CHEMICAL CONSTITUTION 



had they been present in one ring, two fragments would have resulted : 



CO ICeH.a^ > CO j^C,Hi3^ 



\nhJ L— COOMe "^NhJ L — COOMe 



NH2 -| r SO3H 



— ^ yc,K,A 



nhJ Lcooh 



To obtain information regarding the position of the carboxyl 

 group, the diamino-carboxylic acid was oxidised with lead tetra- 

 acetate ; ^^ 2 moles were consumed, one by the sulphur atom, and 

 one either by an a-amino acid, as in (a) or by a i : 2 -diamine, as in (b) : 

 (a) NH2— C^ NH2— C^ NH2— C^ 



I kaHiaS PbAc, I ksH^.SO KMnO. | ksHi^SO, 



I I (2 0) 



NH2— C— COOH CHO COOH 



{b) NH,— C>| PbAc, OHC^ 



I I^CeHiiS . COOH _ ^CeHii(SO)COOH 



NH2— CJ OHC 



KMn04 HOOC^ 



(3 0) 



: 



C6Hii(S02)COOH 



HOOC. 



The decision between these two alternatives was made by further 



oxidation of the aldehydic product with permanganate ; i-8 moles 



were taken up, thus favouring the first of the alternatives. This 



choice was supported by the observation that cold esterification of 



the resulting acid gave a basic ester ; this . could only have been 



derived from reaction [a) and not from (b). Finally the intermediate 



aldehyde was isolated as a 2 : 4-dinitrophenylhydrazone with the 



expected nitrogen content. It appeared to follow from this that the 



urea grouping formed part of a 6- and not a 5-membered ring, and 



a-biotin was therefore assumed to be a derivative of 2-ketohexahydro- 



pyrimidine-4-carboxylic acid. 



NH 



/ \ 

 CO C\ 



NH C J 



\ / 

 CH 



COOH 

 When the amino acid obtained by permanganate oxidation of the 

 aldehyde was esterified in hot solution, ammonia was eliminated and 

 an unsaturated ester was produced : 



411 



