Diastereomeric complexes proposed

Figure 6 Diastereomeric complexes proposed when using dioxaborolane 6,...

In rare cases, enantiomeric mixing enhances gelation—in such cases, a diastereomeric complex is proposed fo preferentially form befween the enantiomer and its antipode, with this complex being better able to subsequently assemble into nanoscale fibrils fhan fhe individual enantiomeric building blocks. ... 

Pure material is recovered from each peak after neutralization and optical rotation is taken to determine the conformation by comparison with authentic samples. Finally, NMR investigation was used to deduce the structure of the complexes and two diastereomeric complexes were proposed ... 

Fig. 9 Structure of (a) protonated t rt-butylcarbamoylquinine CSP and (b) the proposed interaction model for the 1 1 diastereomeric complex form by DNB-(5)-leucine and P-chloro-tert-butyl carbamoyl quinine, an analogue of the chirtil selector of /ert-butylcarbamoylquinine CSP. The arrows point to the binding interactions contributing to the chiral recognition (adapted from [118])...

Thus, in both the cases (Figures 15.2 and 15.3), H-bond plays a role in the overall stability of the diastereomeric complex except that the site of FI-bond is different. The three-point rule [29] proposed for resolution of enantiomers considers H-bond as one of the important factors along with jv-tt interactions and steric repulsions between the CSP and one of the enantiomeric forms to distinguish between the two enantiomeric forms. In the application of MR the stationary phase is achiral, but the MR being chiral is responsible for diastereomeric formation and the differential interaction of the diastereomers with the ODS causes separation. 

Efficiency and selectivity are the two keywords that better outline the outstanding performances of enzymes. However, in some cases unsatisfactory stereoselectivity of enzymes can be found and, in these cases, the enantiomeric excesses of products are too low for synthetic purposes. In order to overcome this limitation, a number of techniques have been proposed to enhance the selectivity of a given biocatalyst. The net effect pursued by all these protocols is the increase of the difference in activation energy (AAG ) of the two competing diastereomeric enzyme-substrate transition state complexes (Figure 1.1). 

Cram and co-workers have experimented extensively with chiral recognition in crown ethers derived from various 3-binaphthols (73). In nonpolar solvents, these chiral ethers complex salts of PEA and various chiral a-amino esters (with fast exchange), inducing nonequivalence in their NMR spectra. The senses of proton nonequivalence induced in these solutes have been used to support proposed structures of the diastereomeric solvates (74). 

The resolution of tris(catecholato)chromate(III) has been achieved by crystallization with L-[Co(en)3]3+ the diastereomeric salt isolated contained the L-[Cr(cat)3]3 ion.793 Comparison of the properties of this anion with the chromium(III) enterobactin complex suggested that the natural product stereospeeifically forms the L-cis complex with chromium(III) (190). The tris(catecholate) complex K3[Cr(Cat)3]-5H20 crystallizes in space group C2/c with a = 20.796, 6 = 15.847 and c = 12.273 A and jS = 91.84° the chelate rings are planar.794 Electrochemical and spectroscopic studies of this complex have also been undertaken.795 Recent molecular orbital calculations796 on quinone complexes are consistent with the ligand-centred redox chemistry generally proposed for these systems.788... 

Distinction between enantiodiscrimination by complexation and by alkylation of equilibrating intermediates is less clear in a number of related cases. It is likely that more than one type of chiral discrimination may be involved. For example, when a conformational ly flexible four-membered ring substrate is used for the same reaction, the enantioselectivity was only 56% ee (Eq. 8E.15) [175]. In this case, it has been proposed that equilibration via a tertiary e-palladium species may be possible, switching the origin of enantio-discrimination to the alkylation step. A more contrasting example involves the formation of an asymmetric diene via selective P-elimination of similar diastereomeric Jt-allyl intermediates (Eq. 8E.16). Evidence suggests that the enantio-determining elimination process occurs after the equilibration of the 7t-allyl intermediates [176]. 

Another breakthrough came several years later, when the photoadduct of trans stilbene with chiral bomyl methyl fumarate 82 was obtained with a hi diastereomeric excess [60]. Here again, a model involving an approach of t reagents in parallel planes was proposed to explain the observed stereoselectivi (Scheme 19). In an attempt to increase the observed de, the cycloaddition reacti of dibomyl fumarates was examined, but a far lower selectivity was observe On this basis, a multistep process was proposed with control of the asymmet induction by the rate of cyclization of the 1,4-biradical intermediates. The natii of the substituents, however, the complexity of the reaction mixture, and the lc chemical yields of the chiral adducts are major limitations for synthetic applic tions [61]. 

A number of enantiomerically pure complexes have been made, and this chemistry has been used in several natural product syntheses. Enantiopure complexes are readily available from the corresponding vinylic epoxides, and in cases where diastereoselective complexation is possible, diastereoselectivities tend to be moderate (typically 3 1 -4 1). The rationale for the origin of this diastereoselectivity has been proposed to derive from a preferential complexation of a Fe(CO)4 fragment to the alkene anti to the epoxide. Since the initial vinyl epoxide is conformationally flexible, four diastereomeric itt-complexes would be produced as a consequence of anti or syn complexation to the s-trans or s-cis conformers. Isomerization of these initial 7r-complexes to alkoxy- 7r-allyl species would then enable interception of an iron-bound carbonyl ligand by the alkoxide to afford diastereomeric lactone complexes. Fortunately, equilibria between the two possible trans itt-allyl complexes and their more stable cis Tr-aUyl analogs simplifies the outcome significantly. Thus, for trans vinyl epoxides, the major diastereomer typically is the one designated as endo cis (the C-1 substituent points toward the iron atom) the minor diastereomer corresponds to the exo cis isomer (the C-1 substituent points away from the iron atom) (Scheme 51). For cis vinyl epoxides, this outcome is reversed - the exo cis isomer is the major product. 

---