A convenient procedure for the synthesis of tetrathia-[7]-helicene and the selective α-functionalisation of terminal thiophene ring

This paper describes a convenient preparation of tetrathia-7]-helicene (TH7]), the generation of the α-anion on the terminal thiophene ring, and the synthesis of the 2-formyl-tetrathia-7]-helicene (2-CHO-TH7]). The key intermediate trans-1,2-dibenzodithiophene-ethene, prepared in 97% yield by McMurry coupling of the 2-formyl-benzo1,2-b;4,3-b′]dithiophene, was transformed into TH7] using a known procedure. The described method affords TH7] in 46% overall yield, which is more than four times the yield previously reported in the literature. The α-anion of TH7], which is easily generated on the α-position of one of the terminal thiophene rings, reacts with electrophilic reagents such as D₂O and DMF. The latter reaction proved to be the best way to prepare 2-CHO-TH7], a key intermediate for the preparation of new substituted heterohelicenes.