Ytterbium(III)-Catalyzed Synthesis of 2-Aryloxazoles

Significance

Reported is the Yb(OTf)₃-catalyzed regioselective synthesis of di- and trisubstituted 2-aryloxaxoles 3 by the reaction of propargylic alcohols 1 with aryl amides 2. Although evidence is not provided, the reaction is proposed to proceed through an allenic cation, which is attacked by the aryl amide at the less sterically hindered carbon to give an intermediate, which undergoes a 5-exo-trig cyclization to give product 3. The reaction was shown to proceed for propargylic alcohols 1 bearing both EWG and EDG aryl moieties R^¹ and R^². No products were formed for the reaction of secondary alcohols and alkyl amides, whereas the reaction works with R^³ = H or n-Bu. The regiochemistry of the oxazoles 3 was determined by ^¹H NMR spectroscopy and confirmed for one product by X-ray crystallographic analysis.