Stable Configuration of Ester-Stabilized Azomethine Ylides. Stabilization of<i>anti</i>-Form by 1,5-Dipolar Interaction and of<i>syn</i>-Form by Hydrogen Bonding
Stable configuration of ester-stabilized azomethine ylides is found to depend upon the substituent on the ylide nitrogen. Thus, N-substituted azomethine ylides derived from N-substituted 2-aminoacetates and carbonyl compounds undergo cycloaddition with olefinic dipolarophiles predominantly in anti forms, while N-protonated (or N-unsubstituted) azomethine ylides generated from 2-iminoacetates exclusively in syn forms.
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 ylidegeneration, while syn ylides from N-unsubstituted 2-amino esters. Relative stability among all possible ylide isomers has been