One-pot synthesis of precocene I and II and a formal synthesis of robustadial A and B via 2-phenyl-4H-1,3,2-benzodioxaborin

The reaction of a phenol, and α,β-unsaturated aldehyde, and phenylboronic acid yields a 2-phenyl-4H-1,3,2-benzodioxaborin. Upon heating, this compound decomposes to an orthoquinone-methide intermediate, which undergoes an electrocyclization reaction to a chromene ring system. This method has been applied to the synthesis of precocenes I and II and the robustadial A and B.