Equatorial orientation

Bonds 1 and 3 are in axial positions in the helix, with bond 2 oriented equatorially. 

Fig. 4 shows c.d.-difference spectra that are due to the presence of a hydroxymethyl group in one member of the pair. The pairs of pyranoses have the same conflguration about each asymmetric carbon atom that is part of the ring. Three related pairs have the same conflguration at the carbon atoms near the hydroxymethyl group and give very similar difference spectra. The similarity indicates that the rotameric distribution for the hydroxymethyl group is similar for each of these pairs. This is to be expected, as all three sugars have 4-hydroxyl groups oriented equatorially, but the relationship is certainly not obvious from the c.d. spectra themselves (see Fig. 3). In contrast, when the conflguration near the hydroxymethyl group...

It was also observed that an electropositive group, particularly nitrogen-containing heterocycles, at the anomeric center tends to be oriented equatorially. This phenomenon is referred to as reverse anomeric effect. However, it is accepted that the reverse anomeric effect is not generalized [1,2]. 

Fig. 3-5. Structure of cellulose. Note that the /S-D-glucopyranose chain units are in chair conformation ( Ci) and the substituents HO-2, HO-3, and CH2OH are oriented equatorially.

An isolated example of the use of x-ray diflraction analysis is provided by the work of Camerman and coworkers. Hydroformyl-ation of 3,4,6-tri-O-acetyl-D-glucal gave 4,5,7-tri-0-acetyl-2,6-anhydro-3-deoxy-D-gIuco- and -D-manno-heptitols, which were converted into the corresponding 1-p-bromobenzenesulfonates. One of these sulfonates was shown hy x-ray analysis to have the CJ(d) conformation in which all of the substituents are oriented equatorially hence, this sulfonate was the D-gluco isomer. 

Only c/5-1,3-dienes can attain this geometry, and only cis dienes isomerize by this mechanism. If R and R differ in size, the most stable transition state is that in which the bulkier group is oriented equatorially. This accounts for the observation that isomerization of m-1,3-hexadiene gives predominantly cw,/rflAi -2,4-hexadiene. ... 

In the presence of the isopropyl group, the cyclohexane ring exists preferentially in the chair conformation in which this group is oriented equatorially... 

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