Absolute asymmetric reaction definition

The p-hydroxyamines are a class of ccxnpounds which fall within the generic definition of equation (8). When the alcohol is secondary, the possibility for kinetic resolution exists if the titanium tartrate complex is ct able of catalyzing the enantioselective oxidation of the amine to an amine oxide (or other oxidation product). The use of the standard asymmetric epoxidation complex, i.e. Ti2(tartrate)2, to achieve such an enantioselective oxidation was unsuccessful. However, modification of the complex so that the stoichiometry lies between Ti2(taitrate)t and Ti2(tartrate)t.s leads to very successful kinetic resolutions of p-hydroxyamines. A representative example is shown in equation (13).The oxidation and kinetic resolution of more than 20 secondary p-hydroxyamines provides an indication of the scope of the reaction and of some structural limitations to good kinetic resolution. These results also show a consistent correlation of absolute configuration of the resolved hydroxyamine with the configuration of tartrate used in the catalyst. This correlation is as shown in equation (13), where use of (+)-DIPT results in oxidation of the (S)-P-hydroxyamine and leaves unoxidized the (R)-enantiomer. 

The enantioselectivity associated with quaternary allylation is connected with scenario 5 above (one of the five points associated in the catalytic cycles shown by Schemes 12.10a and b where chirality could be induced), which is where enantioselection of one of two faces of the nucleophile (the enolate ion) occurs. Theoretical studies of the transformation using the PHOX ligand have shown support for an inner sphere mechanism, where nucleophilic attack of the enolate onto the rf-allyl ligand occurs from the Pd-bound enolate and not from an external nucleophile.74 These studies have not been able to definitively determine the step that defines the enantioselectivity of the reaction, and it is not clear how these results would carry over to reactions involving the Trost ligands. At this time, selection of which ligand one should use not only to induce enantioselectivity but also to predict the sense of absolute configuration of any asymmetric Tsuji-Trost allylation is mostly based on empirical results. Work continues on this... 

The discovery that -imines obtained from a variety of isopropylidene protected sugar open-chain aldehydes and ketenes, or ketene equivalents afford cz5-substituted 8-lactam adducts usually with a high asymmetric induction and definite relative geometry depending on the absolute configuration of the stereogenic center next to the imine carbon atom (Scheme 1) [21-24], prompted many laboratories to exploit this reaction in a number of syntheses. Imines derived from both easily available enantiomeric forms of 2,3-0-isopropylidene-glyceraldehyde are particularly attractive. 

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