# Barrelene

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**Barrelene** is a [bicyclic](/source/Bicyclic) [organic compound](/source/Organic_compound) with [chemical formula](/source/Chemical_formula) C8H8 and [systematic name](/source/Systematic_name) bicyclo[2.2.2]octa-2,5,7-triene. First synthesized and described by [Howard Zimmerman](/source/Howard_Zimmerman) in 1960, the name derives from the resemblance to a [barrel](/source/Barrel), with the staves being three [ethylene](/source/Ethylene) units attached to two [methine](/source/Methine) groups. It is the formal [Diels–Alder](/source/Diels%E2%80%93Alder) adduct of [benzene](/source/Benzene) and [acetylene](/source/Acetylene). Due to its unusual [molecular geometry](/source/Molecular_geometry), the compound is of considerable interest to theoretical chemists.

[Iptycenes](/source/Iptycene), with the alkene groups part of an [arenes](/source/Arene), are related compounds. It is also a starting material for many other organic compounds, such as [semibullvalene](/source/Semibullvalene).

## Synthesis

The original Zimmerman synthesis modified in 1969[2] starts from [coumalic acid](/source/Coumalic_acid):[note 1]

Many alternative routes have been devised since then, one of them starting from [benzene oxide](/source/Oxepin):[3][4]

An alternate route that allows synthesis of the parent barrelene system and a variety of substituted barrelenes has also been reported.[5]

## Barrelene reactions

Barrelene is [hydrogenated](/source/Hydrogenation) with hydrogen gas and [Adams' catalyst](/source/Adams'_catalyst) in [ethanol](/source/Ethanol) to the fully saturated [bicyclo\[2.2.2\]-octane](/source/Bicyclooctane). [Bromination](/source/Bromination) with [bromine](/source/Bromine) in [tetrachloromethane](/source/Tetrachloromethane) gives a di-bromo adduct because a [coupling reaction](/source/Coupling_reaction) intervenes:

[Epoxidation](/source/Epoxidation) of barrelene with [oxone](/source/Oxone) gives the trioxatrishomobarrelene[6] which on [rearrangement](/source/Rearrangement_reaction) with [boron trifluoride](/source/Boron_trifluoride) (driving force:relief of [strain energy](/source/Strain_energy)) converts into the trioxatrishomocubane:[7]

This compound can be envisioned as a [cubane](/source/Cubane) with three oxygen atoms inserted into three opposite edges or as [9-crown-3](/source/9-crown-3) capped by two [methine](/source/Methine) units. The molecule is [chiral](/source/Chirality_(chemistry)) and the separate [enantiomers](/source/Enantiomer) have been isolated.

Certain barrelenes have been used as a [monomer](/source/Monomer) in a [ring opening metathesis polymerization](/source/Ring_opening_metathesis_polymerization):[8][9]

The catalyst is a [Schrock carbene](/source/Schrock_carbene) (a *molybdenum bis-(hexafluoro-tert-butoxy) carbene catalyst*) and the long alkyl chain attached to the monomer is required for solubility. Oxidation of the polymer with [DDQ](/source/2,3-Dichloro-5,6-dicyano-1,4-benzoquinone) affords the naphthalene pendant of [poly(p-phenylene vinylene)](/source/Poly(p-phenylene_vinylene)).

[Isopentane](/source/Isopentane) solutions of barrelene undergo [photolytic](/source/Photolysis) [isomerisation](/source/Isomerisation) when [acetone](/source/Acetone) is added as a [photosensitizer](/source/Photosensitizer) to produce [semibullvalene](/source/Bullvalene#Semibullvalene). Prolonged irradiation results in further isomerisation to form [cyclooctatetraene](/source/Cyclooctatetraene).[10]

## Notes

1. Reaction scheme: [decarboxylation](/source/Decarboxylation) of [coumalic acid](/source/Coumalic_acid) (1) takes place at 650 °C with [copper](/source/Copper) to give α-[pyrone](/source/Pyrone) (2). The reaction with [methyl vinyl ketone](/source/Methyl_vinyl_ketone) (3) is a tandem [Diels–Alder](/source/Diels%E2%80%93Alder)/retro-Diels–Alder/Diels–Alder sequence, which yields di-[ketone](/source/Ketone) 5 as a mixture of two [isomers](/source/Isomer). It is possible to convert the [endo isomer](/source/Endo_isomer) 5b to the exo isomer 5a by an [epimerization](/source/Epimerization) process through the [enol](/source/Enol). The ketone groups are converted to [oxime](/source/Oxime) groups in 6 by reaction with [hydroxylamine](/source/Hydroxylamine) and then to the [tosylate](/source/Tosylate) groups in 7 by reaction with [tosyl chloride](/source/Tosyl_chloride). A basic [Beckmann rearrangement](/source/Beckmann_rearrangement) takes the scheme to give [amide](/source/Amide) 8 and its [hydrolysis](/source/Hydrolysis) to the di-[amine](/source/Amine) 9 takes place with [sodium hydroxide](/source/Sodium_hydroxide). Finally, a [Hofmann elimination](/source/Hofmann_elimination) through [ammonium salt](/source/Ammonium_salt) 10 gives the barrelene 11.

## References

1. [International Union of Pure and Applied Chemistry](/source/International_Union_of_Pure_and_Applied_Chemistry) (2014). *Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013*. [The Royal Society of Chemistry](/source/Royal_Society_of_Chemistry). p. 1257. [doi:10.1039/9781849733069](https://doi.org/10.1039/9781849733069). ISBN 978-0-85404-182-4.

1. Zimmerman, Howard E.; Grunewald, Gary L.; Paufler, Robert M.; Sherwin, Maynard A. (April 1969). ["Synthesis and physical properties of barrelene, a unique Moebius-like molecule"](https://pubs.acs.org/doi/abs/10.1021/ja01037a024). *Journal of the American Chemical Society*. **91** (9): 2330–2338. [doi:10.1021/ja01037a024](https://doi.org/10.1021/ja01037a024). [ISSN 0002-7863](https://www.worldcat.org/issn/0002-7863)

1. Cossu, Sergio; Battaggia, Simone; De Lucchi, Ottorino (1997-06-13). ["Barrelene, a New Convenient Synthesis"](https://pubs.acs.org/doi/10.1021/jo962267f). *The Journal of Organic Chemistry*. **62** (12): 4162–4163. [doi:10.1021/jo962267f](https://doi.org/10.1021/jo962267f). [ISSN 0022-3263](https://www.worldcat.org/issn/0022-3263)

1. Step one in this reaction between [oxepin](/source/Oxepin) (one of the possible [tautomers](/source/Tautomer)) with (*Z*)-1,2-bis(phenylsulfonyl)ethylene is a [Diels–Alder reaction](/source/Diels%E2%80%93Alder_reaction). The reagents for [de-epoxidation](/source/De-epoxidation) are [tungsten hexachloride](/source/Tungsten_hexachloride) and [butyllithium](/source/Butyllithium). The second [elimination reaction](/source/Elimination_reaction) takes place with [sodium amalgam](/source/Sodium_amalgam) in [Julia olefination](/source/Julia_olefination) style.

1. Wagaman, Michael W.; Bellmann, Erika; Cucullu, Michèle; Grubbs, Robert H. (1997-12-01). ["Synthesis of Substituted Bicyclo\[2.2.2\]octatrienes"](https://pubs.acs.org/doi/10.1021/jo971039y). *The Journal of Organic Chemistry*. **62** (26): 9076–9082. [doi:10.1021/jo971039y](https://doi.org/10.1021/jo971039y). [ISSN 0022-3263](https://www.worldcat.org/issn/0022-3263)

1. endo, exo,syn-3,7,10-Trioxapentacyclo[3.3.3.02,4.06,8.09,11]undecane

1. Kozhushkov, Sergei I.; Preuß, Thomas; Yufit, Dmitrii S.; Howard, Judith A. K.; Meindl, Kathrin; Rühl, Stephan; Yamamoto, Chiyo; Okamoto, Yoshio; Schreiner, Peter R.; Rinderspacher, B. Christopher; de Meijere, Armin (June 2006). ["4,7,11‐Triheterotrishomocubanes – Propeller‐Shaped Highly Symmetrical Chiral Molecules Derived from Barrelene"](https://chemistry-europe.onlinelibrary.wiley.com/doi/10.1002/ejoc.200600019). *European Journal of Organic Chemistry*. **2006** (11): 2590–2600. [doi:10.1002/ejoc.200600019](https://doi.org/10.1002/ejoc.200600019). [ISSN 1434-193X](https://www.worldcat.org/issn/1434-193X)

1. Pu, Lin; Wagaman, Michael W.; Grubbs, Robert H. (1996-01-01). ["Synthesis of Poly(1,4-naphthylenevinylenes): Metathesis Polymerization of Benzobarrelenes"](https://pubs.acs.org/doi/10.1021/ma9500143). *Macromolecules*. **29** (4): 1138–1143. [doi:10.1021/ma9500143](https://doi.org/10.1021/ma9500143). [ISSN 0024-9297](https://www.worldcat.org/issn/0024-9297)

1. Wagaman, Michael W. & Grubbs, Robert H. (1997-07-01). ["Synthesis of Organic and Water Soluble Poly(1,4-phenylenevinylenes) Containing Carboxyl Groups: Living Ring-Opening Metathesis Polymerization (ROMP) of 2,3-Dicarboxybarrelenes"](https://pubs.acs.org/doi/10.1021/ma9701595). *Macromolecules*. **30** (14): 3978–3985. [doi:10.1021/ma9701595](https://doi.org/10.1021/ma9701595). [ISSN 0024-9297](https://www.worldcat.org/issn/0024-9297)

1. Zimmerman, H. E. & Grunewald, G. L. (1966). "The Chemistry of Barrelene. III. A Unique Photoisomerization to Semibullvalene". *[J. Am. Chem. Soc.](/source/J._Am._Chem._Soc.)*. **88** (1): 183–184. [doi:10.1021/ja00953a045](https://doi.org/10.1021/ja00953a045)

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