Synthesis of trans-3,4-dihydroxy-3,4-dihydrophenanthro[3,2-b][1]benzothiophene, a potentially carcinogenic metabolite of sulfur heterocycle phenanthro[3,2-b][1]benzothiophene

Abstract

The present study describes the synthesis of trans-3,4-dihydroxy-3,4-dihydrophenanthro[3,2-b][1]benzothiophene (3), which is a potential proximate carcinogen of the environmentally occurring potent carcinogen phenanthro[3,2-b][1]benzothiophene (1). Three approaches were investigated for the synthesis of 3. In the first approach, the diarylalkene 9, which was prepared by the Wittig reaction of ylide 7 and aldehyde 8, was subjected to an oxidative photocyclization reaction to produce a mixture of compounds from which 5, a synthetic precursor to 3, was obtained only in trace amounts. The major cyclized product was 10. The second approach entailed the Suzuki cross-coupling reaction of 2-(dihydroxyboryl)-5-methoxybenzaldehyde (12) with 3-bromodibenzothiophene (11) to produce 13 in 92% yield. The aldehyde function of 13 was elongated with trimethylsulfonium iodide in the presence of KOH to generate the ethylene oxide 14, which, after methanesulfonic acid treatment, produced a 1 ∶ 1 mixture of 3-methoxyphenanthro[3,2-b][1]benzothiophene (6) and 3-methoxyphenanthro[1,2-b][1]benzothiophene (15). Only the undesired compound 15 could be isolated in pure form by extracting the mixture with hot ethanol, leaving behind a 7 ∶ 3 mixture of 6 and 15. In the third approach, the Wittig reaction of 7 with 2-bromo-5-methoxybenzaldehyde (16) produced 17 predominantly as the Z-isomer in quantitative yield. Cyclodehydrobromination of 17 with KOH–quinoline at elevated temperature produced a mixture from which 6 and 3-methoxyphenanthro[3,4-b][1]benzothiophene (18) were easily separated by column chromatography in 23 and 51% yields, respectively. The intermediate 6 was conveniently processed to 3 following the reaction sequence: methoxy phenol→o-quinone→dihydrodiol. A relatively polar dihydrodiol obtained by similar processing of the mixture of 6 and 15 was identified as 3.