C-allyl glycosidation

C-Allyl glycosides can be prepared by the reaction of glycal epoxides with allyltributyltin in the presence of tributyltin triflate as a Lewis acid,281 and aldonitrones can be allylated with trimethylsilyl triflate as a catalyst (Equations (101) and (102)).282... 

Araki, Y, Kobayashi, N, Watanabe, K, Ishido, Y, Synthetic studies by the use of fluorinated intermediates. Part 2. Synthesis of glycosyl cyanides and C-allyl glycosides by the use of glycosyl fluoride derivatives, J. Carbohydr. Chem., 4, 565-585, 1985. 

Isomerization of the C-allyl glycoside 302 provides the corresponding C-(l-propenyl) derivative 303 as a major E-isomer. Cross metathesis proceeds in high yield only with the Grubbs II catalyst to deliver the homodimer 304, transformed to C-disaccharide analog 305 of the mycobacterial motif. 

Other interesting results pertain to 1,2- and 1,3-asymmetric inductions of C-allyl glycosides, homoallylic esters, and 4-methylene-US-dioxanes. The homologation with participation of an internal hydrosilane leads to cyclic siloxane derivatives. ... 

P-C-Allyl glycosides have been prepared by the ring-opening of epoxides by allylstannanes in the presence of a Lewis acid.54... 

Exposure of an allylstannane to halopyranoses in the presence of AIBN enables access to the corresponding allyl adducts [95] this procedure is useful for C-allyl glycosidation [96]. To enhance the practical utility of C-glycosidation, the effect of conformational restriction on a- and / -selectivity was studied in free-radical allylation at the anomeric positions of phenylselenoxylose derivatives of fixed conformation (Scheme 12.37) [97]. Treatment of a phenylthionocarbonate of 2 -deoxynucleo-side [98] and an iodopyrrolidine ]99] to allyltributyltin/AIBN resulted in substitution of the allyl moiety. 

Scheme 5. Preparation of C-allyl glycosides by intermolecular addition to allyl tin reagents...

It was found that the stronger acid, SnCU, provided a trace amount (3 %) of two isomeric C-allyl glycosides in the ratio of 1.4 1 (P a). Tributylstannyl triflate, the weakest among the Lewis acids studied, afforded 57 % of the product as a mixture of p- and a-isomers in a ratio 95 5. The author s explanation [217] of the Lewis acid effect on stereoselectivity is illustrated in Fig. 5.62. 

Penta-O-acetyl-6-D-glucopyranose with allyltrimethylsilane and boron trifluoride gave the corresponding a-C-allyl glycoside in 8l% yield and high selectivity when the reaction was carried out in acetonitrile in 1,2-dichloroethane, however, the a- and 3-anomers were produced in equal proportions. ... 

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]. 

Guijar et al. converted a C-allyl glycoside to a CAA by hydroxylation of the double bond followed by Jones oxidation [38]. An acid ftmction was also introduced by reacting the free anomeric center with methyl(triphenylphosphor-anylidene) acetate in a Wittig reaction and followed by conversion to a C-glycoside by intramolecular Michael addition, cf. Scheme 5 for the preparation of 36 from 2,3 5,6-di-O-isopropylidene-D-mannofuranose (34) [6]. A disadvantage of this... 

The most important of such processes is the reaction of glycals with alcohols catalyzed with Lewis acids known as Ferrier-I rearrangement [1,2] an example is shown in Figure 10.1. Reaction of glycals with allylsilanes catalyzed by Lewis acids is also a convenient method for the synthesis of C-allyl glycosides (e.g., 3) [2,3],... 

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