Opening of the Ring

Reactions resulting in opening of the pyrone ring without transformation into a different cyclic product are relatively uncommon, although some of them are quite important. 

1 is opened to ethyl 3,5-dioxohexanoate by the action of ethanol (48JBC485). Dehydroacetic acid can be efficiently converted into methyl 

Two reactions with nucleophiles which occur at C6 are the reduction of 272 into 273 (83CB3366) with sodium borohydride and opening of the ring with pyrrolidine in 152, followed by decarboxylation to afford 274 (70JOC258). 

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Ring-opening polymerizations are catalyzed by a wide variety of substances, including the bases OH and RO and the acids H and BF3 water is also used as a catalyst. The reactions proceed by the opening of the ring by the catalyst to form an active species. 

The photolysis of the diazopyrazolone (369 X = N2, R = Me) in methanol yields two isomeric forms of methyl 3-phenylazo-2-butenoate (394) (80CC1263). The azo esters may arise via protonation of the carbene (395) with a concurrent opening of the ring by the nucleophilic solvent. 

Some small-ring compounds containing geminal fluorines suffer hydrolysis of fluorine only after initial opening of the ring For example, alkaline hydrolysis of... 

Furthermore, the strongly metallic character of selenium weakens the C-Se bond and thus favors reactions involving opening of the ring. The basicity of the three heterocycles is approximately in the same order, the nitrogen atom of selenazole and thiazole possessing much the same properties as the heteroatom of pyridine. Of the two carbon atoms ortho to nitrogen, that is, the 2-carbon and the 4-carbon, only the one in the 2-position is fairly active as a result of its interaction with selenium or sulfur. The 4- and 5-positions of thiazole and selenazole are more susceptible to electrophilic substitution than the 3- and 5-positions of pyridine. This is particularly true of the 5-position of selenazole. Thus it can be said that the 2- and 5-positions of the selenazoles and thiazoles... 

The interaction of cyclic aminomethylphosphines may proceed either with opening of the ring or with its preservation (89IZV1375). 1,3,5-Diaza-phosphorinane (43) with R = C6H4Me-p at the nitrogen atoms, when treated by diborane, opens its ring with rupture of the C—N bond in the N—C—N fragment and coordination of the borane molecule at the phosphorus atom [Eq. (47)]. 

In Section 8, in discussing substitutions at sulfenyl sulfur, we noted some of the uncertainties that can complicate the use of / Nu- and / -values to deduce information about the extent of bond making and bond breaking in the rate-determining transition state. For substitutions at sulfonyl sulfur Deacon et al. (1978) have been able to examine a system, the opening of the ring in sultones [79] by phenoxide ions (159), where the most serious of these... 

Free haem groups are ferroporphyrins (cyclic tetrapyrroles). The first reaction of haem catabolism is the release of iron this is followed by the opening of the ring to produce a linear tetrapyrrole called biliverdin. A molecule of carbon monoxide is released as the ring opens. Biliverdin is converted to bilirubin by reduction. These initial reactions may occur in the liver or in other tissues of the reticuloendothelial system, notably the spleen. 

An ab initio study of the acid hydrolysis of /3-lactams has yielded a value of 14.23 kcal moU for the energy barrier for the opening of the ring. Two theoretical studies of N-methyl-2-azetidinone (149) have been reported. In the first, semiempirical calculations... 

The titanium-mediated photocatalytic oxidation of a pyridine solution was conducted by Low et al. (1991). They proposed that the reaction of OH radicals with pyridine was initiated by the addition of a OH radical forming the 3-hydro-3-hydroxypyridine radical followed by rapid addition of oxygen forming 2,3-dihydro-2-peroxy-3-hydroxypyridine radical. This was followed by the opening of the ring to give At-(formylimino)-2-butenal which decomposes to a dialdehyde and formamide. The dialdehyde is oxidized by OH radicals yielding carbon dioxide and water. Formamide is unstable in water and decomposes to ammonia and formic acid. Final products also included ammonium, carbonate, and nitrate ions. 

As further proof of the course of the reaction between 85 and lithium alkyls, the primary product 86, resulting from the opening of the ring in compound 85 according to Eq. (13), was reacted anew with MeCl (44). At —40°C, formation of the symmetrical n-tetraphosphane 92 takes place, as shown in Eq. (17). This is an equally stable derivative, whose P H -NMR spectrum shows the characteristic resonances of an AA XX spin system. 

Almost all mammals carry out further degradation of uric acid with the help of uricase, with further opening of the ring to allantoin, which is then excreted. However, the primates, including humans, are not capable of synthesizing allantoin. Uric acid is therefore the form of the purines excreted in these... 

THF can be polymerized only with cationic initiators, for example, boron trifluoride or antimony pentachloride. The initial step consists of the formation of a cyclic oxonium ion one of two activated methylene groups in the a-position to the oxonium ion is then attacked by a monomer molecule in an S 2-reaction, resulting in the opening of the ring. Further chain growth proceeds again via tertiary oxonium ions and not, as formerly assumed, via free carbonium ions ... 

Most reactions of oxadiazolinones (62) involve nucleophilic attack at the carbonyl group. This is typically followed by opening of the ring, often with subsequent rccyclization to a different heterocycle. Simple nucleophilic displacement occurred on conversion of oxadiazolinone (62a R = R) into chloro-oxadiazole (63) on treatment with a mixture of phosphorus oxychloride and phosphorus pentachloride <84JIC436) or with thionyl chloride <90AP(323)595>. 

Opening of the ring and expulsion of thiobenzamide and sulfur leads to the thiadiazole (180). The preferential attack of (177) on (178) instead of the excess hydrazine present in the reaction mixture is partially explained by differences in solubilities <88BCJ4043>. 

A mixture of cis and trans-1,2-dim ethyl cyclopentanes will be ormed as the stereochemical integrity of the molecule would be destroyed on opening of the ring. 

Reaction of thiete and its fused derivatives with alkyllithium and alkyl-potassium bases leads to the opening of the ring via two different modes of cleavage. (Scheme 14)... 

Tetrazines (39) react with cyclopropenes (149) via the unisolated tricyclic intermediates (150) to form the diazanorcaradienes (151) by loss of nitrogen. Three reactions of the diazanorcaradienes have been observed (1) opening of the ring to give the 5H-diazepines (152) (2) isomerization by the walk mechanism to the diazanorcaradienes (153), which may be transformed into the 4//-diazepines (154) (3) reaction with another molecule... 

Isotope labelling again proved (21) that the decarboxylation involves only carbon 2 of the oxazolidine-2,5-dione, and studies of the kinetic isotope effect (22) demonstrated that this step is not the rate-determining in the overall process. Therefore, the rate of the overall reaction is determined either by (a) — the formation of the complex, or by (b), viz. by opening of the ring between atoms 1 and 5. This conclusion applies also to the "normal amine-propagated NCA polymerisation. 

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