A three-step synthetic approach to asymmetrically functionalized 4H-cyclopenta[2,1-b3,4-b′]dithiophenes

A convenient and efficient three-step route toward both symmetrically and asymmetrically functionalized 4H-cyclopenta[2,1-b:3,4-b′]dithiophenes has been developed. Using this method a broad collection of functionalized bridged bithiophenes can smoothly be accessed. Starting from 3-bromo-2,2′- bithiophene, prepared by Kumada coupling of 2-thienylmagnesium bromide with 2,3-dibromothiophene under Pd(dppf)Cl₂ catalysis, lithiation and subsequent reaction with dialkyl ketones afforded (a)symmetrically dialkylated tertiary alcohol derivatives. By means of final Friedel-Crafts dehydration cyclization in sulfuric acid medium, these derivatives were converted to 4,4-dialkyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophenes. Upon replacement of the dialkyl ketone reagent by ethyl levulinate, an ester-functionalized 4H-cyclopenta[2,1-b:3,4-b′]dithiophene was prepared, representing an attractive precursor for variously functionalized cyclopentadithiophene compounds.