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. 1H, 13C NMR and GC-MS analyses were consistent with the formulation of the product as a symmetrical dimer of 1, but did not allow unambiguous distinction between two possible isomers, each of which could exist as either syn or anti diastereoisomers. X-Ray diffraction studies identified the product as the anti isomer of 3,3,5a,8,8,10a-hexamethyl-1,2,3,5,5a,6,7,8,10,10a-decahydropyra zino [2,3-g] quinoxaline (2). Compound 2 crystallizes in the tetragonal space group I41/a (No. 88) with a = 12.090(1), c = 22.007 (3) A, V = 3216.8(7) A3 and Z = 8. The solid-state structure also displays extensive hydrogen bonding between molecules of 2. A likely mechanism for the formation of 2 is presented.