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Self-regeneration of chirality

Pattenden s synthesis of (/ )- and (S )-2-methyl-cysteine [12] is based on Seebach s self regeneration of chirality [13]. Scheme 5 shows the synthesis of the (/ )-isomer 5b. [Pg.220]

Intraligand asymmetric induction. An instructive introduction to intraannular alkylations is the self-regeneration of chirality centers concept introduced by Seebach [62-66]. Scheme 3.9 illustrates the concept and Table 3.1 lists several representative examples. A chiral educt, such as an amino acid derivative, is condensed with pivaldehyde. This derivatization creates a new stereocenter selectively, and this second stereocenter then controls the selectivity of the subsequent alkylation by directing the electrophile to the face of the enolate opposite the tert-butyl group, a good example of intraannular 1,3-asymmetric induction. After purification of the alkylation product, hydrolysis affords enantiomerically pure products. [Pg.85]

Table 3.1. Selected examples of Seebach s self-regeneration of chirality centers (Scheme 3.9)... Table 3.1. Selected examples of Seebach s self-regeneration of chirality centers (Scheme 3.9)...
Incorporation of an auxiliary into a cyclic system has been used for the diastereoselective addition of cuprates to unsaturated 6-membered ring dioxinones, which are perhaps less important for their synthetic potential than for the mechanistic insight they provide. The dioxinones shown in Scheme 4.12a were obtained from / -3-hydroxybutanoic acid using the self-regeneration of chirality centers concept discussed in Chapter 3 (c/., Scheme 3.9 and 3.10). After the addition, hydrolytic removal of the achiral auxiliary (pivaldehyde) liberates a 3-alkyl-3-hydroxybutyrate that is essentially enantiomerically pure [134]. [Pg.149]

The application of self-regeneration of chirality to oxazohdinones and imi-dazolidinones 110 (A = protected N, B = 0 or NMe, respectively) became well known as Seebach s amino acid synthesis. Despite the elegancy of this approach. [Pg.141]

The term self-reproduction of chirality has been introduced to describe a process where a center of chirality in a compound is used to induce the formation of a new, second chirality center, which subsequently functions as an auxiliary . Thus, reaction at the first center destroys its chirality, whereupon a second reaction regenerates it through the influence of the chirality at the second center which bears the bulky auxiliary. This auxiliary is subsequently removed1,2. [Pg.816]

Like their ubiquitous 0(O-acetal relatives, N, 0-acetals can exceed the narrow bounds of passive protection and participate in synthetic operations of far greater significance. Wc will illustrate the point by the enantioselective a-alkylation of proline [Scheme 8.163]357 360 without the use of a chiral auxiliary. The procedure is another example of the self-regeneration of stereocentres and begins... [Pg.502]

In the literature, a variety of methodologies have been reported for the asymmetric synthesis of quaternary a-amino acids. [3] Among these methods, Seebach s self-regeneration of stereocenters [5] for the preparation of a-substituted amino acid derivatives (Scheme 3) was most attractive to us. The principal reasons are the ready availability of the required chiral imidazolidinones (up to 90% ds), the stability of the corresponding enolates at higher temperature (up to 0 C), and the predictability of the stereochemical outcome. [Pg.25]

We showed the high ability of PAR to produce various chiral alcohols coupled with NADH self-regeneration in the presence of IPA as a hydrogen donor in the previous section. IPA can also profitably act as a solvent for the PAR substrates that cannot dissolve in aqueous media. However, at high concentrations of IPA, the efficiency of substrate conversion by PAR clearly fell. Consequently, the conversion yields with high substrate concentrations were relatively low and insufficient for industrial application. [Pg.157]


See other pages where Self-regeneration of chirality is mentioned: [Pg.85]    [Pg.154]    [Pg.39]    [Pg.140]    [Pg.140]    [Pg.146]    [Pg.85]    [Pg.154]    [Pg.39]    [Pg.140]    [Pg.140]    [Pg.146]    [Pg.101]    [Pg.127]    [Pg.522]    [Pg.35]    [Pg.84]    [Pg.62]    [Pg.70]    [Pg.364]    [Pg.593]    [Pg.50]    [Pg.59]    [Pg.577]    [Pg.68]    [Pg.844]    [Pg.73]    [Pg.164]    [Pg.969]    [Pg.969]   
See also in sourсe #XX -- [ Pg.85 , Pg.149 ]

See also in sourсe #XX -- [ Pg.140 , Pg.141 , Pg.146 ]




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