Oxazolin-5-ones, tautomerism

Azlactones — see also l,3-Oxazolin-5-ones Erlenmeyer synthesis, 6, 202 hydrolysis, S, 64, 101 tautomerism, 6, 186 unsaturated... 

These results are relevant to the tautomerism which is, in principle, possible between A -oxazolin-5-ones (azlactones) (76) and their meso-ionic isomers (77). Kille and Fleurj interpreted the spectroscopic... 

It was shown previously that saturated 5(4//)-oxazolones or 2-oxazolm-5-ones with only one substituent at C-4 can be considered as the tautomeric form of saturated 5(2//)-oxazolones or 3-oxazolin-5-ones. These compounds can also be considered as amino acid derivatives and, indeed, cyclization procedures are the most commonly used to prepare these compounds. The cyclization reaction employs a variety of cyclodehydrating agents and the general method is shown in Scheme 7.23, with an A-acyl-a-amino acid being the most typical starting material used. In this way, 5(4//)-oxazolones derived from most natural amino acids 99 (R3 = H) have been obtained by heating the corresponding A-acyl derivatives in the presence of acetic anhydride. 

Phenyl-A2-oxazolin-4-one forms the furan (282) on treatment with dimethyl acetyl-enedicarboxylate. The reaction may be interpreted as proceeding through a Diels-Alder addition to the tautomeric oxazole (281) (Scheme 74) (72CC1000). 

Dimethylaminomethyleneaminopyrazine (180) and 2-phenyl-A2-oxazolin-5-one (181) gave 2-phenyl-4- V-(pynizin-2-yl)iminomclhyl]-A2-oxa/olin-5-one that (according to NMR data) exists as the tautomeric 2-phenyl-4-[Ar-(pyrazin-2-yl)aminomethylene]-A2-oxazolin-5-one (182) (Ac20, 70°C, 2 h 46%).299... 

The best known of the potential mercaptoimidazoles are the imidazoline-2- and benz-imidazoline-2-thiones, which resemble imidazolin-2-ones in that the tautomeric form (53 X = S) is the preferred form. The crystal structure and the HNMR spectrum of 1,3-dimethyl-3H-imidazoline-2-thione have been interpreted as showing partial double bond character in the N—C—N system, but no aromaticity (70CC56). However, the preference for a betaine structure (56) rather than (57) or (58) should be accepted with caution since it is really only a resonance structure similar to others which undoubtedly contribute to the overall structures of oxo-, thio- and amino-imidazoles. Measurement of the piSTa values for a series of imidazoline-2-thiones substituted variously on C-4, C-5, N-1 and N-3 by hydrogen, phenyl or methyl shows that all of the values are similar. Approximate Kr values calculated show that these compounds exist even more in the thione forms (53, X = S 58) than do the corresponding thiazoline-2-thiones and oxazoline-2-thiones. The UV spectra in aqueous solution support thione structures, as do dipole moment and X-ray studies (76AHC(S1)280, p. 400). 

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