Prazosin is a sympatholytic drug used to treat high blood pressure and anxiety, PTSD, and panic disorder. It is an alpha-adrenergic blocker that is specific for the alpha-1 receptors. These receptors are found on vascular smooth muscle, where they are responsible for the vasoconstrictive action of norepinephrine. In addition to its alpha-blocking activity, prazosin is an antagonist of the MT3 receptor (which is not present in humans), with selectivity for this receptor over the MT1 and MT2 receptors. Prazosin can be synthesized from 2-amino-4,5-dimethoxybenzoic acid. Reaction with sodium cyanate leads heterocyclization into 2,4-dihydroxy-6,7-dimethoxyquinazoline. Substituting hydroxyl groups of this compound with chlorine atoms by reaction with thionyl chloride, or a mixture of phosphorus oxychloride with phosphorus pentachloride gives 2,4-dichloro-6,7-dimethoxyquinazoline.
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