STRUCTURAL DETERMINATION OF A DIMERIC SIDE-PRODUCT ACCOMPANYING DIHYDROPYRAZINE PREPARATION

The identity of a major side-product of attempted 2,3-dihydropyrazine dehydrogenation has been elucidated using a geminal dimethyl analog [2 ,2,5,6-tetramethyl-2,3-dihydropyrazine (1)] which cannot aromatize. H- 1, C-13 NMR and CC-MS analyses were consistent with the formulation of the product as a symmetrical dimer of 1, but did not allow unambiguou s distinction between two possible isomers, each of which could exist as either syn or anti diastereo-isomers. X-Ray diffraction studies ide ntified the product as the anti isomer of xamethyl-1,2,3,5,5a,6,7,8,10 ,10a-decahydropyrazino [2,3-g] quinoxaline (2). Compound 2 crystallize s in the tetragonal space group I4(1)/a (No. 88) with a=12.090(1), c=2 2.007(3) Angstrom, V=3216.8(7) Angstrom(3) and Z=8. The solid-state st ructure also displays extensive hydrogen bonding between molecules of 2. A likely mechanism for the formation of 2 is presented.