Diastereotopic surfaces

The earliest reaction to be studied showing open-chain diastereoselectivity was nucleophilic attack on a carbonyl group, either a carbonyl group with a stereogenic centre adjacent to it or a carbonyl group like that in 4-fert-butylcyclohex-anone, where the diastereotopic surfaces are distinguished by being axial or equatorial. 

Supplementary support for the interpretation of the temperature-dependent dynamic H NMR spectra of 33 is presented by additional studies of (A,A,A,A)/(A,A,A,A)-[(EtNH3)4n Mg4(L12)6 ] (34). In 33 and 34, the methylene protons of the ligands exhibit identical VT NMR spectra. Moreover, the diastereotopic methylene protons (magenta) of the ethyl ammonium counterions of 34 display similar temperature-dependent coalescence as the ligand vinylether methylene protons (green). This is due to the fact that, even in solution, the ethyl ammonium groups are fixed to the tripodal calix-like surfaces of the [Mg4(L12)6]4 scaffold and therefore the methylene protons are in a chiral environment and display diastereotopicity. 

We must now turn to those cases where the attack on one surface gives one diastereoisomer, and attack on the other surface gives a different diastereoisomer, when the surfaces are said to be diastereotopic. 

We have been concerned so far only with double bonds in which the top and bottom surfaces are either the same or enantiotopic and it has made no difference to the argument which we chose to use to illustrate the principle. We must now turn to those cases where the attack on one surface gives one diastereoisomer, and attack on the other surface gives a different diastereoisomer, when the surfaces are said to be diastereotopic. 

A method for stereoselective synthesis of a-amino acids can be used to illustrate a diastereoselective chemical reaction." " The key is the stereoselective reduction of a carbon-nitrogen double bond in which the hydrogen atom addition is highly preferred from one diastereotopic face. The sequence is shown below for the synthesis of D-alanine. The optical purity observed is 96% in this instance, with optical yields of 92-97% reported for other amino acids. The chirality which is present in the reactant molecule directs the course of the addition of hydrogen in the step in which the new chiral center is created. This occurs as the result of a steric effect. It is easier for the reactant to approach the surface of the hydrogenation catalyst from the less bulky side of the molecule. After the reduction, the newly created chiral product is obtained by a reaction which also releases the original chiral reactant in a form in which it can be reconverted to the chiral hydrazine. 

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