Azlactones, tautomerism

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

In the third transition state (TS3), the neutral catalyst is recovered by transferring the proton back from the catalyst to the substrate. In other words, the (former) azlactone ether oxygen atom deprotonates the tertiary ammonium ion. For proton transfer, again an LBHB is formed (N-0 distance 2.479 A, <(0,H,N)=166.2°). In the product complex, the catayst is neutral and the A-acylamino acid ester is bound in its iminol form to the catalyst (Product(iininol)). Finally, an additional 66.6 kJ moF are gained by the subsequent iminol-amide tautomerization (Product(ainide)) (Fig. 1). 

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... 

Recently a novel reaction between the azlactone 76, R = Ph R = Me, and cinnamylidene aniline yielding the adduct 79 has been described.This result has been interpreted as involving a Diels-Alder cycloaddition between cinnamylidene aniline and the valence tautomeric ketene 78, R = Ph R = Me. 

The cycloaddition reactions of 2-phenyloxazol-4(5H)-one with acetylenic dipolarophiles has been briefly reported. " The formation of 2-phenylfurans may well involve a tautomerism analogous to that exhibited by azlactones (76 77). 

Bergmann and Stern155 found that dehydroazlactones can be obtained from treatment of a-haloacyl a-amino acids with acetic anhydride in presence of a base such as sodium acetate or pyridine. They showed that halo-azlactones of type 8 undergo spontaneous dehydrohalo-genation giving a pseudoazlactone 9 that equilibrates with its tautomeric azlactone 10 (Scheme 3). 

The tautomerization of azlactones (5(4/f)-oxazolones) also gives miinchnones. The crystalline miinchnone 91 has been isolated and fully characterized. The tautomerization favors 91 over 90 in the presence of triethylamine in dimethylformamide (DMF Equation 5) <1997PJC1045>. 

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