Oxazolin-5-onee

In recent years, several interesting developments in the chemistry of pseudooxazolones have been reported. The chemical properties of this system depend to some extent on the type of substitution at the 

2-position of the hetero ring. The discussion, therefore, is divided into two parts. 

The member of this class which has been studied most thoroughly is 

2-benzylidene-4-methyl-5(2Zf)-oxazolone (57). This compound may be prepared by ring closure of either 3-bromo-2-phenylacetamido-propionic acid or A-(a-halophenylacetyl)alanine (56) [Eq. (30)]. These reactions presumably proceed via unstable halogeno-5(4/f)-oxazolones, which rapidly lose hydrogen halide. 

While the possibility of a rapid equilibrium between the 5 2H) and 5(4 f)-isomers in compounds such as 57 exists, Knunyants et cU.  

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Thioacetyl derivatives (155) are obtained by direct heterocyclization reactions (365. 378, 563) and by a sulfur-oxygen exchange" reaction involving thioacetic acid and A-2-oxazoline-5-one (154) or A-2-thiazoline-5-one (156) (Scheme 81) (365, 378, 379). Ra-Ni reduction of 155 affords the 5-unsubstituted thiazole (379). 

A-2-Oxazoline-5-one (2091 when treated with thioacetic acid yields the corresponding thiazoline-5-one (210) (Scheme 107) (458. 461). These results have been questioned recently (365) however, it appears in the later report that a large excess of thioacetic acid was used instead o-f the stoichiometric amount previously used. 

Ring fission occurs readily in many of these compounds. For example, azlactones, i.e. 4JT-oxazolin-5-ones containing an exocyclic C=C bond at the 4-position (508), are hydrolyzed to a-benzamido-a,/3-unsaturated acids (509), further hydrolysis of which gives a-keto acids (510). Reduction and subsequent hydrolysis in situ of azlactones is used in the synthesis of a-amino acids e.g. 508 -> 507). 

Cyclopropenones react with nitrosobenzene by an O-initiated attack at C-1 to produce isoxazolin-5-ones (75TL3283, 78USP4053481), and an isoxazolin-5-one was produced as a by-product in the photolysis of nitroethylene (78AJCU3). Substituted oxazolin-5-ones have... 

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

Phenylmercunc chlonde, conversion to phenyltnchloromethylmercury by reaction with sodium tn-chloroacetate 46, 98 2 Phenyl 2 oxazolin-5 one, 47,... 

Polyamides containing a-aminoacid units are readily obtained by reaction of bisazlactones (2-oxazolin-5-ones) with diamines. When polyamines such as diethylenetriamine (DETA) or triethylenetetramine (TETA) are used as the diamine component, the resultant polyamides readily cyclodehydrate above 200°C to produce polymers containing 2-imidazolin-5-one units in the backbone. Polyamides derived from simple diamines (e.g. 1,6-hexanedi amine) cyclodehydrate only in the presence of a suitable catalyst. Carboxylate salts and certain Lewis acids have been found to be efficient catalysts for this transformation. 

Initial polymerization studies were conducted with 2,2 -tetramethylenebls(4,4-dimethyl-2-oxazolin-5-one) 4 and... 

Photoelimination of carbon dioxide from the 2-oxazolin-5-one 474 in the presence of methyl acrylate affords the cis- and frans-l-pyrrolines 475 and 476.394 A nitrile ylid is believed to be involved in this and other analogous transformations.395... 

C-Alkylation of 2-oxazolin-5-ones under catalysed mildly basic conditions provides a convenient route to a-branched a-amino acids (50-80%) [126]. Similarly, A-alkyloxindoles are mono- and di-alkylated at the 3-position [127], For other examples of the alkylation of heteroaryl systems, see Chapter 5. 

Gu, R.-L., Lee, I.S. and Sih, C.J., Chemo-enzymatic asymmetric synthesis of amino acids. Enantioselective hydrolyses of 2-phenyl-oxazolin-5-ones. Tetrahedron Lett., 1992, 33, 1953-1956 Crich, J., Brieva, R., Marquart, P., Gu, R.-L., Flemming, S. and Sih, C.J., Enzymic asymmetric synthesis of a-amino acids. Enantioselective cleavage of 4-substituted oxazolin-5-ones and thiazolin-5-ones. J. Org. Chem., 1993, 58, 3252-3258. 

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... 

The 5-oxazolones or oxazolin-5-ones are very interesting heterocyclic compounds that have been used as intermediates in the synthesis of a variety of organic molecules. Two structural classes are possible, the 5(27T)-oxazolones (or 3-oxazo-lin-5-ones) and 5(47T)-oxazolones (or 2-oxazolin-5-ones). These structures differ only in the position of the double bond. Apart from the presence of the heteroatoms (N and O), the carbonyl group and the double bond, the 2- or 4-position, respectively, can be saturated or unsaturated. The isomeric 5-oxazolones are... 

A samrated 5(2//)-oxazolone, also known as a 3-oxazolin-5-one or a pseudox-azolone, can be considered as the tautomer of a saturated 5(4//)-oxazolone (or 2-oxazohn-5-one) by a 1,3-prototropic shift (Scheme 7.1). 

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