Stereogenic atoms, absolute configuration

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). 

In a subsequent study a number of fulleroproline-containing dipeptides and other derivatives, such as 201 and 202 (Scheme 1.15), were prepared and isolated as optically pure diastereoisomers or resolved into the enantiomers.241 The most distinctive feature in the CD spectra of these adducts is a sharp extremum at 428 nm, the sign of which is characteristic of the absolute configuration of the newly generated stereogenic center at the a-C-atom of the proline unit. To establish this relationship, the absolute configuration of the isolated diastereoisomers 201 and 202 (Scheme 1.15), synthesized by reaction of azomethine ylides generated from enantiomerically pure 203 and... 

Most commonly, chiral molecules have point chirality, centering around a single atom, usually carbon, which has four different substituents. The two enantiomers of such compounds are said to have different absolute configurations at this center. This center is thus stereogenic (i.e., a grouping within a molecular entity that may be considered a focus of stereoisomerism), and is exemplified by the a-carbon of amino acids. 

In 2000, Nicolaou et al. synthesized 161 by a different route, found its ll- and 13C-NMR spectra to be identical to those of plakoside A, and claimed the structure of plakoside A as 161. 27 We found that both 161 and 161 exhibit entirely identical H- and 13C-NMR spectra. Moreover, they were in complete agreement with those reported for the natural product. Although 161 and 161 are diastereomeric, their stereogenic centers are separated by seven or eight carbon atoms, and they therefore showed identical spectroscopic properties. It was concluded that the absolute configuration of plakoside A could not be... 

Absolute configuration (Section 7.5) The three-dimensional arrangement of atoms or gronps at a stereogenic center. 

If the silicon atom of allylsilane is stereogenic, disappointing selectivities are obtained [1156], On the other hand, if the silicon substituents are chiral, a chirality transfer may be observed if the allylic group itself bears the asymmetry [54, 736] or asymmetric induction may be observed if another substituent of silicon is chiral [737, 1014], Chirality transfer in into-- or intramolecular reactions of allyl-silanes 2.78 with aldehydes is very efficient under TiCl4 catalysis however, from allylsilanes 2.79, a poor selectivity is observed [737, 1014] (Figure 6.53). More useful results were obtained by Nishitani and Yamakawa [1222] in the cyclization of 6.60, whose chiral ester is derived from (l/ ,25,5i )-phenmenthol 1.4 (R = Ph). The chiral auxiliaty is recovered after lactonization, but the absolute configuration of the cis lactones thus formed has not been determined (Figure 6.53). 

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