2,3-Unsaturated allyl glycoside

Fig. 30 Synthesis of 2,3-unsaturated allyl glycosides or glycosyl cyanides...

Hotha and Tripathi [45] reported a practical method based on microwave irradiation for Perrier reaction of per-O-acetylated glycals 49a,b with primary, secondary, allylic, benzylic, and monosaccharide alcohols they used a catalytic amount of niobium pentachloride to give the corresponding 2,3-unsaturated O-glycosides 51 and 53 with a stereoselectivity in high yields and with short reaction times (Scheme 12.24). 

Two groups have reported on the use of the Claisen rearrangement to obtain 2,3-unsaturated C-glycosides. Fraser-Reid and colleagues found the standard reaction applied to derivatives of the allylic alcohol (77) inefficient because of the poor yields of the derived vinyl ethers or because allylic esters e.g. (78) predominated. They were successful, however, in obtaining compounds (79) and (80) by modified procedures (Scheme 5). An alternative synthesis of saturated, and hence unsaturated, C-glycosides was developed as illustrated in Scheme 6. In parallel work Ireland and co-workers used the enol silyl ether approach as shown in Scheme 7 and employed the products in polyether antibiotic syntheses. ... 

C-Glycosyl amino acids of type 68, for example, are robust to degradation by glycosidases with respect to C>-glycosides. They can be obtained directly by CM of C-allyl glycosides with unsaturated amino esters [58]. They were recently produced by CM of vinylcarbohydrate 65 with the amino ester 66. The mercury(II)-mediated cyclization of 67 affords the C-glycosyl amino esters 68 (Scheme 28) [59]. 

For the synthesis of 2,3-unsaturated C-glycosides with carbonyl groups at C-3 of the C-1 substituents the use of allyl silyl ethers together with a Lewis acid catalyst is recommended [74]. 

The Tsuji-Trost reaction is the palladium-catalyzed allylation of nucleophiles [110-113]. In an application to the formation of an A-glycosidic bond, the reaction of 2,3-unsaturated hexopyranoside 97 and imidazole afforded A-glycopyranoside 99 regiospecifically at the anomeric center with retention of configuration [114], Therefore, the oxidative addition of allylic substrate 97 to Pd(0) forms the rc-allyl complex 98 with inversion of configuration, then nucleophilic attack by imidazole proceeds with a second inversion of configuration to give 99. 

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