Bicyclo[3.3.0]octadienes, dibromo-, configuration of
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
Askani'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%).
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