Imidazolin-2-ones, fused

If one excludes zwitterionic structures which appear to have less stability than the other forms, there are four possible structures for the imidazolin-2-ones (52-55 X = O) (Scheme 18). IR evidence points to a carbonyl form (53 X = 0) in the solid state, while the characteristic absorption in the UV region supports their designation in solution as the carbonyl forms. If their melting points are compared with imidazoles (e.g. imidazole, 90 C imidazolin-2-one, 250 °C), the high values may point to the presence in the solid state of salt-like structures. Benzimidazolinones, and other fused-ring imidazolin-2-ones, appear to exist and react as the ketone forms (66RCRI22). 

Tetrahydroquinoxalines such as the ester (20, R = H) have been studied as model compounds for tetrahydrofolic acid. The latter has a vital role in one-carbon metabolism. On treating the ester with formaldehyde, in aqueous dioxan, the fused imidazoline 40 is isolated. Compound 40 undergoes acid-catalyzed rearrangement to the diazepine 42. Using a suitably blocked ester (20, R = Me), it has been possible to isolate the formaldehyde condensation product 41, formed by interaction with nitrogens 4 and 10. It is suggested that a compound of this type is formed prior to the imidazoline 40. ... 

Two examples have been reported of rearrangements that lead to pyrrolo[l,2-a]pyrazines. The Schmidt reaction on the ketone 52 gave only one product, the 3-oxo compound 53, paralleling the behavior of the ketone 54, which gave only the corresponding 3-oxopyrido[l,2-a]pyrazine on treatment with hydrazoic acid. The 1,6-dioxo compound 56 was obtained on refluxing the fused imidazoline 55 in water. 

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