Ring contraction partial

When aromatic hydrocarbons are treated with aluminum chloride the autocondensation is accompanied by side reactions (ring contraction, partial hydrogenation, etc.) reactions of phenyl and naphthyl ethers are, however, smoother, and Scholl and Seer278 obtained good yields of 4,4 -diethoxy-l,l -binaphthalene from 1-ethoxynaphthalene and aluminum chloride in nitrobenzene. 

Cyanogen azide is a useful reagent for conversion of pyrrolidine enamines of 3-keto steroids to A-norsteroids. " Ring contractions can be carried out in the presence of 17j5-hydroxy, 17j -acetoxy, 20-keto groups and isolated double bonds. In a typical procedure, 17j -hydroxy-5a-androstan-3-one (partial formula 8) is converted into the enamine (9) by pyrrolidine in benzene... 

Table 4 shows that benzene is formed at almost identical rates from cyclohexene, hexadiene, methylcyclohexene and cyclohexadiene react under low partial pressure over Ga-HZSM-5. which suggests that over these catalysts benzene is formed from the same intermediate. In contrast over H-ZSM-5, under identical experimental conditions, the rate of benzene formation from the hydrocarbons cited was one to two orders of magnitude lower. These results prove again that gallium plays a decisive role in aromatization. Over H-ZSM-5 the major hydrocarbon formed is methylcyclopentene from cyclohexene (ring contraction)... 

Stable cations, anions and radicals derived from azepines are rare species, although many ring contractions and some valence isomerizations appear to involve prior formation of a cationic species, especially in those partially saturated systems in which an enamine moiety forms an integral part of the azepine ring. It is only recently that azepine anions have been generated and their synthetic potential exploited (Section 5.16.3.6). 

Partially unsaturated azepines, like the fully conjugated systems, tend to be unstable in acid solution and undergo either rearrangement or ring contraction, generally to an aromatic system. Protonation takes place at nitrogen or, as in those hydroazepines in which delocalization of the nitrogen lone pair is possible, at the /3 -carbon of the enamine system. 

Chlorocyclopentanone has been rearranged to methyl cyclobutanecarboxylate (1). however, it was necessary to use electrochemically generated superoxide in the presence of iodomethane partially ring-contracted byproduct was also formed.53... 

Partial racemization, however, occurred in the deamination of optically active 2-aminobicy-clo[2.2.1]hept-5-ene-2-carbonitrile yielding a 1 2 mixture of the ring-contraction products 2-hy-droxybicyclo[3.1.1]hept-3-ene-l-carbonitrile (21, 40% retention of optical activity) and 4-hy-droxybicyclo[3.1.1]hept-2-ene-l-carbonitrile (22, 5% retention of optical activity). The chiral bridged cation gave only 21, while from the achiral allyl cation both 21 and 22 were formed.87... 

One synthesis of cyclopentenone [80], requiring a resolution, involved initial ring contraction of phenol when treated with alkaline hypochlorite (49). Resolution of the resulting cis acid [85] was effected with brucine. The desired enantiomer [86] formed the more soluble brucine salt and was thus obtained from the mother liquors of the initial resolution. Oxidative decarboxylation with lead tetracetate, partial dechlorination with chro-mous chloride, and alcohol protection gave chloro enone [87]. Zinc-silver couple (50) dechlorinated [87] to the desired cyclopentenone [80]. 

Interest in the structure-activity relationships of the gibberellins has led to the partial synthesis of the four stereoisomers about positions 5 and 6 of the hexa-hydrofluorene (57). The unsaturated acid anhydride (56) was prepared by the benzilic acid ring-contraction of the 6,7-diketones obtained from oxidation of desisopropyl dehydroabietic acid, and converted into the saturated acids by reduction and base-catalysed epimerisation. 

Ring contraction of the 3,6-dihydro-2//-thiopyrans 41, which are readily available in two steps from dimethyl malonate, was shown to give the tetrahydrothiophenes 42 upon treatment with A -iodosuecinimide (NIS) in the presence of a carboxylic acid. The reaction was suggested to proceed via a bicyclic thiiranium ion intermediate. Moreover, base induced elimination of HI from 42 (with for example R = Bn) gave the partially unsaturated system 43 <04EJO74>. 

Irradiation (254 nm) of the imide (430) brings about partial isomerization of the starting material to the trans-isomer (431). This product is accompanied by the ring-contracted compounds (432) and (433). Secondary irradiation of the... 

Under the given conditions the main reaction of n-butylbenzene is the partial or complete hydrogenolysis of the side chain to form n-propyl-benzene, ethylbenzene, toluene, and benzene. In addition hydrogenation of the benzene ring, contraction of the hexamethylene cycle to a five-membered one, hydrogen cleavage of the latter, formation of xylenes, methylpropylbenzenes, and diethylbenzenes, and dehydrocyclization of the side chain of w-butylbenzene to naphthalene are also observed. 

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