Benzofurans, hydroxy-, tautomerism

Benzimidazolone, 50 Benz [cd] indole, 5-hydroxy-, tautomerism of, 19 Benzofurans... 

Equilibrium and rate constants for the keto-enol tautomerization of 3-hydroxy-indoles and -pyrroles are collected in Table 32 (86TL3275). The pyrroles ketonize substantially (103-104 times) faster than their sulfur or oxygen analogues, and faster still than the benzo-fused systems, indole, benzofuran, and benzothiophene. The rate of ketonization of the hydroxy-thiophenes and -benzothiophenes in acetonitrile-water (9 1) is as follows 2-hydroxybenzo[b]thiophene > 2,5-dihydroxythiophene > 2-hydroxythiophene > 3-hydroxybenzo[/ Jthiophene > 3-hydroxythiophene. 3-Hydroxythiophene does not ketonize readily in the above solvent system, but in 1 1 acetonitrile-water, it ketonizes 6.5 times slower than 2-hydroxythiophene (87PAC1577). 

Equilibrium and rate constants for the keto-enol tautomerization of hydroxy heterocycles are summarized in Table 38 <1986TL3275>. The pyrroles ketonize (i.e., 226 — 230) substantially faster (103-104 times) than their sulfur or oxygen analogues, and still faster than the benzo-fused systems (indole, benzofuran, and benzothiophene). 

The hydroformylation of the orf/zo-prop-2-enylphenol 81 which contains no benzylic hydroxy group gives a mixture of the open-chain aldehyde 82 and the seven-membered cyclic hemiacetal 83 in a 82 83 ratio of approximately 40 60 (equation 34) °. The ben-zofuran epoxide 85 and its valence-isomeric quinone methide 86, both readily obtainable from benzofuran 84, rearrange thermally above —20°C to form the allylic alcohol 87 and the tautomeric phenol 88 (equation 35) . ... 

---