Naphthoquinones

Naphthoquinones are a class of organic compounds based on a naphthalene ring system bearing two ketone groups. The quinone functionality makes them strongly electrophilic and redox-active, capable of alternating between oxidized and reduced forms. This redox chemistry underpins many of their biological and chemical applications.

Structurally, they are commonly discussed in two principal isomeric forms: 1,4-naphthoquinone (para-naphthoquinone) and 1,2-naphthoquinone (ortho-naphthoquinone). Many natural products are substituted naphthoquinones, including lawsone (2-hydroxy-1,4-naphthoquinone) found in henna, juglone (5-hydroxy-1,4-naphthoquinone) in walnut trees, and plumbagin (5-hydroxy-2-methyl-1,4-naphthoquinone) from the Plumbago genus.

Some naphthoquinones fulfill important biological roles. Menaquinones, or vitamin K2 compounds, are naphthoquinone derivatives with long isoprenoid side chains that act as lipid-soluble electron carriers in bacteria and contribute to human nutrition. Vitamin K3 (menadione) is a synthetic naphthoquinone used in research and, in some contexts, as a supplement. The naphthoquinone core also appears in various plant metabolites that participate in defense and pigmentation.

Biosynthesis occurs primarily in plants and microorganisms via polyketide- and shikimate-derived pathways. In chemistry and industry, naphthoquinones are valued as dyes, pigments, and redox reagents. Many derivatives exhibit biological activities, including antimicrobial, antifungal, and anticancer properties in laboratory studies, though safety and efficacy vary widely among compounds. Some naphthoquinones can generate reactive oxygen species under light exposure, contributing to phototoxic effects.