Tetronic acids, tautomerism

For 3-acetylthiotetronic acid the same type of tautomerization process (90 91, 92 93 as a result of prototropy between internal tautomers 90, 91 92, 93, rotation of the side-chain group, between external tautomers) was observed as for the corresponding 3-acetyl-tetronic acids ( H NMR, NMR) [76JHC533 78BCJ651 79JCS (P2)1605],... 

Tetronic acids exist predominantly in the dioxo form (32) in solvents of low polarity, while the existence of the monoenol form (33) has been established in other solvents by infrared " and ultraviolet spectral comparisons- and from dipole moment data. " Haynes and Pliramer " have recently reviewed the structure of these compounds [see also reference 28(a)], and the tautomerism of vitamin A (34), which has a related structure, has also been surveyed.- Analogous compounds carrying an amino group in the 3-position are also know n. ... 

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... 

Tautomerism in the tetronic acid nucleus concerns only the vinylogous acid and never includes the methylene group [but note obtusifolin (127)]. Two isomers have been detected in 2-acylated tetronic acids15 0a (63) that are... 

A. S. Shawali, I. M. Abbas, N. F. Abdelfattah, and C. Parkanyi, A theoretical study of tautomerism in dehydroascorbic acid osazone and related tetronic acid derivatives, Carbohydr. Res., 110 (1982)... 

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