Azulene ring

Azulyl sulphoxides 127 have also been prepared by a reaction involving a direct electrophilic substitution on the azulene ring by alkane- or arenesulphinyl chlorides186 (equation 69). Preparation of the methyl and phenyl sulphoxides of 4,6,8-tri-methylazulene and 4,6,8-tri-isopropylazulene by this method resulted in fair yields (57— 72%). However, the substitution on azulene itself gave only low yields of the corresponding sulphoxides. 

The title compound 183, a new therapeutic agent154 for stomatitis, pharyngitis and ophthalmia, has been labelled155 with 14C in the ethyl group attached to the azulene ring (equation 65) for the study of metabolism in animals. 

The Friedel-Crafts acylation at the 3-position of the azulene ring was possible due to the effect of the electron-withdrawing 1-methoxycarbonyl group. 183 has been prepared previously in an eight-step synthetic route in an unsatisfactory reaction yield156. 

We have reported the synthesis of a series of azulene-substituted methyl cations, i.e., tri(l-azulenyl)methyl, di(l-azulenyl)phenylmethyl, and (1-azul-enyl)diphenylmethyl cations (2a+, 3a+, and 4a+) (Figure 6) (6). In order to examine substituent effects on the azulene rings and to enhance their stabilities, a series of the cations (2b-d+, 3b-d, and 4b-d+) bearing tert-buty groups on each azulene ring were also synthesized (7). 

Table I summarizes the pKR+ values and redox potentials for the tri(l-azulenyl)methyl cations. The oxidation exhibited a barely separate two-step, two-electron oxidation wave. This wave is ascribed to the oxidation of two azulene rings to generate a tricationic species. The reduction showed a one-electron wave, which is ascribed to the formation of a neutral radical.

In the phenylazoazulene [52], the more basic of the two nitrogens is the nitrogen in the j3-position to the azulene ring (Gerson and Heilbronner, 1959), as predicted from molecular orbital calculations. In jO-dimethylaminophenylazoazulene [53], the same nitrogen re... 

About 25 diterpenes (107-130) of the dolastane-type featuring a hydroben-zo[/]azulene ring system have been isolated exclusively from marine sources (Fig. 14) [61-76]. 

The synthesis and characterization of a series of azulenyl-substituted cations (223) showed468,469 that these are extremely stable ions, which is attributed to the dipolar structure of the azulene rings. 

We conclude this article with a brief summary of our recent work on fulvene cycloadditions, an area in which a theoretical prediction led to the discovery of a new type of cycloaddition, and eventually to a viable method for the synthesis of the azulene ring skeleton. 

Stabilization by the introduction of bulky /er/-butyl groups into the azulene rings should be attributable to not only their steric and inductive electronic effects but also C-C hyperconjugative effects induced by the o-bond of the tert-butyl groups. The p/ R value of 14.3 for methyl cation 2b with six tert-butyl groups is the highest value ever reported for a methyl cation substituted with only hydrocarbon groups. 

Polycyclic aromatic hydrocarbons can be bent by bridging in a similar fashion. Two recent examples of such [n] cydophanes are azulenophane 116 [77] and 1,8-dioxa[8](2,7)pyrenophane 117 [78]. X-ray structure analysis revealed an angle of 3.7° between the azulene ring planes, whereas the pyrene moiety is bent by nearly 90°. 

Electrophilic reactions at ring atoms 4.2.1 Substitution at the azulene ring system... 

It is possible to selectively reduce an unsaturated side-chain catalytically without reducing the azulene rings (66MI1). Recently, an effective metal-free method has been published for the selective reduction of the C = C double bond even in azulenic enones, for instance, thiophenes 54 (Scheme 81) or 55a and 55b, using the hydride donor cycloheptatriene and protic acid (06OL3137). 

Reduation. Reduction of azulene by sodium in liquid ammonia gives 1,6-dihydroazulene [62] azulenes are also readily reduced by sodium and ethanol, or by catalytic hydrogenation, to give polyhydroazulenes [5,6,63]. It is possible, however, to reduce an unsaturated side--chain catalytically without reducing the azulene ring [64],... 

Similarly side-chain carbonyl groups can be reduced by means of the Wolff-Kishner reaction, and azulene carboxylic acids and-esters have been reduced to alcohols by means of lithium aluminium hydride, in each case with retention of the unreduced azulene ring [65]. 

Milder conditions may result in oxidation of side-chains without destruction of the azulene ring system. Thus it proved possible to oxidise guaiazulene with permanganate to give a mixture of... 

There is an example of electrophilic substitution occurring in a side-chain as well as in the azulene ring. Sulphenylation of guaiazulene with trifluoromethylsulphonyl chloride leads to substitution in the 4-methyl and 7-isopropyl groups as well as at the 3-position [138]. 

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