Stereogenic atoms, absolute

A-trityl-(l/ ,2S)-norephedrine (58), the corresponding allyl phosphonates 62a,b were obtained via the Arbusov rearrangement of 2-ethoxy-1,3,2-oxazaphospholidine 60. The absolute configuration of the major diastereomer, 62a was determined by X-ray as (2S1,45,51 ).The reaction of the major diastereomer of allyl phosphonates 61a and 62a with DBU afforded the corresponding vinylphosphonates 63a,b (Scheme 21) [48], Nucleophilic addition to these resulted in induction of chirality at the [1-position of the stereogenic phosphorus atom in the initially produced diastereomeric phosphonates 64 or 65 (Scheme 21) [48],... 

Some generic structures of /3-amino acids are shown in Fig. 6.40. Since, in /3-amino acids, two C-atoms separate the amino and carboxylate groups, there are two possible locations for attachment of a single side chain (i.e., /32 and /33), or even two or more side chains (e.g., /32,3 and /32,2,3, respectively). In a /3-peptide, these symbols can be used as prefixes, e.g., the /33/32-dipeptide in Fig. 6.40 becomes /33-HAla- /32-HVal for R=Me and R = i-Pr. The stereodescriptors (R) and (S) should be used to specify the absolute configuration at the stereogenic centers. The same rules apply to y-amino acids and y-peptides. 

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... 

In addition reactions to C=C double bonds, two new sp3-hybridized C atoms are produced. Each of them can be a stereogenic center (stereocenter) if properly substituted. If stereocenters are produced, their configuration must be specified. The first question is which absolute configuration is produced at any new stereocenter. Then the question arises about the configuration of the new stereocenters relative (a) to each other or (b) to additional stereocenters the molecule contained before the addition reaction occurred. 

Scheme 6.12) [39]. Cyclobutane 33 was formed with perfect stereocontrol over the two stereogenic centers in a- and [3-position to the carbonyl group. The concave shape of the substrate forces a highly selective approach of the olefin. The relative and absolute configuration at the chlorine-bearing carbon atoms was not relevant, as chlorine was subsequently eliminated under reductive conditions. Compound 33 was further elaborated into (i)-sterpurene (34). 

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