Hydrogen atoms, prochiral pairs

Another example is provided by malic acid, a chiral molecule which also contains a prochiral center (see Eq. 9-74). In this case replacement of the pro-R or pro-S hydrogen atom by another atom or group would yield a pair of diastereoisomers rather than enantiomers. Therefore, these hydrogen atoms are diastereotopic. When L-malic acid is dehydrated by fumarate hydratase (Chapter 13) the hydrogen in the pro-R position is removed but that in the pro-S position is not touched. This can be demonstrated by allowing the dehydration product, fumarate, to be hydrated to malate in 2HzO (Eq. 9-74). The malate formed contains deuterium in the pro-R position. If this malate is now isolated and placed with another portion of enzyme in H20, the deuterium is removed cleanly. The fumarate produced contains no deuterium. 

Ln(tfc)3 chelates were used to probe the meso structure of dieldrin (32) " and meso and chiral isomers of cis- (32 ) and frawi-4,5-dihydroxy-4,5-dihydroaldrin (32") . For the me o-compounds 32 and 32" in the presence of Ln(tfc)3 chelates, prochiral pairs of hydrogen atoms such as H4-H5 and H3-H6 exhibit distinct resonances. The H4-H5 pair also couple to each other. For each enantiomer of the chiral compound (32c), the H4-H5 pair is equivalent and these appear as a singlet. In the presence of Ln(tfc)3, two singlets are observed, one each for the H4-H5 pair of the (R)- and (5 )-isomers. A similar analysis of the meso and rfZ-isomers of cis- and frawi-2,3-butylene oxide was facilitated through the use of Ln(tfc)3 chelates. ... 

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