Glycals allylic ethers

TABLE 37.4. Ferrier Rearrangement of Glycal Allylic Ethers... 

An early example utilizing an allylic ether, shown in Scheme 7.39, was reported by Herscovici et al. [142], and is applicable to various glycals. As shown in Scheme 7.40, the mechanism of these transformations involves formation of the carbocation followed by the evolution of its equilibrium with the illustrated 0-glycoside. As the carbocation reacts with the allylic ether, the reaction is driven to provide the desired product. 

SCHEME 7.39 Reactions of glycals with allylic ethers. 

Unlike nitroalkylation reactions, the use of allylic ethers as reagents in C-glycoside chemistry has proven to be substantially more useful. These reactions, illustrated in Figure 2.7.1 and alluded to in Scheme 2.3.16,41 42 show a propensity for a selectivity and are able to provide additional asymmetric centers. These reactions, however, procede in an S -like fashion and are limited to glycals. Consequently, a wide variety of deoxy sugar derivatives are available. 

Scheme 2.7.2 Reactions of Glycals with Allylic Ethers...

In a final example, summarized in Scheme 2.7.4, Herscovici, et al.,120 utilized several silyl ethers in reactions with various glycals to prepare the products shown. In the same report, a study was carried out involving the use of deuterated allylic ethers. This experiment, shown in Scheme 2.7.5, was designed to prove the involvement of a pinacole-type mechanism in the transformation. The 1,2-deuteride shift supported this hypothesis. 

In the reaction of allylic ethers with aryl halides, elimination of /t-alkoxy group occurs in some cases. Intramolecular reaction of the glycal 127 afforded the cis-fused pyrano[2,3-c]pyran 129 via deoxypalladation as shown by 128 [74]. On the other hand, ring cleavage of 130 occurs by deoxypalladation to give 132 via 131 [75]. Furthermore, the fcjiy-annulated pyranoside 134 was obtained by domino HR without elimination of the allyl ether group in 133 [76]. 

Sulfonamides can also be alkylated by support-bound electrophiles (Table 8.10). Polystyrene-bound allylic alcohols have been used to N-alkylate sulfonamides under the conditions of the Mitsunobu reaction. Oxidative iodosulfonylamidation of support-bound enol ethers (e.g. glycals Entry 3, Table 8.10) has been used to prepare /V-sulfonyl aminals. Jung and co-workers have reported an interesting variant of the Baylis-Hillman reaction, in which tosylamide and an aromatic aldehyde were condensed with polystyrene-bound acrylic acid to yield 2-(sulfonamidomethyl)acrylates (Entry 4, Table 8.10). 

Activation of 26 with AgBF, led to the intermediate 1,2-A-sulfonylaziridinc, which reacted with the tributyltin ether glycal 27 to afford the dimer 28. The second glycal was activated with dimethyldioxirane (DMDO) to supply the intermediate epoxide that was treated with ZnCl2 and allyl alcohol to afford compound 29. 

Unlike the carbohydrates with double bonds at positions other than between C-1 and C-2 ( isolated alkenes ), which exhibit normal alkene chemistry, glycals are vinyl ethers and therefore undergo a number of highly selective addition reactions due to the strongly polarized double bonds and the presence of bulky substituents at the C-3 allylic centers. Straightforward addition reaction includes initial electrophilic addition at the double bond, followed by the addition of a nucleophile at C-1 to give the 1,2-trans adduct (O Scheme 19). 

Recently, Claisen rearrangement of allyl-vinyl ether prepared from glycal ester with Tebbe reagent was reported [91]. In contrast to the Ireland-Claisen rearrangement, in principle, a non-enolizable ester can be employed (O Scheme 63). This method was applied for the synthesis of C-disaccharide. 

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