92 THE CHEMICAL CONSTITUTION OF THE PROTEINS 



Further proof that isoleucine has this constitution was given by 

 Bouveault and Locquin in 1906. They synthesised it from sec. butyl- 

 acetoacetic ester, which they prepared from sec. butyliodide and sodium- 

 acetoacetic ester : 



CH, CH S 



CO CH. CO CH. 



I I II 



HCNa + I C CH 2 . CH 3 = Nal + HC CH -CH 2 CH 3 



COOC 2 H 5 H COOC 2 H 5 



This compound, when treated with nitrosyl sulphate was decomposed 

 into*acetic acid and a-oximino-sec-butylacetic acid, which, on reduction 

 with zinc dust and hydrochloric acid in alcohol, gave a 60 per cent, yield 

 of dl-isoleucine : 



CH 3 



NOH NH 2 



CO 11 I 



I y CH 8 /CH 3 /CH* 



H C CH/ -C CH( ->H C CH( 



I \C 2 H 5 I \C 2 H 5 \C 2 H 5 



COOC 2 H 5 COOH COOH 



Locquin has since obtained d-isoleucine from this racemic compound 

 which was identical with Ehrlich's natural product, and this therefore 

 has^the above constitution. 



By the same series of reactions which Fischer and Schmitz employed 

 in the preparation of leucine, F. Ehrlich synthesised isoleucine in 1 908 

 from malonic ester and secondary butyliodide, i.e., according to the 

 following scheme : 



C 2 H 5 . CHI . CH 3 + CHNa(COOC 2 H 5 ) 2 -> C 2 H 5 . CH(CH 3 ) . CH(COOC 2 H 6 ) a 

 -> C 2 H fi . CH(CH 3 ) . CH(COOH) 2 - C 2 H 8 . CH(CH 3 ) . CBr(COOH) 2 



-> C 2 H 6 . CH(CH 3 ) . CHBr . COOH -> C 2 H 5 . CH(CH S ) . CH(NH 2 ) . COOH 



The same synthesis has also been carried out by Brasch and 

 Friedmann. 



