Oxidation, oxazole reactions activating group

Meyers and Tavares also investigated radical bromination as a means to effect net oxidation of activated oxazolines. They found these reaction conditions were acceptable for preparing 2-alkyl-4-oxazolecarboxylic acid esters only if the 2-alkyl group was methyl or primary, i.e., 65 Rj = R2 = H or Rj = n-C4, R2 = H. However, these conditions failed completely if the 2-alkyl group was secondary, e.g., isopropyl or cyclohexyl. In these cases, the desired oxazole 65 was isolated in <1% yield. Instead, the sole product was 66, the result of oxidation with concomitant side chain bromination (Scheme 1.20, Table 1.1, entries 7 and 8). 

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