Reactions of Azomethine Ylides Derived from Aldimines

1 Reactions of Azomethine Ylides Derived from Aldimines 

Cycloaddition reactions of azomethine ylides with electron-deficient alkenes fiimish pyrrolidine derivatives, whereby up to four stereocenters are formed. Besides Re/Si discrimination endo/exo stereoselection has to be rationalized in these HOMO/LUMO controlled 1,3-dipolar cycloadditions. 

Azomethine ylides are generated e.g. in situ starting from the corresponding Schiff bases of a-amino acids by deprotonation with an amme base (DBU or triethylamine) in the presence of metal salts such as LiBr or AgOAc, or by treatment of the starting materials with titanium salts. In general, the ylides are prepared in THF at -78°C/room temperature or in acetonitrile at higher temperature in the presence of the dipolarophile.  

The obtained azomethine ylides (M = Ag(I), Li(I), T1(I), Ti(IV)) have been reacted with electron-deficient alkenes. Reversal of regioselectivity was demonstrated by appropriate choice of the Lewis acid. Rate acceleration was achieved by addition of strong bases. Alternatively azomethine ylides are obtained from aziridines by thermolysis or photolysis.  

Besides the dipolarophiles discussed above, chiral unsaturated (R)s-p-tolyl vinylsulfoxides A have been studied, too. In addition, chiral unsaturated bicyclic lactams B have been applied successfully by Meyers et al. as dipolarophiles in cycloadditions with azomethine ylides.  

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