Optimised preparation of 1,5‐dialkylsemibullvalenes

Abstract

AbstractAskani's synthesis of the 1,5‐dialkylsemibullvalenes 1a and c was optimised on a gram scale. The as yet unknown trimethylenesemibullvalene 1b was obtained according to the same protocol which also greatly improved a recent synthesis of barbaralane (2). Allylic brominations of the dienes 5 + 6 with N‐bromosuccinimide yielded complex mixtures of 7–10. Depending on the substitution pattern at the apical carbon atoms C‐1 and C‐5 of the bicyclo[3.3.0]octadiene system, the intermediate allylic radicals are attacked preferentially either from the exo (R = Me) or from the endo face [R, R = (CH2)3, (CH2)4]. Reductive cyclisations of the dibromides 9 and exo,exo‐15 were carried out with a sonicated suspension of an activated zinc‐copper couple in tetraglyme. Sweeping of the volatile products from the reaction mixtures with nitrogen at elevated temperatures allowed the convenient isolation of the semibullvalenes 1 and barbaralane (2) in reasonable purities (92–97%) and yields (57–82%).