Absolute configuration nomenclature

Identify what appears to be the compound s central molecular scaffold and estimate how flexible or inflexible that system may be in terms of adopting different shapes or movements in 3D space locate any asymmetric centers and if stereochemical information is displayed in the structure then discern the R or S absolute configuration nomenclature for each of these sites. 

Implicit ia the base names are the absolute configurations at carbons 8 and 12 and the iadicated numbering systems. Derivatives of these parent stmctures are named according to terpene and steroid nomenclature rules (see Steroids Terpenoids). The lengthy and awkward nature of the chemical abstract systematic nomenclature (12) for these compounds has resulted ia the development (13) and use of simplified nomenclature based on common names. 

This chapter has provided a general introduction to stereochemistry, the nomenclature for chiral systems, the determination of enantiomer composition and the determination of absolute configuration. As the focus of this volume is asymmetric synthesis, the coming chapters provide details of the asymmetric syntheses of different chiral molecules. 

The P-atom in sarin (9.84), soman (9.85), and tabun (9.87) is a stereogen-ic center, allowing for stereoselective enzymatic hydrolysis [162], This aspect has been extensively investigated for soman, which exists as four stereoisomers by virtue of the presence of a second stereogenic center (C-atom). These stereoisomers are usually designated as C(+)P(-), C(-)P(+), C(+)P(+), and C(-)P(-), where C(+/-) refers to the 1,2,2-trimethylpropyl moiety and P(+/ ) to the P-atom. Such a nomenclature may be convenient but has no implication for the absolute configuration. The C(+)P( ) and Cf-)P(-) epimers are the more active toward acetylcholinesterase and, hence, the more toxic ones. In contrast, the C(+)P(+) and C(-)P(+) epimers are preferentially hydrolyzed... 

Since the first reaction undoubtedly proceeds with inversion of configuration at sulfur, and since additional experiments demonstrated that the formation of chlorosulfurane 177 from sulfoxide 180 takes place with retention at sulfur, (5>chirality was assigned to (+>177. As for the designation of absolute configuration, Martin and Balthazor (195) proposed a system of nomenclature for optically active pentacoordinate species. 

Figure 3.8 The two enantiomers of a-aminoacids. Here we follow the classic nomenclature of l- and D-aminoacids for indicating the two chiral forms. In terms of the S, R nomenclature, L-aminoacids correspond to the S absolute configuration -except for cystein, which is R.

Throughout this article, the heteroring will be referred to as either piperazine-2,5-dione or occasionally as cyclodipeptide. The earlier practice of referring to the ring as diketopiperazine has been avoided. The numbering of the piperazinedione ring is as shown in (1). In the cyclodipeptide nomenclature, the common three-letter code for the two amino acids, with the necessary prefix to indicate absolute configuration, will be used. 

Special nomenclature has been developed to specify the absolute configuration of the four substituents... 

FIGURE 5.11 Absolute configuration of l,2 -disubstituted metallocenes with the nomenclature (solid line is upper ring substituent and dotted line is lower ring substituent). 

The accepted conventional nomenclature based on the cepham (the fused /3-lactam-perhydrothiazine system) is used throughout this chapter. Carbon atom bonded to C-3 has been numbered as C-10 (or C-3 ). Stereochemistry at C-7 is specified either as absolute configuration R/S, or as a//3 depending on the orientation of the substituent, below or above the plane, respectively. The above abbreviated common names and numbering for the cephalosporins should not be confused with the IUPAC systematic nomenclature as used by Chemical Abstract, which, for example, designates 7-ACA as (6/3,7/3)-3-(acctoxymcthyl)-7-amino-8-oxo-5-thia-l-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid. 

Macias, F. A., Varela, R. M., Torres, A., and Molinillo, J. M. G. 2000. Potential allelopathic activity of natural plant heliannanes a proposal of absolute configuration and nomenclature. J. Chem. Ecol. 26, 2173-2186... 

Deduce the absolute configuration of L-cysteine according to the R/S nomenclature. 

A useful atlas of chiral molecules has appeared the major monoterpenoids of known absolute configuration are illustrated, with a literature coverage to the end of 1971.3 The reader should beware of printing errors e.g. (+)-/ -irone lacks a methyl group at C-2, (—)-(R)-a-cyclogeraniol lacks a double bond between C-2 and C-3, and (+)-S-carotene has only 39 carbon atoms (p. 131) carvotanacetone is incorrectly indexed and the nomenclature and presentation of thujane monoterpenoids is different from that used in these Reports. 

The absolute configuration at a chiral tetracoordinate center can be described using the D and l nomenclatures (but there are drawbacks, see [25]), or with the R and S nomenclature to be summarized below. 

The R and S nomenclature has also been extended to the chiral plane (see for example [44]). Thus, structure XXXV has the (S) absolute configuration. Some confusion between chiral axes and chiral planes has led to the proposal of a new definition of the chiral plane and of a new procedure for specifying planar chirality [45]. 

The molecular helices and propellers discussed above contain no center of chirality, and the P and M nomenclature is thus the only way of describing their absolute configuration. This nomenclature, however, is also applicable to some series of chiral compounds which display several centers of chirality. As will be discussed in Section 6, the presence in a molecule of two or more centers of chirality usually implies the existence of several stereoisomers, but steric reasons may reduce down to two the possible number of stereoisomeric forms. Thus, 2,3-epoxycyclohexanone contains two asymmetric carbon atoms, but for steric reasons only two stereoisomers, namely the (2S 3S)-(—)- and the (2/ 3/J)-( + )-enantiomer, exist the former is depicted in diagram XL [49]. 

---