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 Tropone or 2,4,6-cycloheptatrien-1-one is an organic compound with some importance in organic chemistry as a non-benzenoid aromaticity, aromatic. The compound consists of a ring of seven carbon atoms with three conjugated system, conjugated alkene ...

, 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 In chemistry, dehydrohalogenation is an elimination reaction which removes a hydrogen halide from a substrate (chemistry), substrate. The reaction is usually associated with the synthesis of alkenes, but it has wider applications. Dehydrohalogen ...

at elevated temperatures, while another uses

acyloin condensation Acyloin condensation is a Redox, reductive coupling of two carboxylic esters using Impurity, impure metallic sodium to yield an α-hydroxyketone, also known as an acyloin. The reaction is most successful when ''R'' is Aliphatic compound, aliphat ...

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 ...

pimelic acid Pimelic acid is the organic compound with the formula HO2C(CH2)5CO2H. Pimelic acid is one unit longer than a related dicarboxylic acid, adipic acid, a precursor to many polyesters and polyamides. However compared to adipic acid, pimelic acid is ...

the

acyloin In organic chemistry, acyloins or α-hydroxy ketones are a class of organic compounds of the general form , composed of a hydroxy group () adjacent to a ketone group (). The name ''acyloin'' is derived from the fact that they are formally deri ...

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 In organic chemistry, a cycloaddition is a chemical reaction in which "two or more Unsaturated hydrocarbon, unsaturated molecules (or parts of the same molecule) combine with the formation of a cyclic adduct in which there is a net reduction of th ...

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 In organic chemistry, a ketene is an organic compound of the form , where R and R' are two arbitrary valence (chemistry), monovalent functional group, chemical groups (or two separate Substituent, substitution sites in the same molecule). The na ...

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 An acid is a molecule or ion capable of either donating a proton (i.e. hydrogen cation, H+), known as a Brønsted–Lowry acid, or forming a covalent bond with an electron pair, known as a Lewis acid. The first category of acids are the ...

, 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 In chemistry, resonance, also called mesomerism, is a way of describing bonding in certain molecules or polyatomic ions by the combination of several contributing structures (or ''forms'', also variously known as ''resonance structures'' or '' ...

with the carbonyl group, as a

vinylogous In organic chemistry, vinylogy is the transmission of electronic effects through a conjugated organic bonding system. The concept was introduced in 1926 by Ludwig Claisen to explain the acidic properties of formylacetone and related ketoald ...

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

s. 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 In chemistry, tautomers () are structural isomers (constitutional isomers) of chemical compounds that readily interconvert. The chemical reaction interconverting the two is called tautomerization. This conversion commonly results from the reloca ...

: 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 In organic chemistry, polyketides are a class of natural products derived from a Precursor (chemistry), precursor molecule consisting of a Polymer backbone, chain of alternating ketone (, or Carbonyl reduction, its reduced forms) and Methylene gro ...

pathway, which affords a

ic intermediate that undergoes ring expansion. They are especially found in specific plant species, such as ''

Cupressaceae Cupressaceae or the cypress family is a family of conifers. The family includes 27–30 genera (17 monotypic), which include the junipers and redwoods, with about 130–140 species in total. They are monoecious, subdioecious or (rarely) dioecio ...

'' and ''

Liliaceae The lily family, Liliaceae, consists of about 15 genera and 610 species of flowering plants within the order Liliales. They are monocotyledonous, perennial, herbaceous, often bulbous geophytes. Plants in this family have evolved with a fai ...

'' 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 ''Thujopsis'' () is a genus of conifers in the cypress family (Cupressaceae), the sole member of which is ''Thujopsis dolabrata''. It is endemic to Japan, where it is known as ''asunaro'' (). It is similar to the closely related genus ''Thuja'' ...

'', ''

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 ''Chamaecyparis taiwanensis'' (Taiwan cypress; ) is a species of cypress, native to the mountains of Taiwan, where it grows at altitudes of 1300–2800 m.Rushforth, K. (1987). ''Conifers''. Helm . Description It is a slow-growing coniferous tree ...

'' and ''

Juniperus thurifera ''Juniperus thurifera'', the spanish juniper, is a species of juniper native to the mountains of the western Mediterranean region, from southern France (including Corsica) across eastern and central Spain to Morocco and locally in northern Alger ...

'' were in the list of trees from which the first tropolones were identified. The first synthetic tropolones were

thujaplicin Thujaplicin (isopropyl cycloheptatrienolone) is any of three isomeric tropolone-related natural products that have been isolated from the softwoods of the trees of ''Cupressaceae'' family. These compounds are known for their antibacterial, antifu ...

s derived by

Ralph Raphael Ralph Alexander Raphael (1 January 1921 – 27 April 1998) was a British organic chemist, well known for his use of acteylene derivatives in the synthesis of natural products with biological activity. Early life and education Ralph Raphael w ...

Tropolone derivatives

References

{{Monoamine metabolism modulators Dopamine beta hydroxylase inhibitors Wood extracts