Ring contraction benzene rings

A third approach to 3-amino-/3-lactams is by Curtius rearrangement of the corresponding acyl azides. These are readily prepared from r-butyl carbazides, available via photochemical ring contraction of 3-diazopyrrolidine-2,4-diones in the presence of f-butyl carbazate (c/. Section 5.09.3.3.2). Thus treatment of (201) with trifluoroacetic acid followed by diazotiz-ation gives the acyl azide (202) which, in thermolysis in benzene and subsequent interception of the resulting isocyanate with r-butanol, yields the protected 3-amino-/3-lactam (203) (73JCS(P1)2907). 

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... 

In quinoline and isoquinoline, the benzene ring is more receptive to fluorma-tion, its double bonds being saturated and the hydrogen atoms replaced in preference to those in the pyridine ring As with pyridine and its homologues, ring contraction takes place during fluorination with cesium tetrafluorocobaltate at... 

Phenyloxazole, irradiated in benzene at 80°C, gave two products 61, derived from a ring contraction, and 4-phenyloxazole (62), obtained in very low yields (Scheme 25) [73JCS(CC)539 77JCS(P1)239]. The authors thought that 4-phenyloxazole was ascribable to a Dewar intermediate. 

Ring contraction to 3 6 fused, e.g. 17, rather than 4 5 fused ring systems, accompanied by phenylation, occurs on treating l//-azepine-l-carboxylates, e.g. 16. with pailadium(II) acetate in benzene solution.242 In solvents other than benzene, ring-scission products result (see Section... 

Treatment of ethyl 1 W-azepine-l-carboxylate with palladium(II) acetate in benzene, or in an aprotic solvent, results in ring contraction (see Section 3.1.2.4.) or ring opening (vide infra), respectively, however, with palladium(II) acetate in acetic acid ethyl 2,3-diacetoxy-2,3-dihydro-l//-azepine-l-carboxylate (6) is formed as the major product along with ( , )-hexa-2,4-dienedial.243... 

Nitroso-5//-dibenz[/>,/]azepine (9, R = NO) is relatively stable to photolysis under argon, whereas in benzene solution in the presence of oxygen, irradiation induces an oxidative Fischer -Hepp-type rearrangement to 2-nitro-5//-dibenz[6,/]azepinc (10, R = N02), accompanied by ring contraction to acridine-9-carbaldehyde184 (see also Section 3.2.2.4.). 

The synthesis of li7-cyclopropa[/]phenanthrene (142) presented unexpected difficulties and met many failures. Early approaches used a variety of schemes which were not adequate for this highly reactive compound and invariably produced ring-opened products. Thus irradiation of the substituted indazole 138 resulted in nitrogen extrusion and formation of the biradical 139, which reacted with the solvent, benzene, to form 140. The desired cycloproparene 141 was not formed. Ring contraction of 144, in turn, produced derivatives of 9-phenanthroic acid, the formation of which was shown not to involve phenanthrocyclopropenone (143). °° The attempted 1/3/elimination of 145 was similarly unsuccessful and afforded no 142. ... 

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)... 

Thietanes can also be prepared by ring contraction of higher-membered heterocyclic rings. Thermolysis of the dithiolane dioxide 129 in benzene leads to trans-2-phenyl-3-benzoylthietane (130) and benzalacetophenone (Eq. 20). 

The 1,5-benzodiazepine 40 on irradiation in benzene under oxygen undergoes oxidative ring contraction to 2-benzoyl-3-methyl-quinoxaline.42 Similarly, photolysis of 7-chloro-2-methylamino-5-phenyl-3//-l,4-benzodiazepine-4-oxide (41) in benzene yields the N-benzoylquinoxaline 42. Related ring contractions of diazepines to reduced quinoxalines have also been observed.43... 

The dimethyl-1,5-benzodiazepine 43 on attempted nitration with H2S04-HN03 at 0°-5° gave, by ring contraction, 2-acetyl-3-methyl-quinoxaline (26%), together with some of the AVV -diacetyl derivative of 1,2-diamino-4-nitro benzene.44... 

Epoxides derived from indene,21 1,2- and 1,4-dihydronaphtha lenc 1702 a,4-benzo-1,S cycloheptadicne, 68 and 3,4-benzo-l,3-cycIo-octodlene841 are a)) reported to undergo isomerization to ketones in the presence of magnesium bromide (Eq. 450). That the carbonyl function is separated in every instance from the benzene ring by a methylene group may be taken to indicate probable existence of an open car-bonium ion in these processes. Bing contraction has been noted in one... 

According to the IUPAC rules for naming polycyclic compounds, when a benzene ring is ortho-fused to another ring the prefix benzo is attached to the name of the parent ring. This is contracted to benz" when preceding a vowel, as in benzanthracene. 

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