Polycyclic Aromatic Heterocycles with a Benzo[c][1,2]azaborinine Core

2-Formylphenylboronic acid condenses with active methylene reagents like alpha-amino esters, ketones, and lactams directly to give benzo-fused 1-hydroxy-2,1-azaborines in good yields within minutes under microwave-accelerated conditions. The new stable boron heterocycles prepared to demonstrate this approach were characterized by NMR spectroscopy. Three members of a new diboron heteropentacyclic ring system, 5a,12a-diaza-5,12-dibora-5,12-dihydroxypentacene-6,13-dione, were prepared, and one of these highly fluorescent compounds was characterized by X-ray crystallography. Some key solid-state structural features revealed by the X-ray analysis provide a basis for explaining the limited solubility of these pentaheterocycles in common organic solvents.