Deuteration, oxazole reactions

The ease of replacement of halogen atoms attached to the oxazole ring by nucleophiles follows the order Hal-2 Hal-4 > Hal-5. Halogen-metal exchange of 4- or 5-bromooxazoles with butyllithium, followed by quenching with D20, affords the corresponding deuterated oxazoles, but other replacement reactions are difficult. The 4-bromooxazole (139) gives a low yield of mandelic acid on alkaline hydrolysis (equation 6). The substitution of all three chlorine atoms in (140) by piperidine has been observed. 

Venkatasubramanian and Krishnamachari required deuterated oxazoles for a study of the flash photolysis of oxazole 1. They obtained (2-D)-oxazole 593 exclusively from 1 using CF3COOD and D2O (Scheme 1.162). The authors did not speculate about this surprising result. The reaction is selective for monodeu-teration, but 1 also suffers decomposition (up to 40%) under the reaction conditions leading to a mixture of 593 and deuterated pyrazines 594 n = 0-4. Trifluoroacetic acid-D was much more efficient than deuterosuUuric acid for this reaction. 

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