Stereochemical Studies on the Intramolecular Imino Diels-Alder Reaction

The intramolecular Diels-Alder cycloaddition of some N-acylimines has been shown to be a totally stereoselective process producing tetrahydropyridine systems having a trans relationship of hydrogens a to nitrogen. Thus, thermolysis of 13 or 14 led to a 55:45 mixture of trans-6,5-fused cycloadducts 16 and 17, respectively. The stereostructures of these compounds were established by direct correlation with authentic methyl dihydropalustramate diastereomers 3, 4, and 6. Similarly, precursor 29 cyclized to give exclusively trans-6,5-cyclo adduct 30, whose structure was established by X-ray crystallography on the derived dihydro acid 32. The homologous Diels-Alder precursor 36 also cyclized stereoselectively to give the trans-6,6-fused system 37, which was characterized by X-ray crystallography on acid 38. These stereochemical results have been rationalized on the basis of an azafumarate-type of acylimine intermediate 40 which cyclizes via transition state C, having the N-acyl group endo and the carbomethoxyl group exo.