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

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.