Chiral glycine compounds

Halogenation in captodative position is efficient and selective. For example, NBS bromination of the chiral glycine compound 44 gives a quantitative yield (as a 1 1 diastereoisomeric mixture) of 45 [32]. 

The oxazinones 74 and 79, already described as chiral glycine templates in Section 11.11.6.3, have been prepared by the PET cyclisation of 252 by irradiation in the presence of 1,4-dicyanonaphthalene as the electron acceptor and methyl viologen as electron-transfer mediator. When the reaction was carried out under strictly anhydrous conditions, compound 79 was isolated, whereas when the reaction was carried out in wet MeCN, compound 74 was the exclusive product (Scheme 33). In any case, the products were obtained with high stereoselectivity, which is the condition required to use them as chiral auxiliaries <2000EJ0657>. 

Mukaiyama-Michael addition of a chiral ketene acetal to nonprochiral vinyl ketones gives products of 72-75% ee.145 A chirally modified glycine derivative (Schiff-base) adds to vinylic phosphorus compounds to yield, after hydrolysis, products with 54-85% ee.146 Another chiral glycine equivalent was used for the preparation of homochiral proline derivatives via diastereoselective addition to a,3-unsatu-rated aldehydes and ketones.147-148... 

Recently, a conceptually different synthesis of MeBmt using an asymmetric glycine aldol reaction was reported by Evans and Weber [29]. The key step consists in the stereochemically controlled condensation of the chiral glycine enolate synthon (23) with the (R)-aldehyde (24) mediated by stannous triflate (tin salt of trifluoromethanesulphonic acid). The desired syn-aldol adduct (25) was isolated in form of the heterocyclic compound (26). The sense of asymmetric induction in the aldol reaction was established by conversion of (26) over three steps into uniform MeBmt (3). 

A synthesis of all four stereoisomers [(15,25)-, (1R,2R)-, (15,2/ )- and (l/ ,25)-] of l-amino-2-(hydroxymethyl)cyclobutanecarboxylic acid was presented. The synthesis is based on the chiral glycine equivalent, employed in both enantiomeric forms. The key step involves the cyclization of the silyl-protected iodohydrins to the corresponding sprro derivatives with the aid of the phosphazene base Bu-P4. The final compounds were found to display moderate potency as ligands for the glycine binding of the A-methyl-D-aspartate (NMDA) receptor [43] (Scheme 5.25). 

Preparation of Enantiomerically Pure Compounds 573 a-Alkylations of Linear Chiral Glycinates... 

In all the amino acids shown in Table 23.3 except glycine, the a-carbon is chiral. This means that these compounds are optically active. With alanine, for example, there should be two optical isomers ... 

Amino acids are characteristic examples of compounds with an asymmetric carbon atom, with the exception of glycine which, since its a-carbon carries two hydrogens, is often said to be without an asymmetric carbon atom. As a typical C(abc2) system, glycine can be used (Schafer et al. 1984G) to illustrate the conformationally dependent chirality of tetrahedral carbon atoms with two substituents of identical constitution. That is, in compounds containing the glycine residue, some conformations usually exist in which the a-carbon is asymmetric and others in which it is not. 

Compound 72 was shown to display enantioselectivity in the extraction of chiral potassium salts from water into the organic phase.105 The supramolec-ular polymer possesses a homochiral helical architecture onto which one of the anionic enantiomers preferentially binds. Intriguingly, for some of the anions the octamer and polymer showed opposite selectivity, illustrating the difference in supramolecular chirality of the two systems. Furthermore, the polymer was capable of inducing a Cotton effect in the achiral compound potassium A-(2,4-dinitrophenyl)glycinate. Since the apolar side chains would... 

Trifluorothreonine is one of the rare fluorinated compounds found in nature (cf. Chapter 4). The best method for the synthesis of fluorothreonines is the acylation of an equivalent of glycinate anion by a fluoroacetic derivative. The four stereoisomers of monofluorothreonine have been prepared. A completely stereoselective chiral approach involves the alkylation of the Seebach imidazolidinone by fluoroacetyl chloride (Figure 5.16). ... 

Many types of chiral stationary phase are available. Pirkle columns contain a silica support with bonded aminopropyl groups used to bind a derivative of D-phenyl-glycine. These phases are relatively unstable and the selectivity coefficient is close to one. More recently, chiral separations have been performed on optically active resins or cyclodextrins (oligosaccharides) bonded to silica gel through a small hydrocarbon chain linker (Fig. 3.11). These cyclodextrins possess an internal cavity that is hydro-phobic while the external part is hydrophilic. These molecules allow the selective inclusion of a great variety of compounds that can form diastereoisomers at the surface of the chiral phase leading to reversible complexes. 

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