Conformation eclipsed, definition

As in the definition of rotations for the backbone, the sense of rotation is such that a right-handed rotation about any side-chain bond, looking in the direction of numbering, is taken to be positive. The choice of zero angle of rotation is such that the planar eclipsed (cis) conformation is denoted by Xj — 0 (Fig. 4). For the C —bond, the N—C and the —Cy bonds are used to define the angle xi In branched chains, Branch 1 is always used to define the angle of rotation. 

Unfortunately, there is more than one definition for threo- and erythro. Winstein and co-workers gave the following definition of these diastereomers In a compound with two asymmetric carbons that has two common ligands and a third that differs, the isomers that would be meso if the third ligand were identical are erythro diastereomers.An alternative definition is If two asymmetric carbons have only one ligand in common, then the other four ligands are paired in the same commonsense way and isomers that would have equal pairs eclipsed in any conformation are erythro. Eliel, Mislow and their co-workers defined erythro and threo in terms of Fischer projections. The aldol products (see sec. 9.4.A) 68 and 69, and the ester-aldehyde condensation products 70 and 71.30 shown with the erythro and threo notation. 

Figure 3-5 shows that isotactic polymers in certain projections definitely do not have their substituents always on the same side as is frequently stated. Such an assumption is only correct for the two types of projections in the eclipsed conformation. In projections of trans-staggered conformations differences exist according to the monomeric unit. 

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