257 



Ladenburg 1 ) drew attention to the fact that the trans-modifi- 

 cation of the diketo-piperazines (A), is not resolvable into enantiomor- 

 phous isomerides. He, and later Groth 1 ), explained this by indi- 

 cating the existence of a "plane of indirect symmetry" ; however, 

 the symmetry-operation corresponding to it is nothing but an 

 inversion, as has already been indicated. The trans-iorm of 3-6- 

 dimethyl-2-^-diketo-piperazine (irans-alanyl-anhydride) (A), has an 

 inversion-centre, and thus shows the symmetry of the group /. The 

 as-form (B), however, evidently has a single polar binary axis A%. t 

 perpendicular to the plane of the ring; it possesses, therefore, only 

 axial symmetry (C 2 ): 



.CO 



^ ^ 



Km 



NK. 1 



A. 



(trans-iorm) 



B. 



(cts-i orm) . 



and accordingly, its arrangement must differ from its mirror-image. 

 Indeed, Fischer and Raske 2 ) have shown that these compounds 

 exist in four isomeric forms : a laevo- and a dextrogyratory isomeride 

 and a racemic compound, all three corresponding to the cis- 

 form, and one inactive ^<ms-isomeride, which can not be resolved 

 into optically active antipodes. 3 ) 



Pope and Read 4 ) tried to demonstrate the same with 3-6- 



1 ) A. Ladenburg, Ber. d. d. Chem. Ges., 28, 1955, 3104, (1895) ; P. Groth, 

 ibid., 28, 2510, (1895). Cf. the note" on page 254. 



2 ) E. Fischer and K. Raske, Ber. d. d. Chem. Ges., 39, 467, 3981, (1906); 

 In the original paper the substances are discerned as: l-4-diketo-2-5-dimethvl-pipe- 

 razines. 



8 ) In the same way trans-cyclopentane-diol has recently been separated into 

 its optically active components by Ch. van Loon (Proefschrift Delft, (1919), 

 p. 49.) by means of l-menthyl-isocyanate, after the method indicated by 

 R. H. Pickard, Journ. Chem. Soc., 85, 685, 1904); 89, 93, 467, 1254, (1906). 



4 ) W. J. Pope and J. Read, Journ. Chem. Soc., 101, 2325, (1912); C. Stoehr 

 and collaborators, Journ. f. prakt. Chemie, (2), 55, 49 77, (1897). 



17 



