Synthesis of dibenz[a,c]anthracene 1,2-oxide and 3,4-oxide from dibromo-trifluoroacetates: concomitant formation of oxepines

Abstract

The synthesis and spectral properties of 1α-acetoxy-2β,4α-dibromo-,4α-acetoxy-1α,3β-dibromo-, 2β,4α-dibromo-1α-trifluoroacetoxy- and 1α,3β-dibromo-4α-trifluoroacetoxy-1,2,3,4-tetrahydrodibenz[a,c]anthracene are described. The reaction of the two dibromo acetates with anhydrous sodium methoxide produces labile dibenz[a,c]anthracene 1,2-oxide and dibenz[a,c]anthracene 3,4-oxide concomitantly with relatively major amounts of comparatively stable benz[1′,2′:3,4]anthra[1,2-b]oxepine or benz[1′,2′:3,4]anthra[2,1-b]oxepine, respectively. No improvement in the formation of the two arene oxides is noted when a similar reaction is carried out with the two dibromo trifluoroacetates in which the ester group is much more labile than that in the dibromo acetates. Under milder reaction conditions, the 2β,4α, dibromo-1α-trifluoroacetate is hydrolysed to the 2β,4α-dibromo-1α-alcohol which, on subsequent treatment with sodium methoxide, also produces benz[1′,2′:3,4]anthra[1,2-b]oxepine as one of the major products. These studies show for the first time that the use of dibromo esters with a labile ester group has no advantage in the synthesis of the two aforementioned arene oxides because dibromo alcohols produced in these reactions are the intermediates for the formation not only of arene oxides but also of oxepines as well.