A bicyclic indolizidinone carboxylic acid and a tricyclic constrained analogue of l-proline were synthesized and evaluated for their ability to induce helix formation as l-Ala tetrapeptides. Variable-temperature NMR, DMSO titration, CD spectra, and X-ray structure analyses, in conjunction with molecular modeling, confirmed the existence of 3(10)-helical motifs with di- and tetrapeptides of l-Ala.
. 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.
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