A ring transformation of 6,7-dihydro-4H-pyrido[1,2-a]pyrimidin-4-ones
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Keywords:
Moiety
Formylation
Hydrazone
Reactivity
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Formylation
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Abstract Thujopsene (1) can be formylated in the 3‐position in 80% yield by use of one equivalent of the Vilsmeier reagent. The resulting 3‐formylthujopsene (4) is useful for the synthesis of other 3‐substituted derivatives (CH 2 OH, COOH, COCH 3 ). With an excess of the reagent higher formylation products are formed. Two such products have been isolated and characterized and a mechanism for their formation is put forward.
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Acetylations of 3-ferrocenyl-1-methylpyrrole as well as 3-cyano-4-ferrocenylpyrrole and 3-cyano-4-ferrocenyl-1-methylpyrrole were performed. The course of the acylation is highly dependent on the acylation agent, that is acetyl chloride/aluminum chloride (method A ), trifluoroacetic anhydride-acetic acid mixture (method B ) or acetic anhydride/Sc(OTf) 3 (method C ). Method A gives the acetylation on ferrocene moiety, method B affords the trifluoroacetylation on pyrrole moiety and method C affords pyrrole moiety acetylation. Vielsmeier-Haack formylation gives the products of substitution on pyrrole moiety.
Formylation
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Trifluoroacetic anhydride
Pyrrole
Acetyl chloride
Trifluoroacetic acid
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There has been decades of research on determining and predicting acid dissociation constants (pKa) and the tautomer ratios both experimentally and theoretically. However, the lack of an extensive publicly available database of measured tautomeric ratios in water and nonaqueous solvents poses a challenge for the researchers interested in theoretical studies related to tautomers. Hereby, we present Tautobase, to date and to the best of our knowledge, the first extensive open-source tautomer database of measured and estimated tautomer ratios mainly in water, containing 1680 unique tautomer pairs.
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Hydrazone
Molecular spectroscopy
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The tautomeric behavior of 5-nitro-2-anilinopyridine and 2-anilinopyrimidine was studied. 2-(N-Methylanilino) and l-methyl-2-(phenylimino)-1,2-dihydro derivatives were prepared as models for the amino and imino tautomers, respectively, and their spectra were compared. The results and the assumed tautomeric equilibrium constants strongly suggest the equilibria to be favorable to the amino tautomers, the Kt values being considerably smaller than that of the parent 2-anilinopyridine.
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Families of tautomers in the Cambridge Structural Database (CSD) have been identified, analysed and classified. We identified 108 molecules that crystallise in two different tautomeric forms. Most commonly, pairs of tautomers crystallise together in the same crystal structure. Tautomeric polymorphs—pairs of tautomers crystallising in different crystal structures with no other or identical components—are very rare. The calculation of the relative stabilities of the different pairs of tautomers mirrors the relative frequencies with which tautomeric forms are observed in the CSD. Our improved understanding of the factors influencing tautomeric preferences in crystal structures may allow the prediction and design of crystal structures containing tautomeric forms that are, as yet, unobserved in the solid state.
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Abstract Tautomerization is the key feature of pyrazoles, which is connected with the hydrogen transfer in pyrazoles between two nitrogens. It is scholarly accepted that tautomers of 3(5)‐substituted pyrazole derivatives cannot be isolated as individual compounds with the exception of certain extreme conditions. Here using the fact that N‐substitution suppresses tautomerization and allows their separation and isolation, the individual isomers of N‐propanoates of 3(5)‐pyrazole were synthesized, isolated, and further modified several times without change of initial 3‐or 5‐methyl configuration. However, once the group attached to the nitrogen was removed, two individual isomers gained back their tautomeric properties and became the same mixture of tautomers, as expected. Close monitoring by NMR spectroscopy vividly demonstrates the loss and then reverting of tautomeric properties of 3(5)‐pyrazole derivatives along the set of chemical transformations aimed at possible isolation of two tautomers. Failure to isolate them once again proves that the dynamic equilibrium of tautomers in solution is an inherent property of 3(5)‐methylpyrazole.
Pyrazole
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