Structural Differentiation in Asymmetric Reactions 1(33 



same applies of course to the b groups. The molecule contains two 

 planes of symmetry bisecting the a and b groups respectively, a twofold 



b' b" 



i i 



I i 



i i 



a'-^«C^^- a" a"-^«C^^- a' 



I I 



I I 



I I 



b" b' 



Xa Xb 



simple axis and no meso carbon atom. The result therefore is pre- 

 dicted by rule 2 and not inconsistent with rules 1 and 3. 



Example 2, Caaab (XI). Successive rotations of XIa through 120° 

 around the axis Cb convert it into XI6 and XIc which are indistin- 



c 



a' a'" a" a" 



XIa 



guishable from the original and which contain either a" or a'" in place 

 of a'. Hence the three a groups cannot be differentiated. All a groups 

 lie in planes of symmetry, the molecule contains a threefold simple 

 axis and no meso carbon atom. The result therefore is again predicted 

 by rule 2 and is not inconsistent with rules 1 and 3. 



Example 3, Caabc. {XII). The two a groups can react at different 

 rates with an asymmetric agent since it is impossible to superimpose 

 simultaneously a" with a', b with b, and c with c (see, e.g., Xlla, XII6, 



b b c 



i 1 



I 





i T i 



c c b 



Xlla Xllb XIIc 



and XIIc) . The molecule has one plane of symmetry but the two a 

 groups do not lie in it; it possesses no axis greater than one and con- 

 tains a meso carbon atom. Hence the result is predicted by rules 

 1 to 3. 



Example 4, Caa (-)-£>) ( — 6) (XIII). In this case the two b groups 

 are assumed to be structurally identical asymmetric substituents that 



