Stannyl ethers 0-benzylation

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). 

The benzylidene derivative (94) has been converted into the alcohol (95) [97, 98] using the diborane — trimethylamine — aluminium chloride reagent [99] and into the diol (96) [95, 100, 101]. Veyrieres has converted methyl P-lactoside (and the corresponding allyl lactoside) into the 3-O-allyl ether (97) in good yield [102] by alkylation of the dibutylstannylene derivative in the presence of tetrabutylammonium iodide [103, 104], and this was converted into the alcohol (98) and the triol (99) [105]. Veyrieres [106] has also converted (97) into the per-p-bromobenzyl derivative and deallyl-ated the product to give a derivative with a free 3 -hydroxyl group. The diol (100) [107] has been converted by the stannylation procedure [108, 109] into the alcohol (101) [110, 111]. The partially acetylated benzyl P-lactoside (103) [101,112, 113] has been converted into the alcohol (104) via the orthoacetate [113]. 

In contrast to the reaction of y-stannyl alcohol 50, the y-stannyl benzyl ether 53 results in selective cleavage of the butyl-tin bond by reaction with (PhIO)n 18/DCC/BF3 and, after quenching of the reaction mixture with aqueous NH4C1, afforded the chlorostannane in high yield. Interestingly, the chlorostannane both in solution and in the solid state adopts a 1,3-diaxial conformation through Sn-O hypervalent interaction [82]. 

The effect of solvent was also studied and complexing solvents such as THF or Et20 inhibited the cyclopropanation reaction. Furthermore, the presence of an unprotected allylic alcohol was found to be essential, since the methyl or benzyl ether derived from cinnamyl alcohol afforded almost racemic cyclopropanes. This method has also been extended to the enantioselective cyclopropanation of vinylsilanes and -stannanes (Scheme 4) [13]. The corresponding optically active silyl- and stannyl-substituted cyclopropyhnethanols were obtained in the presence of the chiral N,iV-bis(p-nitrobenzenesulfonyl)-l,2-cyclohexane-diamine 9. 

The choice of conditions for alkylation (or any protection method) is dictated by the existing functionality within the carbohydrate. Where there are alternatives within a category, the specific choice is usually made on the basis of previous experience or if the method offers a desired level of selectivity for protection of a particular site. For example, the alkylation conditions within category (a) above [with the exception of (vi)] and that under (c) lead to more or less equivalent levels of selectivity (summarised in section 2.2, Hydroxyl group reactivity) and will not be discussed further. However alkylations and acylations proceeding via 0-stannyl intermediates (b) and via copper chelates [(a)(vi)] follow set patterns that may differ from these general trends. These alternatives are discussed within sections 23.1, Benzyl ethers and 2.3.2, Allyl ethers. 

Other Alkyl and Aryl Ethers.- Two references to benzylation and allylation ire mentioned in the section on methyl ethers. Partial stannylation of methyl a-D-glucopyranoslde with trlbutyltin oxide,... 

Paul E. Floreancig of the University of Pittsburgh showed (Angew. Chem. Int. Ed. 2008, 47, 4184) that Prins cyclization could be also be initiated by oxidation of the benzyl ether 19 to the corresponding carbocation. Chan-Mo Yu of Sungkyunkwan University developed Organic Lett. 2008,10,265) a stereocontrolled route to seven-membered ring ethers, by Pd-mediated stannylation of allenes such as 21, followed by condensation with an aldehyde. 

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