Thermal [2 + 2] cyclodimerisation of strained olefins. Cis,trans-1,5-cyclo-octadiene and trans-cyclo-octene

The title reaction in the absence of catalysts has been investigated both with the racemic and with the optically active cycloolefins. Both title olefins form 2 + 2] dimers of the trans double bonds with complete retention of configuration (trans-syn-trans2 by R + S and trans-anti-trans3 by R + R combination) and with mono-inversion (cis,trans4, both by R + S and R + R combinations), but almost none with di-inversion (cis-syn-cis5 and cis-anti-cis6), together with trans-cis isomerised starting olefins.