Reversible ring opening

On neutralization, the ring-opened species slowly ring-closes again. At 20°, for the ring-opened substance from pteridine, the first-order rate constant is about 10 sec.  

6-Amino- and 6-chloro-pteridine also give this reversible ringopening reaction in acid solution. 

Rate Constants fob the Ring-Opening Reaction OF Hydbated Cations  

Although ring-opening also occurs in the hydrated cations of 1,3,5-, 

8-triazanaphthalene, the reaction, which is much slower than the water-addition step, appears to be irreversible, because decomposition of the ring-opened species supervenes. 

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The parent compounds undergo facile hydrolysis to aminoaldehydes subsequent to the covalent hydration and reversible ring-opening as described above for pyrido[4,3-d]pjrrimidines (Section IV, B). 2-(3-Pyridyl)pyTido[2,3-d]pyrimidine undergoes hydrolysis to yield 2-aminonicotinaldehyde and nicotinamide when treated with N—HCl under reflux for 3 hours. This mechanism also probably involves a covalent hydrate. 2-Methylpyrido[4,3-d]pyrimidin-4(3H)-one, although much more stable than the parent compound, is readily hydrolyzed with dilute acid, whereas the isomeric compounds from the other three systems are stable under such conditions. 

Flash vacuum thermolysis (FVT) of 2-substituted 4//-pyrido[l,2-n]pyrimidin-4-ones 126 above 800 °C afforded (2-pyridyl)iminopropadie-none (130) (99JCS(P2)1087). These reactions were interpreted in terms of reversible ring opening of 4//-pyrido[l,2-n]pyrimidin-4-ones to imidoyl-ketenes 127. A 1,5-H shift in 127 generated the N(l)H-tautomeric methylene ketene 128, in which facile elimination of HX took place via a six-membered cyclic transition state 129 to yield 130. In the case of 2-methoxy derivative 126 (X = OMe) another competing pathway was also identified at lower temperature, which resulted in the formation C3O2 and 2-methylaminopyr-idine via mesoionic isomer 131 (Scheme 9). The products were identified by IR spectroscopy. 

Mutarotation occurs by a reversible ring-opening of each anomer to the open-chain aldehyde, followed by reclosure. Although equilibration is slow at neutral pH, it is catalyzed by both acid and base. 

Thus, the frontier-orbital and Hiickel-Mobius methods (and the correlation-diagram method as well) lead to the same conclusions thermal 2 + 4 cycloadditions and photochemical 2 + 2 cycloadditions (and the reverse ring openings) are allowed, while photochemical 2 + 4 and thermal 2 + 2 ring closings (and openings) are forbidden. 

This type of orientation of the newly formed bonds is called antarafacial, and the reaction would be a [ 2s + 4a] cycloaddition (a stands for antarafacial). We can easily show by the frontier-orbital method that this reaction (and consequently the reverse ring-opening reactions) are thermally forbidden and photochemically allowed. Thus in order for a reaction to proceed,... 

The pyrimido[2,T ][l>3]oxazines 301 are sensitive to water and suffer reversible ring opening to the lactams 302. The equilibrium favors the ring-opened product (Equation 31) <20050BC1964>. 

Avogadro s number and aRi is the symmetry number of the xr-meric cyclic oligomer, which represents a statistical correction for the number of equivalent bonds that can undergo the reverse ring-opening reaction. For Gaussian chains (57) takes the form (58) which is identical to (56), apart from the pres-... 

N. Inotsume, M. Nakano, Reversible Ring-Opening Reactions of Nimetazepam and Nitrazepam in Acidic Media at Body Temperature , J. Pharm. Sci. 1980, 69, 1331 - 1334 N. Inotsume, M. Nakano, Reversible Ring-Opening Reactions of Triazolobenzo- and Triazolothienodiazepines in Acidic Media at Around Body Temperature , Chem. Pharm. Bull. 1980, 28, 2536 - 2540. 

Although reversible or equilibrium polymerizations would almost always be carried out in an irreversible manner, it is interesting to consider the kinetics of polymerization for the case in which the reaction was allowed to proceed in a reversible manner. (The kinetics of reversible ring-opening polymerizations are discussed in Sec. 7-2b-5). 

In TP-1 oxides (31), a reversible ring-opening (Scheme 16) was detected by NMR spectroscopy [76JCS(P1)2166],... 

A crystal structure determination has confirmed that A,A-[HO(en)2CrOHCr(en)2OH]-(C104)3-H20 formed in the reversible ring opening of A,A-[(en)2Cr(OH)2Cr(en)2]4+ has the structure inferred from its chemical properties.446 Analogous complexes are apparently formed by tmd.447... 

Although pyridones are usually resistant to alkali, pyrone rings are often easily opened. Pyran-2-ones are reversibly ring-opened by aqueous alkali to acid anions (222). Hydroxide ions convert coumarins (223) reversibly into salts of coumarinic acids (224) which can be converted into the trans isomers (225), and chromones (226) into 3-dicarbonyl compounds (227). 

Confirmation of the reversible ring opening of pyrylium salts to the pseudobases is provided by the incorporation of lsO into 2,4,6-trimethylpyrylium when the salt is treated with lsO-enriched water (67MI22401). 

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