Condensed thiophen ring systems. Part XI. Synthesis of 2-aryl[1]-benzothieno[3,2-c]pyrazoles and 2-aryl[1]benzothieno[2,3-c]pyrazoles
Abstract
2-Aryl[1]benzothieno[3,2-c]pyrazoles were prepared by reductive cyclisation of the corresponding N-(3-nitrobenzo[b]thiophen-2-ylidene)anilines with triethyl phosphite (20–30% yields) and by thermolysis in bis-(2-methoxyethyl) ether of the corresponding N-(3-azidobenzo[b]thiophen-2-ylidene)anilines (70–75% yields). Treatment of N-(2-nitrobenzo[b]thiophen-3-ylidene)anilines with triethyl phosphite gave mixtures of the corresponding 2-aryl[1]benzothieno[2,3-c]pyrazoles and benzo[b]thiophen-3-carbonitrile, whilst pyrolysis of N-(2-azidobenzo[b]thiophen-3-ylidene)aniline in bis-(2-methoxyethyl) ether gave a mixture of 2-phenyl[1]benzothieno[2,3-c]pyrazole and the anil of 2,3-dihydro-2-oxobenzo[b]thiophen-3-carbonitrile. Mechanistic aspects are discussed.