Reactions of Aromatic Aldehydes with Boron Halides

Recently, during studies directed toward the total synthesis of the antibiotic frustulosin 1 1,2 we encountered some problems in the removal of methoxyl groups during the preparation of the phenolic aldehyde 2. Our efforts to effect this transformation led us to investigate the reactions of the boron trihalides with aromatic aldehydes. Although the methoxyl group has many desirable properties for hydroxyl protection its stability to both acids and bases often presents difficulties at the time of removal. Of the many reagents and methods devised 3 for the removal of methoxyl and other alkoxyl groups boron tribromide can be considered as one of the most widely applicable reagents for this purpose. However, as a fairly powerful Lewis acid, complications can arise when acid.