A- -C-disaccharides

Fig. 14 Synthesis of a C-disaccharide exploiting the Sml2 catalyzed reaction of a glycosyl pyridylsulphone.

Applying the procedure just described, Rouzaud and Sinay (65) described a stereospecific synthesis of a C-disaccharide in which a methylene group takes the place of the interunit oxygen atom. Starting from 51a and the dibromoalkene 59, the hemiacetal 60 was obtained. Reduction of 60, followed by catalytic hydrogenolysis, gave the /J-(l — 6)- C-disaccharide 61. 

The reaction of type i is utilized for the preparation of a variety of dicarboxylic acids. Scheme 25 shows the synthesis of a C-disaccharide (73) from a carbohydrate carboxylic acid (72) [98]. 

C-Glycoside synthesis via the intermolecular addition of anomeric radicals to activated alkenes has been pioneered by Giese and co-workers [120]. A wide variety of acceptors have been used in this area and both simple [131,132] and more complex (a C-disaccharide 290) [133] examples are shown (Scheme 77). 

E. Caneva, Synthesis of C-disaccharides through dimerization of exo-glycal, J. Org. Chem. 57 1304 (1992) A. Boschetti, F. Nicotra, L. Panza, G. Russo, and L. Zucchelli, Easy synthesis of a C-disaccharide, J. Chem. Soc. Chem. Commun. p. 1085 (1989). 

A similar process is possible using samarium diiodide. With this reagent carbohydrate radicals are generated through a one-electron reduction of, for example, anomeric phenyl sulfones. The so-induced cyclization of the tethered sugar 59 gives compound 60, which has been transformed to a C-disaccharide [31]. 

C-Glycosidation of Pyranosyl Compounds Forming a C-Disaccharide [2-4], To a boiling solution of the methylene lactone 66 (610 mg, 1.37 mmol) in 5 mL of 1,2-dimethoxyethane under nitrogen, were added, within 30 min, a solution of tributyltin hydride (873 mg, 3.0 mmol) in 10 mL of 1,2-dimethoxyethane containing AIBN (49 mg, 0.3 mmol) and, simultaneously, a solution of the a-D-glucopyranosyl bromide 47 (1.15 g, 2.80 mmol) in 18 mL of 1,2-dimethoxyethane. After completion of the addition, the mixture was allowed to stir for 15 min at boiling temperature. The solvent was evaporated under reduced... 

Intramolecular C-Glycosidation Forming a C-Disaccharide with Samarium Diiodide [31]... 

B. A Radical Route to C-Branched. Aldehydo Sugars An Example in the Synthesis of a-C-Disaccharide... 

VauzeiUes, B, Cravo, D, Mallet, J M, Sinay, P, An expeditious synthesis of a C-disaccharide using a temporary ketal connection, Synlett, 522-524, 1993. 

C-Glycosidation of Pyranosyl Compound Forming a C-Disaccharide [34]. To a boiling solution of the a-D-glucopyranosyl bromide 47 (748 mg, 1.82 mmol), methylidene-... 

Weinstein, D.S. Nicolaou, K.C. Synthesis of the namenamicin A-C disaccharide towards the total synthesis of namenamicin. J. Chem. Soc., Perkin Trans. 1 1999, 545-547. 

As a natural extension to the utility of silicon tethered radical acceptors for the preparation of C-glycosides, Xin, et al.,27 applied this technology to the preparation of a C-maltose derivative. This example, centering around the preparation of a C-disaccharide, will be discussed in detail in Chapter 8. 

In 1983, the first synthesis of a C-disaccharide was reported by Sinay, et al.,1 and involved the addition of an acetylenic anion to a sugar lactone. The... 

Returning to Diels-Alder methodology, Danishefsky, et al.,8 demonstrated its use in the preparation of interesting C-disaccharides. The key synthetic reaction, illustrated in Scheme 8.3.2, proceeded in a chelation controlled manner giving a 79% yield of a C-disaccharide precursor. The resulting dihydropyranone unit was recognized as suitable for subsequent conversion to various sugar derivatives. 

As already alluded to, the dimerization of sugar units has provided some conceptually useful methods for generating C-disaccharides. As shown in Scheme 8.7.3, Boschetti, et al.,20 effected the dimerization of two furanose units thus providing a one-step synthesis of a C-disaccharide. The reaction was... 

---