Ring Opening and Rearrangement

Benzofurazan (8), on irradiation in benzene solution, undergoes a complex sequence of ring openings and rearrangements to yield, in situ, the cyanobutadienylcarbonylnitrene 9 which is trapped by the solvent to give ultimately the 1 //-azcpinc 10 as the major product.125... 

De Kimpe and Boeykens (22) reported synthesis of the p-lactam derivatives 107 via cycloaddition of azides with 2-methyleneazetidines (104) (Scheme 9.22). Because of electronic control, the intermolecular cycloaddition of the azide with the enamine double bond resulted in the formation of the triazoline intermediate 105, ring opening and rearrangement of which gave the imino lactam 107. Although all attempts to convert compound 107 to the corresponding p-lactam 108 under acidic conditions were unsuccessful, under basic conditions compound 107 was converted into the p-amino amides 109. 

Pyrolysis of some Ai-aiyl five-membered-ring heterocycles causes ring opening and rearrangement to the quinoline ring <99H(51)2377,99JOC3608>. 

Azepin-2-ones are prepared by the ring expansion of 2,6-dialkylphenolates with ethereal chloramine at -70 °C (Scheme 28) (B-69MI51600), and by the mechanistically similar base-promoted ring opening and rearrangement of spiroquinol ethers (234) (81JOC4077). 

Alkynes react as a two-carbon unit with CSI to give 1,2,3-oxathiazine dioxides. The reaction involves initial addition of CSI to form a four-membered ring which then ring opens and rearranges as shown in Scheme 18 (72JOC196). 

The hydrolytic studies are usually performed under acidic conditions, the major disadvantage with cycloartane saponins being cyclopropane ring opening and rearrangement. 

Both tram- and c7r-[l,2,5]trithiepan-4,6-dicarboxylates 402 and 403 were obtained by the reductive ring opening and rearrangement of l,2,3-thiadiazole-4-carboxylates 401 upon reaction with samarium and iodine (Scheme 84). The expected thiadiazoline 404 was not obtained in this reaction <20040BC2870>. 

Pyrazine-2,3-dicarboxylic anhydride gave no products of the reaction of the corresponding heteroaryne with pyridine however, it gave a small but definite amount of phenylpyrazine with benzene, suggesting the formation of pyrazyne. This anhydride is readily converted by ring opening and rearrangement to succinonitrile (Brown et al., 1966). 

A synthetic method for the synthesis of 8-vinyl adenosine 5 -di and triphosphate (243a b) has been developed. This procedure eliminates unwanted depurination side-reactions under acidic conditions by introduction of acetyl protecting groups at the 2 and 3 positions of adenosine. The di- and triphosphate esters were obtained by treatment of the adenylic acid with phosphate and pyrophosphate anions.6,6-Bicyclic pyiimidopyridazin-7-one nucleoside triphosphate (244) was synthesized by nucleophilic ring-opening and rearrangement of a furanopyridine nucleoside in the presence of anhydrous hydrazine. 

Trimethylenemethane has been postulated to be the reactive intermediate in a number of reactions involving the ring-opening and rearrangement of methylenecyclopropanes In fact, methylenecyclopropanes are known to undergo degenerate thermal rearrangements. An early example, the ethylidenecyclopropane-methyl methylenecyclopropane interconversion, is shown in equation 20. ... 

C. Ring Openings and Rearrangements 1. Ring Opening without Reclosure... 

Conversion of monocyclic and polycyclic vinylcyclopropanes with low-valent transition-metal complexes, (e.g. iron, rhodium, zirconium, cobalt, nickel, palladium) mainly leads to ring opening and rearrangement products. A typical reaction pathway of vinylcyclopropanes with transition-metal complexes leads to f/Calkyl-j/ -allyl complexes, which as homodiene complexes exhibit interesting reaction patterns (e.g. carbonylation) leading to organic products. ... 

There have been reports on the reaction of oxazoles with heterodienophiles, including N=N, C=N, C=0, and types, to give new heterocycles <89X3535, 88JOO 663>. In most cases, the products may be envisioned to arise from ring opening and rearrangement of Diels-Alder adducts followed by reclosure, but so far no proof of this mechanism has appeared. A reaction of 5-ethoxy-... 

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