Relative configuration nomenclature

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). 

In response to this nomenclature dilemma, the Cahn-Ingold-Prelog (IUPAC, International Union of Pure and Applied Chemistry) system of nomenclature was developed and is now the standard mediod to specify the relative configuration of chiral centers in molecules. Each chiral center will have two possible mirror-image configurations, which are designated as eidter R or S. 

E3 = — + and E4 = - 7 where the + and - define the relative configuration of two adjacent monomer units in the polymer chain. In accordance with the nomenclature of Bovey (7), we call the sequence (+ +) isotactic, (+ -), or (- +) heterotactic, and (- -) syndiotactic. The transition probabilities between various states were defined for the four-parameter model (1). 

Concerning the relative configuration of stereocenters in the products, the nomenclature used here is (usually) the syn/anti convention currently used for acyclic aldols and related compounds (6-8). To apply this convention, the carbon backbone is drawn in its longest extended (zigzag) form such as for 1.1 (Scheme 1). If both substituents on the chain project in the same direction from the plane of the carbon backbone as in 1.2 or 1.3, then the descriptor is syn. If the two substituents project in opposite directions from the plane of the carbon backbone as in 1.4 or 1.5 the descriptor is anti.f... 

We will use the syn/anti nomenclature [5] to describe the relative configuration of aldol stereoisomers, and the Ik/ul nomenclature [6] to describe the topicity of the reaction. For definitions, see glossary. Section 1.6. 

When a prochiral acceptor (RiCH=A) and a prochiral donor (R2CH=D) react, the stereoisomers are labeled as either syn or anti based on the relative configurations of Ri and R2 when the Michael adduct is drawn in a zig-zag projection, as shown in Scheme 5.28. Using the Re/Si nomenclature and assuming that the CIP rank is A>Ri>H and D>R2>H, the syn adducts arise from Ik topicity and anti adducts arise from ul topicity. 

Scheme 6.25. Diastereoselective cyclopropanations of vinyl carbenoids [97]. For disubstituted carbenes, cis/trans nomenclature is used to describe relative configuration, referring to Ri relative to the carbonyl moiety, as shown in bold.

Scheme 6.31. Diastereoselective cyclopropanation of olefins with vinyl carbenes [116]. Note that only two of the four possible stereoisomers were found in the product mixture. The trans nomenclature refers to the relative configuration of R and CO2R, consistent with that of Scheme 6.24.

In all the listed amino acids, with the exception of glycine, the a-carbon is bound to four different substituents hence it is a stereogenic center. From this it follows that every amino acid can appear in the form of two enantiomers. In the following example, both the enantiomers of alanine are represented together with their absolute configurations. However, enantiomers of amino acids can also be represented by the traditional notation of chiral molecules that is called the relative configuration. This nomenclature for configuration was proposed Emil Fischer in the nineteenth century for the representation of the stereochemistry of carbohydrates. 

The / u system of nomenclature, introduced by Seebach and Prelog to describe the relative configuration of diastereomers, can be extended to specify the relative topicity of diastereoseiective reactions. W This involves comparing the/ /5 mdiRe/Si specifications of the stereogenic units and/or enantiotopic faces or groups involved. The combinations (R, Re), (5, Si), (Re, Re) and (Si, Si) are denoted Ik (like) while (R, Si), (S, Re), (Re, Si) and (Si, Re) are denoted ul (unlike). To illustrate the use of this system we will again take the examples of section 1.11 which are representative of the two major types of diastereoseiective reaction. 

Differently from what commonly reported for the nomenclature of cyclic compounds bearing doubly substituted stereocenters [Cross, L. C. Klyne, W. PureAppl. Chem. 1976, 45, 11-30], for our convenience the cis/trans notation wiU he hereafter employed to indicate the relative configuration of the methoxy groups in 41a and its achiral congeners. 

FIGURE 1.1 Chemical and stereochemical nature of amino acids. Substituents in (a) and (b) are on opposite sides of the plane N-Ca-C, the bold bond being above the plane. Interchange of any two substituents in (a) changes the configuration. For the Cahn-Ingold-Prelog system of nomenclature, the order of preference NH2 > COOH > R2 relative to H is anticlockwise in (a) = (S) and clockwise in (c) = (R). 

In systems in which the vehicle configuration is volume limited, theoretical performance comparisons, using density impulse (Isd) are also necessary. This nomenclature, which is the product of the Is and the bulk density of the propellants, defines the amount of thrust available in a unit volume of the propellant. The relative importance of Isd to Is must be defined in the mission analysis. 

For this set of activities let s assume that the first marble we are working with is dark colored. Figure 8.5 shows the three possible distinct distributions for the first marble. We use the same nomenclature for the relative weights of j = 0 and 1 as before. We will not breakdown the j = 0 list to tell whether the marble is in the left sub-box or the right one, we just define a configuration by the total number in each box, summing over the sub-boxes. The results are the same as before W(l, 0) = 2 and W(0, 1) = 1. This time the areas of the boxes did not matter, but the number of sub-boxes in each box (or looking ahead, in each level ) did. We denote the number of sub-boxes in each box j as gj. So, as before, go = 2 and gi = 1 (see Fig. 8.4). 

The cis and trans nomenclature for alkenes is an old method of classifying the configurational isomers of alkenes and is still in common use. However, it is only suitable for simple 1,2-di-substituted alkenes where we can compare the relative position of the two substituents with respect to each other. When it comes to trisubstituted and tetrasubstituted alkenes, a different nomenclature is needed. 

The compound has two chirality centres and three pseudo chirality centres. There is however, only one (achiral) diastereomer of the compound shown in the question. The two isomers can be distinguished from one another solely on the relative position of the chlorine or bromine atoms which lie in a plane which also happens to be the plane of symmetry of the molecule (this is the only symmetry element present, therefore the symmetry point group is Cs). It is possible in this instance to specify the configuration unequivocally using the descriptors E and Z. However, in systematic nomenclature the complete configuration of all the stereogenic centres is specified. Thus the (so-called) Z isomer is (ls,3r,5 ,6r,7S)-l,6-dibromo-3,6-dichloroadamantane and the isomer is (ls,3r,5 ,6s,7S)-l,6-dibromo-3,6-dichloroadamantane, i.e. the two isomers can be distinguished simply by the descriptor used for position 6. 

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.

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