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.