Gauche orientations, and

Note that the otCH-pCH torsion angle is trans in all cases (D. W. Urry and N. G. Kumar, Biochemistry, 13, 1829-1831, (1974). A gauche orientation gives so much steric crowding that the complex will not form... 

Starting from ( )-enolates, the corresponding argument predicts that transition state 10a is lower in energy than the alternative model which suffers from mutual steric hindrance of the substituent R1 and the methyl group in a gauche orientation thus, stereoconvergence results. 

In Fig. 11, experimental results are shown for the PET films, together with two calculated relationships between B20/a2 and P20o°v< rall> based on the alternative hypotheses that the NMR anisotropy arises either from wholly trans oriented material, according to Equation (40b) or from wholly gauche oriented material. It is very satisfactory to note that the experimental results are very close to the first hypothesis. 

P2X Z = 2 D = 1.30 R = 0.10 for 995 intensities. The enopyranoside has the °HS conformation, with Q = 47 pm, 0 = 51°, = 326°. The orientation of the primary alcohol group is gauche-trans, and the a-glyco-sidic bond has the usual + syn orientation. The bond lengths given, which are of low accuracy, do not differ significantly from normal values. The hydrogen-atom positions were not determined. 

Nonadditivities of individual effects of vicinal substituents were recognized early. Dalling and Grant (100,101) reported that upheld (i.e., negative) a-ICSs of 2.5 to 3.5 occur at signals of substituted carbon atoms in 1,2-dimethylcyclohexanes if the methyl groups are in gauche orientation to each... 

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