Homoannular dienes, cisoid

However, an important difficulty is still present in fact, while for heteroannular cisoid dienes the allylic axial contributions are opposite in sign to the intrinsic, as depicted in Figure 6, in the case of the homoannular cisoid compounds, the two contributions have the same sign, as pointed out already by Burgstahler. 

Steroidal A2-4- and A5 7-dienes form tricarbonyliron complexes in 30-70% yielt when treated with iron pentacarbonyl. Transoid A3 5- and A4-6-diencs under the same conditions give the complex of A2-4-dienes. The cisoid A2,4-diene can be recovered by treatment with ferric chloride. It is thus possible to convert heteroannular steroidal dienes into the less stable homoannular isomer.7... 

---