R stereocenters

Hilker et al. [59] studied the Novozym 435-catalyzed copolymerization of racemic a,a -dimethyl-l,4-benzenedimethanol with secondary hydroxyl groups with dimethyl adipate. Due to CALB enantioselectivity, hydroxyl groups at (R) stereocenters preferably reacted to form ester bonds with liberation of methanol. The reactivity ratio was estimated as (R)/(S) = 1 x 106. In situ racemization of monomer stereocenters from (S) to (R) by ruthenium catalysis allowed the polymerization to proceed and reach high functional group conversations. Readers should also refer to Chapter 11 for more information on chiral discriminations by lipases. 

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

Figure 2-71. The ordered list of 24 priority sequences of the ligands A-D around a tetrahedral stereocenter, The permutations can be separated into two classes, according to the Cl P rules the R stereoisomer is on the right-hand side, and the S stereoisomer on the left.

A molecule editor can draw a chemical structure and save it, for example as a Molfile. Although it is possible to include stereochemical properties in the drawing as wedges and hashed bonds, or even to assign a stereocenter/stereogroup with its identifiers R/S or E/Z), the connection table of the Molfile only represents the constitution (topology) of the molecule. 

Absolute Stereochemistry Absolute stereochemical assignment of each stereocenter (R vs S) Cahn-Ingold-Prelog Convention (sequence rules)... 

These strategies guide the retrosynthetic conversion of 272 to 278 and the further conversion of 278 via 279 to 282. The r-butyl substituent actuates the clearability of the stereocenters in 279. Further retrosynthetic simplification as dictated by basic FG-, stereochemical and topological strategies then leads from 280 to 281 and to 282, a previously described substance. The successful synthesis followed closely the above outlined retrosynthetic scheme. An enantioselective process was devised for the synthesis of 281 from 282 (see Section 10.12).67, 83... 

Examine three different strands ofpolypropylene. For each strand, assign R/S stereochemistry to each stereocenter. (All three strands have as their terminal monomer perfluoropropane in order to facilitate assignment of stereochemistry.) Which of the three strands corresponds to atactic polypropylene, isotactic polypropylene and syndiotactic polypropylene ... 

Recendy, Darzens reaction was investigated for its synthetic applicability to the condensation of substituted cyclohexanes and optically active a-chloroesters (derived from (-)-phenylmenthol). In this report, it was found that reaction between chloroester 44 and cyclohexanone 43 provided an 84% yield with 78 22 selectivity for the axial glycidic ester 45 over equatorial glycidic ester 46 both having the R configuration at the epoxide stereocenter. 

Figure 5.1 Ternary organolithium(RLi) sparteine epoxide complex in which the proton on the (R)-epoxide stereocenter is held closer to the organolithium.

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. 

Configurations and d.r. s or ee s of remaining stereocenters presumed to be identical with that of starting complex 16. h Determined by H-NMR spectroscopy. c Enantiomerically pure compound. J Diastereomerically pure compound. Stereochemistry of minor isomer not reported. 

Enantiomerically pure compounds. b The d.r. s of remaining stereocenters presumed to be identical with that of starting complex 18. 

More recently, the addition of cyanide ion, generated from TMS cyanide and cesium fluoride, to a-aziridino N-siflfinyl imines, being chiral either at the a position or at sulfur, has been examined [87] (Scheme 28). The configuration of the newly formed stereocenter was determined only by the chiral (S)-sulfinyl group. In fact, the R configuration (diastereomeric excess, de, 98%) was obtained from either the Q -(ii)-imine 186 or the a-(S)-imine 188, giving 187 and 189, respectively. Acyclic 2,3-diaminonitriles can be obtained... 

Stereoisomerism indicates whether double bonds are cisitrans, and indicates stereocenters (R, S), which we will cover in the chapter on configuration. 

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