Twisted amides: crystal and optimized structures, and molecular geometry analysis of 1-acetyl-3,4,7,8-tetramethylglycoluril and 1,6-diacetyl-3,4,7,8-tetramethylglycoluril

The crystal structures of racemic 1-acetyl-3,4,7,8-tetramethylglycoluril (3 ) and 1,6-diacetyl-3,4,7,8-tetramethylglycoluril (4) were determined by X-r ay diffraction. Compound 3 forms dimers in which two complementary N-H ... O=C hydrogen bonds link a pair of enantiomeric molecules, while 4 crystalli zes in two very similar but symmetry unrelated geometries. Unlike the paren t 3,4,7,8-tetramethylglycoluril (1), both 3 and 4 are asymmetric: they exhi bit a large dihedral angle between the two bridgehead-to-bridgehead substit uent bonds (angle Me-C-C-Me similar to -23 and -24 degrees, respectively). Further, the plane through one acetyl group in 4 is twisted with respect to the plane through the tetrahydroimidazolone ring to which it is attached, while the other acetyl group is close to coplanar with its corresponding ri ng, as is the case for the acetyl group of 3. The experimental structures w ere used as a basis for evaluating optimized geometries at four levels of t heory: restricted Hartree-Fock (ab initio) using 3-21G and 6-31G* basis set s and semiempirical AM1 and PM3 methods. General and specific geometric cri teria of comparison are defined and used to evaluate the optimized structur es. Using this scheme for comparison, the 6-31G* geometries are closer to t he experimental geometries than the 3-21G geometries, and both are found to reproduce the X-ray geometries far better than the two semiempirical metho ds. The agreement between the conformations of the ab initio and the experi mental geometries supports the hypothesis that the asymmetry in these molec ules is dominated by intramolecular effects, as opposed to those of crystal packing. The results are consistent with an inherent asymmetry of the elec tron density between N1 and N6 in 3, and a corresponding induced effect in 4 as a result of the twisting in the amide moiety. (C) 2000 Elsevier Scienc e B.V. All rights reserved.