Thermodynamic parameters for the conformational equilibria of seven 4-methyl-4-R-I ,1 -dimethoxycyclohexanones (R = Et, Prn. Pr'. Bun, and But) were obtained from temperature-adapted shift values. It w a s assumed that bulkier alkyl groups will take the equatorial position rather than the methyl group. AAG" values are found to be larger than expected from the additivity principle, except for But where the contrary i s found. For all derivatives AAH' = ca. 485 cal mol-l. Unfavourable entropic contributions are found for axial a-branched substituents.SHIFTS for methoxy-groups are different when in either the axial or equatorial position,l allowing this group to be used as a possible probe for conformational studies. A systematic study of some cyclohexanone dimethyl acetals, substituted in the 3-and/or 4-position, and of some bicyclic derivatives, has shown,, however, that the interpolation of absolute shift values, as observed in conformational heterogeneous systems between those OMe Xa R' OMe & R2 OMe IIe Ie C.found in model ' compounds, gives unreliable results. Shift values of the methoxy-signals are influenced by substituents on C(3) and C(4), and attempts to predict this influence by comparison with suitable model compounds were unsuccessful. Thus, we attempted to predict the influence of the Me-4eq or Me-4ax group on the shift of the methoxy-group by comparison of Av in 1,1-dimethoxy-3-rnethylcyclohexane with Av in cis-1 ,ldimethoxy-3,5-dimethylcyclohexane. From this, a correction (0-92 in CS, and 0432 Hz in CDClJ might be introduced for the Av values observed in cis-and transl,l-dimethoxy-3,4-dimethylcyclohexane leading to Av(Me-4ax) (2.44 in CS, and 3.73 Hz in CDC13) and to Av(Me-4eq) (3.53 and 4-80 Hz). Finally from the value observed in 1,1-dimethoxy-4-rnethylcyclohexane (3.36 in CS, and 4-50 Hz in CDCl,) one would predict Ke(CS2) = (2.44 + 3-36)/(3.53 -3.36) = 33 [AG",,(Me) = 2-08 & 0.2 kcal mol-l] and Ke(CDC13) = (3.73 + 4-50)/(4.80 -4-50) = 27 [AGoW1(Me) = 1-96 & 0.13 kcal mol-l] and these values are certainly too high, the expected value being 1.69 kcal m0l-1.~94The absolute shift method definitely fails when trying to evaluate the position of equilibrium as a function of temperature. Thus it was found that even for anancomeric models Av, changes with temperature, the variation being of the same magnitude as that caused by the t

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Shift values of the methyl and methylene protons of dimethylketals and dioxolanes of cyclic and acyclic carbonyl compounds are affected by substitution, f.i. the ring size in 1,3 dioxaspiranes. No simple relation could be obtained, and the influence of ring strain (bond hydridization) is not important. The difference in the general trend between dioxolanes and dithiolanes is observed. No simple theory concerning Fhift dependence on fairly remoted magnetic anisotropy seems possible.

Solvent shift criteria can be used for the cis-trans structural assignment in glycidic esters. Protons of substituents which are trans to the alkoxycarbonylgroup are shifted towards higher field in benzene than in carbon tetrachloride. Definitive proof of the structures were obtained by Nuclear Overhauser Effect experiments.

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