Oxazolylboronic acids

Attempts to make oxazolylboronic acids failed probably due to the equilibrium between cyclic and acyclic valence bond tautomers of the lithiooxazoles. A somewhat relevant Suzuki coupling involved the Pd-catalyzed cross-coupling of 6-bromo-2-phenyloxazolo[4,5-6]pyridine with phenylboronic acid to provide 6-phenyl-2-phenyloxazolo[4,5-6]pyridine [15]. 

Preparation of C2 oxazolylboronic acids is troublesome, likely due to the ring-opening liability of 2-lithiooxazoles. The C4 position can be made into a oxazolylboronate by palladium-catalyzed reaction of a C4 triflate with bis(pinacolato)diboron/ The C4 boronate can also be prepared through lithium-halogen exchange of a C4 bromide and reaction with B(0/-Pr)3 and pinacol. C2-TIPS protected oxazole can be lithiated at the C5 position and trapped with B(OzPr)3 to form the C5 boronic acid. °... 

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