Pyridine-Substituted Hydroxythiophenes. I. Preparation of o-(2-, 3- and 4-pyridyl)-3-hydroxythiophenes.
作者:Salo Gronowitz、Yihua Zhang、Anna-Britta Hörnfeldt、Erik Högfeldt、P. Spielbüchler、Jørgen B. Pedersen、Povl Krogsgaard-Larsen
DOI:10.3891/acta.chem.scand.46-0654
日期:——
2-(2-, 3- and 4-pyridyl)-3-hydroxythiophenes and 4-(2-, 3- and 4-pyridyl)-3-hydroxythiophenes dagger have been prepared by hydrogen peroxide oxidation of the corresponding boronic esters. In the former case the boronic esters were obtained in three steps from 2,3-dibromothiophene via the corresponding 3-bromo-2-pyridylthiophenes synthesized by Pd(0)-catalyzed coupling between 3-bromo-2-trimethylstannylthiophene and the corresponding bromopyridines. In the latter case the known isomeric pyridylthiophenes were converted into the corresponding boronic esters in three steps via tribromo- and 3-bromo-4-pyridylthiophenes successively. 4-(3- and 4-pyridyl)thiophen-2(5H)-ones were also obtained in the syntheses of 4-(3- and 4-pyridyl)-3-hydroxythiophene. They are suggested to arise from rearrangement during the halogen-metal exchange. Spectroscopic investigations by H-1 NMR and IR show that these hydroxythiophene systems exist exclusively as enol forms.