X-ray diffraction examination of 4-acetyl-10-methyl-2-phenyl-1,4,4a,5,10,10a-hexahydro-1,3,4-thiadiazino[5,6-b]quinoxaline has shown that the 4a-H and 10a-H hydrogens attached to the carbon atoms common to both heterocycles are cis-oriented, as in the annelation of five-membered heterocycles to the tetrahydropyrazine ring, but in this case the torsion angle H(4a)C(4a)C(10a)H(10a) is much greater, having a value of 60.