Conformational analysis. XXII. Conformational equilibria in 2-substituted 1,3-dioxanes

Nuclear magnetic resonance characterization of the ring conformations of monosubstituted derivatives of 1,3-dioxa-5,6-benzocycloheptene

“…It is interesting to compare the trends for AG:, in Table 2 with AGO values obtained for BF, catalysed equilibrations of derivatives 13 to 1 7 of 4,6-dimethyl-l,3-dioxane in diethyl ether at 25°C (10). Although these values correspond to the C-e 5 C-a process, there exists a remarkable parallel to the trend of AG;,, listed in Table 2 for solutions in dimethyl ether for the C-e = T B process of the analogously 2-substituted seven-membered acetals.…”

The conformational preference and barrier to internal rotation of an equatorial 3,5-dichlorophenyl group by the J method. Derivatives of cyclohexane, 1,3-dithiane, 1,3-dioxane, and 1,3-dioxolane

“…Compounds 6-9 were prepared by standard procedures (34,(36)(37)(38)(39)(40)(41)(42) and gave satisfactory mass and 'H nmr spectra (28,(43)(44)(45)(46)(47). These were repeatedly calibrated at 5-Hz intervals in the frequency sueep mode of a n HA100 spectrometer at a probe temperature of 305 5 1 K. The spectral dispersion was 1 Hz:' cm and sweep rates were 0.02 and 0.01 Hz/s.…”

Synthesis and stereochemistry of some new 1,3,5-tris(1,3-dioxan-2-yl)-benzene derivatives

“…The A values (A = free conformational enthalpy [7]) of aryl groups located in the acetal part of the 1,3-dioxane ring are high (e.g. A P h = 13.04 kJ/mol [8]) and 2-aryl-1,3-dioxanes are anancomeric compounds. They show high preference for the conformer presenting the aromatic group (R 1 = aryl, R 2 = H, Scheme 2) in equatorial orientation [1-3, 9, 10].…”

“…They show high preference for the conformer presenting the aromatic group (R 1 = aryl, R 2 = H, Scheme 2) in equatorial orientation [1-3, 9, 10]. At position 2 of the 1,3-dioxane ring the A-value of methyl group (A Me = 16.63 kJ/mol [8]) is higher than the A-value of phenyl group and the thermodynamic measurements [11] showed that the equatorial preference of methyl group in 2-methyl-2-phenyl-1,3-dioxanes is three times higher than expected by the simple addition of the A values of the two substituents (ΔG o exp = 10.11 kJ/mol; A Me -A P h = 3.63 kJ/mol) and the conformational equilibrium of the corresponding 2,2-disubstituted-1,3-dioxanes (R 1 = CH 3 , R 2 = aryl, Scheme 2) is strongly shifted towards the conformer exhibiting the aryl group in axial position [4,6,12].…”