Synthesis and Conformational Analysis of Sterically Congested (4R)-(-)-1-(2,4,6-Trimethylbenzenesulfonyl)-3-n-butyryl-4-tert-butyl-2-imidazolidinone: X-Ray Crystallography and Semiempirical Calculations

The crystal structure of (4R)-(-)-1-(2,4,6-trimethylbenzenesulfonyl)-3-n-butyryl-4-tert-butyl-2-imidazolidinone (3) was determined by single-crystal X-ray diffraction. Compound 3 crystallizes in triclinic system in space group P1 (not equal 1). The crystal data a = 10.6216(5)angstrom, b = 16.532(1)angstrom, c = 8.9572(9)angstrom, proportional to= 91.193(6)degrees, beta = 93.849(6)degrees, gamma = 88.097(4)degrees, V = 1568.2(2)angstrom(3), Z = 3, D-calc = 1.253 g/cm(3), mu(CuK alpha) = 15.98 cm(-1), F-000 = 636.00, T = 20.0 degrees C, and R = 0.037. The crystal structure confirmed the occurrence of three molecules of 3A, 3B, and3C in which the n-butyrylmoiety adopted the s-transoid conformation. Crystal structure also revealed that the conformation of 2,4,6-trimethylbenzenesulfonyl groups was in anti-position relative to tert-butyl group. The crystal packing showed that three molecules of compound 3 are stacked as a result of intermolecular pi-pi interactions between the phenyl ring of one molecule and the phenyl ring of the other molecule by approaching each other to an interplanar separation of 5.034 angstrom. Interestingly, these stacked molecules are also connected by intermolecular CH-pi. interaction. The conformational analysis of the s-transoid 3A, 3B, and 3C was separately performed by molecular mechanic MM+ force field. Additionally, computational investigation using semiempirical AM1 and PM3 methods was performed to find a correlation between experimental and calculated geometrical parameters. The data obtained suggest that the structural data furnished by the AM1 method is in better agreement with those experimentally determined for the above compound. It has been found that the lowest energetic conformer computed gives approximate correspondence with experimental solid state data.