Reaction with mesoionic oxazoles

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. ... 

Y-Acylprolines, in a reaction with trifluoroacetic anhydride and catalytic base, rearrange to oxazoles via mesoionic oxazolium-5-olates (168) <93TL859>. The yields are good for N-benzoyl, N-cinnamoyl, and 7V-pivaloylprolines, but no oxazole is formed with N-acyl groups bearing a hydrogens on an sp carbon. A possible mechanism is shown in Scheme 78. 

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. 

Among other reactions, the bis-metallated species (151) derived from nitroalkanes condense with dialkyl carbonates to give comp>ounds (152), in 60—80% yield, which can serve as precursors of both a-amino-acids and a-hydroxyamino-esters as well as a-keto-esters. Oxazolin-5-ones (153) can be alkylated at the 4-position by alkyl halides in hot DMF containing HMPA and ethyldi-isopropylamine. Yields are good (60—90%) for allylic, benzylic, and propargylic halides but otherwise poor (e.g. 32% with EtI) under these conditions acid hydrolysis of the products affords substituted a-amino-acids. Mesoionic l,3-oxazol-5-ones (154), obtained from imidoyl chlorides and acyl-tetracarbonylferrates, react with alcohols to give N-acyl-a-amino-acid esters. ... 

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