Stannylene acetals 0-allylation

Alkylation requires more vigorous conditions. These reactions were originally performed on the stannylene acetal with the alkylating reagent in DMF at elevated temperatures (45°C for methyl iodide or 100°C for benzyl bromide)66 or on the tributylstannyl ether in neat benzyl bromide or allyl bromide at 80-90°C.67 It was then discovered that the presence of added nucleophiles markedly accelerates the reactions, so that alkylation of both tributylstannyl ethers and dibutylstannylene acetals in benzene, which is very slow at reflux with benzyl bromide alone, occurs at a reasonable speed at reflux in the presence of added tetrabutylammonium halides.57,63 Many other nucleophiles are also effective, including A-methylimidazole68 and... 

An interesting development is the observation that alkylation of dibutyl-stannylene acetals of this type in the presence of cesium fluoride in DMF gives different regiochemistry than that obtained under all other conditions. For instance, as shown in Fig. 23, the reaction of the dibutylstannylene acetal of methyl 4,6-O-benzylidene-a-D-glucopyranoside with benzyl bromide yields ratios of 0-2 to 0-3 products of 74 8, 41 15, and 46 19, in neat benzyl bromide,110 in toluene containing tetrabutylammonium iodide,91 and in DMF,91 respectively, but 25 52 in DMF containing cesium fluoride.91 Similar reversals are obtained in allylation reactions on the same substrate.91109... 

An alternative activation of the anomeric hydroxyl makes use of 1,2-0-dibutylstannylene acetals [382]. Thus, for instance condensation of the stannylene acetal of 3,4,6-tribenzylman-nose with methyl iodide, allyl or benzyl bromide afforded the corresponding /3-mannosides in almost quantitative yields (O Scheme 66). 

A study on the selectivity of stannylene acetal-mediated alkylation of methyl 4,6-0-benzylidene-a-D-glucopyranoside has concluded that increased bulk of the alkyl groups on tin and a non-polar or no co-solvent increase the proportion of 0-2 mono-ethers formed.The tin-mediated allylation and benzylation of 1,2-0-... 

The use of stannylene acetals and stannyl ethers is widely used for achieving selective benzylation, the selectivity paralleling that observed for allylation (see section 2.3.2, Allyl ethers). 

Achieving selective acetylation by standard methods tends to be unreliable and reagent specific (as exemplified in Figure 2.34) [49], however, reliable selectivity can be achieved via the prior preparation of stannylene acetal intermediates (see section 2.3.2, Allyl ethers) or, complementarily, via orthoesters derived from 1,2-cis-dio pairings (Figure 2.35) [50]. 

Stannylene-mediated glycosylation was first applied to alkyl mannosylations. Condensation of the stannylene acetal of 3,4,6-tri-O-benzyImannopyranose (44) with methyl iodide and allyl bromide in DMF afforded the desired methyl and allyl P-mannopyranosides, respectively, in almost quantitative yields [20]. Benzyl p-mannopyranoside was obtained from the same stannylene acetal by use of BU4NI in benzene [21]. The electrophilic leaving group is responsible for the reactivity and stereoselectivity. Methylation with methyl tosylate and dimethyl sulfate gave methyl mannopyranosides as anomeric mixtures at temperatures above 75 °C [20]. 

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