Catechol Ring

A 6-carbon benzene ring with hydroxyl (–OH) groups at positions 3 and 4 (3,4-dihydroxybenzene)
The two hydroxyl groups are critical for receptor affinity

β-Phenylethylamine

The backbone of all sympathomimetics: catecholamines, amphetamines, antidepressants, bronchodilators
No hydroxyl groups → highly lipid soluble → crosses the BBB freely
Has essentially no intrinsic pharmacological activity on its own — it’s purely a scaffold to hang functional groups on

Beta-Carbon

Modification	Effect
–OH group added	↓ lipid solubility → poor CNS penetration
Any substituent here	Greatly ↑ α and β receptor agonist activity

Alpha-Carbon

Modification	Effect
Any substituent here	Blocks MAO → ↑ half-life
	Drug lingers longer at synapse → indirect sympathomimetic effect

Amine tail group

Amine substituent	Receptor preference	Example
None (–NH₂)	α-selective	Noradrenaline
Short alkyl (–CH₃)	Balanced α + β	Adrenaline
Longer alkyl	β-selective	Isoprenaline, salbutamol

Bigger alkyl substituent on the amine = more β-receptor preference

Aromatic Ring Hydroxyl Groups

Pattern	Effect
Two –OH groups (positions 3,4)	Maximum receptor affinity, but ↓ lipid solubility → excluded from CNS
No –OH groups	Good CNS penetration (e.g. phenylethylamine, amphetamine)
–OH at positions 3,5 (with large amine group)	β₂ selectivity
The catechol 3,4-hydroxyl pattern = maximum receptor binding, minimum CNS entry

Clinical Correlates

Drug	Key structural feature	Consequence
Noradrenaline	No amine substituent	Potent α agonist
Adrenaline	N-methyl group	Mixed α + β agonist
Salbutamol	Large N-alkyl + 3,5-OH pattern	β₂ selective, no CNS effect
Amphetamine	α-methyl group, no ring –OH	MAO resistant, CNS penetrant, indirect
Ephedrine	α-methyl + N-methyl	Indirect + direct, CNS penetrant

Summary

β-carbon OH → blocked from brain, boosted at receptors
α-carbon substituent → avoids MAO, acts indirectly
Bigger amine tail → more β, less α
Two ring –OHs (3,4) → high affinity, but lipophobic; move to 3,5 → β₂ selectivity