Stannylene ether, formation

In favourable cases, stannylene formation can be conducted in refluxing methanol as solvent with no special provision being made for the removal of water. In the example shown in Scheme 4.164, the crude stannylene derivative obtained on removal of the methanol was treated with benzyl bromide and cesium fluoride as the activator in DMF at room temperature to give the benzyl ether in 81% yield.307... 

Acid-catalysed alkylation of an alcohol with O-alkyl trichloroacetimidate prepared from allyl alcohol and trichloroacetonitrile is readily accomplished Scheme 4.233]440 as previously discussed for the preparation of benzyl and tert-butyl ethers.311 However, these conditions are not compatible with many of the protecting groups employed in oligosaccharide synthesis. For such cases, two methods for 0-allylation under essentially neutral conditions have been devised. The first method takes advantage of the mild conditions and regioselectivity of stannylene alkylations (see section 4.3.3). The method is illustrated by the selective O-allylation of o-lactal, which began with stannylene formation on an 0.8 mole scale [Scheme 4.234].441... 

Glycosylation of 3,4,6-tri-O-benzylglucopyranose (42) via stannylene acetal with methyl iodide resulted in the production of the 2-0-methyl ether (70%) and a-methyl glycoside (30%) [20], presumably as a result of the formation of stannylene acetal on axial 0-1 and equatorial 0-2, The equatorial 0-2 is more reactive during stannylene complexation, which leads the 2-0-methyl ether. Thus stannylene-mediated alkylation of 6-0-tritylmannose and methyl a-mannopyranoside afforded 3-0-alkylated products because of the greater reactivity of the equatorial oxygen at the 3-position [20]. The reaction mechanism is discussed in detail in Section 8.3.4. 

Isomerization of the stannylene acetal from 1,2-0 to 2,3-0 and the resultant equatorial 3-0 activation rationalize the formation of the /t.vctA/o-disaccharide as shown in Scheme 17. The epimerization of the stannylene acetal seems much faster than alkylation. The use of the analogous 3-0-benzyl ether effectively prevented the unfavorable coupling. Thus, generation of the etheric by-product was suppressed in the reaction with the same primary triflate (57). 

Regioselective Alkylation, Acylation, and Sulfonyla-tion. Synthetic applications of stannylenes have followed the elegant studies of Moffatt and Ogawa, who showed that the inherent differences in the nucleophilicities of carbohydrate hydroxys can be amplified by the formation of trialkyltin ethers. Several selective acylations and alkylations could thus be accomplished. Further it was noted that while acylation proceeds... 

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