Equilibria ring-chain tautomerism

Equilibrium Ring-Chain Tautomeric Ratios for 4-Imino-4//-PYRIDO[l,2-a]PYRIMIDINE-3-CARBONITRILE 167 168 (R = R1 = H) IN DIFFERENT... 

An interesting ring-chain tautomerism between 2-azidothiazole (328) and thiazolotetrazole (328a) has been reported (597, 618, 619), the 328 structure predominating (Scheme 190). The solvent polarity and basicity influences this equilibrium constant significantly (1592). 

Thiol-thione tautomers have not been extensively studied, but UV and IR evidence show that 5-phenylisothiazole-3-thiol exists in the SH form. Ring-chain tautomerism of 2,3-dihydro derivatives of 1,2-benzisothiazole can occur (26a 26b) and the position of equilibrium depends very much on the solvent, physical state and nature of the substituents (69JOC919, 81KGS1209). 

Ring-chain tautomerism of derivatives of 1,3,4-triazolidines 283 involves the equilibrium of three isomeric forms (Scheme 102) [90TL3927 96AHC(66)1, p. 33], In DMSO solution, the predominant form (about 70%)is 283c, the content of 283a and 283b varied between 13-25% and 5-17% respectively. 

For the isoxazolines 284 substituted at position 3, ring-chain tautomerism is depicted by the equilibrium 284 and 285-287 (Scheme 103). In general the cyclic tautomers 284 are strongly preferred. The ring-opened forms exist in equilibrium with 284 in rare cases [95ZOB705 96AHC(66)1, p. 21]. The equilibrium of the oxazolidinones 288 [78MI1, p. 107] is affected by the nature of the solvent. 

Tautomerism of 2-substituted hexahydropyrimidines has been studied (980PP53), and free energies, enthalpies, and entropies of activation for this ring-chain tautomeric equilibrium have been measured [97JCS(P2)169]. 

Ketal 73 can be formed in a yield of about 60 % by refluxing a solution of tetronic acid (36), ethylene glycol, and a catalytic amount of para-toluenesulfonic acid in benzene for approximately 12 hours. With only one electrophilic site, 73 reacts smoothly with Dibal-H to give lactol 35 in 84% yield. Compound 35, a participant in a ring-chain tautomeric equilibrium process,18 should be regarded as a latent aldehyde. This substance can, in fact, serve as... 

The NMR spectroscopy has been widely used in the studies of different types of equilibria like ring-chain tautomerism, racemisation or stereomutation and proton transfer equilibrium in Schiff bases. 

Ring-chain tautomeric equilibrium between 39a and 39b and the pyrimido[l,6-z pyrimidines 40a and 40b has been revealed. Equilibration was fast in DMSO- 6 (DMSO - dimethyl sulfoxide) and favored the bicyclic form (Equation 2) <1996ACS1137>. 

Section 4.2.1 will be devoted to heterocycles, section 4.2.2 will cover other kinds of protomeric tautomeric equilibria (e.g., enol/ketone, formic acid, formamidine, etc.), and section 4.2.3 will discuss an example of a ring/chain tautomeric equilibrium. The order of presentation will be approximately by increasing molecular weight within each section. A review by Kwiatkowski et al. [267] covers work on formamide, pyridines, pyrimidines, purines, and nucleic... 

Crystal-structure-sensitive bond-forming and -breaking processes other than ring-chain tautomerism are also known. For example, acetylation of 4-pyridone in pyridine gives rise to crystalline A-acetyl-4-pyridone. When this is dissolved in CH2C12 it gives a nearly 1 1 equilibrium mixture of this acetyl pyridone and 4-acetoxypyridine. The A-acetyl derivative is obtained by crystallization (79). 

Ring-chain tautomerism caused by the migration of an azido group has been observed rather rarely. By means of IR spectroscopy, the equilibrium 19A 19B was detected [81JCS(P1)2884] in benzene solutions of N,N-disubstituted tetrahalogenophthalamic acid azides. Mixtures of open-chain and cyclic isomers of 2-benzoylbenzoyl azide were obtained [88JCS(P1)2149], from which only isolation of the cyclic isomer succeeded. 

The tetrahydro-l,2,4-triazine 4-oxides (49a) in tetrachloromethane exist in equilibrium with the open-chain structures (49b) (77ZOR2617), and equilibria between triazinones (50a) and the open-chain form (50b) were also observed (80ZOR2297). Ring-chain tautomerism was also found with the aldehydes (51a 51b) here the triazine ring is present throughout (77CB1492). 

A ring-chain tautomeric equilibrium between the pyranopyrandione (254) and the 4-pyranonecarboxylic acid (255 equation 9) in CDC13 solution has been investigated by H NMR spectroscopy (73AC(R)29l). The equilibrium constant Ki was calculated as 1.7 at 298 K and AH° for the equilibrium reaction as 16.2 kJ mol-1. 

The existence of a tautomeric equilibrium involving prototropic, ring-chain tautomerism was established on the basis of the u.v., i.r., and 1 H-n.m.r. spectra of the synthetic compounds 55 and 56. 

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