Vinyl azides in heterocyclic synthesis. Part 3. Isolation of azirine trimers (1,3,8-triazatricyclo[4.3.0.03,5]non-7-enes) and intramolecular interception of nitrile ylides by neighbouring π-bonds or nucleophiles

Photolysis of azidocinnamates (1a–e) at 300 nm in light petroleum gives exclusively the triazatricyclononenes (7a–e) in a highly stereospecific trimerisation of the corresponding azirine (2). A mechanism is proposed involving the sequence: azide (1)→ azirine (2)→ nitrile ylide (8)→ dimer (9)→ azomethine ylide (10)→ trimer (7)(Scheme 2). This mechanism not only rationalizes the formation of the trimers (7), rather than dimers or oligomers, but also their stereochemistry. Further evidence for it is provided by photolyses of the azidocinnamates (1f–h) which give the ‘dimer’(13), the cyclopropaisoquinoline (16), and the imidazopyridine (20), respectively; these three products result from intramolecular interception of the azomethine ylide or nitrile ylide intermediates.