Pyridine-substituted hydroxythiophenes. V. Preparation of 5-(2-, 3-and 4-pyridyl)-2-hydroxythiophenes

5-(2-, 3- and 4-Pyridyl)-2-t-butoxythiophenes have been prepared in very good yields by Pd(0) catalyzed cross-coupling of the three isomeric bromopyridines with 5-trimethylstannyl-2-t-butoxythiophene derived from 2-bromothiophene via 2-t-butoxythiophene. Dealkylation of 5-(2-, 3- and 4-pyridyl)-2-t-but-oxythiophenes with boron trifluoride etherate in dichloromethane at room temperature led to predominant formation of rearranged products, 5-(2- and 3-pyridyl)-3-t-butyl-3-thiolene-2-ones, together with a small amount of 5-(2- and 3-pyridyl)-2-hydroxythiophenes as a mixture of two tautomeric keto forms in the case of the 2-pyridyl and the 3-pyridyl isomers, and exclusive formation of rearranged product in the case of the 4-pyridyl isomer. However, dealkylation of 2-methoxy-5-(2-, 3- and 4-pyridyl)thiophenes, prepared similarly to the 5-(2-, 3- and 4-pyridyl)-2-t-butoxythiophenes, with boron tribromide under the same reaction conditions as above resulted exclusively in the tautomeric mixture of 5-(2- and 3-pyridyl)-3-thiolene-2-ones and 5-(2- and 3-pyridyl)-4-thiolene-2-ones in the case of the 2-pyridyl and 3-pyridyl isomers. In the case of the 4-pyridyl isomer polymerization took place.