Synthesis of thiophenes from allenyl sulfones involving α,β‐unsaturated sulfines as intermediates

The synthesis of thiophenes starting from allenyl sulfones, via intermediate formation of α,β‐unsaturated sulfines, is described. The allenyl sulfones were synthesized by a [2,3]‐sigmatropic rearrangement of appropriately substituted prop‐2‐ynyl sulfinates. Treatment of the allenyl sulfones with n‐butyllithium and chlorotrimethylsilane gave α‐silylated allenyl sulfones in almost quantitative yield, which, on heteroconjugate addition of organolithium reagents, gave the desired α‐silyl carbanions. These reacted with sulfur dioxide to give the α,β‐unsaturated sulfines, which underwent in‐situ rearrangement to 2‐sulfonylthiophenes. The yield of the thiophenes depends on the organolithium reagents used. The use of exocyclic allenyl sulfones was more troublesome, and a thiophene was obtained for only one example.