Synthesis and Reactions of Oxazoles
ABSTRACT The chemistry of oxazoles continues to be an important focus of academic and industrial laboratories around the world. This small-ring heterocycle has elicited extraordinary creativity from medicinal and process chemists, polymer chemists, materials scientists, photographic dye chemists, and natural products chemists engaged in basic and applied research.Synthetic strategies for almost any oxazole-substitution pattern are available from classical methods or have been developed as new methodologies. Oxazoles are readily prepared from cyclic precursors such as oxazolines, N-acyltriazoles, N-acylaziridines, isoxazoles, N-acylisoxazolones, and imidazoles. On the other hand, virtually any type of functionalized acyclic moiety has been converted to an oxazole. For example, α-diazoketones, -esters, -nitriles, -sulfones, and -phosphonates have been converted to oxazoles using rhodium-carbene methodology. Acetylenes, acyl cyanides, amides, amino acids, amino nitriles, azides, enamines, hydrazones, imidates, imines, isocyanides, ketones, oximes, nitriles, thioimidates, and vinyl halides are all useful precursors to oxazoles. Oxazoles are now routinely prepared very efficiently via solid-phase methodologies that have been adapted for combinatorial libraries and parallel syntheses.Oxazoles serve as versatile precursors to a remarkable variety of heterocyclic ring systems via nucleophilic addition, ring opening, and recyclization as well as [2 + 2], [3 + 2], and [4 + 2] cycloaddition reactions. The oxazole ring has found significant use as a precursor to aminoketones, amino acids, dipeptides, and triacylamines. The predictable and regioselective metallation chemistry of oxazoles has been combined with transition metal-catalyzed cross-coupling reactions to provide synthetic approaches to a vast array of increasingly complex natural products. In addition, the complexity and sensitivity of oxazole natural products, particularly those of marine origin, has fostered several novel, efficient, and mild syntheses of mono-, bis-, and tris-oxazoles. Oxazoles have served as scaffolds from which macrocyclization reactions have produced a variety of interesting and novel depsipeptides and macrolactones. Despite these significant advances, challenges remain. For example, a mild, efficient, and general method to oxidize 4-alkyloxazolines to 4-alkyloxazoles is still needed.The chapter describes the major developments in this field from 1983 to 2001.The chapter is divided into the following sections: “Introduction,” “Synthesis of Oxazoles,” “Reactions of Oxazoles,” and “Oxazole Natural Products.” The introduction is a brief discussion of the numbering and lists the major 1H, 13C, and 15N resonances of a few selected examples.The second section describes the common methods of synthesis. No attempt has been made to describe every monocyclic oxazole synthesized of all synthetic methods. The most common and useful synthetic methods and some particularly novel methods have been included together with tables of some representative examples. In addition, this section and the reactions of oxazoles are not necessarily organized in the same manner as in the previous volume. This reflects the changing emphasis on new methods applicable to complex natural product synthesis.The third section presents important reactions of oxazoles but again no attempt has been made to describe every reaction of an oxazole or every derivative prepared. There, the aim is to convey the wealth of chemistry available by the selected examples.The last section includes syntheses of some naturally occurring oxazoles. The choice of these natural products is arbitrary and selected from the recent literature to demonstrate the versatility of oxazoles in synthesis.
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ABSTRACT: An efficient and general preparation of several chiral N-protected 5- (aminomethyl)oxazoles has been accomplished by treatment of N-protected α- amino esters with α-lithiated isocyanides, obtained by metalation of methyl and benzyl isocyanides with BuLi or of ethyl isocyanide with lithium diisopropylamide.01/1998;