Erythro and Threo

Erythro and threo are common terms in stereochemistry used for naming molecules with two stereogenic centers. The names derive from the saccharides erythrose and threose, so let’s draw their Fischer projection to understand the basis of this notation:

What we see is that when two common substitutes, in this case the H and OH groups, of the stereogenic centers are on the same side, we have the sugar erythrose and when they are on opposite sides, it is the threose. Threose and erythrose exist in two enantiomeric forms which are designated as D and L enantiomers. There is a separate post about the D and L notation in carbohydrates, however, as far as today’s topic, the names erythrose and threose are applicable to both enantiomers:

This approach for naming chiral compounds became quite general and spread outside of carbohydrates. The configuration of any molecule with two stereogenic centers can be classified as erythro or threo.

For example, the following halide is said to be erythro when the Cl and Br groups on the neighboring carbon atoms are on the same side, and threo, when they are on the opposite sides:

Erythro or threo configurations can also be evident in sawhorse projections. If the two identical groups on both chiral centers can be eclipsed, then we have the erythro configuration:

However, some uncertainty arose when the erythro/threo nomenclature was applied to systems with only one common substituent on the two neighboring chiral centers. This led to the use of more common and modern sin and anti nomenclature.

In this case, we draw the carbon chain as a zig-zag, and if the substitutes lie on the same side of the plane, the stereochemistry is syn and when the groups are on the opposite sides of the plane, it is anti: