The Role of Dissymmetry 81 



Thus the photochemical asymmetric decomposition of camphor and lactic 

 acid [72] and the decarboxylation of acids having the formulae 



CH3 



I 

 C2H5— C— COOH 



I 

 CN 



and 



HOOC COOH 



\ / 



CI— C — C— CI 



/ \ 



CH3 CH3 



were undertaken. 



The negative results of these attempts are explained by the absence of any 

 circular dichroism or any photochemical dissociation in those parts of the 

 absorption spectrum which were being studied. An asymmetric effect may, 

 accordingly, be expected in substances which are highly susceptible to photo- 

 chemical decomposition taking place imder the influence of light with waves of 

 the same length as those which manifest dichroism. Under these conditions 

 selective absorption of one component of circularly polarized light occurs and 

 this should lead to asymmetric synthesis when the reaction proceeds with the 

 activation (or leads to the appearance) of centres of dissymmetry. 



In 1929, Kuhn & Braun [74], bearing this in mind, submitted the ethyl ester 

 of a-bromopropionic acid to photochemical decomposition by the action of 

 circularly polarized light. 



The major component of the light brought about a greater decomposition of 

 the antipode of the opposite sign. The product contained an excess of the (+)- 

 ester with a = +0.05° when right circularly polarized light was used. 



A relatively high degree of asymmetric resolution was obtained (rotation, 

 a = 1-04°, degree of resolution 0-5%) by the use of the dimethylamide of a- 

 azidopropionic acid (Kuhn & Knopf, 1930 [75]). 



As in the previous case, the right-handed component of the light brought 

 about greater decomposition of the (— )-antipode. 



The optical activation (a = ±0-21°) of the nitrosite of humulene (a-caryo- 

 phyllene [76]) by the action of circularly polarized light was carried out by 

 Mitchell [77]; left-handed light brought about decomposition of the (+)- 

 antipode. 



Special hopes were entertained of being able to carry out asymmetric photo- 

 chemical decomposition of the light-sensitive salts of cobalt and rhodium which, 

 in the optically active form, have a high specific rotation [78]. However, only 

 the racemic complex K3{Co(C204)4} could be optically activated by the selective 

 decomposition, under the influence of right circularly polarized light, of the 

 (— )-antipode [79]. 



As the cases which have been discussed are not really cases of asymmetric 

 synthesis, but they were the first examples of the photochemical resolution of 

 racemates. 



