Oxazoles, mesoionic, cycloaddition

The mesoionic oxazol-5-one 245 is in equilibrium with an acylaminoketene 246, which undergoes a [2+2] cycloaddition reaction with diisopropylcarbodiimide to give the cycloadduct 247, mp 159-160 °C in 63 % yield. ... 

Part A is devoted specifically to the synthesis, reactions, and spectroscopic properties of oxazoles and encompasses four chapters Chapter 1—Synthesis and Reactions of Oxazoles Chapter 2—Spectroscopic Properties of Oxazoles Chapter 3— Diels-Alder and Cycloaddition Reactions of Oxazoles and Chapter 4—Synthesis and Reactions of Mesoionic Oxazoles. 

Mesoionic oxazolium-5-oxides 49 react with aminomalonic ester to give pyrrolidinones 50 as the major or exclusive products <99H(50)71> and the oxazolamine 51 is converted by sodium acetate in acetic acid into the hydantoin 52 <99JHC283>. The intramolecular Diels-Alder cycloaddition of the oxazole 53 and related compounds has been used as a route to substituted isoquinolines <99JOC3595>. ... 

There are two common forms of mesoionic compound that belong to the oxazole ring system mlinchnones (1,3-oxazolium-5-olates) 86, named by Huisgen for the city Miinchen, and isomilnchnones (l,3-oxazolium-4-olates) 87 (Figure 4). These are highly versatile compounds which are often used as 1,3-dipoles in cycloaddition chemistry <2002HC(59)681>. 

Oxazole derivatives have been thoroughly studied and display a wealth of synthetic opportunities. The mesoionic miinchnones undergo 1,3-dipolar cycloadditions across the C(2)—C(4) positions. The 5(4H)-oxazolones have excited interest for a long time because they undergo fission of the C(5)—O bond to afford a-amino acids. Appropriately substituted dihydrooxazoles and oxazolidines are effective carriers of asymmetry, and they have only begun to be exploited in this role although numerous possibilities have been demonstrated. 

Diaryl or arylalkyl imidoyl chlorides react with the acyltetracarbonylferrate anions (67) at room temperature in CH2CI2 to give the mesoionic l,3-oxazol-5-ones (68) in isolated yields of 37—65%. The anion (67) is generated in the normal way by reaction of [Fe(CO)s] with sodium amalgam followed by reaction with the appropriate acid chloride at -30 °C. The mesoionic compounds (68) are useful intermediates since they react with alcohols to give a-amino-acids and undergo a variety of cycloaddition reactions to form new heterocycles. 

---