Enantiotopic interconversion

The enzyme-catalyzed interconversion of acetaldehyde and ethanol serves to illustrate a second important feature of prochiral relationships, that ofprochiral faces. Addition of a fourth ligand, different from the three already present, to the carbonyl carbon of acetaldehyde will produce a chiral molecule. The original molecule presents to the approaching reagent two faces which bear a mirror-image relationship to one another and are therefore enantiotopic. The two faces may be classified as re (from rectus) or si (from sinister), according to the sequence rule. If the substituents viewed from a particular face appear clockwise in order of decreasing priority, then that face is re if coimter-clockwise, then si. The re and si faces of acetaldehyde are shown below. 

Interestingly, Peters (1988) notices that the methylene protons are enantiotopic for the light lanthanides (R = Ce-Dy) and become diastereotopic for R = Ho-Yb which implies a dynamic intramolecular interconversion between the two helical enantiomers P- R(Ll-2H)3]3 M-[7 (L1-2H)3]3 occurring at a moderate rate on the NMR time scale. The... 

La(L4)]3+ isomerisation process on the NMR time scale. Compared to the parent complexes [La(L3)3]3+ which display enantiotopic methylene protons for T > 233 K (see sect. 3.1.3), the introduction of the covalent TREN tripod in [La(L4)]3+ significantly slows down the helical interconversion process. Protonation of the apical nitrogen atom produces the C3-symmetrical podates [R(L4+H)]4+ which display only marginally faster racemisation processes. Although some steric constraints induced by the tripod limit the thermodynamic stability of the podates [f (L4)]3+ and [/J(L4+H)]4+, stability constants confirm their quantitative formation for a total ligand concentration of 0.05 mol dm-3 and a stoichiometric ratio R L4 = 1.0 (Renaud etal., 1999 fig. 17). 

If only the presence of the surface itself destroys the mirror symmetry of the free species, i.e., even without any distortion of the molecular backbone, the two resulting enantiomers cannot be superimposed by translation and rotation within the plane. The resulting absolute configuration depends on which enantiotopic face of the molecule is turned towards the substrate. Interconversion is only possible by reflection with the mirror plane perpen-... 

If the interconversion of the Jt-allyl intermediates 34 and 35 is much slower than nucleophihc attack, the product distribution depends on the nature of the substrate. In this case the two enantiomeric chiral substrates 30 and ent-30 are converted to the corresponding product enantiomers 36 and ent-36 with overall retention of configuration. Starting from a racemic mixture of 36 and ent-36, the two product enantiomers 36 and ent-36 are formed in a 1 1 ratio and, therefore, a chiral catalyst cannot induce enantioselectivity (except for kinetic resolution). However, the analogous reaction of the hnear, achiral substrate 31 can be rendered enantioselective if a chiral catalyst is used that adds preferentially to one of the enantiotopic faces of 31 to give either complex 34 or 35. In this case, the enantioselectivity is determined in the oxidative addition of the substrate to the catalyst while nucleophilic addition to the 7i-allyl intermediate is irrelevant for the enantiomeric excess of the overall reaction. The relative rates of k-O-k isomerization and the other processes shown in Scheme 15 strongly depend on... 

The enantiotopic alkene approach was used in a synthesis of vernolepin 5.128, with p-hydride elimination proceeding away from the original alkene site, leading to a ketone 5.124 after tautomerization (Scheme 5.38). After a series of functional group interconversions and protecting group manipulation steps, a known intermediate 5.127 for vernolepin could be synthesized. When taken through to the end, this work also served to determine the absolute stereochemistry of this natural product. 

Fig. 8. The process of fc fc interconversion 11, leading to enantiomerization. The different methylene protons are denoted by letters (a-e). Homotopic sites are denoted by identical letters, enantiotopic sites are denoted by the same letter with a bar. As a result of the dynamic process the following exchanges take place a f, e b, a f and e b. Note that the two protons within a given methylene group do not mutually exchange.

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