Azulene from cycloaddition reactions

The 1,2-cycloaddition reaction can take place in an intramolecular manner (equation 63), although in this example the initial excitation involves the aromatic group . A reaction of a different type is thought to be involved in the first stage of the formation of azulene or naphthalene photodimers from diphenylacetylene (equation 64), though here it is claimed that an intermediate benzocyclobutadiene species has been detected . The intermediate isomer of diphenylacetylene is formed via the triplet state and is relatively long-lived at — 10 °C. The major dimers formed are 1,2,3-triphenylazulene and 1,2,3-triphenylnaphthalene hexaphenylbenzene and octaphenylcubane are also produced . 

There are general reviews on heterocyclic syntheses by cycloaddition reactions of isocyanates and on the use of heterocyclic cations in preparative organic chemistry. More specific topics are 5-hydroxymethylfuran-2-carb-aldehyde, isobenzofurans and related ort/io-quinonoid systems, the conversion of 2//-cyclohepta[Zj] furan-2-one (1) into derivatives of azulene, the synthesis of indoles from o-alkylphenyl isocyanides, and abnormal Fischer indolization reactions of o-methoxyphenylhydrazones. Two reviews on isoindoles have appeared and a lecture on highly conducting charge-transfer complexes that are based on heterocyclic selenium and tellurium donors has been reprinted.Recent advances in the chemistry of imidazole and in the use of nitro-imidazoles as radiosensitizers have been summarized. There have been reviews on benzimidazole A -oxides and on dihydrobenzimidazoles, benzimidazolones, benzimidazolethiones, and related compounds. Other topics are synthetic applications of 1,3-dithiolium and 1,3-oxathiolium salts and of isoxazoles, the chemistry of benzisoxazoles, 2-amino-oxazoles, 5-oxazolones (2), furoxans, benzofuroxans, and related systems, the synthesis of five-membered meso-ionic compounds, and tetrazoles. ... 

Photolysis of the triazepine products produces 2,2-dimethylpropanenitrile and the corresponding pyrazole in quantitative yield <1997BSF927>. Reaction of sydnone 89 with fulvene 91 proceeds by [ji4s + jt6s]-cycloaddition followed by spontaneous loss of carbon dioxide and a molecule of dimethylamine or acetic acid from the pseudo-azulene , cyclopentaMpyridazine 92 (Equation 9) <1996CC1011, 1997T9921>. 

Kato et al. (151,152) explored the chemistry of 2-ferf-butylMvenes with isomiinchnones, as well as with several other mesoionic compounds, in a novel approach to pseudo-hetero-azulenes. Thus, isomtinchnone 51a, generated as before in situ from A-benzoylphenylglyoxyanilide 253 with triethylphosphite, reacts with 2-ferf-butyl-6-(dimethylamino)fulvene to give the [47i+6ti] adduct diphenylcyclo-penta[c]pyran in low yield. Likewise, reaction of 51a with dimethylfulvene gave a mixture of two adducts, one of which arises from a [47i+2ti] cycloaddition. 

Wittig reactions with pyrrole-2-aldehyde led to the esters (79) which were cyclisized to 3a-azaazulen-4-ones (80).104,105 4-Methylene-3a-aza-azulenes (81) have been obtained from 80 with stabilized phos-phoranes.36 Reaction of dimethyl acetylenedicarboxylate with 81 could not be achieved. A similar cycloaddition was successful in the synthesis of cycl[3,3,3]azines (2) (Section V). 

Azulene (201) reacts with benzyne to give cycloadduct 202, the yield being best (33%) when 1 was generated via Pb(OAc)4 oxidation of 1-aminobenzotriazole . The product is derived from addition across the cyclopentadiene moiety of 201. Although a concerted cycloaddition is possible, reaction may take place via electrophilic attack of benzyne at Cl of azulene (known to be prone to electrophilic attack) to form zwitterionic intermediate 203, followed by ring closure at C3a. 

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]. 

The first attempt to construct a condensed core was a unique silicon-initiated (CO-SiCaT) reaction of carbonylated carbotricyclization of enediynes 2.314 [154, 161]. The 5-7-5 fused tricyclic products can be produced from the same type enediynes and CO through also by the rhodium-catalyzed intramolecular [2+2+2+1] cycloaddition [154, 160]. Reaction of dodeca-ll-ene-l,6-diyne 2.314 or heteroatomic analogues with a hydrosilane catalyzed by Rh(acac)(CO)2 at ambient temperature and pressure affords the corresponding fused 5-7-5 tricyclic system with a seven-membered lactone ring, 5-oxo-l,3a,4,5,7,9-hexahydro-6H-cyclopenta[e]azulenes 2.315 or related heteroanalogs in a yield from good to excellent (50-98%) (Scheme 2.104) [154]. 

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