Erythro nomenclature

It is found in lichens and in some algae. It has m.p. 120 C, is very soluble in water and is about twice as sweet as sucrose. It is a reference compound upon which the erythro nomenclature is based. 

Organic chemists use an informal nomenclature system based on Eischer projections to distinguish between diastereomers. When the caibon chain is vertical and like substituents ar e on the sane side of the Eischer projection, the molecule is described as the erythro diastereomer. When like substituents are on opposite sides of the Eischer projection, the molecule is described as the three diastereomer. Thus, as seen in the... 

Erythro/threo Terms derived from carbohydrate nomenclature used to describe the relative configuration at adjacent stereocenters. Erythro refers to a configuration with identical or similar substituents on the same side of the vertical chain in Fischer projection. Conversely, a threo isomer has these substituents on opposite sides. These terms came from the nomenclature of two carbohydrate compounds, threose and erythrose (see Fig. 1-35). 

Figure 6 Structures of the four stereoisomers of sphingosine. Sphingosine has two chiral carbon atoms (C-2 and C-3). The Fischer projection formula of each structure is also shown, with C-1 at the top, to illustrate the D/L and erythro/threo stereochemical nomenclature. C-3 has an erythro orthreo configuration as it relates to C-2, depending on whether the similar groups (amino and hydroxy) are on the same or opposite side of the Fischer projection. D versus L refers to the configuration at C-2 relative to the configuration of D-glyceraldehyde versus L-glyceraldehyde.

In addition to the R and S designations, compounds with two chiral centers may also be identified by stereochemical nomenclature that describes the entire system. For example, the erythro and threo nomenclature derived from carbohydrate chemistry may be employed to describe the relative positions of similar groups on each chiral carbon. Thus, the ephedrines are designated as erythro forms since the similar groups (OH and NHCH3) are on the same side of the vertical axis of the Fischer projection, and the pseudo-ephedrines are designated as threo forms since like groups are on opposite sites of the vertical axis of the projection (Fig. 10). 

In reading Nef s description of these substances and their preparation, it must be borne in mind that the available, naturally occurring D-xylose was at that time called i-xylose. Moreover, Rosanoff s convention for assigning configurational prefixes was then relatively new and was not utilized by Nef. Accordingly, Nef s 1-xylose and 1-arabinose, and d-erythro-, l-threo-, l-erythro-, and d-[Pg.41]

Heathcock et al. described the process that formed diastereomers in the aldol condensation, from precursors that do not contain a chiral center, as simple diastereoselectivity.209,210 Naming protocols to describe the diastereomers produced in the aldol condensation include the erythro/threo nomenclature, as well as the syn/anti nomenclature was discussed in Section I.4.B. Using this latter convention, diastereomers 340 and 343 were designated as anti and diastereomers 341 and 342 were designated as syn. 

EXAMPLE 3.4 The aldo-octose shown in Fig. 3-4 has an L-galactose-like configuration in the first five carbon atoms, and in the last three carbon atoms a configuration like the lower three of D-erythrose. Accordingly it is called D-erythro-L-galacto-octose. This is a rarely encountered monosaccharide and is only given here as an example of the use of the standard nomenclature for carbohydrates. 

Erythro and threo nomenclature. The terms erythro and threo are used with dissymmetric molecules whose ends are different. The erythro diastereomer is the one with similar groups on the same side of the Fischer projection, and the threo diastereomer has similar groups on opposite sides of the Fischer projection. The terms meso and ( ) [or (d,/)] are preferred with symmetric molecules. 

Explain mechanistically the stereochemical course of the reaction. Note Don t be concerned in case you are unfamiliar with the erythro/threo nomenclature we are asking you to simply account for the E or trans stereochemistry of the alkene product. 

The 3 different systematic names have different numbering schemes numbering shown is for the furan name erythro and threo stereodescriptors depend on which nomenclature is used. 

For stereoregular insertion there are two modes to consider—cis insertion and trans insertion. For both isotactic and syndiotactic production, the cis mechanism has been determined to be in operation. This was established by polymerizing with cis-, and trans-l-deuteriopropylene or related monomers. The expected stereochemistry was demonstrated when deuteriopropylene was polymerized. The cis monomers produce erythro monomer imits whereas the trans monomer yields the threo units when cis- and tra/is-l-d-propylene is polymerized. In some cases the nomenclature appearing in the literature can be confusing and contradictory, but all indicate cis insertion. To be specific, as defined below, stereochemical structures from cis and trans addition to the double bond of cis-(l-di) and trans-(l-di)-propylene to isotactic polypropylene are as follows (229) ... 

The bromination of alkenes is an example of an electrophilic addition reaction. (See Experiments [A2b] and [D2] for detailed discussions of the mechanism involved in this reaction. In particular, refer to Experiment [D2], which very closely resembles this reaction, for a discussion of the erythro and threo nomenclature used in this experiment.) In the present reaction, bromination of 4-nitrochalcone yields eiyfl2ro-2,3-dibromo-3-(4-nitrophenyl)propiophenone. 

When neither the enolate 29 nor the aldehyde contains stereogenic units, both reactants have enantiotopic faces and 30a and 30b are enantiomers. The same is true for the pair 31a and 31b. However, 30 and 31 form a pair of diastereomers. When an aldol addition leads to an excess of one of these diastereomers 30 or 31, it is said to exhibit simple diastereoselectivity. Several notations that assign descriptors to diastereomeric aldols are found in the literature. The classical erythro/threo nomenclature, which is based on Fischer projection formulas [62], will not be used in this chapter, because it can cause considerable confusion with branched carbon chains. Among the... 

---