The cyclization of 2‐acetonylthionaphthalene, prepared from 2‐mercaptonaphthalene (1) and chloro‐acetone in the presence of sodium hydroxide, with polyphosphoric acid gave 1‐methylnaphtho[2,1‐b]thiophene (4) in 64% overall yield from 1. By bromination with N‐bromosuccinimide, 4 was converted in 40% yield into l‐bromomethylnaphtho[2,1‐b]thiophene (8). Treatment of 8 with potassium cyanide in a phase‐transfer medium gave l‐cyanomethylnaphtho[2,1‐b]thiophene (10) in good yield. Compound 10 was reduced to the corresponding aldehyde 11 and then cyclized with polyphosphoric acid to phenaleno[l,9‐bc]thiophene (12) in 24% overall yield from 10.
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Tominaga et al. (1981) studied this question.
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