Enantiomers experimental protocols

The second finding was a disclosure from the laboratory of Paul Hopkins (2) which provided experimental protocols wherein either the S or R enantiomers of compound 5 could be obtained from ethyl lactate. Thus the combination of the discoveries of Pearson (4) and Hopkins provided the basis for a synthesis of both the naturally occurring (8R) NeuSAc and (7R) KDO. 

A Reservation. Inasmuch as the starting material was raodipalmitoyl phosphatidylcholine, the data from the above experimental protocol tells only that the enzyme has stereochemical preference for the sn-3 enantiomer. Even though it is easy to show that there was an equimolar release of free fatty acid and formation of a lysophosphatidylcholine, it is not possible to tell whether the enzymatic attack occurred at the sn- or the sn-2 acyl ester bond. In any event, the stereospecificity of phospholipase A2 has been established by this experimental approach, and with this information a route to proof of specific positioning of fatty acyl substituents on naturally occurring phosphoglyc-erides is accessible. 

As can be nicely seen in Fig. 24, the configuration of two enantiomers is found to be unique in all structure calculations using the distance geometry, floating chirality protocol, whereas a quasi-homogenous distribution of chiralities is found without experimental restraints. Thus, it is possible to determine relative configurations in rather complicated molecules by this approach. 

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