Ring transformations of heterocyclic compounds. XXV . Phenanthrene aldehyde imines via ring transformation of pyrylium salts with methylenedihydroisoquinolines—A novel access to the phenanthrene skeleton

The ring transformation of 2,4,6‐triarylpyrylium salts 1 with 2‐methylenedihydroisoquinolines 6 , generated in situ from the related 2‐methylisoquinolinium salts 2 , in the presence of bases is reported. Whereas the transformation of 1 with 2 ( 6 ) and sodium methoxide in methanol leads to 2‐(2,4,6‐triarylphenyl)isoquinolinium salts, with sodium ethoxide in ethanol the aryl substituted phenanthrene‐9‐carbaldehyde imines 4 are obtained, the structure of which was confirmed by an X‐ray structure determination of the 1‐(4‐methylphenyl) substituted derivative. Acid catalyzed hydrolysis of the imines 4 gives rise to the parent phenanthrene aldehydes. The transformation 1 + 2 ( 6 ) → 4 represents a novel access to the phenanthrene skeleton.