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Zinc Acetate-Promoted Buchwald−Hartwig Couplings of Heteroaromatic Amines

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Regioselective manipulation of hydroxyl groups in polyols has been critical to carbohydrate chemistry.

A robust selective allylation of 1-methyl-L-α-rhamnose was developed using n-Bu2SnO as the catalyst and proton sponge as optimal base for high regioselective control. The optimized condition afforded the 3-O-allylated rhamnose in excellent regioselectivity (> 20:1) and 82 % isolated yield on a 50 g scale (scheme).

A scalable isolation/purification process was developed which afforded the desired product in 72 % yield with 1530 ppm of Sn. The residual Sn level can be further reduced to < 20 ppm by treatment with SiliaMetS Cysteine & TAAcONa (Table 1).

Residual Sn (ppm)
Experimental StepStarting LevelEnd LevelRecovery (%)
Extractive Work Up-1,53072
1st treatment 20 wt % w/ SiliaMetS Cysteine1,53027392
2nd treatment 20 wt % w/ SiliaMetS Cysteine27331.993
1st treatment 20 wt % w/ SiliaMetS TAAcONa1,53026791
2nd treatment 20 wt % w/ SiliaMetS TAAcONa26719.595

This catalytic system also showed outstanding potential in the selective benzylation of the same substrate.
The isomeric purity remained unchanged after the scavenger treatment.

Li, X. et al. Org. Process Res. Dev., 2017, 21, 1653 - 1658

 

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