2,3,4,5-Tetrahedro-1-benzoxepin (8) and its 3-substituted derivatives (9–13) have been studied by 1H and 13C dynamic nuclear magnetic resonance in a few solvent systems. The results show that, while 8 and its methyl derivative 9 exist solely in chair forms (C for 8 and Ce:Ca (96:4) for 9), the twist-boat (TB) conformation contributes significantly to the conformational equilibria of the derivatives 3-methoxy 10 (Ca:Ce:TB, 70:20:10), 3,3-dimethyl 11 (C:TB, 90:10), 3,3-methylmethoxy 12(Ca:Ce:TB, 89:4:7), and 3,3-dimethoxy 13 (C:TB, 92:8). The analysis of several factors (steric, electrostatic, and electronic) on the conformational behaviour of these molecules shows why the TB form is a viable conformational alternative to destabilized chair forms in this benzoxepin system.
The conformational properties of 8,11-dimethyl-5,6-benzo-2-oxo-1,3,2-dioxathiepin (5) and of 4,4-dimethyl-5,6-benzo-2-oxo-1,3,2-dioxathiepin (6) have been investigated using 1H and 13C dnmr as well as ir spectroscopy. Analysis of low temperature nmr spectra (down to −148 °C) and of S=O stretching vibrations in room temperature ir spectra indicates that the most stable conformations detected for solutions in CHF2Cl/CD2Cl2 (80:20) are TB (83%) and C-a (17%) for 5 and only the TB 7 for 6. Substituent effects of methyl groups at the peri and the benzylic positions on the conformational properties of the seven-membered sulfites are discussed.
The conformational and dynamic properties of the stereoisomers of 4-methyl-5,6-benzo-2-oxo-1,3,2-dioxathiepin (5) and of the 4,7-dimethyl derivative (6) have been investigated by 1H and 13C dnmr methods, as well as by ir spectroscopy. Analysis of the nmr spectra at low temperature (−125 °C), together with the conformational interpretation of the stretching vibration bands of the S = O bond in the ir spectra, indicate that the most stable conformations detected for solutions in CHF2Cl are C-a (82%) and TB-t-Mec (18%) for 5a, TB-c-Mec, (96%) and TB-c-Mea, (4%) for 5b, C-a for 6a, TB-t-Mec, c-Mec, for 6b and C-e for 6c. The substituent effects of a methyl group at the benzylic position on both the conformational and dynamic properties of the seven-membered sulfite are discussed.
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