Aromatics ring enlargement

PFAU - PLATTNER Cyclopropane synthesis DIazoalkane insertion Into olelins wtth lormation of cyclopropanes or ring enlargement ol aromatics to cydoheptatnenes see also lormation ol pyrazoHnes (von Pechman). 

The use of the stannylquinones 81 results in the regioselective formation of 1,4-naphthoquinones or 9,10-anthraquinones 82 [40]. Highly-oxygenated angularly-fused polycyclic aromatic compounds are prepared by the ring enlargement [41]. (Scheme 29)... 

The ring enlargement of five-membered rings containing a quaternary carbon atom occurs predominantly with its participation (5). This can be judged from the isomer composition of aromatics from 1,1,3-trimethylcyclo-pentane (Scheme IXB) (/M-xylene p-xylene toluene = 62 34 4). With palladium, the tertiary C atom is more reactive 87). 

Kazansky et al. (5) estimated the role of C5 cyclic intermediates in aromatization to be about 5% over platinum on carbon. Dautzenberg and Platteeuw found about 11% C5 cyclic pathway with nonacidic platinum on alumina (23). 2,2,4-Trimethylpentane is forced to produce aromatics via C5 cyclization because of its structure here the quaternary carbon atom facilitates ring enlargement (5, 23). 

Although the gem-dihalocyclopropanes are fairly stable compounds, they can participate — as has been shown in the above sections — in quite a number of chemical transformations. Several reactions between dihalocarbenes and alkenes have been described in which no dihalocyclopropane formation could be observed that these intermediates might have been produced was only inferred from the type of products finally isolated. A typical process of this type is the e/ufo-addition of dihalocarbenes to norbomene and norbomadiene as discussed above. Comparable rearrangements have been observed, when dichlorocarbene additions either lead to aromatic products or when they cycloadd to rather inert aromatic systems. In the latter case a ring-enlargement takes place. A reaction related to the concerted opening of two cyclopropane rings in a bicyclopropyl system as discussed above takes place when dichlorocarbene is added to spiro[2.4]hepta-4,6-diene [227]. 

Additions to carbon-carbon double bonds have already been mentioned. Carbenes also add to aromatic systems, but the immediate products rearrange, usually with ring enlargement (see 5-50). Additions of carbenes to other double bonds, such as C=N (6-61 and 6-62), and to triple bonds have also been reported. 

Evidence also suggests, however, that alkanes with only five carbon atoms in a linear chain may undergo aromatization via 1,5 ring closure followed by ring enlargement. Numerous mechanisms were put forward to rationalize these transformations.209... 

Fig. 8.16. Aromatic ring part of the COSY spectra of the complex shown in the inset. (A) Magnitude mode spectrum (B) phase-sensitive spectrum, absorption mode (C) phase-sensitive spectrum, dispersion mode (D) ISECR [20] spectrum (sequence in Fig. 8.2D). Sin2 weighting functions have been used for spectra (A)-(C) and a cos2 weighting function for spectrum (D). Peaks in (A) and (D) are in phase and positive. The positive components of the 6-5 peak in (B) and (C) are shown in the enlargements.

Aromatization and ring enlargement of propylcyclobutane were observed over Pt/Si02, whereas only aromatization took place on Pd/Si02 and Rh/ Si02.514 Aromatization was also found during the hydrogenolysis of bicy-clo[3.1.0]hex-2-ene-endo-6-carboxylic acid (40), where cyclohexane carboxylic acid and benzoic acid were the products (Scheme 4.135).515... 

Publications from two laboratories (55, 72) have argued that aromatization to xylenes of 1,1,3-trimethylcyclopentane on platinum films at 300°-330°C can be explained only by a ring enlargement at the quaternary carbon atom ... 

It is evident that this addition/ring enlargement sequence has to be tested with other heterocumulenes too. So far only phenylthioisocyanate has been used, but completely analogous to CS2 dihydropyrrole derivatives 227 are now generated, which are either isolated or directly aromatized to pyrrole-3-carboxylates 228 after acid treatment (Eq. 93)97). 

First example the Yg-increment of the group -C(CFl3)2-0- is 8.5 (according to Table 6.1.). This value is valid for purely flexible chains. If the same group is found between two aromatic rings, its free rotations are suppressed, the chain is locally stiffened and the increment is enlarged to 15.0 (Table 6.1A) ... 

Aromatic ketones of the a-tetralone type 11/74 can be converted by a Wittig reaction to compounds of type 11/75, Scheme 11/11. Under Prevost reaction conditions (AgN03,I2,CH30H) two ring enlargement products are formed, 11/77 and 11/78 (both together in 67 % yield), which by hydrolysis are converted to the a-cyano ketone 11/79 [56], This procedure has been applied successfully to... 

An interesting approach to macrocyclic benzolactones was discovered by treating 2-nitrocycloalkanones with 1,4-benzoquinone (VII/133) in the presence of catalytic amounts of l,8-diazabicyclo[5.4.0]undec-7-ene (DBU). The transformation involves a Michael reaction, aromatization, and ring enlargement via... 

Scheme VIII/32. Synthesis of macrocyclic lactones with an annelated aromatic ring by ring enlargement [100] [101].

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