Simple generation of ester-stabilized azomethine ylides from 2-amino esters and carbonyl compounds. Stereochemistry of their cycloadditions

Condensation of 2-amino esters with carbonyl compounds leads to simple generation of ester-stabilized azomethine ylides which are trapped by olefinic dipolarophiles as cycloadducts. Anti ylides are exclusively involved in the cycloadditions when N-substituted 2-amino esters are employed for the ylide generation, while syn ylides from N-unsubstituted 2-amino esters. Relative stability among all possible ylide isomers has been inspected and the selective involvement of particular ylidic forms has been explained on the ground of 1,5-dipole interaction or hydrogen bonding stabilization. Endo- and exo-selectivity of the cycloadditions is also discussed.