Synthesis of a benzene ring with a ten-membered para-bridge.

Abstract

WE have recently investigated the synthesis of para-bridged benzene rings by dehydrogenation of maleic anhydride adducts of large ring dienes1. As this method is very laborious, we decided to find the lowest possible number of carbon atoms necessary to form a para-bridge starting from a preformed benzene ring, and then to use our dehydrogenation method systematically only for smaller bridges, which cannot be synthesized by ring closure. We have used the acyloin ring closure as described by Prelog and collaborators2, and cyclized the diester prepared according to Ruzicka and collaborators3 from pure p-xylene. Surprisingly, we obtained the acyloin in 70 per cent yield (b.p. 150–160°, 0.3 mm. mercury). After standing in the icebox, it solidified to a very low-melting crystalline solid (found: C, 78.07; H, 8.93; calculated for C16H22O2; C, 78.01; H, 9.00 per cent). In alcohol with platinum oxide, this compound took up one mole of hydrogen and the corresponding diol was obtained (m.p. 102–103°; found: C, 77.39; H, 9.73 per cent; calculated for C16H24O2: C, 77.37; H, 9.74 per cent). The diol consumed the theoretical amount of periodic acid. The high yield of the pure acyloin is interesting, as under identical conditions the cyclization reaction gives a yield of only 58 per cent cyclotetradecoin. It seems that the shortest para-bridge will have less than ten and more than six carbons, which is the length of the shortest bridge in the meta-position4.