Imidazoles 2-silyl groups, active

Imidazole or pyridine mediated silylation of l,2-0-[l-exo-ethoxy )ethylidene]-oc-D-glucopyranose failed to give a high yield of the 6-silyl ether due to some polymerization and side reactions. Activation of hydroxyl groups via a tributylstannyl intermediate followed by the tetrabutylammonium bromide catalyzed reaction with tert-butyl-chlorodiphenylsilane was more successful [231], the 6-0-silyl derivative being isolated in 87 % yield.. The lability of this protecting group under benzylation with benzyl bromide and sodium hydride at 0 °C has been observed [449]. 

Table 16 shows the influence of reaction conditions and of activation reagents such as TMSC1, hexamethylphosphoric triamide (HMPA), M-methyl-imidazole (NMI), saccharin and tetrabutylammonium fluoride (TBAF) on the DS of silyl dextran. For complete silylation, a mixture of 1.8 mol HMDS and 0.2 mol chlorotrimethylsilane (TMSC1) per mol OH groups have to be used. The more reactive TMSC1 reacts in a first step with hydroxyl groups to trimethylsilyl dextran and HC1. Subsequently, the HC1 cleaves HMDS in TMSC1 and NH3 [218]. Complete silylation of dextran could also be carried... 

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