Wittig reactions of 1,2-dihydro-1,2-bis(triphenylphosphoranylidene) benzocyclobutene. Synthesis of all-cis- and mono-trans-1,2,3,4,6,7,8,9-octahydrodibenzo[b,h]benzo[3,4]cyclobuta[1,2-e]thionin, analogues of biphenylene containing a thionin ring

Abstract

A Wittig reaction between 1,2-dihydro-1,2-bis(triphenylphosphoranylidene)benzocyclobutene (5) and 2,2′-thio-biscyclohex-1-enecarbaldehyde (4) gave both all-cis-(7) and mono-trans-(8) 1,2,3,4,6,7,8,9-octahydrodibenzo[b,h]benzo[3,4]cyclobuta[1,2-e]thionin. Oxidation of the thionins with hydrogen peroxide gave the corresponding sulphoxides. In the case of the mono-trans-thionin (8), two isomeric sulphoxides were isolated, which differ only in the relative orientation of the oxygen atom and the trans-double bond. The barrier to ring inversion in these compounds must, therefore, be relatively high.