxy-2,3-dihydro-5H-l,4-dioxepine (6), and 6,7-dihydro-5H-l,4-dioxepine (7) have been theoretically studied by molecular mechanics, indicating a preference for a twist-boat conformation in 5 and for a chair in 6 and 7. The stability of the different conformations is governed by the conjugation of the oxygen atoms with the ir system, in 5 and 7, and by this conjugation and the anomeric effect in 6. The barrier for the chair twist boat interconversion is 2.50 kcal/mol in 5 and 3.78 kcal/mol in 6. The concordance between calculated and experimental coupling constants of 5 and 6 upholds these results. © 1989, American Chemical Society. All rights reserved.