Non-Aromatic Heterocycles 157Cycloaddition reactions
Many methods have been developed for -lactam synthesis, the most widely used methods being two-component
couplings that occur either via concerted cycloaddition or two-step mechanisms.
The cycloaddition of azomethine ylides to alkenes is an elegant entry to pyrrolidines. The required 1,3-dipoles can
be produced in a number of ways; the example below is one of the simplest wherein a trimethylsilylmethylamine, an
aldehyde and the alkene are simply heated together.
Metathesis
A very useful general method for the synthesis of fi ve- to seven-membered, and higher, partially unsaturated hetero-
cycles is Grubbs’ olefi n metathesis applied to acyclic bis(alkenyl)-amines, -ethers and so on, as illustrated below by
dihydropyrrole and tetrahydropyridine syntheses.