Structural Differentiation in Asymmetric Reactions 167 



gives XVII6, which is indistinguishable. The result with the meso 

 compound is predicted by rule 1. However, as structure XVI has two- 

 fold alternating axes, the differentiation of the substituents is an 

 exception to rule 2 unless its authors intended to exclude alternating 

 axes. The result is not predicted by rule 3 since the compound contains 

 no meso carbon atoms but two enantiomorphic asymmetric carbon 

 atoms. Our results present an interesting paradox which may be 

 exemplified as follows. If tartaric acid is considered as an intermediate 

 in an enzymatic process which results in differential labeling of the 

 carboxyl groups, the symmetric meso compound can qualify as a pos- 

 sible intermediate but the dyssymmetric optically active forms cannot. 

 If one considers the types of structures which permit the differentia- 

 tion of identical substituents and those which do not, one can think 

 of innumerable reactions which link the two types in either direction. 

 Since the conversion of a structure which does not permit differentiation 

 by any process into one which admits this possibility might be con- 

 sidered a contradiction, it is perhaps not superfluous to show that this 

 is not the case. For example, the olefinic carbon atoms as well as their 

 identical substituents in structure XVIIIa cannot be differentiated 

 since the structure is indistinguishable from XVTII6 which results from 

 it by rotation. However, if this substance undergoes cis addition of 

 two c groups, a meso structure XIX results which as set forth in 



.a" a' I 

 ,C'=C" c c' 



XVIIIa XVIII b 



~v 



a' 



\ 



a" \ 



c 



XIX a XIX b 



example 7 permits the differentiation of the central carbon atoms and 

 their substituents. Since arrangements XVIIIa and XVIIIb are in- 

 distinguishable, there will be an equal chance that the molecule presents 

 itself either way to an asymmetric reagent. Hence even a unidirec- 

 tional vis addition (broken arrow) will produce the superposable struc- 

 tures XlXtt and XIX6 in equal amounts. Therefore, even if a subse- 

 quent reaction at the left of the two central carbon atoms in XlXa 

 and XIXt> proceeds at a rate different from that at the right, this 



