Synthesis and Pharmacology of trans‐4‐n‐Propyl‐3,4,4a,10b‐tetrahydro‐2H,5H‐1‐benzopyrano(4,3‐b)‐1,4‐oxazin‐7‐ and ‐9‐ols: The Significance of Nitrogen pKa Values for Central Dopamine Receptor Activation.