Reaction of phenolic mannich bases with enamines. General synthesis of pyran‐containing fused ring systems

Abstract
The reaction of o‐phenolic Mannich bases with enamines yields cyclic O,N‐acetals having a 2‐aminodihydropyran or a spiropyranopiperidine structure in common. The 2‐aminodihydro‐pyrans are easily converted to 2‐hydroxydihydropyrans and to pyrans. The preparation of a large variety of pyran‐containing fused ring systems is reported.