SYNTHESIS AND TOPOCHEMISTRY OF 2,5-BISACRYLATE-SUBSTITUTED 1,4-BENZOQUINONES

The 2,5-bisacrylate-substituted 1,4-benzoquinone bisketal 7a was synth esized by electrochemical oxidation of the corresponding dimethoxybenz ene 6. The methyl ester 7a was transesterified to the corresponding et hyl, n-propyl and isopropyl esters 7b-7d by Ti(OEt)(4) catalysis and t o the corresponding n-butyl and benzyl esters 7e-7f by the Li alkoxide s. The bisketals 7a-7f were hydrolyzed to the corresponding 2,5-bisacr ylate-substituted 1,4-benzoquinones 1a-1f. The crystal structure of th e ethyl ester 1b was determined by X-ray diffraction, which revealed s hort intermolecular contacts of 3.463 and 4.051 Angstrom between vinyl groups and quinone double bonds, respectively, related by twofold sym metry. Because of the shorter distance and the smaller shift in projec tion (vinyl: 1.002; quinone: 2.107 Angstrom), only the vinyl group is photoactive in the crystal. In the solid state, topochemically control led [2+2] photocycloadditions take place at the vinylic groups of the bisvinyl quinones 1a-1f, to afford dimers and oligomers (n(max) = 7), under topochemical control. The cyclobutane units generated from 1b an d from 1c-1f have twofold and centric symmetry, respectively, as deter mined by H-1-NMR spectroscopy and simulation thereof. The twofold symm etry in the cycloaddition products of 1b is in agreement with the crys tal structure of the monomers.