Superarmed Glycosyl Donors in Glycosylation Reactions

The armed-disarmed concept, although discovered with 0-pentenyl glycosides, has been found to be applicable to many other classes of glycosides such as thioglycosides [6], selenoglycosides [39], fluorides [40], phosphoroamidates [41], substituted thioformimidates [42], glycals [21], y-benzoxazolyl (SBox), and 5-thiazolinyl (STaz) [43, 44]. 

5 Armed-Disarmed Concept in the Synthesis of Glycosidic Bond 

As expected the superarmed galacto derivative 770 was found to be significantly more reactive than the armed galacto derivative 772. Thus, disaccharides 111 and 775 were formed in 5 min (92 %) and 40 min (85 %), respectively. As in the case of D-glucose, no reaction took place with the perbenzoylated galactoside 114. Similar observations were made with mannosides 115-119 disaccharides 116 and 77S were formed in 50 min (79 %) and 90 min (79 %), respectively, whereas no glycosidation with the disarmed donor 119 took place. To this end, it was determined that not only did the 2-0-benzoyl-3,4,6-tri-0-benzyl donors 96,110, and 775 readily react, while disarmed glycosyl donors 94, 114, and 779 did not, but also, as postulated, they proved to be more reactive than their armed counterparts 93,112, and 777. 

Mydock and Demchenko [48] studied whether the enhanced reactivity of superarmed donors 96, 110, and 775 was sufficient to allow for direct chemoselective couplings. As acceptors were chosen armed benzylated glycosides 725,124, and 125, all bearing the same leaving group (SBox) (Fig. 5.30). 

The superarmed concept was validated by direct coupling of the superarmed glycosyl donor 96 and benzylated ( armed ) acceptor 7 01 whereby the corresponding disaccharide 103 was obtained in 70 % yield (Fig. 5.31). No self-condensation products were detected. 

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Superarmed Glycosyl Donors in Glycosylation Reactions Table 5.6 1,2-Migration in carbohydrates... 

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