α‐Pyrones. II . Synthesis of tetrahydroquinoline derivatives of 4‐aminobutanoic and 2‐amino‐2‐pentenedioic acids

Ethyl 3‐benzoylamino‐2‐oxo‐6‐triphenylphosphoranylidenemethyl‐2 H ‐pyran‐5‐carboxylate ( 1 ) reacts with 2‐ nitrobenzaldehydes ( 2 ) to give 6‐(2‐nitrostyryl)‐2 H ‐pyran‐2‐ones ( 3 ), as the E stereoisomers, in good yields. The reduction of compounds 3 , performed with hydrogen over Pd/C at room temperature and 1 atmosphere, leads to a mixture of 2‐amino‐4‐tetrahydroquinolinylidene‐2‐pentenedioic acid derivatives 5a‐d as the main products, the corresponding 3‐butenoic acid derivatives 6 and a minor amount of pyrano[2,3‐ c ]benzazocines 9a‐c. At 40 atmospheres and 90°, the reduction gives 4‐amino‐2‐tetrahydroquinolinylbutanoic acid derivatives 8a‐d as the main products and their precursors 7a,b,d as the minor ones. Amines 4c,d are isolated by stopping the reduction after the uptake of 3 equivalents of hydrogen.