Two-stage activation procedure

Figure2.7 Two-stage activation procedure using glycosyl fluoride and thioglycoside as glycosyl donor.

G. Oligosaccharide synthesis from glycosyl fluorides and sulfides and the two-stage activation procedure 317... 

H. Application of two-stage activation procedure to complex molecule synthesis 321... 

G. Oligosaccharide Synthesis from Glycosyl Fluorides and Sulfides and the Two-Stage Activation Procedure... 

The two-stage activation procedure for the synthesis of complex oligosaccharides developed in these laboratories [23] combines the chemistries of glycosyl fluorides and sulfides. It is, therefore, instructional to discuss first, the use of these two classes of glycosyl donors in oligosaccharide synthesis, before describing the two-stage activation method. 

Scheme 9 The two-stage activation procedure for glycoside bond formation [23].

H. Application of Two-Stage Activation Procedure to Complex Molecule Synthesis... 

Numerous applications of the two-stage activation procedure have already been recorded. In this chapter, we highlight schematically the synthesis of the following complex mole-... 

Nicolaou used glycosyl fluorides as donors and thioglycosides as acceptors, and the procedure took advantage of the ready conversion of thioglycosides into glycosyl fluorides with NBS and DAST [17]. The two-stage activation procedure is illustrated by the synthesis of a phytotoxic oligosaccharide (78), a member of rhynchosporosides [18] (Scheme 5.13). 

The two-stage activation procedure for oligosaccharide synthesis, which was developed by Nicolaou and applied to his synthesis of avermectin Bia [10]> was also employed in his total synthesis of efrotomycin [15] (Scheme 12.11). The chemo- and stereoselective glycosidation of the... 

Even greater decolorizing power can be developed on alkali-roasted chars if they are given a brief activation with steam at 800° C, followed by further activation with air at 500° to 600° C (Table 8 5). Curiously enough, this two-stage activation procedure, which is so effective for alkali-roasted chars, is of no practical value for other chars thus far studied. 

The combination of the chemistry of thioglycosides with that of glycosyl fluorides, known as two-stage activation procedure , proved to be an efficient strategy for the synthesis of oligosaccharides and it has been successfully employed for the preparation of many complex molecules, [22] such as Avermectin Bu [108] or sulfated Le tetrasaccharide [109] (Scheme 11). 

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