THE CRYSTAL-STRUCTURE OF 3,4,7,8-TETRAMETHYLGLYCOLURIL

The crystal structure of 3,4,7,8-tetramethylglycoluril (5) was determi ned by X-ray diffraction. The structure reveals a hydrogen-bonding mot if in the crystal lattice that differs from that present in related gl ycolurils. The two sides of each molecule form part of two independent , but parallel, infinite helical chains. These chains are formed by th e NH donor and C=O acceptor on one side of a glycoluril molecule, form ing H-bonds to two different molecules at adjacent positions within th e helix. On the other side of the same molecule, a similar motif gener ates another helix of opposite helicity to the first. The molecule has a crystallographic plane of symmetry through the two bridgehead carbo n atoms and the two bridgehead methyl groups, which are syn-periplanar . Thus, 5 is similar to 3,4-dimethylglycoluril (3), but differs from s ome glycolurils, where there is a significant dihedral angle between t he two bridgehead-to-bridgehead substituent bonds. Bond lengths and an gles in 5 resemble those reported for 3, but bond lengths around the b ridgehead positions are slightly lengthened relative to 3.