Tropolone is an

organic compound Some chemical authorities define an organic compound as a chemical compound that contains a carbon–hydrogen or carbon–carbon bond; others consider an organic compound to be any chemical compound that contains carbon. For example, carbon-co ...

with the chemical formula . 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 chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...

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 In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...

of pimelic acid the acyloin again followed by oxidation by

bromine Bromine is a chemical element; it has chemical symbol, symbol Br and atomic number 35. It is a volatile red-brown liquid at room temperature that evaporates readily to form a similarly coloured vapour. Its properties are intermediate between th ...

. :

An alternate route is a +2 cycloaddition of

cyclopentadiene Cyclopentadiene is an organic compound with the chemical formula, formula C5H6. It is often abbreviated CpH because the cyclopentadienyl anion is abbreviated Cp−. This colorless liquid has a strong and unpleasant odor. At room temperature, ...

with a ketene to give a bicyclo .2.0eptyl structure, followed by

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

and breakage of the fusion bond to give the single ring: : OS tropolone from CpH

Thy hydroxyl group of tropolone is acidic, having a p''K''_a of 7, which is in between that of

phenol Phenol (also known as carbolic acid, phenolic acid, or benzenol) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile and can catch fire. The molecule consists of a phenyl group () ...

(10) and

benzoic acid Benzoic acid () is a white (or colorless) solid organic compound with the formula , whose structure consists of a benzene ring () with a carboxyl () substituent. The benzoyl group is often abbreviated "Bz" (not to be confused with "Bn," which ...

(4). The increased acidity compared to phenol is due to resonance stabilization with the carbonyl group, as a vinylogous carboxylic acid. The compound readily undergoes ''O''-

alkylation Alkylation is a chemical reaction that entails transfer of an alkyl group. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion, or a carbene (or their equivalents). Alkylating agents are reagents for effecting al ...

to give cycloheptatrienyl derivatives, which in turn are versatile synthetic intermediates. With metal cations, it undergoes deprotonation to form a bidentate ligand, such as in the complex. The carbonyl group is also highly polarized, as common for tropones. There can be substantial

hydrogen bonding In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, Covalent bond, covalently b ...

between it and the hydroxyl group, leading to rapid tautomerization: the structure is symmetric on the NMR timescale. : TropoloneTaut

Natural occurrence

Around 200 naturally occurring tropolone derivatives have been isolated, mostly from

plants Plants are the eukaryotes that form the kingdom Plantae; they are predominantly photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with cyanobacteria to produce sugars f ...

and

fungi A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...

. Tropolone compounds and their derivatives include (such as and derivatives and others. Tropolone arises via a polyketide pathway, which affords a

ic 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 ''Thuja plicata'' is a large evergreen coniferous tree in the family Cupressaceae, native to the Pacific Northwest of North America. Its common name is western redcedar in the U.S. or western red cedar in the UK, and it is also called pacific re ...

'', '' Thujopsis dolabrata'', ''

Chamaecyparis obtusa ''Chamaecyparis obtusa'' (Japanese cypress, hinoki cypress or hinoki; or , ) is a species of cypress native to central Japan in East Asia, and widely cultivated in the temperate climate, temperate northern hemisphere for its high-quality timber ...

'', '' 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.

Tropolone derivatives

References

{{Monoamine metabolism modulators Dopamine beta hydroxylase inhibitors Wood extracts