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Optimised preparation of 1,5‐dialkylsemibullvalenes
Author(s) -
Quast Helmut,
Dietz Thomas,
Peters EvaMaria,
Peters Karl,
von Schnering Hans Georg
Publication year - 1995
Publication title -
liebigs annalen
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 0947-3440
DOI - 10.1002/jlac.1995199507157
Subject(s) - allylic rearrangement , chemistry , radical , nitrogen , zinc , decomposition , sonication , medicinal chemistry , organic chemistry , chromatography , catalysis
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 = (CH 2 ) 3 , (CH 2 ) 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%).