Thermal and Electron Transfer Induced Reactions of Enediynes and Enyne-Allenes, 5. First Isolation of Two Rotamers About the PC Bond in a Phosphane Oxide†

The thermal cyclization of the phosphane oxide substituted enyne-allene 1d results in the formation of 4d, an unknown double-bond shift isomer of the 11H-benzo[b]fluorene system. Because of the completely hindered rotation about the PC bond, two rotamers could be isolated and identified on the basis of their spectral data. No thermal interconversion between the two rotamers was observed.