SYNTHESIS, STRUCTURES AND TOPOCHEMISTRY OF 2-MONOVINYL-SUBSTITUTED 1,4-BENZOQUINONES

In the course of topochemical investigations of substituted quinones t he 2-monovinyl-substituted 1,4-benzoquinones 1 were synthesized by ele ctrochemical oxidation of the corresponding 1,4-dimethoxybenzene deriv atives 3 to the quinone bisketals 4 and subsequent hydrolysis. In solu tion, the aryl-substituted vinylquinones 1a-1t underwent unexpected Di els-Alder additions. The stereochemically unique course of the reactio n was proved spectroscopically and by X-ray structure analysis of the dimer 8k. As a basic requirement for the topochemical studies, the cry stal structures of eight quinones 1 were determined by X-ray diffracti on. In the crystals of 1a, 1c, 1e, 1f, 1i and 1n with an alpha-type pa cking arrangement, the vinylic double bonds have short contacts (< 4.4 Angstrom) across a centre of symmetry. The quinone 1k crystallizes in a beta arrangement with short distances (< 4.0 Angstrom) between mole cules related by translation and in a gamma arrangement without suitab le contacts. The crystals of all quinones 1 were irradiated with UV li ght to perform [2+2] cycloadditions. The photoreactive quinones dimeri zed exclusively at the vinylic double bonds, resulting in the correspo nding cyclobutanes 9. Topochemically controlled the dimers 9a-9c, 9e, 9l-9n, 9q, 9r and 9u with molecular C-i symmetry were formed whereas t he dimers 9k and 9t have C-s symmetry. The structures of the cyclobuta nes were determined by spectroscopical investigations and in the case of 9b, 9e and 9r additionally by X-ray analysis. Despite short contact s, crystals of 1f and 1i were photostable. This is probably because of insufficient lattice flexibility indicated by relatively high densiti es.