S stereocenters

Clearable Stereocenter(s). Stereocenter(s) which can be eliminated retrosynthetically by application of a transform with stereocontrol (stereoselectivity). 

It should be noted that the sense of asymmetric induction in the lithiation/ rearrangement of aziridines 274, 276, and 279 by treatment with s-butyllithium/ (-)-sparteine is opposite to that observed for the corresponding epoxides (i.e. removal of the proton occurs at the (S)-stereocenter) [102], If one accepts the proposed model to explain the selective abstraction of the proton at the (R) -stereo-center of an epoxide (Figure 5.1), then, from the large difference in steric bulk (and Lewis basicity) between an oxygen atom and a tosyl-protected nitrogen atom, it is obvious that this model cannot be applied to the analogous aziridines. 

The high enantioselectivity again can be rationalized by enantioface-selective alkene coordination in 63 (Fig. 35). The olefin moiety is expected to bind trans to the upper imidazoline moiety [70,73] thereby releasing the catalyst strain. Coordination at this position may, in principal, afford four different isomers assuming the stereoelectronically preferred perpendicular orientation of the alkene and the Pt(II) square plane. In the coordination mode shown, steric repulsion between both olefin substituents and the ferrocene moiety is minimized. Outer-sphere attack of the indole core results in the formation of the product s stereocenter. 

Electrophilic substitution of indole. acidity of the LiC104-Et20 system. It can Diels-Alder reactions and cyclocond up to S stereocenters are created by this m action. Camphorsulfonic acid is also pm condensation involving an aldehyde and catalyzed. ... 

Figure 7 Permutation group and cycle index for stereo carbon atoms in alkyl groups. All permutations maintain the R or S stereocenter.

The racemic cyclohexyl enol ester shown in Scheme 2.58 was enzymatically resolved by CRL to give a ketoester with an (S)-stereocenter on the a-position... 

Theabsolute stereoregularity ( Fabs) of a polymer can be defined by quantifying the likelihood that identical stereocenters occur in succession along a polymer chain. Mathematically, there are several possible ways to proceed, but T abs in Table 2.1 averages the probability that adjacent (a,P and P,y) and nearly adjacent (a,y and P,S) stereocenters are identical. For the hemiisotactic continuum, the probability that two adjacent stereocenters are the same is simply [m]. The probability that nearly adjacent stereocenters are the same is either 1.00 (for a,y) or [m] -I- (1 — [m]) (for p,8), depending on which pair of stereocenters is considered. Thus Tabs = 1 + [m] /2 and will vary between 0.50 and 1.00. 

The central carbon between the amine and carboxylic acid is R while the side chain contains an S stereocenter for both amino acids. 

---