Oxazoles and oxazolines

Imidazoles can also be made by heating 4-tosyloxazolines in saturated methanolic ammonia or monoalkylamines. These reactions proceed through intermolecular condensation of a-aminoketones and amidines and intramolecular cyclization of a-ainidinoketones, respectively [49] (see Section 4.2). When A -unsubstituted 4-oxazolin-2-ones are added to isocyanates, the 2-oxo-4-oxazoline-3-carboxamide products cleave under the influence of strong acids and heat. Subsequent ring closure gives 4-imidazolin- 

---

Keywords Diels-Alder reactions of isoxazoles, isoxazolines, isoxazolidines, oxazoles and oxazolines... 

B. E. Maryanoff, Oxazoles and Oxazolines in Organic Synthesis, Oxazolinones, in J. Turchi, The Chemistry of Heterocyclic Compounds, 45, 963 (1986). 

Their presence has only been reported in a limited number of foods coffee, cocoa, meat products, barley and soy sauce. Very few sensory properties have been reported for oxazoles and oxazolines. 

Oxazoles and oxazolines in organic synthesis 86CH(45)963. Oxazolidin-3-yl as a protecting group in asymmetric synthesis 92 PAC1889. 

The cyclamides are small cyclic peptides that characteristically contain multiple thiazole, thiazoline, oxazole, and oxazoline rings, which are derived from cysteine, serine, and threonine residues. Some of the first examples of this class to be described were the patellamides (53-55) from the tunicate Lissoclinum patella, although it was later determined they were produced by the symbiotic cyanobacterium Prochloron sp. The structures were solved by a combination of acid hydrolysis and GC analysis, coupled with 2D NMR. Smaller cyclic peptides from this class include the hexapeptides westiellamide (56) and microcyclamide (57) from M. aeruginosa Many members of the class possess cytotoxic properties, although their biological function or mechanism of action is not fully understood. In contrast to many cyanobacterial peptides, aside from the unusual heterocyclic residues, these peptides generally contain only ribosomal amino acids. 

The latter authors also found it when serine and/or threonine reacted with sucrose. It was previously identified in other model reactions by treating furfural with hydrogen sulfide and ammonia (Shibamoto, 1977) or by heating rhamnose with ammonia (Shibamoto and Bernhart, 1978). Ho and Hartman (1982) proposed a plausible mechanism for the formation 2,4,5-trimethyloxazole from the reaction of dl-alanine or L-cysteine and 2,3-butanedione. Ho el al. (1982) also studied the formation of oxazoles and oxazolines in the Strecker degradation of dl-methionine and L-cysteine with 2,3-butanedione. In their proposed mechanisms, the authors suggested the formation of intermediate 3-oxazolines. 

Lee M.H., Ho C.T. and Chang S.S. (1981) Thiazoles, oxazoles, and oxazolines identified in the volatile flavor of roasted peanuts J. Agric. Food Chem. 29, 684-6. 

Maga J.A. (1978c) Oxazoles and oxazolines in foods. J Agric. Food Chem. 26, 1049-50. 

Maga J.A. (1981a) The chemistry of oxazoles and oxazolines in foods. CRC Crit. Rev. Food Technol. 14, 295-307. 

CycUzations. A one-step eycUzation of A(-hydroxyethyl amides to oxazoles and oxazolines is mediated by DAST. P-Hydroxy carbamates usually undergo fluorination (OH — F), but the N-Boc derivatives afford oxazolidin-2-ones. ... 

Maga (17D06) has reviewed the occurrence of oxazoles and oxazolines in a variety of processed food systems. Most of them possess green, sweet, and nutty aroma qualities and have been identified in coffee, soy sauce, wheat, and cooked beef and some of them have very low odor thresholds. In 1976, LefflngweU discussed how certain flavorants (reaction products) can be produced in tobacco and tobacco smoke (2337). 

With the exception of the report by Stoffelsma and Pypker [65], oxazoles and oxazolines (Figure 5.6f and Figure 5.6g, respectively) have been found only in food systems that have undergone Maillard reaction[66,67]. The oxazoles typically have green, sweet, floral, or vegetable-like aromas [68,69]. 

---