Ring-chain tautomerism aldehydes

The hydrochloride of (3) holds water rather tenaciously, and the infrared spectrum indicates that the water is covalently bound. Mild oxidation of the cation (3) gives 4-hydroxyquinazoline in high yield and ring-chain tautomerism is excluded on the grounds that quinazo-line does not give a positive aldehyde test in acid solution, 2-Methyl-quinazoline also has an anomalous cationic spectrum and a high basic strength (see Table I), but 2,4-dimethylquinazoline is normal in both these respects, which supports the view that abnormal cation formation entails attack on an unsubstituted 4-position. ... 

The spectra of 1,3,5-, 1,3,7-, and 1,3,8-triazanaphthalerie cations revealed that they were predominantly hydrated, and mild oxidation to the corresponding 4-oxo compounds indicated that hydroxylation took place on C-4. Ring-chain tautomerism in the cation was excluded, at least during the first 45 min after mixing, because negative aldehyde tests were obtained with p-nitrophenylhydrazine. 1,3,6-Triazanaphthalene, under the same conditions, gave a yellow precipit-... 

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... 

Ring-chain tautomerism occurs in sugars (aldehyde vs. the pyranose or ftira-nose structures). In benzamide carboxaldehyde (125), whose ring-chain tautomer is... 

Because of their ring-chain tautomeric character, tetrahydro-l,3-oxazines can be used as aldehyde sources in acid-catalyzed condensation reactions involving the carbon transfer via the open forms. This approach is especially advantageous in those cases where the aldehydes required for the condensations are unstable or difficult to access <2003EJ03025>. 

The oxazolidine system proved a good protecting group with which to mask the ethanolamine moiety in the a-formylation and a-benzoylation of 558, and it could also be used as an aldehyde donor in the rearrangement, based on the ring-chain tautomeric character of 559, under acidic conditions to yield 3-(2-hydroxyethyl)-substituted 1,3-oxazin-4-ones 560 (Scheme 106) <1996JOC3358>. 

Methyl hydrazones of a wide range of aldehydes and ketones (380) undergo addition-cyclization to give 1,2,4,5-tetrazines (382) via 381 (216-219). Substituent effects on the ring-chain tautomerism between 381 and 382 were studied by NMR spectroscopy. 

The photoaddition of aldehydes to o-quinones is a general reaction leading to the formation of products which are to be regarded as derivatives of methylene ethers of o-dihydroxy compounds (A).198 194 247 Such compounds exhibit ring-chain tautomerism and are generally assigned open-chain structures (B).188 242... 

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). 

The ratio of 33A and 33B proved to be slightly solvent-dependent (Table V). The reactions of 3-amino-l,2-propanediol with substituted aromatic aldehydes in CDC13 resulted in five-component ring-chain tautomeric equilibria. Besides the open-chain form 34A, two epimeric oxazolidines (34B and 34B ) and two epimeric tetrahydro-l,3-oxazines (34C and 34C ) were identified in the tautomeric mixture. The proportions of the tautomers in the equilibrium for X = p-NC)2 were [34A] [34B] [34B ] [34C] [34C ] = 40.7 7.0 5.9 9.7 36.7 (94MI1, 94MI2). 

Ring/chain tautomerism The tautomeric interconversion between the closed ring form and the open chain form of a molecule, e.g. in sugars, the interconversion between the closed ring hemiacetal and the open chain aldehyde. 

Various methods are available for the preparation of the foregoing precursors to V-acyliminium ions. The most straightforward synthesis of V-(l-hydroxyalkyl)-amides or -carbamates (2 X = OH) involves the addition of primary or secondary amides (carbamates) to aldehydes or ketones. This reaction is an equilibrium process and usually disfavors the adduct except for two special cases. The first involves very reactive aldehydes like formaldehyde, chloral and glyoxylic acid and its esters (equation 4) compounds (4)-(6) are fairly stable compounds that are frequently used in amidoalkylation reactions. The second special case is the intramolecular variant (ring-chain tautomerism), if it leads to five- or six-membered rings (equations 5 and 6). Thus, for n = 1 or 2, (7) and (8) cyclize to (9) and (10), respectively, although the nature of R has an influence on this process. " ... 

Stability differences between cis and trans cyclopentane-fused 1,3-oxazines were also observed in the case of the ring-chain tautomeric mixtures 119-121, obtained by reacting cis- and trans-2-hydroxymethylcyclopentylamine with aromatic aldehydes [149]. The ring form of the trans derivatives is present in fairly low amount as compared with the cis compounds [149, 150]. jV-Methyl substitution stabilizes the ring forms in both the cis and trans cases [149]. For a detailed discussion of the ring-chain tautomerism of alicyclic 1,3-amino alcohols, see, for example, refs. [150, 151]. 

There are cases of other types of tautomerism, such as the ring-chain tautomerism in aldehyde and ketone alcohols (especially the... 

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