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 Step | Starting Level | End Level | Recovery (%) |
Extractive Work Up | - | 1,530 | 72 |
1st treatment 20 wt % w/ SiliaMetS Cysteine | 1,530 | 273 | 92 |
2nd treatment 20 wt % w/ SiliaMetS Cysteine | 273 | 31.9 | 93 |
1st treatment 20 wt % w/ SiliaMetS TAAcONa | 1,530 | 267 | 91 |
2nd treatment 20 wt % w/ SiliaMetS TAAcONa | 267 | 19.5 | 95 |
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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