Anhydro-2-Thio-Glycosyl Donors

Tatsuta and Toshima [73] designed conformationally rigid 2,6-anhydro-2-thio sugars for the stereocontrolled synthesis of 2,6-dideoxy-a- and -P-glycosides which are very commonly found in bioactive natural products. These new donors have a very rigid structure (boat conformation) and the stereoselectivity of the glycosylation should not be affected by the anomeric effect in the same manner as the more usual chair conformers. 

Coupling of the armed phenylthio glycoside 121 with the disarmed acceptor 122 gives the a-disaccharide 123 in 89% yield. Activation of 123 by lithium aluminohy-dride (LAH) reduction of the sulfoxide group and subsequent reacetylation allows to glycosylate an alcohol in an a-selective procedure (124 is obtained in 98% yield when R = cyclohexyl). 

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Figure 5. Repulsive interactions in glycosylation with a 2,6-anhydro-2-thio glycosyl donor.

Scheme 27. 2,6-Anhydro-2-thio sugars as glycosyl donors for the stereospecific synthesis of 2-deoxy-a- and fl-glycosides.

Figure 6. 2,6-Anhydro-2-thio-cladinose as a glycosyl donor in erythromycin A synthesis.

Figure 1. Synthesis of the CDE trisaccharide of olivomycin A with 2,6-anhydro-2-thio sugars as glycosyl donors.

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