Glycals unsaturated sugar formation

Alternatively, Pd(0) adds oxidatively to the double bond of a glycal derivative resulting in the formation of a ir-allyl complex, which may react with carbon nucleophiles to give C-glycosides with a double bond between C(2) and C(3).26 A rt-allyl complex may also be formed starting from a Ferrier rearrangement product (2,3-unsaturated sugar derivative).22... 

Easily prepared from the appropriate monosaccharide, a glycal is an unsaturated sugar with a C1-C2 double bond. To ready it for use in polysaccharide synthesis, the glycal is first protected at its primary -OH group by formation of a silyl ether (Section 17.9) and at its two adjacent secondary -OH groups by formation of a cyclic carbonate. Then, the protected glycal is epoxidized. 

Formycin and formycin B with modified sugar subunits have also been prepared using, mostly, the nucleoside-nucleoside approach. In one instance, however, 2 -deoxyformycin B (458) was prepared by regio- and stereospecific palladium-mediated C—C bond formation between the fu-ranoid glycal 454 and protected 3-iodopyrazolo[4,3-d]pyrimidine. The C-nucleoside 455 having an unsaturated sugar moiety was subsequently elaborated to 458 (92JOC4690) (Scheme 125). 

The formation of C-S bonds by thiol radical additions to unsaturated sugars has also been reported, such as the addition of thiolacetic acid to glycal 167 [111]. Giese... 

When considering sugar ring unsaturation removed from the 1,2 position, many methods are available in addition to methods analogous to those used in glycal formation. As shown in Scheme 6.39, the direct elimination of sulfonates under basic conditions provides a useful strategy [20]. Moreover, in the case of m/i5 -2,3-bis-sulfonates, initial displacement of one sulfonate with lithium iodide can be followed by metallation and tandem elimination, as shown in Scheme 6.40 [69]. 

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