Phenolic cyclization of epinephrine, metaproterenol, metaraminol, phenylephrine, and terbutaline with formaldehyde

An exploratory study of the rates of cyclization of the title compounds, all 3-hydroxyphenalkanolamines, with formaldehyde to 1,2,3,4-tetrahydroisoquinolines (THIQs) was performed using high-performance liquid chromatographic (HPLC) methods. Reactions occur quantitatively and practically instantaneously at room temperature and neutral pH; thus, rates were measured at acid pH. Cyclization occurs ortho or para to the 3-phenolic function, so that all but the 3,5-dihydroxyphenyl derivatives, metaproterenol and terbutaline, gave two THIQs. Terbutaline reacted significantly slower than the other compounds. Formaldehyde occurs in pharmaceutical systems and it serves as a model for other aldehydes that occur in sugars and flavors. The pharmaceutical implications of the reaction are discussed.