Darstellung und Konformationsanalyse von 9,11‐Diphenyl[5](2,6)‐pyranophan‐10‐on

Preparation and Conformational Analysis of 9,ll‐Diphenyl[5](2,6)‐pyranophan‐lO‐one Phanone 5 has been generated by pyrolysis of bis(sulfone) 4 . This reaction is the first example of a combined allylic‐aliphatic pyrolysis of bis(sulfones); it follows a two‐step mechanism, as is concluded from the absence of cross products and the formation of mono(sulfone) 5a as a by‐product. The flipping of the pentamethylene chain of 5 is unhindered at room temperature as shown by 1 H NMR. At 213 K this flipping process is slowed on the NMR time scale, and rotation about the C‐2C‐3 axis is no longer possible. The crystal structure of 5 shows the expected tub‐like conformation of the heterocycle.