Lewis acids silver perchlorates

The combination of dicyclopentadienylzirconium dichloride and silver perchlorate activates armed glycosyl sulfoxides in dichloromethane between -20 °C and room temperature, but only very simple acceptors were studied [335]. Other Lewis and Bronsted acids studied include the environmentally benign europium, lanthanum and ytterbium triflates [336], certain polyoxometallates [337], sulfated zirconia [338] and Nafion H [338]. 

Compounds containing carbon-carbon double bonds m y also act as Lewis bases. The rather high solubilities of the inorganic Lewis acids, boron trifluoride, aluminum bromide, and silver perchlorate, in benzene derivatives and in olefinic compounds suggest acid-base interactions. Addition compounds formed from silver perchlorate and a number of such hydrocarbons have been isolated it has been shown that the silver ion is not bonded to any single carbon atom but sits astride the two -electron dumbbells as shown in Figure 5-1. Thus, such adducts are... 

Trimethylsiloxy)allyl cations (5 Y = OSiMea) are obtained by the reaction of 2-(trimethylsi-loxy)allyl chlorides (4 X = Cl, Y = OSiMea) with silver perchlorate in nitromethane (Scheme 2). lliese intermediates are also obtained with the aid of a Lewis acid. It has been reported that 2-siloxyacrolein (12a R = H), but not the less electrophilic (12b R = Me), when activated by an equimolar amount of SnCl4 serves as a l-hydroxy-2-oxidoallyl synthon (13) (Scheme 5). ... 

Mukaiyama, T, Katsurada, M, Takashima, T, The evaluation of catalysts generated from Lewis acids and silver perchlorate in a highly stereoselective glycosylation of 1-0-acetyl-D-glucose, Chem. Lett., 985-988, 1991. 

So far, the only examples of O-C migrations have applied to pyranose sugars or derivatives thereof. However, it should be noted that such reactions are applicable to furanose sugars. One particular example, reported by Matsumoto, et al.,101 was used in the total synthesis of gilvocarcin M. The specific reaction, shown in Scheme 2.4.25, utilized the acetoxy activated furanoside shown. Treating this compound with the phenol and using the hafnocene dichloride-silver perchlorate complex as the Lewis acid, the desired C-arylglycoside was obtained in 87% yield with the a anomer favored in a ratio of 8 1. 

In typical procedures for glycosyl fluoride glycosylation reactions, the glycosyl fluoride is mixed with an aglycon alcohol and a Lewis acid promoter. As noted above, Mukaiyama s original disclosure described the use of a combination of tin(II) chloride and silver perchlorate as the activator [20], but since then, many other... 

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