Stereoselectivity configuration retention

Base-induced ring opening may also lead to stereospecific ester formation. Stereoselectivity with retention of configuration suggests that the carbanion formed upon cleavage does not undergo inversion prior... 

Likewise, the reaction of vinylzirconium derivatives 305 with (diacetoxyiodo)benzene followed by anion exchange affords alkenyl(phenyl)iodonium salts 306 stereoselectively with retention of configuration (Scheme 2.88) [442]. 

Among the available methods for the stereoselective synthesis of quaternary proline analogues (7)/ the direct a-functionalisation of (5 )-proline-derived oxazolidinone 5 proposed by Seebach et al. in 1983 has been extensively used in the last thirty years for the production of many bioactive compounds (Scheme 11.1)/ Both a-allq lation and a-condensation reactions proceed stereoselectively with retention of configuration, exemplifying the concept of self-reproduction of chirality . More recently, Germanas and coworkers introduced the cheaper oxazolidinone 6, derived from the condensation of (S)-proline 1 with trichloroacetaldehyde (Scheme 11.1). ... 

Scheme 1. Stereoselectivity of the metathesis reaction explanation of the configuration retention...

The continuity of the M-H-C interaction all along the above mechanism results in a syn stereoselectivity with retention of configuration on both carbon atoms of the inserted double or triple bond. 

The C—C double bond in the cyclopentene ring can be cleaved by the osmium tetroxide-periodate procedure or by photooxygenation. The methoxalyl group on C-17 can, as a typical a-dicarbonyl system, be split off with strong base and is replaced by a proton. Since this elimination occurs with retention of the most stable configuration of the cyclization equi-hbrium, the substituents at C-17 and C-18 are located trans to one another. The critical introduction of both hydrogens was thus achieved regio- and stereoselectively. 

An enantioconvergent transformation leads to a single enantiomeric product from a racemate [51]. Each enantiomer is transformed via independent pathways by the same catalyst or by two different catalysts (Figure 6.6). For example, the hydrolysis of epoxides may proceed with high regio- and stereoselectivity vdth inversion or retention of configuration. Several enantioconvergent transformations of epoxides are reported in the last section of this chapter. 

Typically, lyases are quite specific for the nucleophilic donor component owing to mechanistic requirements. Usually, approach of the aldol acceptor to the enzyme-bound nucleophile occurs stereospedfically following an overall retention mechanism, while the facial differentiation of the aldehyde carbonyl is responsible for the relative stereoselectivity. In this manner, the stereochemistry of the C—C bond formation is completely controlled by the enzymes, in general irrespective of the constitution or configuration of the substrate, which renders the enzymes highly predictable. On the other hand, most of the lyases allow a reasonably broad variation of the electrophilic acceptor component that is usually an aldehyde. This feature... 

We discovered a complementary procedure for conversion of OMen to other functional groups. The ester P-OMen bond was shown to be cleaved in a stereoselective manner reductively [85,86]. The cleavage takes place with almost complete preservation of stereochemical integrity at phosphorus. The reducing agents are usually sodium or Hthium naphthalenide, lithium biphenyUde, and Hthium 4,4 -di-fert-butylbiphenyl (LDBB). The species produced is then quenched with an alkyl hahde or methanol to afford tertiary or secondary phosphines, respectively (Scheme 5b). Overall, the displacement reaction proceeds with retention of configuration. 

The anomeric configuration is set in the reductive lithiation step, which proceeds via a radical intermediate. Hyperconjugative stabilization favors axial disposition of the intermediate radical, which after another single electron reduction leads to a configurationally stable a-alkoxylithium intermediate. Protonation thus provides the j9-anomer. The authors were unable to determine the stereoselectivity of the alkylation step, due to difficulty with isolation. However, deuterium labeling studies pointed to the intervention of an equatorially disposed a-alkoxylithium 7 (thermodynamically favored due to the reverse anomeric effect) which undergoes alkylation with retention of configuration (Eq. 2). 

The stereochemistry of the C(3) hydroxy was established in Step D. The Baeyer-Villiger oxidation proceeds with retention of configuration of the migrating group (see Section 12.5.2), so the correct stereochemistry is established for the C—O bond. The final stereocenter for which configuration must be established is the methyl group at C(6) that was introduced by an enolate alkylation in Step E, but this reaction was not very stereoselective. However, since this center is adjacent to the lactone carbonyl, it can be epimerized through the enolate. The enolate was formed and quenched with acid. The kinetically preferred protonation from the axial direction provides the correct stereochemistry at C(6). 

Analogous stereoselective addition reactions of triorganogermanes to double bonds have teen described 63,64) they proceed with retention of configuration. 

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