Other Types of Protecting Group

In this chapter I shall select examples to illustrate other types of protecting groups and more examples will appear throughout the rest of the book. 

Completely stable toward adds or bases. Orthogonal to other types of protecting groups. Attachment via activated esters and cleavage via Pd(0) catalyst. 

The other type of protective groupings comprises substituted hydroxy-methylene derivatives. The first of them was the methylanilinomethylene grouping (59), formed from the hydroxymethylene derivative (57) by treating it with methylaniline. After methylation by the usual method, the removal of the protective grouping in the methylated derivative (62) was originally [43] carried out in two stages treatment with acid for the re-... 

C2 halogenated tryptamine precursor 261, followed by intramolecular aza-Michael addition. In this example, by Stewart and Pfeifer, the order of these reaction was controlled by the type of protecting group on the nitrogen, while a range of acrylate systems were reported in good to excellent yields. Others have also reported similar Heck/Michael processes [129-131]. 

In contrast, selective protection of 1,2-diequatorial diols in sugars, in the presence of other hydroxyl group combinations, had proved to be an especially difficult task. Until a few years ago, the protection could only be achieved using disilox-anylidene acetals [24]. However, this type of protecting group is frequently not sufficiently stable as to withstand normal glycosylation conditions. The final solution to the protection of frans-l,2-diols [25] was provided in the 1990s by Ley and co-workers with the development of 1,2-diacetals (Fig. 1). 

Treatment of iodo-hept-l-enitols like 76 with BujSnH afford cyclitols 77. " Several other examples are given in which seven-membered rings are observed. It appears the product ratios depend on the disposition and type of protecting group used. 

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