Preparation of Glycals

Zinc catalyzed reductive elimination of acetobromoglucose to give triacetyl D-glucal [64] is the most usual entry into glycal chemistry. Usefully the product can be deprotected imder Zemplen conditions and the resulting trihydroxyglucal [65] can be elaborated e.g. fiiUy protected, as its trisilylether by treatment with an excess of rcrt-butyldimethylsilylchloride and imidazole in DMF [66]. 

Notes and discussion. The traditional (and indeed the original) method of the synthesis of glycals is via reductive elimination of an acylated glycosyl haUde. This reaction leads to the formation of the acetylated glycal, which can then be de-acetylated and further functionalised. A variety of reducing conditions have been reported zinc/acetic acid with or without platinic chloride has been most commonly 

Two round-bottomed flasks (250 and 1000 ml) with mbber septum and magnetic stirrer bar Magnetic stirrer 

Source of dry argon (or nitrogen) as inert gas Measuring cylinder 

---

Scheme 5.43 Preparation of glycals via base-induced eliminations.

G. Bellucci, G. Catelani, C. Chiappe, and F. D Andrea, A simple and highly diastereoselective preparation of glycal epoxides using the MCPBA-KF complex, Tetrahedron Lett., 35 (1994) 8433-8436. 

Furanoid and pyranoid glycals. The classical method for preparation of glycals is reduction of pyranosyl halides with zinc and acetic acid. A more recent method is lithium-ammonia reduction of furanosyl and pyranosyl halides with a blocking acetonide group. ... 

Deoxy-sugars. Part XVII. An Investigation of the Glycal Method for the Preparation of Derivatives of 2-Deoxy-D-galactose, W. G. Overend, F. Shafizadeh, and M. Stacey, /. Chem. Soc., (1951) 992-993. 

E. Wieczorek and J. Thiem, Preparation of modified glycosyl glycerol derivatives by glycal rearrangement, Carbohydr. Res., 307 (1998) 263-270. 

Typical Procedure for the Preparation of Selenoglycosides from Glycals... 

The latter elimination route to D-ribal is illustrative of the approach necessary for the preparation of the sensitive furanoid glycals, which are not accessible via the... 

Scheme 5.53 Preparation of glycosyl donors from glycals using PhSCI and PhSeCI.

These methods complement other approaches [159] to the preparation of 2-deoxy-2-aminoglycosides and serve to further emphasize the rich diversity of products available from the glycals. 

---