Facile Synthesis of 3-Nitro-2-substituted Thiophenes
摘要:
A new approach to 3-nitro-2-substituted thiophenes has been developed. Exposure of commercially available 1,4-dithane-2,5-diol to nitroalkenes in the presence of 20% triethylamine results in a tandem Michael intramolecular Henry reaction to form the corresponding tetrahydrothiophene. Subsequent microwave irradiation on acidic alumina in the presence of chloranil effects the solvent free dehydration and aromatization to form 3-nitro-2-substituted thiophenes cleanly and rapidly. A simple workup procedure removes the requirement for purification by chromatography in most cases.
Conjugate addition of various Grignardreagents to 1-alkyl-2-nitropyrroles and to 2-nitrothiophene has been investigated. 1-Alkyl-2-nitropyrroles undergo alkylation at 3 and 5 positions with prevalence of the latter isomer. On the contrary, in 2-nitrothiophene system, formation of the 3-isomer prevails. In both systems, a bulkier Grignardreagent favours the 5-isomer formation. This trend can be reversed