Inter‐ and intramolecular hetero diels‐alder reactions, 41. Unusual stereocontrol in intramolecular hetero diels‐alder reactions of 2‐Aza‐1,3‐butadienes. A stereoselective sequential synthesis of annulated tetrahydropyridines

Condensation of aldehydes 1a–e with 5‐amino‐3‐methylisoxazole ( 4 ) gives the corresponding imines 5a–e with a 2‐aza‐1,3‐butadiene moiety, which cyclize selctively, e.g. 5a to form the trans ‐fused tetrahydropyridine 7a and 5c to yield the cis ‐fused cycloadduct 8c . The astounding difference in the selectivity of these reactions is explained by electronic effects and suggests a change in the dominating interactions of the orbitals in the transition structure. The structure of 7a is elucidated by an X‐ray analysis.