Baldwin's rules

Endo-trig reactions (5-endo-trig)

The problem with 5-endo-trig cyclizations is that the nitrogen's lone pair has problems reaching round the π* orbital of the Michael acceptor. There is no problem reaching as far as the electrophilic carbon in the plane of the substituents but, if it bends out of this plane, which it must if it is to overlap with the π* orbitals, it moves too far away from the methylene carbon to react. It's like a dog chained just out of reach of a bone.

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