Synthesis and isomerization of arene oxidemetabolites of phenanthrene, triphenylene, dibenz[a,c]anthracene and dibenz[a,h]anthracene

Abstract

Dibenz[a,h]anthracene 3,4-oxide 5A_RS, synthesised from the enantiopure dibromoMTPA precursor 9A_RRS*, was found to have totally racemized and to be accompanied by benz[5,6]anthra[1,2-b]oxepine 11A. Phenanthrene 3,4-oxide 5B_RS, obtained from the enantiopure bacterial metabolite cis-3,4-dihydroxy-3,4-dihydrophenanthrene 12B by a modified synthetic approach involving the chlorohydrin ester 16B, was observed to racemize spontaneously at ambient temperature. Dibenz[a,h]anthracene 3,4-oxide 5A_RS/5A_SR, phenanthrene 3,4-oxide 5B_RS/5B_SR, triphenylene 1,2-oxide 5C_RS/5C_SR, and dibenz[a,c]anthracene 1,2-oxide 5D_RS/5D_SR, obtained from the corresponding racemic cis-tetrahydrodiol precursors 14A–14D by the new method, were obtained without any evidence of the formation of benz[5,6]anthra[1,2-b]oxepine 11A, naphth[1,2-b]oxepine 11B, phenanthro[10,9-b]oxepine 11C, or benz[3,4]anthra[1,2-b]oxepine 11D isomers respectively. The total racemization of arene oxide 5A_RS and formation of oxepine 11A from the bromoMTPA precursor 8A_RRS* are in accord with earlier PMO predictions based on resonance energy considerations. Photoisomerization of arene oxides 5A_RS/5A_SR, 5C_RS/5C_SR, and 5D_RS/5D_SR was found to yield the corresponding oxepines 11A, 11C, and 11D.