Biological evidence versus structural bias - phenolic Mannich bases in medicinal chemistry

Phenolic Mannich bases (pMb's) are a distinct subclass of C-Mannich bases in which aminomethylation occurs at an aromatic carbon ortho or para to a phenolic hydroxyl group, typically combining a basic amine with a redox-active phenolic unit. The amine can enhance aqueous solubility, while intramolecular hydrogen bonding between the phenolic proton and the tertiary amine may modulate conformational bias, apparent polarity, and membrane permeability, with potential consequences for pharmacokinetics. At the same time, the phenolic Mannich bases motif is frequently flagged by PAINS filters due to context-dependent assay interference and/or the capacity of some pMb's to generate reactive ortho-quinone methide intermediates. This review provides an updated and focused analysis of pMb's in medicinal chemistry, restricted to phenolic Mannich bases bearing at most one benzylic proton at the aminomethyl position, to better resolve the functional contribution of the pMb motif from that of intrinsically bioactive phenols. We summarize recent advances across multiple therapeutic areas, dominated by antitumor investigations but extending to antimicrobial, antiparasitic, antineurodegenerative, anti-inflammatory, and other activities, highlighting series that are prone to interpretable structure-activity relationships. Across indications, recurring structure-activity trends are delineated in which a free phenolic hydroxyl group is frequently required and para-substitution is often associated with improved potency, alongside shared liabilities such as oxidative metabolism and context-dependent off-target effects. Taken together, the evidence supports treating PAINS alerts as prompts for stringent validation rather than automatic disqualification, as numerous pMb series display consistent structure-activity relationships compatible with drug-like optimisation.