SYNTHESIS OF AZULENE

The allenylsilanes are excellent nucleophiles and they can react with a variety of electrophilic species in annulation processes that provide access to diverse products. Allenylsilane 112 (Eq. 13.36) reacts with tropylium fluoroborate 111 to provide azu-lene 113 [35]. The reaction is slow and it is necessary to use an acid scavenger so as to inhibit protiodesilylation by the fluoroboric acid that is generated during the course of the annulation. The excess tropylium salt abstracts a hydride from the reaction intermediate leading to the azulene. There are relatively few direct methods for the synthesis of azulenes. 

Hafner s synthesis of azulene,216 the cyclopentadienyl anion can also be used to introduce the five-membered fragment. This anion reacts with heterocyclic quaternary salts in a complex manner the pseudoazulene, however, is obtained in one step. The total yields are only about 10%, but it is possible to use starting materials that are easily available in large quantities. This variant was applied to prepare 2H-cyclopenta[thiadiazolium salts145 and cyclopenta[c]thiopyranes (31) using N-methyl thiazoliumbromide.90... 

A prototype for the new cyclization reaction is the Ziegler-Hafner synthesis of azulene (J)4 by heating the fulvenoid decapentaene (/). Woodward28 was the first to argue that a thermal, electrocyclic [rr 10 s] ring-closure could be involved in this thermolytic process. Of course it seems likely that the nonisolable intermediate (2) may be formed by an intramolecular electrophilic attack of atom 10 to the electron-rich position 1 in compound (1). Indeed a distinction between the two different reaction modes is mechanistically irrelevant in this case. But only its interpretation as an electrocyclic process allows the generalization to [ir 6 s] cyclizations too. 

Another intriguing example is a synthesis of azulene that utilises the bis(dimethylamine) derivative of glutaconaldehyde produced with loss of 2,4-dinitroaniline from l-(2,4-dinitrophenyl)pyridinium chloride (Zincke s salt). ... 

The thermal automerization and rearrangement reactions of PAHs have been widely investigated during the past two decades (for examples see refs. [31 e, g, 62-64]). The main objective was to understand the processes of formation of aromatic hydrocarbons in fuel rich flames and the mechanisms of transformation of the PAHs that have been observed at these elevated temperatures. In most cases, thermally initiated rearrangement reactions in the carbon skeletons of PAHs require high enthalpies of activation resulting in low product selectivities and poor overall yields. Because the expected products are often more effectively prepared by conventional routes, this approach has been used as a synthetic tool only in a few cases, e.g. the synthesis of azulenes [65] and the rearrangement of bifluorenylidenes to benzenoid hydrocarbons [38]. 

Synthesis of azulenes by condensation of cyclopentadienes with derivatives of glutaric dialdehydes. 

Also, a synthesis of azulene that gives good yields starts from the fulvene derivative860 that is formed quantitatively from cyclopentadiene and penta-nedial A-methyl-A-phenylmonoimide ... 

SYNTHESIS AND DIELS-ALDER REACTIONS OF 3-ACETYL-2(3H)-OXAZOLONE, 62, 149 SYNTHESIS OF AZULENE, 62, 134... 

Sulphur.—Desulphurizations. Pyrolyses of cyclic bis-sulphones can be used to obtain large-ring cyclophanes (e.g. Scheme 24)/ Sulpholenes undergo pyrolysis or LiAlH4-promoted SO2 elimination to give dienes, and this approach has been used in the synthesis of azulenes. A related reaction gives Dewar-benzene derivatives. ... 

An interesting new synthesis of azulenes has been developed that involves the [6 + 4] cycloaddition of NiV-diethyl-l-butadienylamine to fulvenes. The Diels-Alder product of dimethyl acetylenedicarboxylate addition to the azulene formed from dimethylfulvene possesses structure (234), whose n.m.r. spectrum indicated it to be a mixture of conformational isomers. The high barrier (AG ss 18—20 kcal... 

The first chemical synthesis of azulene was reported by Pfau and Plattner in 1937. Their synthesis takes advantage of the ring enlargement of indane 133 on addition of diazoacetic ester to give cycloheptatriene 135. This compound is converted into azulene via a three-step sequence hydrolysis, dehydrogenation, and decarboxylation of the resulting acid. 

For some examples (a) C. Cui, C. Zhu, X.-J. Du, Z.-P. Wang, Z.-M. Li, W.-G. Zhao, Green Chem. 2012, 14, 3157-3163. Ultrasound-promoted sterically congested Passerini reactions under solvent-free conditions, (b) K. Sato, T. Ozu, N. Takenaga, Tetrahedron Lett. 2013, 54, 661-664. Solvent-free synthesis of azulene derivatives via Passerini reaction by grinding. 

Synthesis of Azulenes. Reaction of tropylium cations with allenylsilanes produces substituted azulenes. Typically, commercially available tropylium tetrafluoroborate (2 equiv) is ert5)loyed. The second equivalent dehydrogenates the dihydroazu-lene intermediate to produce the aromatic product. Poly(4-vinyl-P3Tidine) (poly (4-VP)) or methyltrimethoxysilane is used to scavenge the HBF4 produced in the reaction. 

The divinylcyclopropane rearrangement has been used in a model study for the synthesis of tiglianes, daphnanes, and ingenanes [e.g. (152) - (153)], and Scott et al have outlined a new short synthesis of azulene (154) involving intramolecular carbene addition as a key stage. 

In 1977, Houk reported a new general synthesis of azulene 4 by using the [6+4] cycloaddition of thiophene-5,S-dioxides 184 and 109 [112]. This reaction provides the azulene nucleus in a single step. Several azulene derivatives 187 and 188 were prepared the substituents of the formed azulenes are derived from those of the thiophene-5,5-dioxides 185 and 186 (Scheme 6.48) [113]. 

A pioneering study on [6-f4] reactions of diazomethane to dimethylfulvene was reported by Houk [54]. Other advances include the double [6+4] cycloaddition of tropone to dimethylfulvene [55], intramolecular [6+4]cycloadditions of fulvenes [56], selective intramolecular [6+4]cycloadditions of aminodienylfulvenes [57], and the [6+4] cycloadditions of diethylaminobutadiene to fulvenes for the synthesis of azulenes [58]. 

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