Azaazulene ring

Avoidance s. Prevention Axial s. Alcohols, axial Azaazulene ring... 

In 2,6-disubstituted 1,3-diazaazulenes, such as 14a-c (Scheme 5 62BCJ1188), nucleophilic substitution occurs preferentially at C-6, rather than at C-2. Attack at C-2, if possible at all, demands harsher conditions. In 2,8-dichloro-l-azaazulenes only chlorine atoms at position 8 are susceptible to hydrolysis (61YZ1799 62YZ418 77BCJ1184), and in 2-ethoxy-l,3-diazaazulenes 17 and 18 only the substituents at C-6 are attacked (68CPB1308). The reason is that the electron densities at the seven-membered-ring positions 6 and 8 are lower than at C-2. 

Interestingly in these cases protonation takes place at the ring carbon, whereas for 5-azaazulene protonation occurs at the -equivalent nitrogen, thus retaining the aromatic rr-electron structure.239... 

Chapters 2 and 3, both authored by Wilhelm Flitsch (Federal Republic of Germany), deal, respectively, with the chemistry of the azaazulenes (not previously covered since 1958) and of hydrogenated porphyrins, a class of increasing importance in biochemical processes. The reactivity of ring-nitrogen atoms in azines toward electrophiles is covered by M. R. Grimmett (New Zealand) and B. R. T. Keene (England) in Chapter 4. 

Another approach to obtaining spiropyrans whose colored forms absorb in the infrared is to use a heterocyclic base, such as those displayed in Figure 1.4, based upon benz[ctf]indole (121), 2-azaazulene (122),139-141 pyrroloanthrone (123),142 and other ring systems (124, 125)143-... 

From the above structures we noticed all of stemona alkaloids contain a 4-azaazulene or perhydroazaazulene ring and most of them have a-methyl-y-lactone ring linked at C-3 position, which can be detected by IR absorption at around 1760 cm-1, characteristic mass peak at M+ - 99 (M+ -C5H7O2) and doublet methyl signal at around 81.20. Some of these alkaloid structures contain another a-methyl-y-lactone ring in its left side. 

Vinyliminophosphoranes derived from 1,6-methanol[10]-annulenes react with isocyanates to give 1,6-methanol[10]-annulenopyridines. Bis(vinyliminophosphoranes) combine with isocyanates to provide azolofused l,3-diazepines. Differences on the periphery of the fused ring systems occur based on isocyanato (alkyl vs aryl) substituents. The thermal reaction of [(2-azulenyl)imino] phosphoranes on [(1-azaazulen-z-)imino]-phosphoranes with 2-bromotropone allow for the preparation of 6-aza- and 6,7-diazaazuleno[1,2-a]azulenes. °... 

Synthesis of Heterocycles. TMSK has also gained prominence as a useful reagent for accessing various heterocycles. TMSK reacts readily with aA -diarylnitrones to afford N-alkyloxindoles, which can subsequently be converted to oxindoles upon hydrolysis (eq 19). The analogous ring expanded 3,3a-dihydro-3-trimethylsilyl-l-azaazulen-2(lH)-ones can also be accessed via an [8 + 2] cycloaddition of TMSK with 8-azahepta-fulvenes.4 ... 

A short new synthesis of 1-azaazulene derivatives 496 consists in the alkylation oftroponimines 497 with pyrrolidino enamines 498, as reported by Nitta s group. The reaction leads to formal [8+2] cycloadducts 499, and subsequent aromatiza-tion takes place under the reaction conditions (Scheme 6.131) [296]. In this transformation, pyrrolidino enamines are required, probably because of ring strain in the [8+2] cycloadduct 499. 

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