✍ Scribed by Stanislaw Pikul; A. Greg Switzer
No coin nor oath required. For personal study only.
Zirconium tetrachloride was found to catalyze glycosylation of various sterols with peracetylated 5-hydroxymethylene galactosyl fluoride. (~) 1997 Elsevier Science Ltd
In the course of our work on cholesterol-lowering glycosides I we have experienced low yields of the glycosylation reaction using 5-substituted peracetylated galactosyl bromide 1, diosgenin (3) and mercury cyanide in acetonitrile, lb,2 The low yields, attributed to low thermal stability of 1 and general low reactivity of sterols, directed our attention to galactosyl fluorides known for considerably higher stability and good reactivity. 3 In fact crystalline 2, 4 prepared in 75% yield from the corresponding hexaacetate using HF-pyridine, 5 was found to be stable at room temperature for many months. However, when it was used in a BF3*Et20 catalyzed glycosylation reaction 6 with diosgenin or its trimethylsilyl ether 4, several products were formed with the acetylated diosgenin 5 as the main one. This transesterification, frequently a predominant course of reaction, was the apparent reason for rather low yields (20%) of the desired product 6. Other glycosylation promoters offered little improvement except AgC104-SnCI2 which increased the yield by 5-10% but required longer reaction times compared to BF3-Et20 catalyzed reaction. Also, silver based reagents are typically not the first choice due to economic and toxicological reasons. OAc
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