I would like to use 5-bromo-2-hydoxy-4-methoxybenzaldehyde in one of the synthetic procedures, but it's not commercially available. Instead 5-bromo-2,4-dimethoxybenzaldehyde is available.
Hi, there si a paper that describes regioselective demethylation in good yield: http://www.sciencedirect.com/science/article/pii/S0968089607004105
selectively demethylate a 2-methoxyl group in 5-bromo-2,4-dimethoxybenzaldehyde , you has to use HI or HBr to cleavage ether linkage under refluxing condition.
One equivalent of trimethylsilyl iodidie can be used. Since it is a bulkier reagent and it is attacked by the Lewis base -OMe so more electron rich and less hindered -OMe may preferentially attack on TMS-I. Based on this, it may be predicted that if one equivalent of Me3Si-I is used, the 4-methoxy group is likely to be demethylated first to give the major product. Other bulkier silyliodide derivatives may result in more selectivity. TMS-I is available commercially and is used in DCM solvent at low temperature to cleave an ether linkage. Selectively -OMe group is demethylated over other alkoxy groups as the reaction proceeds via SN2 attack of iodide ion on the methyl group of the complex -O+(Me)SiMe3 followed by usual work up.
AlCl3 with ethyl marcaptan might work. The carbonyl group will facilitate the co-ordination of AlCl3 with the 2-methoxy group and thus may preferentially cleave the methocy at 2-position.
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