The structures of the title compounds bearing a five‐, six‐ or seven‐membered A ring have been investigated by uv and 1H and 13C nmr spectroscopy. The imine‐enol‐enamine (I‐II‐III) tautomerism of these compounds depends greatly on the ring size. A significant solvent‐dependence is observed only for the five‐membered‐ring compounds 1 and 2, which in ethanolic solution exist predominantly in the imine form I, and in chloroform solution in the enol form II. The compounds with a six‐membered A ring, 3 and 4, are mainly in the enamine form III. On protonation, 3 and 4 change into the E and Z isomeric mixture of the enol tautomer II. The seven‐membered‐ring compound 5 is a mixture of the imine I and the enamine III tautomers.
ASJC Scopus subject areas
- Organic Chemistry