Design and Synthesis of<i>α</i>‐Diketones. The Cyclobutane‐1,2‐dione Chromophore: Synthesis, Dienophilic Reactivity and Electronic Properties of Cyclobutenedione and Polycyclic Cyclobutanediones [1]

Abstract

Abstract A new synthesis of cyclobutenedione (1) is described. It is found that the tricyclic cyclobutanedione 3 splits into 1 and cyclopentadiene if subjected to flash pyrolysis conditions. The dienophilic reactivity of 1 allows the construction of various polycyclic cyclobutanediones, such as 3–5 and 12–14. Other new polycyclic diones, like 9–11, 15 and 16 are prepared by means of the acyloin condensation. The low stereoselectivity of 1 with cyclopentadiene is compared with the selectivity of other cyclobutenes, and the results are placed in the context of secondary interactions and steric hindrance. The unusual colour properties of these diones, from colourless 1 to blue 6 are investigated by means of photoelectron spectroscopic measurements and semi‐empirical calculations (MNDO, HAM/3, CNDO/S‐CI).