2 -deoxy-P-glycosides

Deoxy-P-glycosides are important structural components of many natural products. Recently, 2-deoxy-2-iodoglycosyl trichloroacetimidates [469] have proved to be... 

Scheme 5.52 Preparation of 2-deoxy-2-iodo-(i-glycosy acetates and their use in formation of 2-deoxy-p-glycosides.

General Method for the Preparation of 2-Deoxy-2-lodoglycosylbenzenesulfo-namides from Glycals and Its Use as Glycosyl Donors in the Synthesis of 2-Benzene-sulfonamido-2-Deoxy-P-Glycosides... 

The most attractive approach for the 2-deoxy-p-glycosides starts with 2-bromo-2,6-dideoxyhexopyranosyl bromides (2 ) such as These are available regio- and stereo-specifically from readily accessible and simple precursors by reaction with dibromomethyl methyl ether (DBE) (22, 2 ). In principle, the formation of related compounds may be anticipated by the addition of bromine to glycal precursors. Previous studies (24-27). however, proved these processes to yield several isomers which renders this approach of little preparative value. 

W. R. Roush, D. P. Sebesta, and R. A. James, Stereoselective synthesis of 2-deoxy-p-glycosides from glycal precursors. 2. Stereochemistry of glycosidation reactions of 2-thiophenyl- and 2-selenophenyl-a-D-g/uco-pyranosyl donors, Tetrahedron, 53 (1997) 8837-8852. 

W. R. Roush and C. E. Bennett, A highly stereoselective synthesis of 2-deoxy-P-glycosides using 2-deoxy-2-iodo-glucopyranosyl acetate donors, /. Am. Chem. Soc., 121 (1999)3541-3542. 

Sulfides are hydrogenolyzed usually by Ra-Ni.383-391 Phenyl and methylthio groups were removed by Ra-Ni (10 mmol Ni/mmol) refluxing in ethanol for 2 hours.392 A thio enol ether was desulfurized with Ra-Ni (100 mmol Ni/mmol) in MeOH at room temperature for 30 minutes.393 During the synthesis of 2-deoxy-P-glycosides the removal of the arylthio group was achieved using Ra-Ni with no problem.394... 

Recently, Biao Yu and Zunyi Yang reported 2,3-0-thionocarbonyl- l-thioglycosidcs as viable glycosyl donors for the construction of 2-deoxy-p-glycosides (Scheme 4.5b)... 

Grewal G, Kaila N, Franck RW (1992) Arylbis(arylthio)sulfonium salts as reagents for the synthesis of 2-deoxy-p-glycosides. J Org Chem 57 2084—2092... 

Ramesh S, Kaila N, Grewal G, Franck RW (1990) Aureolic acid antibiotics a simple method for 2-deoxy-P glycosidation. J Org Chem 55 5-7... 

Ramesh S, Franck RW (1989) Aureolic acid antibiotics synthesis of model 2-deoxy-p-glycosides of a-hydroxytetralone. J Chem Soc Chem Commun 1989 960-962... 

The uniquely generated 2 -hydroxyl group arising from opening of the 1,2-epoxide donor can therefore be exploited in the synthesis of 2-deoxy-P-glycosides [41]. Deoxygenation into 62 was accomplished by free radical reduction of the 2 -... 

Reactions with secondary acceptors proceeded in lower yields and exhibited sharply diminished stereoselectivities. It should be mentioned that 1,2-epoxides can be converted [43] to other glycosylating agents, thiophenyl glycoside 66, pentenyl glycoside 67, fluoride 68, which can be helpful after acylation at C-2 for the synthesis of 2-deoxy-P-glycosides (Scheme 15). 

Scheme 21. Synthesis of 2-deoxy-P-glycosides with cycloadduct glycosyl donors.

Thiem [3] introduced the use of 2-bromo-2-deoxy-a-D-glucopyranosyl bromides 96 for the P-selective glycosylation of complex aglycones involved in aureolic acid antibiotics. These bromides were prepared by the reaction of dibromomethyl methyl ether [67] with a methyl 2,3-0-isopropylidene-a-D-mannopyranoside 95 and gave predominantly (P a 10 l) 2-deoxy-2-bromo-P-glucopyranosides in their silver triflate-promoted reactions with sugar alcohols. Radical debromination afforded the 2-deoxy-P-glycosides 97 in excellent yields (Scheme 23). 

Scheme 26. 1,2-Ethyl (or phenyl) thio group migration for the stereospecific synthesis of 2-deoxy-P-glycosides.

Simple alkoxides react upon these a-adducts to give 2-deoxy-P-glycosides according to a Sn mechanism [97]. Activation by thiophilic promoters such as silver perchlorate in acetonitrile [96], or NIS in dichloromethane, or IDCP in acetonitrile [98] leads with alcohols mainly to 2-deoxy-a-glycosides. 2 -Deoxy-a-disaccharides (147) can thus be prepared in good yields, but with a p ratios better than 9 I only in a few eases (sugar alcohol 146). 

---