trans-Cycloalkenes. Part 12. trans,trans- and cis,trans-Cyclonona-1,5-diene

Abstract

Possible synthetic routes to trans,trans-cyclonona-l,5-diene from the cis,cis-isomer have been investigated using the phydroxyphosphine oxide olefin inversion procedure. A stepwise approach via the epoxytrans-cyclononene (1 0) failed. However consecutive double elimination involving the bis-phydroxyphosphine oxides (1 1 ) and (1 2) gave cisand trans-l,2-divinylcyclopentane which are believed to have been obtained by rapid Cope rearrangement of first formed ' parallel ' and ' crossed ' tramtrans-cyclonona- 1,5-diene (IA) and (1 6) respectively. Arising out of our experiences with trans-cyclonona-l,2,6triene described in the previous paper in this series, we decided to attempt the preparation of trans,trMs-cyclonona-1,s-diene. An additional incentive was provided by a recent statement in the literature that the latter ' should be more stable than the known cis,trans-isomer.' This statement was based on examination of molecular models. It is supported by early forcefield calculations but not by more recent ones." rrms,trans-Cyclonona-l,5-diene might be expected to exist in two diastereoisomeric forms or at least two conformational isomers with a relatively high energy barrier between them. These two forms are the parallel and crossed structures (1A) and (1B) respectively. In designing routes to these, it was decided to utilise the P-hydroxyphosphine oxide elimination since this olefin inversion procedure was known to proceed with high stereospecificity in suitable cases, and by appropriate choice of precursors it might be possible to direct formation of either (1A) or (1B).Clearly one can adopt either a stepwise approach in which the trans-double bonds are introduced one at a time or a simultaneous one in which they are introduced together in a