Ring contraction, cyclohexane

Within the cubane synthesis the initially produced cyclobutadiene moiety (see p. 329) is only stable as an iron(O) complex (M. Avram, 1964 G.F. Emerson, 1965 M.P. Cava, 1967). When this complex is destroyed by oxidation with cerium(lV) in the presence of a dienophilic quinone derivative, the cycloaddition takes place immediately. Irradiation leads to a further cyclobutane ring closure. The cubane synthesis also exemplifies another general approach to cyclobutane derivatives. This starts with cyclopentanone or cyclohexane-dione derivatives which are brominated and treated with strong base. A Favorskii rearrangement then leads to ring contraction (J.C. Barborak, 1966). 

Cyclopentene-l-carboxaldehydes are obtained from cyclohexene precursors by the sequence cyclohexene - cyclohexane-1,2-diol -> open-chain dialdehyde - cyclopentane aldol. The main advantage of this ring contraction procedure is, that the regio-and stereoselectivity of the Diels-Alder synthesis of cyclohexene derivatives can be transferred to cyclopentane synthesis (G. Stork, 1953 G. BUchi, 1968). 

Wiberg and Koch 167) also disagreed with Littler s results, and found that the major product (75%) obtained on treatment of cyclohexanone with aqueous thallium(III) perchlorate was cyclopentanecarboxylic acid (XL). 2-Hydroxycyclohexanone was isolated in only 3 % yield unchanged starting material accounted for the remainder of the product. Wiberg and Koch were unable to detect any cyclohexane-1,2-dione in the product mixture, but did prove that 2-hydroxycyclohexanone did not function as the precursor to the ring-contracted acid. From the results obtained from a study of the oxidation of 2,2,6,6-[Pg.196]

The Prins cyclization can also be coupled with a ring-contraction pinacol rearrangement, as illustrated in Scheme 1.6. This allows a smooth conversion of alkyl-idene-cyclohexane acetal 1-16 to single bond-joined cyclohexane cyclopentane aldehyde 1-17 [le]. 

The zeolite structure also plays a large role in RC product distribution. Weitkamp et al.62 conducted experiments with Pt/HZSM-5 catalysts, which have very narrow pore sizes when compared with other zeolites, such as USY or SAPO. They found that c is I trans-1,3-dime thylcyclopentane was formed, while 1,1 and 1,2-DMCP were not. This indicates that the more oval shaped 1,3-DMCP was able to diffuse through the pores, while the more bulky and spherical isomers were not, and thus not seen in the product distribution. In short, when compared with dealkylation to cyclohexane, ring contraction of MCH is a more effective pathway to yield higher ON products. However, in order to further improve the ON, ring-opening of the RC isomers may be necessary, as shown below. 

The difference in stability between the cychc six- and seven-membered methoxy tel-luroxides is noteworthy. Indeed, while the cyclohexane derivative is stable and isolable, giving ring contraction on treatment with 1 equiv of MCPBA, the cycloheptane derivative is unstable, suffering telluroxide elimination (like cycloheptene formation from cyclohep-tyl phenyl telluride), as will be shown in Section 4.7. 

Surprisingly, the magnesium enolate of 2-methylcyclohexen-l-one reacts with chloro-acetone to give an unexpected product via a cyclohexane ring contraction (equation 74). 

In contrast with these ring expansion processes, photoelimination of nitrogen from ethyl o-azidobenzimidate (114) is followed by cyclization with the formation of 3-ethoxy-IH-indazole (115), and irradiation of the 2-azidopyrazines (116) yields the ring-contracted 1-cyanoimidazoles (117), presumably via the nitrenes (118). Products derived by insertion of (p-toluenesulphonyl)nitrene are obtained on irradiation of p-toluenesulphonyl azide in p-xylene or cyclohexane. "... 

An nnnsnal ring-contraction reaction occnrs on the acid-catalyzed interaction of trimeth-ylhydroqninone 144 with cycloalkane-1,2-diols (e.g. 145) to form the spiro componnds 146 (eqnation 67) . Besides two isomers of cyclohexane-1,2-diols 145, this rearrangement was also described for cyclopentane-, cycloheptane- and cyclooctane-1,2-diols . 

Unlike some other group VIII metals (S), platinum does not promote hydrogenolysis of cyclohexanes, but dehydrogenation to aromatic hydrocarbons. Cycloheptanes undergo ring contraction and aromatization rather than hydrogenolysis (9-72) (Scheme 1). 

A number of oxidative ring contractions of dihydrothiazines are known. For example, when a cyclohexane solution of the compound 117 was exposed to the air at room temperature, a 1 1 mixture of the derivatives 186 and 187 was produced, probably by way of the hydroperoxide 185. As already indicated (Section V,C,l,a), the product of the reaction of the dihydro-thiazinone 97 with hydrogen peroxide and acetic acid was the thiazoline 136 other examples of this oxidative ring contraction have been de-... 

Benzothiazines have been reported to undergo a number of rearrangement reactions, one of the most commonly encountered involving ring contraction. Thus among the products of autoxidation of 1,4-benzothiazines are benzo-thiazoline derivatives.137,141,167 For example, the benzothiazine 135 on treatment with oxygen in acidified cyclohexane gave the acyl derivative 136. 

From the effect of solvent (Table 15) it is evident that the reactions discussed are nitrene reactions hydrogen-rich solvents suppress ring contraction and give rise to solvent dimer (bibenzyl) and/or a yellow nitrene dimer. The structure of the dimer is not known, but one possibility is shown in 144. A similar (colorless) dimer was obtained from 9-phenanthridylnitrene at 500 ° 7). Xhe two dimers formed from 137 and 141 in cyclohexane have nearly identical IR spectra. How could a hydrogen-rich solvent promote dimeriztion There is evidence from aryl azide decomposition in solution that amino radicals are formed first, and these dimerize and dehydrogenate as shown for 1-naphthylnitrene in [Eq. (48)] 82). 

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