Thujaplicin
Thujaplicin 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, antifungal, and antioxidant properties. They were the first natural tropolones to be made synthetically.
History
Thujaplicins were discovered in the mid-1930s and purified from the heartwood of Thuja plicata Donn ex D. Don, commonly called as Western red cedar tree. These compounds were also identified in the constituents of Chamaecyparis obtusa, another species from the Cupressaceae family. C. obtusa is native to East Asian countries including Japan and Taiwan, and is also known as Taiwan hinoki, from which the β-thujaplicin was first isolated in 1936 and received its name, hinokitiol. Thujaplicins were the first natural tropolones to be made synthetically, by Ralph Raphael and colleagues, and the β-thujaplicin was the first non-benzenoid aromatic compound identified, by Tetsuo Nozoe and colleagues. The resistance of the heartwood of the tree to decay was the main reason prompting to investigate its content and identify the compounds responsible for antimicrobial properties. β-thujaplicin gained more scientific interest beginning in the 2000s. Later, iron-binding activity of β-thujaplicin was discovered and the molecule has been ironically nicknamed as “Iron Man molecule”, because the first name of Tetsuo Nozoe can be translated into English as “Iron Man”.Occurrence and isolation
Tjujaplicins are found in the heartwood of the conifer trees belonging to the Cupressaceae family, including Chamaecyparis obtusa, Thuja plicata, Thujopsis dolabrata var. hondai, Juniperus cedrus, Cedrus atlantica, Cupressus lusitanica, Chamaecyparis lawsoniana, Chamaecyparis taiwanensis, Chamaecyparis thyoides, Cupressus arizonica, Cupressus macnabiana, Cupressus macrocarpa, Juniperus chinensis, Juniperus communis, Juniperus californica, Juniperus occidentalis, Juniperus oxycedrus, Juniperus sabina, Calocedrus decurrens, Calocedrus formosana, Platycladus orientalis, Thuja occidentalis, Thuja standishii, Tetraclinis articulata.Thujaplicins can be produced in plant cell suspension cultures, or can be extracted from wood using solvents and ultrasonication.
Biosynthesis
Thujaplicins can be synthesized by cycloaddition of isopropylcyclopentadiene and dichloroketene, 1,3-dipolar cycloaddition of 5-isopropyl-1-methyl-3-oxidopyridinium, ring expansion of 2-isopropylcyclohexanone, regiocontrolled hydroxylation of oxyallyl (4+3) cycloadducts, from --limonene regioselectively by several steps, and from troponeirontricarbonyl complex by few steps. The synthesis pathway of β-thujaplicin from troponeirontricarbonyl complex is found below:The synthesis pathway of β-thujaplicin by electro-reductive alkylation of substituted cycloheptatrienes is shown below:
The synthesis pathway of β-thujaplicin through ring expansion of 2-isopropylcyclohexanone is shown below:
The synthesis pathway of β-thujaplicin through oxyallyl cation cyclization is shown below:
Chemistry
Thujaplicins belong to tropolones containing an unsaturated seven-membered carbon ring. Thujaplicins are monoterpenoids that are cyclohepta-2,4,6-trien-1-one substituted by a hydroxy group at position 2 and an isopropyl group at positions 3, 4 or 5. These compounds are enols and cyclic ketones. They derive from a hydride of a cyclohepta-1,3,5-triene. Thujaplicins are soluble in organic solvents and aqueous buffers. Hinokitiol is soluble in ethanol, dimethyl sulfoxide, dimethylformamide with a solubility of 20, 30 and 12.5 mg/ml, respectively. β-thujaplicin provides acetone on vigorous oxidation and gives the saturated monocyclic diol upon catalytic hydrogenation. It is stable to alkali and acids, forming salts or remaining unchanged, but does not convert to catechol derivatives. The complexes made of iron and tropolones display high thermodynamic stability and has shown to have a stronger binding constant than the transferrin-iron complex.There are three isomers of thujaplicin, with the isopropyl group positioned progressively further from the two oxygen atoms around the ring: α-thujaplicin, β-thujaplicin, and γ-thujaplicin. β-Thujaplicin, also called hinokitiol, is the most common in nature. Each exists in two tautomeric forms, swapping the hydroxyl hydrogen to the other oxygen, meaning the two oxygen substituents do not have distinct "carbonyl" vs "hydroxyl" identities. The extent of this exchange is that the tropolone ring is aromatic with an overall cationic nature, and the oxygen–hydrogen–oxygen region has an anionic nature.
Biological properties
Insecticidal and pesticidal activity
Thujaplicins are shown to act against Reticulitermes speratus, Coptotermes formosanus, Dermatophagoides farinae, Tyrophagus putrescentiae, Callosobruchus chinensis, Lasioderma serricorne.Hinokitiol has also shown some larvicidal activities against Aedes aegypti and Culex pipiens, and anti-plasmodial activities against Plasmodium falciparum and Plasmodium berghei.