Glycosyl dithiocarbonates

Scheme 4.72 Synthesis of S-glycosyl dithiocarbonates (glycosyl xanthates).

Glycosyl dithiocarbonates (xanthates) are attractive glycosyl donors because of their high reactivity in glycosylation reactions. Most procedures for the synthesis of dithiocarbonates involve reaction of per-O-acetylglycosyl halide 3 or 4 with... 

The 0,5-dialkyl dithiocarbonates (Table 4.8) are readily prepared under phase-transfer catalytic conditions by the reaction of an alkylating agent with potassium O-alkyl dithiocarbonate [35, 39], which can be obtained from carbon disulphide and the appropriate potassium alkoxide [cf. 40]. Monosaccharides are converted into 5-glycosyl dithiocarbonates via the in situ formation of the tosylate, followed by reaction with potassium O-alkyl dithiocarbonate (Scheme 4.6) [41], In a similar manner, 5-glycosyl 7V,7V-diethyldithiocarbamates are obtained from the monosaccharide and A.A-diethyldithiocarbamate (see 4.3.2) [42]. 

S -Alkyl thiocarbamates (Table 4.24) have been synthesized [I] by a procedure (Scheme 4.19) which is closely analogous to that employed for the preparation of S-alkyl thiocarbonates (see Section 4.1), S-Glycosyl dithiocarbamates [2], which are useful precursors for thiosugars, have been prepared by simple nucleophilic displacement of the tosyloxy group by the N,N-diethyldithiocarbamate anion (cf preparation of S-glycosyl dithiocarbonates, 4.1.13). 

A solution of 2-azido glycosyl nitrate 6 (2.5 mmol) and O-ethyl S-potassium dithiocarbonate 5 mmol) in acetonitrile (20 ml) was kept for 5 h at room temperature, diluted with dichloromethane (200ml), washed with water (30ml), dried (MgS04) and concentrated. Column chromatography of the residue (4 1 hexane-ethyl acetate, 0.3% triethylamine) gave the title compound in 97% yield. 

Glycosyl halides, reaction with ethyl potassium dithiocarbonates, 111 —, poly-O-acetyl-, solvolysis of, 39 Glyculosonic acids, 3-deoxy-, 253, 254 Glyoxal, bis(phenylhydrazone), 81 Guanosine, 5-(fucosyl dihydrogen pyrophosphate), 205 5-phosphate, 227 trihydrogen pyrophosphate, 205 —, 2-deoxy-, 5-triphosphate, 231 Guaran, 71, 75... 

A new route to 1,2-tronr-glycosyl thiocyanates, such as the glucose derivative 5, involved opening of 1,2-anhydrosugars with ammonium thiocyanate and phase transfer catalyzed reaction of glycosyl halides with < -ethyl-S potassium dithiocarbonate proved to be an efficient new method for the preparation of the known 5-glycosyl xanthates 6. ... 

The 5-glucopyranosyl phosphorodiamidimidothioate (116) afforded good yields of disaccharides when coupled with several sugar mono-ols in the presence of lutidinium tosylate, the a, S-ratios being about 10 1. Likewise, the A -acetylneuraminic acid derivative (117), made by use of the corresponding dithiocarbonate, acts as a glycosyl donor when and with silver triflate. ... 

The 1-thio-D-glucose derivative (15) has been synthesised from the corresponding free sugar (12) by treatment with 5,5 -bis( 1-phenyl-l f-tetrazoI-5-yl)dithiocarbonate, As indicated in Scheme 3, a thiocarbonate derivative (14) was formed initially with concomitant liberation of thiol (13). Direct attack by this thiol at C-1 (path a) gave the main product (15) in 63% yield, whereas benzyloxy participation in the displacement at C-1 with attack by the nucleophile at C-6 (path b) gave its regio-isomer (16) which was isolated in 33% yield. The use of compound (15) in silver triflate promoted glycosylations is referred to in Chapter 3. 

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