Of 4-azaazulenes

The frontier orbitals of 4-azaazulen-5-one (6) have been compared with those of pyrrolizin-3-one (9) (78CB2407). 

Protonation of 4-azaazulenones and tautomerism of 4-azaazulenes, which have been studied mainly by H-NMR spectroscopy, are covered in Sections III,B and III,A, respectively. 

Ultraviolet spectra of 4-azaazulenones 5-8 (shown in Fig. 2), have been compared with Pariser-Parr-Pople (PPP) calculations (85MI2,85UP1). The special role of 4-azaazulen-3-one (5) deduced from the lH-NMR spectrum is reflected here in a long-wave maximum at 540 nm (see Table I for Hiickel HOMO-LUMO gaps and REPE values). 

Treatment of perhydro-4-azaazulene (3) with mercuric acetate produces a mixture of dehydro derivatives 31a and 31b, which, with acids, yields homogeneous salts of structure 32 (56JOC344). The enamine 34, which was obtained by the same route from 33, served as a model compound in a study of the synthesis of cephalotaxine (72JOC3691). A reaction with ethyl y-bromo-acetoacetate surprisingly yielded the quinolizidine 36, which was formed by rearrangement of the intermediate annellation product 35 (Scheme 3) Phthalimides 38 were obtained from a Baeyer-Villiger oxidation of 4-azaazulen-3-ones 37 (77JOC1093). 

The synthesis of 4-azaazulen-9-one (8) is shown in Scheme 19. The final cyclization could be achieved in boiling quinoline as well as by flash vacuum pyrolysis. Interestingly, only the parent compound 8 was obtained. Attempts to synthesize a methyl derivative of 8 by thermolysis of 172b led to the pyrrole 173 (78CB2407). 

Flitsch, W., The Chemistry of 4-Azaazulenes, 43, 35 Hydrogenated Porphyrin Derivatives Hydroporphyrins, 43, 73. 

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