Amine Exchange and Unexpected Ring‐Opening Reactions of Pyranone Derivatives: Synthesis of 3‐Amino‐Substituted Oxonaphthopyran‐carbaldehydes and Tetrahydropyrimidinethanones as New Potential Oligonucleotide Stabilization Agents

3‐(3‐Aminopropyl)amino]‐1‐oxo‐1H‐naphtho2,1‐b]pyran‐2‐carbaldehyde ( 10 ) was synthesized by nucleophilic substitution reaction of 2‐(3‐dimethylamino)‐1‐oxo‐1H‐naphtho2,1‐b]pyran‐2‐carbaldehyde ( 9 ) and the monoprotected propane‐1,3‐diamine. The reaction with the unprotected reagent led to the unexpected 1‐(2‐hydroxynaphthalen‐1‐yl)‐2‐(tetrahydropyrimidin‐2(1H)‐ylidene)ethanone ( 6 ). Extension of this reaction to chromone 16 gave 1‐(2‐hydroxy‐3‐isopropyl‐6‐methylphenyl)‐2‐(tetrahydropyrimidin‐2(1H)‐ylidene)ethanone ( 7 ). The X‐ray crystal structures of 6 and 7 were also determined.