. Can. J. Chem. 65, 1761 (1987). The I3C and 'H nuclear magnetic resonance data of 3-oxycyclohexenes, 2-oxymethylenecyclohexanes, and 1-oxytetralins show (i) that the pseudoaxial preference is larger for the acetoxyl group than for the methoxyl group in 3-oxycyclohexenes, (ii) that the methoxyl and the acetoxyl groups have the same pseudoaxial preference in I-oxytetralins, and (iii) that in 2-oxymethylenecyclohexanes, the acetoxyl group prefers the equatorial orientation whereas the methoxyl group prefers to be axial. These results are interpreted in terms of the orbital picture (T-u* stabilization of the axial conformer) of the generalized anomeric effect to which is opposed a conformational effect termed the "unsaturation effect" in 2-acetoxymethylenecyclohexane and probably also, but to a much smaller extent, in I-acetoxytetralin. The results also show that cis and trans tert-butyl analogues of 3-oxycyclohexenes do not represent the true limit conformers whereas cis and trans 4-tert-butyl derivatives of 2-oxymethylenecyclohexanes do not differ drastically from the limit conformers.ADAMA OU~DRAOGO, MINH TAN PHAN VIET, JOHN K . SAUNDERS et JEAN LESSARD. Can. J. Chem. 65, 1761 (1987) Les donnees de la resonance magnetique nuclCaire du I3C et du 'H montrent (i) que la preference pseudo-axiale d'un groupe acktoxyle est supkrieure t i celle d'un groupe mithoxyle dans les oxy-3 cyclohexenes, (ii) que les groupements methoxyle et acktoxyle des oxy-1 tttralines ont la mCme preference pseudo-axiale et (iii) que, dans les oxy-2 mCthylknecyclohexanes, le groupement acetoxyle prefkre l'orientation tquatoriale tandis que le groupement mkthoxyle prefkre l'orientation axiale. Ces rksultats sont interpretts en termes des interactions orbitalaires (stabilisation T-u* du conformere axial) de l'effet anomkre generalist auquel s'oppose un autre effet appele << effet d'insaturation s dans l'acitoxy-2 mCthylknecyclohexane; cet effet serait present aussi dans l'acktoxy-l tttraline mais serait beaucoup moins fort. Les rCsultats montrent encore que les derives tert-butyles cis et trans ne representent pas les vrais conformkres des oxy-3 cyclohexknes alors que les derives tert-butyles en position 4 des oxy-2 mCthylenecyclohexanes ne diffkrent pas trop des vrais conformkres.
CHFCl? solvents show that the axial preference increases with electron withdrawal in the aryloxy group: from 79% (AGoE +A = -0.4 kcal mol-I) (4-OCH,) to 90% (AGoE+A = -0.7 kcal mol-I) (4-NO2) in CF2Br2. The axial preference (anomeric effect) is smaller in the more polar CHFC12 solvent, as expected, and the substituent effect is smaller also: change in AGoE+A from -0.3 (4-OCH,) to -0.5 (4-NO2) kcal mol-I. However, the axial preference of 2-acetoxytetrahydropyran is shown to be smaller than that of 2-phenoxytetrahydropyran in CF2Br2 solvent although the acetoxy group is expected to be more electronegative than the phenoxy group. Furthermore, the polarity of the solvent has no effect on the conformational equilibrium of 2-acetoxytetrahydropyran. The results are interpreted in terms of both orbital and electrostatic interactions. The ring-inversion barrier of 2-alkoxy and 2-aryloxytetrahydropyrans, determined by the temperature-coalescence method, is lower than that of tetrahydropyran by about 1.5 kcal rnol-I .Key words: 2-substituted tetrahydropyrans, conformational analysis, anomeric effect, ring inversion.ADAMA OUEDRAOGO et JEAN LESSARD. Can. J. Chem. 69.474 (1991).Les donntes de la resonance magnktique nuclCaire du I3C a 156 K, dans les solvants CF2Br2 et CHFCl,, montrent que la prCfCrence axiale d'une serie de tetrahydropyranes portant, en position 2, un groupement phCnoxy substitut a la position par0 croit avec le pouvoir Clectroattracteur du substituant : de 79% (AGOE-,~ = -0.4 kcal mol-') pour un groupement 0CH3 a 90% (AGoE +A = -0.7 kcal mol-I) pour un groupement nitro dans CF2Br2. La prCfCrence axiale (l'effet anomere) est moindre dans CHFCl?, solvant plus polaire, tel qu'attendu, et l'effet de substituant est tgalement plus faible : variation de AGoE + A de -0.3 (OCH,) a -0.5 (NO2) kcal mol-I. Toutefois, dans CF2Br,, la proportion de conformere axial est moins ClevCe pour le 2-acCtoxytCtrahydropyrane que pour le 2-phtnoxytetrahydropyrane m&me si le groupement acCtoxy est plus Clectrontgatif que le groupement phCnoxy. De plus, la polarit6 du solvant n'a pas d'influence sur l'tquilibre conformationnel du 2-acCtoxytttrahydropyrane. Les rCsultats sont interprttCs en termes d'interactions orbitalaires et Clectrostatiques. La dktermination de la barriere d'inversion de cycle d'alcoxy-et d'aryloxytCtrahydropyranes par la mCthode de coalescence de tempkrature montre que la barritre est plus faible que celle du tttrahydropyrane d'environ 1,5 kcal par mole.Mots elks : tCtrahydropyranes substituts en position 2, analyse conformationnelle, effet anomere, inversion de cycle.
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