Synthesis and Properties of 5,7-Diazaullazines

Abstract

Hitherto unknown 5,5,6ij]pyrrolo [2,1,5-de]quinolizines (5,7-diazaullazines) were prepared by a three-step synthesis via Clauson-Kaas, Sonogashira, and cycloisomerization reactions with diverse functionalization. The properties, including cyclovoltammetry and UV−vis and fluorescence spectroscopy, as well as solvatochromism, were studied for selected derivatives and supported by density functional theory calculations. Results were compared in detail with previously reported 5-and 6-azaullazines, and the impact of introduced nitrogen atoms was analyzed.

Divergent Synthesis of 5,7-Diazaullazines Derivatives through a Combination of Cycloisomerization with Povarov or Alkyne–Carbonyl Metathesis

“…Both compounds exhibit an irreversible oxidation potential, while 5a is slightly more easily oxidized (0.95 V; onset potential of 0.85 V) than compound 6a (1.08 V; onset potential of 0.96 V), which corresponds to the experimentally deduced HOMO energies of −5.65 eV (5a) and −5.76 eV (6a) [42]. As expected, the presence of a withdrawing benzoyl group on the ullazine scaffold leads to a lower oxidation potential compared to diazaullazine 8 [29]. Similarly, the exchange of a pyridine ring of 7 with a more electron-poor pyrimidine ring (5a) leads to a reduced oxidation potential (Table 3) [30].…”

“…In contrast, 6a features a completely different structure of the absorption spectrum and strongly blue-shifted absorption and emission spectra with higher extinction coefficients of the lowest energy band (Figure 4). The fine structure and the location of the lowest energy band is very similar to that of the symmetrically 3,9-substituted 5,7-diazaullazines (8) [29]. However, 6a has a much lower fluorescence quantum yield compared to 8.…”

“…In addition to the extension of the π-system and variations in the substitution pattern, doping of the scaffold by heteroatoms has been used to alter the inherent properties of ullazines. Several examples of N-, O-, and B-doped ullazines are known in the literature [9,[22][23][24][25][26][27][28][29].…”

“…Recently, we studied the synthesis of 5,7-diazaullazines [29]. The incorporation of a pyrimidine ring into the ullazine scaffold leads to strongly altered optical properties through the stabilization of the HOMO and LUMO energies with strong intramolecular charge transfer (ICT) properties and improved quantum yields.…”

“…During our studies, the group of Chen reported a related approach for the selective synthesis of pharmaceutically relevant naphthyridinones and quinolinones via either Povarov or ACM reaction, respectively, starting from formyl-phenylpropialamide (Scheme 1) [32]. Recently, we studied the synthesis of 5,7-diazaullazines [29]. The incorporation of a pyrimidine ring into the ullazine scaffold leads to strongly altered optical properties through the stabilization of the HOMO and LUMO energies with strong intramolecular charge transfer (ICT) properties and improved quantum yields.…”