Differentiation diastereotopos

A complete set of examples is given in Section 1.2.4.2. Occasionally, an ambiguity can arise when the starting material is not clearly defined. For instance, the following reaction" may be classified as diastereotopos- or diastereoface-differentiating depending on whether the lactone or the enolate is considered to be the starting material ... 

Diastereodifferentiating Reactions (Diastereoface-, Diastereotopos-, and Diastereomer-Differentiating Reactions)... 

When the configuration at (at least) one chiral unit is known in a nonracemic product, then the relative configuration needs to be determined. This situation occurs in substrate-induced reactions with many classical examples in steroid, terpene, and carbohydrate chemistry, such as the reduction of a carbonyl group (diastereoface-differentiating reaction), the transformation —CH2------> —CHBr— by radical bromination (diastereotopos-differentiating reaction), or dif-... 

These concepts have been described here since they do offer a way out of the specific-selective dilemma, and they could be applied in biochemistry. For instance alcohol dehydrogenase differentiates between the enantiofaces of acetaldehyde, 63, and, in addition, utilizes HA of NADH, 64, thereby showing a diastereotopos-differentiating capability as well. 

Haller tried to model the diastereotopos-differentiation in the deprotonation step by MOPAC/PM3 calculations [64,66]. The results were not really convincing, but give an idea why the a s-annulated ring in (-)-sparteine is so important Close agreement between theoretical and experimental results was achieved for deprotonations, promoted by (J ,J )-l,2-bis(dimethylamino)cyclohexane 145 (Fig.2) [85]. The carbamates 146a, 149a and 150a, derived from ethanol, 2-... 

Two different reaction conditions for the synthesis of the diastereomerically enriched benzyllithium compounds have been used in the study a kinetically controlled route, diastereotopos-differentiating deprotonation at low temperatures, mostly observed in nonpolar solvents (e.g. in toluene, n-pentane) or a thermodynamically controlled route, epimerization between two diastereomeric lithium alkyls at higher temperatures, mostly observed in polar solvents (e.g. in THF) (Scheme 1). 

In the model system for (/ ,5)-2 in a polar solvent with a coordinating dimethyl ether molecule at the lithium center (Epimerization 2 in Scheme 1), the energy difference between MIN-7 and TS-4 is smaller by 25 kJ/mol. Furthermore, in contrast to the nonpolar case described, there is an energy difference of 9 kJ/mol between the two diastereomeric minimum structures MIN-7 and MIN-8. The conclusion from these studies is that from ether solution, a highly diastereomerically enriched (,R,S)-2 should be obtained by epimerization at room temperature, while a diastereotopos-differentiating deprotonation without noticeable epimerization should be possible in nonpolar solution (under the reaction conditions described). 

Norder A, Warren SA, Herdtweck E, Huber SM, Bach T. Diastereotopos-differentiation in the Rh-catalyzed amination of benzylic methylene groups in the a-position to a stereogenic center. J m Chem Sac. 2012 134 13524-13531. 

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