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Continuing structure-activity studies on the anticonvulsant activity of analogs of N-(benzyloxy)-2-azaspiro[4.4]nonane-1,3-dione (2a), which displayed anti-electroshock seizure (MES) activity and a protective index (TD50/ED50) of > 4.5 are reported. An in-depth analysis of this moiety was studied employing the Topliss structure activity and the Craig plot analytical approaches as well as a semiempirical method. CLOG P analysis was also applied to this series after experimentally determining the NOR fragment. All compounds were minimized and these physicochemical parameters correlated to anticonvulsant activity. Several interesting substituted benzyloxy compounds emerged from this study: the 2',4'-dichloro (2b), 4'-(trifluoromethyl) (2c), 2'-bromo (2d), 3'-chloro (2o), 2'-chloro (2r), 2'-fluoro (2p), and 3'-fluoro (2w) analogs, all of which had comparable, or better activity than the parent unsubstituted analog (2a). X-ray crystal analysis of the active 2a versus inactive N-benzyl-2-azaspiro[4.4]nonane-1,3-dione (10) is discussed.

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1-Benzyl-5,5-diphenyl-2,4-dioxo-3-imidazolidineacetic acid (AC), M(r) = 400.434, triclinic, P1, a = 8.7640 (3), b = 11.112 (1), c = 11.323 (2) A, alpha = 102.10 (2), beta = 95.44 (5), gamma = 109.12 (1)., V = 1002.65 (30) A3, Z = 2, Dx = 1.33 g cm-3, F(000) = 420, mu(Mo K alpha) = 0.852 cm-1, T = 293 K, R = 0.066 for 5551 unique observed reflections. The compound crystallizes from ethanol at room temperature in the form of colourless prism. 3-(2,4-Dichlorobenzyl)-5,5-diphenyl-2,4-dioxo-1-imidazolidineacetic++ + acid (AD), M(r) = 469.323, monoclinic, P2(1), a = 8.0399 (7), b = 9.7237 (6), c = 26.9768 (12) A, beta = 94.281 (4)., V = 2102.92 (96) A3, Z = 4, Dx = 1.48 g cm-3, F(000) = 968, mu(Mo K alpha) = 3.417 cm-1, T = 293 K, R = 0.066 for 5677 unique observed reflections. The compound crystallizes from ethanol at room temperature in the form of colourless prisms. Two approaches are employed in trying to understand the known differences in pharmacological activity: an analysis of the molecular geometries, and electronic structure calculations. A detailed analysis is made of the molecular geometries both from the X-ray diffraction results, and following energy minimization with molecular mechanics. The ab initio calculations employ the energy-minimized conformations. Several electronic properties are intercompared for AC, AD and their common parent molecule diphenylhydantoin (DPH). The analyses of geometry and electronic structure indicate dissimilarities between active and inactive compounds which may be linked to differences in the activity.