STaz derivatives

The STaz moiety was found stable toward common protecting group manipulations involving basic and acidic conditions, for example, acetylation, benzylation, acetal formation and cleavage, etc. (77). The STaz derivatives were found to be stable toward hydrolysis in the presence of acidic thiophilic reagents. Thus, comparative hydrolytic stability studies showed that STaz glycosides are even more stable than their 1-5-ethyl and 1-5-phenyl counterparts in the presence ofNBSorNlS/TfOH. 

Additionally, the arming and disarming properties of the STaz glycosides were evaluated. It was demonstrated that activated (benzylated) STaz derivatives could be activated over electronically disarmed (partially acylated) STaz glycosyl acceptors in die presence of either AgOTf or Cu(OTf)2 78). For example, a classic armed-disarmed approach (79) allowed chemoselective activation of 54 over disarmed acceptor 61 in the presence of Cu(OTf)2to afford... 

S-Benzoxazolyl (SBox) and, especially, S-thiazolinyl (STaz) moieties are sufficiently stable for use in anomeric protection. These derivatives can, however, be activated under mild conditions using silver triflate [114-118]. 

Demchenko investigated a series of per-acetylated and per-benzoylated thioimidates that include S-oxazolinyl (83), S-oxazinyl (84), and S-thiazinyl (85) derivatives (Figure 2). The glycosyl donor properties of these compounds and their hydrolytic stability were determined to be similar to those of the SBox and STaz glycosides, yet slightly lower glycosylation yields were obtained (82). A... 

A number of different classes of glycosyl thioimidates have been discussed herein. Due to a fairly broad diversity of these derivatives, generalization is often impractical. So far, only S-pyridyl, SBox, and STaz glycosides have been more or less systematically investigated. The following conclusions are primarily focused on the properties of the SBox and STaz glycosides developed by the authors of this overview. 

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