Stereochemical Implications from the Conformational Analysis of the Seven-membered Ring in 5-Oxabenzocycloheptene

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. 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...

Section: Dynamic Propertiesmentioning

confidence: 99%

“…After 8 h, most of the starting material had disappeared (less -30%); compounds 8c and 8e were then isolated and purified from the crude mixture by vpc under the conditions given above. Each product was characterized by 'H and 13 C spectra (Tables 1 and 2) …”

Section: 2-mentioning

confidence: 99%

Conformational consequence of stereoelectronic effects in seven-membered cyclic acetals: nuclear magnetic resonance investigation of 1,3-benzodioxepin and its 2-substituted derivatives

St-Amour¹,

Viet²,

St‐Jacques³

1984

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“…Our results seem to show that the same trend exists in seven-membered rings. A previous report (13) has shown that while A6 for the benzylic methylene protons is -0.106ppm in benzocycloheptene, introduction of a p oxygen atom moves the equatorial proton at 0.504ppm upfield from the axial proton. Assuming that the same trend applies qualitatively to 2 (or 6) we would expect the two oxygens P to the protons on C-3 to shift the equatorial proton at about 0.4 ppm upfield from the axial proton since in benzocycloheptene A6 for the corresponding protons in 0.41 ppm (14).…”

Section: Conformations Of the Seven-membered Ringsmentioning

confidence: 84%

Conformational dynamic investigation of 1,5-benzodioxepin and its dimethyl derivative by ¹H and ¹³C nmr

Menard¹,

St‐Jacques²

1981

The conformational and dynamic properties of 1,5-dioxa-6,7-benzocycloheptene (2; 1,5-benzodioxepin) and its dimethyl derivative (7) have been investigated by dynamic 1H and 13C nmr methods. Analysis of the spectra at low temperatures (below coalescence) indicates that the most stable seven-membered ring conformations detected for solutions in CHF2Cl are a mixture of the chair (C, 80%) and the twist-boat (TB, 20%) for 2 and a mixture of TB (64%) and C (36%) for 7. Free energy barriers were determined for the chair inversion of 2 (6.5 kcal/mol) and 7 (7.5 kcal/mol) as well as for the twist-boat pseudorotation of 7 (6.8 kcal/mol). The conformational properties of these seven-membered heterocycles are discussed and compared to those known for 2,4-benzodioxepins.

“…It is important to point out that each of the compounds prepared actually contains essentially equal amounts of syn and anti isomers, but because the 'Hmr signals of each side of the ring are essentially independent (14,16), only the anti isomers have been shown in the schemes.…”

Section: D(h)mentioning

confidence: 99%

“…The key reaction involved a catalytic reduction of double bonds with deuterium gas; it is expected to place vicinal deuterium atoms in a predoniinalltly cis arrangement (14,16). As these compounds were prepared at different times, the isotopic purity varied according to the experimental conditions.…”

mentioning

confidence: 99%

Nuclear magnetic resonance investigation of the conformational dynamic properties of benzocycloheptenones and related compounds

Bodennec¹,

St‐Jacques²

1977

Free energy barriers for ring inversion were determined from dnmr studies of 3,4-benzo-cycloheptenone (4) and for deuterated derivatives 6 and 7 of 4,5-benzocycloheptenone (5). The investigation of 2,3-benzocycloheptenone (3) revealed negative results. The magnitude of ΔG≠ for 4 and the trend observed for 5, 6, and 7 strongly suggest that the chair conformation of these molecules interconverts through a wagging motion of the C(5) end of the benzocycloheptene seven-membered ring.