by a sequence that includes citric acid (94) as an intermediate.
When 93 is labeled with 14C at the 4 position, the label is found only at C-1 of
95, despite the fact that 94 is not chiral. The two CH2COOH groups of 94 are
enantiotopic and the enzyme easily discriminates between them.253 Note that
the X atoms or groups of any molecule of the form CX2WY are always
enantiotopic if neither W nor Y is chiral, although enantiotopic atoms and
groups may also be found in other molecules, for example, the hydrogen
atoms in 3-fluoro-3-chlorocyclopropene (96). In this case, substitution of an
H by a group Z makes the C-3 atom asymmetric and substitution at C-1 gives
the opposite enantiomer from substitution at C-2.