Tropolone

Tropolone is an organic compound with the formula C7H5(OH)O. It is a pale yellow solid that is soluble in organic solvents. The compound has been of interest to research chemists because of its unusual electronic structure and its role as a ligand precursor. Although not usually prepared from tropone, it can be viewed as its derivative with a hydroxyl group in the 2-position. 
Synthesis and reactions.
Many methods have been described for the synthesis of tropolone. One involves bromination of 1,2-cycloheptanedione with "N"-bromosuccinimide followed by dehydrohalogenation at elevated temperatures, while another uses acyloin condensation of the ethyl ester of pimelic acid the acyloin again followed by oxidation by bromine.
The compound readily undergoes O-alkylation to give cycloheptatrienyl derivatives, which in turn are versatile synthetic intermediates. With metal cations, it undergoes deprotonation to give chelate complexes, e.g., Cu(O2C7H5)2.
Natural occurrence.
Around 200 naturally occurring tropolone derivatives have been isolated, mostly from plants and fungi. Tropolone compounds and their derivatives include dolabrins, dolabrinols, thujaplicins, thujaplicinols, stipitatic acid, stipitatonic acid, nootkatin, nootkatinol, puberulic acid, puberulonic acid, sepedonin, 4-acetyltropolone, pygmaein, isopygmaein, procein, chanootin, benzotropolones (such as purpurogallin, crocipodin, goupiolone A and B), theaflavin and derivatives, bromotropolones, tropoisoquinolines and tropoloisoquinolines (such as grandirubrine, imerubrine, isoimerubrine, pareitropone, pareirubrine A and B), colchicine, colchicone and others. Tropolone arises via a polyketide pathway, which affords a phenolic intermediate that undergoes ring expansion.
They are especially found in specific plant species, such as "Cupressaceae" and "Liliaceae" families. Tropolones are mostly abundant in the heartwood, leaves and bark of plants, thereby the essential oils are rich in various types of tropolones. The first natural tropolone derivatives were studied and purified in the mid-1930s and early-1940s. "Thuja plicata", "Thujopsis dolabrata", "Chamaecyparis obtusa", "Chamaecyparis taiwanensis" and "Juniperus thurifera" were in the list of trees from which the first tropolones were identified. The first synthetic tropolones were thujaplicins derived by Ralph Raphael.
Biological effects.
It is an inhibitor of grape polyphenol oxidase and mushroom tyrosinase.