Iodo- or 2-bromopyridine N-oxides were readily magnesiated with i-PrMgCl x LiCl via the iodine or bromine-magnesium exchange. The bromine adjacent to pyridine N-oxide (at the 2- or 6-position) can be regioselectively magnesiated in the presence of other position substituted halogens. This method was tested in various substituted pyridine N-oxide systems, and has been successfully applied to the total synthesis of caerulomycins E and A.
A new, mild, convenient method allows the preparation of a wide range of functionalized pyridine N-oxides. 2-, 3-, and 4-iodopyridine oxides as well as 2-bromo analogues undergo halide-magnesium exchange. 3-and 4-Bromo groups are not attacked which allows regioselective reaction of di-and tribromopyridine oxides. This approach can be applied to a facile total synthesis of caerulomycins such as (XVIII).
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