2,3-Unsaturated 1-thioglycosides

Thio>gIycal derivatives act as useful donors in a palladium-activated coupling with acceptor alcohols (Scheme 5). 2,3-Unsaturated thioglycosides are also donors. ... 

The use of the unsaturated thioglycoside (30) as a starting materia) for the preparation of spiro-sugars is described in Chapter 24. The preparation of a,P-unsaturated acyclic derivatives such as (31) from C-glycosides is mentioned in Chapter 16. 

Dithiocetals, thioglycosides preparation, 181 DNA, disaggregation, ultrasonic irradiation, 11 Double bonds, unsaturated monosaccharides,... 

Thiols, thioglycosides preparation, 183 2,3,4-Tri-O-ethylamylose, structure, 412 Triple bonds, unsaturated monosaccharides, 95-96... 

As previously stated, a distinct feature of this rearrangement is the retention of the anomeric information. This advantage is nicely illustrated by the direct synthesis of a carba-disaccharide from a disaccharide [65]. As shown in Scheme 8.35, maltose was first transformed into the thioglycoside 131. Manipulation to the free primary alcohol 132 was achieved using a classical sequence of reactions. At this point, glycosylation of methanol was realized in the presence of NCS to afford 133, and the unsaturated derivative 134 was obtained after iodination and elimination. Triisobutylaluminum induced rearrangement nicely afforded the carba-disaccharide precursor 135 in a yield of 89% as a mixture of two isomers. 

However, 2,3-anhydro-thioglycosides undergo—under basic conditions—other undesired reactions. If the anion is generated at the a-position to the anhydro ring, the -elimination occurs readily providing the unsaturated carbohydrates [41]. Furanoses, under the same conditions undergo aromatization (O Fig. 14)... 

Epoxidation. With Mn-salen as a catalyst, enol derivatives give 2-hydroxy acetals. Unsaturated acids form lactones during oxidation in the presence of an iron porphyrin. Glycosylation. The action of a PhlO-MejSiOTf combination on thioglycosides consists of oxidation and Lewis acid catalysis, thus allowing the synthesis of disaccharides. 

When a 2, 3 -unsaturated pentafuranose thioglycoside is utilized allylic displacement is observed and the product is the 3 -isomeric nucleoside (eq 28). ... 

For thioglycosides with low donor activity, DMTST is less effective than promotion of the glycosylation reaction with the NIS/AgOTf system. DMTST is useful for the reaction of thioglycosides with acceptors containing unsaturation (eq 11). In some cases, acid-sensitive protecting groups (e.g., sUyl ethers) may transfer or be cleaved under reaction conditions that include DMTST. ... 

A number of hexopyranose epoxides have been treated with UBr to furnish ring-contracted dihydrofuran aldehydes. Epoxides (58) and (59) afforded unsaturated aldehydes (60) and (61), respectively. Conjugate addition of LiMeCuCN to aldehyde (61) with m situ trapping of the enolate produced the enol silyl ethers (62). The branched nitro compound (63) has been converted separately into the aldehyde (64) and the thioglycoside (65). Whereas previous applications of the Tebbe reagent have used specially purified material, apparently the crude reagent system can be employed on sugar esters and lactones CHoOR... 

Unsaturated phenyl 1-thioglycosides were intermediates in the synthesis of the core trisaccharides of esperamycin which is referred to in Qiapter 19. The rearrangement of a thioglycoside on exposure to trimethyl orthobenzoare is covered in Qiapter 7. 

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