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...

. 62, 2830 (1984).The conformational and dynamic properties of 1,3-dioxa-4,5-benzocycloheptene (8) and several 2-substituted derivatives were investigated by 'H and "C low temperature nmr methods. The results are strikingly different from those reported for the analogous symmetrical dioxepin isomers derived from 2. The unexpectedly large preference of the 2-methoxy derivative (8c) for the C, conformation is explained in terms of deformed torsional angles in the seven-membered ring relative to those in the analogous six-membered one. These results therefore help delineate the limitations to the conformational biasing normally associated with the anomeric effect.ROBERT ST-AMOUR, M. T. PHAN VIET et MAURICE ST-JACQUES. Can. J. Chem. 62, 2830 (1984.Les propriCtCs conformationnelles et dynamiques du dioxa-1,3 benzo-4,5 cycloheptene (8) et de quelques dtrivCs en position 2 ont Ct C CtudiCes par les techniques de rmn 'H et I3C a basse tempkrature. Les rtsultats obtenus sont trks difftrents de ceux prCcCdemment rapportCs pour les isombres analogues de la benzodioxtpine symmttrique dtrivts de 2. La grande preference du derive mCthoxy-2 (8c) pour la forme C, est expliquCe par une dtformation des angles de torsion du cycle a sept chainons comparativement aux dCrivCs analogues cycliques 5 six chainons. Ces rCsultats permettent de mieux circonscrirc les consequences conformationnelles associCes a I'effet anomkre.Stereoelectronic effects ( I ) in acetals and related functions have recently brought into focus the important relationship between conformation and reactivity for several classes of molecules. Studies on the simplest acetal, dimethoxymethane (2), indicate that the g t g arrangement is most stable in conformity with observations made on carbohydrates (3) and 2-substituted tetrahydropyrans (4). This preference constitutes the basis of the anomeric and the exo-anomeric effects extensively investigated for these molecules (5). ' The relative stabilities of the various arrangements possible for a -CH2-0-CH2-0-CH?-fragment have been difficult to assess (1) and the need for a quantitative evaluation based on experimental measurements led to the recent investigation (6) of a broader range of stereochemical arrangements as exist in bicyclic acetals such as 1. By comparison, studies of the cyclic acetal 1,3-dioxane are limited because of the relatively high conformational energy of the twist-boat (TB) conformation (7) and the fact that only highly substituted derivatives have been found to exist in this form.The two possible TB forms of 1,3-dioxane (4 and 5) are characterized by -CH2-0-CH2-0-CH2-fragments in the gtgt disposition and slightly deformed gig-disposition respectively. Previous studies (7) could not clearly distinguish experimentally between these forms because they interconvert easily as a result of the low energy barrier for the pseudorotation motion involved, although calculations suggest that 5 is more stable than 4 (7). In contrast, the dioxepins 2 and 3 provided an opportunity (8, 9) to investigate experimenta...

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The conformational properties of 2-alkyl (Me, Et, i-Pr, and t-Bu) and 2-phenyl derivatives of 1,3-dioxa-5,6-benzocycloheptene (1) were studied by 13C dnmr. Analysis of slow exchange spectra at 100.6 MHz indicates that all derivatives except tert-butyl exist in an equilibrium of chair (major) and twist-boat (minor) conformations. Substituent effects on the position of the equilibrium are rationalized in terms of steric effects.

. Can. J. Chem. 64, 500 (1986). Low temperature X-ray studies of 1,3-dioxa-5,6-benzocycloheptene ( 1 ) and its 2-methyl ( 2 ) and 2,2-dimethyl derivatives (3) show that 1 and 2 exist in the crystal state exclusively in the chair ( C ) form while 3 exists exclusively in the twist-boat (TB) form.The final values of the agreement index, R,,, are 0.048, 0.059, and 0.046 for 1, 2 , and 3 , respectively. Significant structural differences between the g+g+ arrangement of the acetal moiety with the TB form and the g+g-disposition in the C form are revealed. These differences can be interpreted as arising from a less efficient back-donation of an antiperiplanar lone pair on the oxygen atom in the g+g+ relative to the g'g-arrangement. Furthermore, the C-0 bond lengths shed some light on the hybridization state of the lone pairs of electrons on oxygen. Finally the results are used to rationalize nrnr observations in terms of molecular conformations in solution.ROBERT ST-AMOUR, MARC J. OLIVIER, MAURICE ST-JACQUES et FRANGOIS BRISSE. Can. J. Chem. 64, 500 (1986).L'Ctude par diffraction des rayons X A basse temperature du dioxa-1,3 benzo-5,6 cycloheptene (1) et de ses derives methyl-2 ( 2 ) et dimethyl-2,2 ( 3 ) montre que 1 et 2 existent A 1'Ctat solide seulement sous la forme chaise ( C ) alors que 3 n'existe que sous la forme bateau-croisk (TB). Les valeurs finales du facteur d'accord R,,, sont 0,048,0,059 et 0,046 pour 1, 2 , et 3 respectivement. Des diffkrences structurales significatives entre l'arrangement g+g+ du groupe acetal dans la forme TB et la disposition g+g-dans la forme C sont mises en Cvidence. On interprkte ces differences par une retrodonation moins efficace du doublet antiperiplanaire de I'oxygkne dans I'arrangement g+g+ relativement i l'arrangement g+g-. De plus, les longueurs des liaisons C--0 clarifient le type d'hybridation des doublets de l'oxygkne. Finalement, ces resultats permettent de rationaliser les observations de rmn sur les conformations des molCcules en solution. IntroductionIn recent years, we have studied the conformational properties of 1,3-dioxa-5,6-benzocycloheptene 1 and several derivatives in solution (1, 2) by means of 'H and 13C dnrnr (dynamic nuclear magnetic resonance) methods. They reveal the existence of a mixture of the chair form 1A and the twist-boat form, l B , in a ratio of 79:21 (in CHF2C1, at -130°C) (1).