Synthesis of Benz[a]azulenes Substituted at the Benzo Ring

Abstract

It is shown that the thermal electrocyclic ring-closure reaction of 1,2-di[(E)-prop-l-enyl]benzene to yield 2,3-dimethylnaphthalene (cf: Scheme I ) [lo] can successfully be applied also to the synthesis of benz[a]azulenes (cf: Schemes 2 and 3). Starting materials are methyl 4,6,8-trimethylazulen-2-y1 ketone (6) and the corresponding 2-carbaldehyde 5, which, in a Horner-Emmons reaction, are transformed into the (azulen-2-yl)-acrylates (E)-8 and (I9-7, respectively. Vilsmeier formylation of these compounds, followed by the Horner-Emmons reaction leads to the formation of the bisacrylates (E,E)-11 and (E,E)-12, respectively. In an alternative reaction, (E)-8, on treatment with dirnethyl acetylenedicarboxylate (ADM) in the presence of [RuH,(PPh,),], can be transformed into the methoxycarbonyl-substituted bisacrylates (E,E)-and (E,Z)-17. All three bisacrylates, on heating at 180-190° in p-cymene, undergo cyclization to yield the corresponding dihydrobenz[a]azulenes 13, 14, and 18, respectively, which could easily be dehydrogenated on heating in the presence of Pd/C. The new benz[a]azulenes 15,16, and 19 are fully characterized.

An Analysis of the Bonding Properties of Benz[a]azulene by X‐Ray, NMR, and Computational Studies

“…[24]) and of perfectly planar cycloheptatrienes (11.8-12.6 Hz; cf. [25]) point again to the fact that a localized heptafulvene substructure is not fully developed in benz[a]azulenes, ie., there is still a certain degree of azulene conjugation recognizable in 3,4, and their derivatives. The magnitude of the measured 1J(13C,'3C) values of 3 and (DJ-4 fully supports this view (cf.…”

Formation and Thermal Rearrangement of Dimethyl Tricyclo[6.2.2.01,7]dodeca‐2,4,6,9,11‐pentaene‐9,10‐dicarboxylates

“…[49]), in full agreement with the predictions of Salem and Rowland (cf [4? ] That subtle substituent effects govern also the thermal behavior of tricyclic compounds of type 2 is demonstrated by the following experiment, The Rh'-catalyzed reaction of the benz[a]azulene-dicarboxylate 37, the synthesis of which we have recently described [50], leads in MeCN at 100" to the benzo-annelated tricyclic compound 38 (Scheme 13). Whereas tricyclic compounds of type 38, having no substituent at the benzo ring, can successfully be rearranged to the corresponding benzo[a]heptalene-6,7-dicarboxylates (cf [6] [7] [lo]), the thermal reaction of 38, with the two strong n-and a-acceptor substituent at the benzo ring, gave only the starting materials as retro-DielsAlder products.…”

Synthesis of Benzo[a]heptalenes via Benzanellation–Aromatization Sequence

“…It has been shown that the thermal electrocyclic ring-closure reaction of azulenebis[prop-2-enoates] can successfully be applied to the synthesis of benz[a]azulenes [8]. We report here on the first results of the utilization of the later benzanellation concept to the synthesis of new functionalized benzo[a]heptalenes from the corresponding bis[prop-2-enoates].…”

“…± So far, benzo[a]heptalenes have been synthesized by degradation of colchicinoids [1], by application of Hafner×s heptalene synthesis to benz[a]azulenes [2] [3], or by benzene-ring-forming reaction of heptalene-4,5-dicarboxylates and their derivatives [4] [5]. The benzanellation procedures, which have already been tested, are the following: i) Bergman cyclization of corresponding bis(ethynyl)heptalenes [6] to give benzo[a]heptalenes in an overall yield of 6%; ii) Diels-Alder reaction with heptaleno[1,2-c]furans, followed by ring opening of the formed oxabicyclo[2.2.1]heptane substructure [5b] [7]; iii) −one-pot× benzanellation procedure, starting directly from heptalene-4,5-dicarboxylates and lithiated methyl sulfones, leading to 3-sulfonylsubstituted benzo[a]heptalene-2,4-diols in moderate-to-excellent yields [5].It has been shown that the thermal electrocyclic ring-closure reaction of azulenebis [prop-2-enoates] can successfully be applied to the synthesis of benz[a]azulenes [8]. We report here on the first results of the utilization of the later benzanellation concept to the synthesis of new functionalized benzo[a]heptalenes from the corresponding bis [prop-2-enoates].…”

“…The main disadvantage of the novel synthetic pathway is the incomplete dehydrogenation reaction that leads to mixtures of benzo[a]heptalenes and their dihydro precursors, which are difficult to separate. This problem may be solved if alkyl substituents are present at C(1) and C(4) of the benzene ring, as has been observed for the dehydrogenation of benz[a]azulenes [8]. The synthesis of suitably substituted benzo[a]heptalene precursors can be envisaged from the readily available heptalene diesters by MeLi addition, followed by dehydration, and is just under investigation.…”