Tautomeric mixture

Ethyl malonate, like ethyl acetoacetate, exists as a tautomeric mixture of keto and enol forms, although in the case of ethyl malonate... 

In a neutral azole, the apparent rate of formation of an A-substituted derivative depends on the rate of reaction of a given tautomer and on the tautomeric equilibrium constant. For example, with a 3(5)-substituted pyrazole such as (199), which exists as a mixture of two tautomers (199a) and (199b) in equilibrium, the product composition [(200)]/[(201)] is a function of the rate constants Ha and fcs, as well as of the composition of the tautomeric mixture (Scheme 16) <76AHC(Si)l). 

In the absence of oxygen, these thiolene-2-ones are rather stable and have been kept at 0°C for several months. 3-Hydroxythiophene, on the other hand, which has been prepared in low yield from 3-thio-phenemagnesium bromide in the same way as the 2-isomer, or through decarboxylation of 3-hydroxy-2-thiophenecarboxylic acid, "" is very unstable. Its IR spectrum indicates that it also exists as a tautomeric mixture largely in its enolic form. ... 

E. Influence of External Factors on the Equilibrium Position of a Tautomeric Mixture... 

Accurate, quantitative spectroscopic data for the simple alkyl-substituted derivatives of isoindole are not available, cither because of their inherent instability or tendency to tautomerize to isoindole-nines. However, the presence of an isoindole in a tautomeric mixture can be readily discerned by means of ultraviolet spectroscopy. ... 

The 0,N-dideuterated enol was formed by hydrolysis of the O-trimethylsilyl ether 123 (R = TMS) (in 80% [D6]DMSO/20% D2O with 5. lO " M DCl). N-Methylindoxyl (formed by hydrolysis of its acetate) exists in the solid state as a mixture of the enol and the keto tautomers (34% enol/66% keto). The NMR spectrum of freshly prepared solution in DMSO demonstrated signals of both enol and keto forms. However, at equilibrium (reached in 18 h at RT) the ratio of enol to ketone depends strongly on the polarity of the solvent used thus, in [Dg]DMSO the tautomeric mixture contains 92% enol, while in CDCI3 the keto form predominates (97%). A solution with 100% enol could be generated by hydrolysis of its O-trimethylsilyl ether [conditions 80% [Dfi]DMSO/20% D2O with 5 10" M DCl at 32°C (86TL3275 87PAC1577 88TL250)]. 

It was shown by UV spectroscopy that the protonation of 106 in EtOH-HCl or in CF3COOH resulted in a 1 1 tautomeric mixture of 114a and 114b (78BSF273). For 9- and 1-N-methylated derivatives of 106, the formation of 115a and 115b in a 1 3 ratio and the exclusive existence of 116a has been demonstrated. 

Unlike the products of condensation of 1,2-diaminoethane and 1-amino-2-hydroxyethane with aminobutynones, which exist almost exclusively in the enole form, benzoxazoles 359,361 and benzothiazoles 360,362 are present in the solution as a tautomeric mixture of the ketone (359, 360) and enole (361,362) forms. This seems to be related to weakening of the imine nitrogen basicity due to adjacent oxygen and sulfur atoms. 

Tautomerism of simple monosubstituted 1,4-dihydropyrimidines in solution has been studied on an example of 2-phenyldihydropyrimidine 48, prepared by condensation of benzamidine with acrolein [84H(22)657]. IR and H and NMR spectra at -60°C in specially purified solvents showed that this compound exists as a tautomeric mixture of 1,4- and 1,6-dihydro tautomers (Scheme 17), with the relative amount of 1,4-dihydro isomer 48a increasing with the polarity of the solvent. 

Cyclocondensation of 8-hydroxyquinolin-2(l//)-one and chloroacetone in the presence of K2CO3 in dry DMF at ambient temperature for 24 h afforded 2,3-dihydro-3-hydroxy-3-methyl-5//-pyrido[l,2,3- /e]-l,4-benzoxa-zin-5-one (240, R = Me) (97HCA1161). A tautomeric mixture of ring-opened... 

Discussion. Diphenylthiocarbazone (dithizone) behaves in solution as a tautomeric mixture of (C) and (D) ... 

When the valence tautomeric mixture of oxepin and benzene oxide is treated with singlet oxygen, the primary product is the 1,4-endoperoxide 3 which has proven to be too labile for isolation.219 Its formation can be rationalized by a 1,4-addition across the diene system of the benzene oxide structure 3 then rearranges to ba s-3,6,9-trioxatetracyclo[6.1.0.02 4.05 ]nonane (transbenzene trioxide, 4). 

A similar sequence of reactions on l,2-dihydroindeno[l,2-tetrahydro compound 10 with NBS in refluxing dibro-momethane results in bromination of the indene ring and formation of the 10-bromo derivative... 

Bick and co-workers (96) have described the C-acylation of tropinone (124) with 3-methoxymethacryloyl chloride leading to a tautomeric mixture of 187a and 187b. Acid-induced cyclization of the mixture produces bellendine (113) (Scheme 14). 

Oxidation with m-CPBA of monocyclic isoxazolidines (56) without H at a-C gives a tautomeric mixture of acyclic nitrones (57) and six-membered cyclic hydroxylamines (58), with their proportion depending on the substituents... 

Another synthetic approach for generating sugar-containing nitrones is by initial treatment of sugars with unsubstituted hydroxylamine. The resulting cyclic hydroxylamine of the tautomeric mixture (68a) and of the open chain oxime (68b) react with aldehydes to give the corresponding nitrones (69) (Scheme 2.24) (216-220). 

The organozinc reagents 123, which were generated from 1-phenyl-l-alkyne 122 with nBuLi in the presence of 1.5 mol% of HgCl2 followed by addition of 1.5 equiv. of ZnBr2, exists as a tautomeric mixture of an allenylzinc and a propargylzinc species (Scheme 3.62) [103]. 

The H NMR spectra of la-f and 3d-c, 3d-a, 3d-b, 3e-b, 3e-f proved that, in DMSO solution. Each set of compounds formed three-component ring-chain tautomeric mixtures, containing epimeric naphthoxazines together with the open-chain tautomer. Compounds displayed singlets at 8 5.40-6.32 due to NphCHArNH 8.41-8.73 due to NH = CHAr. 

The reaction of metal ion M"+ with the keto, enol tautomeric mixture of acetylacetone (acacH) in acidic aqueous solution has been treated by a similar approach to that outlined above (see Prob. 16). 

The introduction of a p-amino-substituent enhances the basicity of azobenzene very considerably. The value of p/iTa changes from —2-90 (Yeh and Jaffe, 1959a) to -i-2 82 (Bascombe and Bell, 1959). The controversy over the site of protonation of p-aminoazobenzene has been resolved in favour of a tautomeric mixture of the ammonium [48] and azonium [49] forms on the basis of an analysis of... 

This isomerization was confirmed by means of a detailed kinetic NMR study (90T633), but is not detrimental to the synthesis of substituted 1,2,4-triazoles, because in all cases removal of the protecting group leads to a tautomeric mixture of 3- and 5-substituted products. The methods available for removal of the animal function actually depend upon the nature of the added C-5 substituent, with acid hydrolysis occurring only in some cases. More commonly, treatment with NaBH in refluxing ethanol is the method of choice (90T633). Lithiation and derivatization of the SEM protected compound (entry 6) can be achieved without the isomerization shown by the animal derivative, and deprotection can be achieved with either aqueous acid or anhydrous fluoride ion [92H(34)303], However, overall reaction yields are not as high as those for the aminal system. 

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