Additional Stereochemical Nomenclature

The r (for reference ) system was developed to serve as the reference for the stereochemical placement of substituents and is particularly useful for diastereomers. In the case of 48, for example, the carboxylic add function is taken as the reference, and the other substituents are indicated as being either 

For further discussion of the r notation, see (a) reference 97, p. 112 ff (b) Orchin, M. Kaplan, F. Macomber, R. S. Wilson, R. M. Zimmer, H. The Vocabulary of Organic Chemistry John Wiley Sons New York, 1980 p. 139. 

The E or Z nomenclature can also replace the older S)m-anti terminology. Compound 51 has been called sj/n-propiophenone oxime because the OH and the phenyl are on the same side of the double bond, while 52 has been called the anti isomer because these two larger groups are on opposite sides. The distinction between syn and anti depends on the size or complexity of the substituents, and that may not always be unambiguous. Moreover, many authors reserve the terms syn and anti to describe the stereochemical pathway of a reaction, not the stereochemistry of molecules. With the E or Z system, 51 is unambiguously the Z isomer, while 52 is the E oxime. 

The R or S and the E or Z nomenclature systems can be combined as needed to name specific compounds. For example, the lUPAC name of (-)-matsuone (53), the primary sex attractant pheromone of a red pine scale, is (2E,4E,6E,10R)-4,6,10,12-tetramethyl-2,4-tridecadien-7-one. °° 

The R and S system is widely accepted for most organic compoimds, but for historical reasons we often use the d and l system for carbohydrates and amino acids. By convention, a Fischer projection is drawn vertically with the 

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Added in proof) With the recent additions to stereochemical nomenclature lw, it becomes possible to name the faces of compound 62 in rejsi terminology. The cyclobutanone is opened with a double complementation of C(l) yielding (i) in which the left branch has auxiliary descriptor R0, the right S0. The top face is thus si and the bottom face re 178>. 

The scope of this problem can be indicated by a brief consideration of the stereochemical nomenclature of hydroxy derivatives of the bicyclic terpenes. For the parent bicyclic hydrocarbon compound known in the literature at present as cam-phane, the name "bomane is recommended in this report. (Reasons for choosing "bornane are discussed later under "The Bornane Hydrocarbons .) A mono-hydroxy derivative of bornane, on the basis of IUPAC rules (41), would be named as a bornanol. Two such derivatives have the common terpene names, bomeol and isobomeol these names, on the basis of recommendations in this report would both become 2-bornanol. Obviously, this is inadequate and additional designations are necessary to distinguish between die two stereoisomers. 

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). 

A first attempt towards an asymmetric synthesis by the lithiation reaction used (— )-spartein as a chiral additive [12,13], but the enantioselectivity was disappointing. With the chiral (S)-M-ferrocenylmethyl-2-methylpiperidine, a high diastereoselec-tivity was observed [12, 13, 112] (note that the polemic about the structure of the product is due to different stereochemical nomenclature, i.e., central vs. planar , as discussed in Sect. 4.1). A breakthrough was achieved by the readily resolvable... 

The Re-Si nomenclature enables the faces of a carbonyl group to be differentiated stereochemically however, the carbonyl group itself is achiral. Moreover, the Re-Si designation is not indicative of the stereochemistry of the chiral center produced by addition. In the above example hydride addition to the Si face gives... 

CAS now registers and names substances with partially defined stereochemistry. Previously, partial stereochemistry was generally ignored. The presence of unknown chiral centers is indicated by the addition of the term [partial]- to the end of the normal stereochemical descriptor. When the reference ring or chain has incompletely defined chiral atoms/bonds, the format cites the stereochemistry using R and S terms with their nomenclature locants for all known centers. If this method is used to describe a substance for which only relative stereochemistry is known, rel is added to the stereochemical descriptor. Any stereochemical descriptor marked as rel always cites the first centre as R-. 

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) ... 

Two other nomenclature terms that have been used to store stereochemical information in connection tables are the labels aor b indicating whether a substituent points below or above a parent (ring) system, respectively. This method is frequently used for steroids. The actual structure can differ from the parent not only by additional substitution but also by, for example, ring contraction, which is coded as modifier to the parent. Stereoparent descriptors lack the generality of the CIP rules as a separate reference model has to be defined for each ring system. The prime benefit of this system is the ease with which the descriptor can be assigned by humans given a known reference template. 

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