Synthesis and Adrenergic Activity of Ring-Fluorinated Phenylephrines

Article abstract of DOI:10.1021/jm00160a030

2-Fluoro-, 4-fluoro-, and 6-fluorophenylephrine (6-FPE) were synthesized from the corresponding fluorinated 3-hydroxybenzaldehydes.New routes to 2-fluoro- and 6-fluoro-3-hydroxybenzaldehydes were developed based on regioselective lithiation of 2- and 4-<(dimethyl-tert-butylsilyl)oxy>fluorobenzene ortho to fluorine.As with norepinephrine and isoproterenol analogues, the adrenergic properties of phenylephrine were markedly altered by ring fluorination.The order of potency of the fluoro analogues as α₁-adrenergic agonists in the stimulation of contraction of aorticstrips and of phosphatidylinositol turnover and potentiation of cyclic AMP accumulation in guinea pig synaptoneurosomes was 6-FPE > PE > 4-FPE > 2-FPE.The same pattern was observed for the displacement of radioligands specific for α₁- and α₂-adrenergic receptors on brain membranes.The order of potency for the displacement of <³H>dihydroalprenolol, a β-specific adrenergic ligand from brain membranes, was 2-FPE > 4-FPE = PE >> 6-FPE. 6-FPE was much more selective for α-adrenergic receptors compared to β-receptors than was phenylephrine.A rationale for the observed fluorine-induced alterations in potency and selectivity of the FPEs for α- and β-adrenergic systems is presented based on fluorine-induced conformations due to electrostatic repulsion of fluorine and the benzyl hydroxyl group.