R. ST-AMOUR and M. ST-JACQUES. Can. J. Chem. 59,2283(1981.1,3-Dioxacycloheptene and its 2-CH,, 2-OCH,, 2-t-Bu, and 2,2-dimethyl derivatives were studied by 'H and 13C dnmr methods. Substituent effects and the interpretation of spectral changes in 13C nmr at low temperature allow the determination of the most stable conformation of these compounds (TB for 1-4b and C for 5b) and the characterization of the dynamic processes observed for 2b, 4b, and 5b. The differences between the conformational and dynamic properties of the above compounds and their benzo analogs are rationalized in terms of different torsional interactions in the two series.R. ST-AMOUR et M. ST-JACQUES. Can. J. Chem. 59,2283(1981.La dioxa-1,3 cycloheptene (dioxkpine-1,3) et ses derives substitues en position 2 par les groupements -CH3, -OCH3, t-Bu et (CH3)* ont fait I'objet d'une etude de rmn dynamique 'H et 13C. Les effets de substituants en rmn 13C et la comprehension des changements spectraux A basse temperature ont permis de determiner les conformations adoptees preferentiellement par tous les composes etudies (bateau-croise pour 1-4b et chaise pour 5b) ainsi que de caracteriser la nature et les energies des processus dynamiques observes pour les derives 2b, 4b et 5b. La difference des comportements conformationnels et dynamiques entre les composes derives de la dioxkpine-1,3 et ceux de la serie analogue derives de la benzodioxepine-1,3 est expliquee par la difference des interactions torsionnelles prisentes dans les deux series.The conformational analysis of 1,3-dioxacycloalkanes has received considerable attention over the years and, among this group of molecules, 1,3-dioxanes have been most studied (1). As it turns out, six-membered heterocyclic conformations other than the chair form assume relatively little ' importance (2) (except possibly for some substituted derivatives exhibiting strong syn-1,3-diaxial interactions in the chair form (2b)) and consequently the effects of substituents on C-2 can be explained in terms of an axial-equatorial conformer equilibrium. On the other hand, substituted derivatives of 1,3-dioxacyclopentanes (3), 1,3-dioxacycloheptanes (4), and 1,3-dioxacyclooctanes (5) potentially exhibit a more complex behaviour because of the possible existence of several lowenergy ring conformations which can interconvert by low-energy (pseudorotational or other) processes.Previous nmr studies on derivatives of cycloheptene and benzocycloheptene have provided unique information regarding the conformational changes produced in the seven-membered ring (6-10) by certain functional groups. Significant stabilization of the twist-boat conformation was observed in some cases. 1,3-Dioxabenzocycloheptene (la) and its derivatives (2a to 4a) revealed a most interesting behaviour (7) whereby signals of molecules existing in the chair. and/or the twist-boat forms were detected. Enhanced amounts of the twist-boat form 'Author to whom correspondence may be addressed.were explained in terms of effects associated with steric and electronic int...