Dialkylstannylene acetals

Dialkoxydialkyl stannanes cross-couple with derivatives of chiral tartaric acid, preserving the original chirality, as shown in reaction 32. The dialkylstannylene acetals produced in this reaction are useful reagents for synthesis of chiral compounds28611. 

Typical 119Sn NMR Chemical Shifts for Dialkylstannylene Acetals... 

When the two oxygen atoms involved in the stannylene acetal are diaste-reotopic, three dimers, two with C2 symmetry, can be formed, as shown in Fig. 7 for those obtained from 1,2-propanediol. Dimers are named by means of the numbers of the tricoordinate oxygen atoms.44 Steric effects appear to be the most important factor in determining the relative populations of the three dimers. In particular, stannylene acetals derived from fram-diols with one adjacent axial substituent exist in solution, to the level of detection of 119Sn NMR spectroscopy, as the symmetric dimer in which the tricoordinate oxygen atom is not adjacent to the axial substituent.44 Similarly, dialkylstannylene acetals from carbohydrate-derived terminal 1,2-diols exist predominantly as symmetric dimers with the primary oxygen atoms tricoordinate (Fig. 8).19,37... 

Although the factors that influence the regioselectivities obtained from dialkylstannylene acetals are considerably more complicated than with tributylstannyl ethers, the outcome in most cases is surprisingly similar. The factor influencing regioselectivity in the absence of added nucleophiles will be discussed first. 

If p-toluenesulfonylation occurred under these conditions, the product distribution would depend on [R X]. Changing the concentration of p-toluenesulfonyl chloride from 0.10 M to 1.2 M did not affect the product ratio in a reaction with a dialkylstannylene acetal of methyl 2,3-0-isopropylidene-D-mannofuranoside. This result provides additional evidence that this reaction and all slower reactions occur under conditions where equilibration is faster than reaction. 

Two types of tin derivatives have been used mainly to achieve these reactions dialkylstannylene acetals are formed by reaction of dialkyltin oxides with diols tributyltin ethers are formed by reaction of bis(tributyltin) oxide with alcohols. Some aspects of the regioselectivity achieved with these two types of intermediates are the same, while others are different, as will be outlined in the sections to follow. Where the reaction outcomes are similar, the dialkyltin reagents are strongly preferred because of their lower toxicity. As these reagents are heavily employed, the topic has been reviewed several times, but siguilicant uew observatious continue to be made. This chapter is not intended to be a comprehensive summary of all applicatious of these intermediates, but will highlight examples of each type of selectivity. 

Scheme 5.1.12 The influence of the structure of the alkyl group on the regioselectivity of p-toluenesulfonation reactions on dialkylstannylene acetals of a hindered terminal-1,2-diol" ...

Selective activation of hydroxyl groups by the stannylation procedure is an established practice to efficiently manipulate diols and polyols [53, 54]. Among the various reagents and methods, activation through dialkylstannylene acetals is noteworthy. This two-step protection method involves the conversion of dihydroxy substrates into cyclic dialkyl dioxastan-nanes which, in turn, are treated with suitable nucleophiles, to regioselectively afford the corresponding mono alkyl, silyl or acyl derivatives under neutral conditions. 

Dialkylstannylene acetals which are usually prepared by azeotropic removal of water in the presence of dibutyltin oxide, can be used to selectively protect only one of two secondary hydroxyl groups or a primary, in the presence of a secondary, hydroxyl group. When more than two free hydroxyl groups are present on the same molecule, the tin atom can bridge several different pairs of oxygen atoms. Nevertheless only one derivative is generally obtained [53]. 

Many aspects of the regioselective manipulation of polyols through dialkylstannylene acetals have been studied and some interesting modifications have improved this procedure [53, 55], For example, the regioselective formation of monobenzyl, monoallyl and monomethyl ethers, which normally proceeds at very slow speed, is markedly enhanced when the reaction of benzyl and allyl bromides or methyl iodide on dialkylstannylene derivatives of polyhydroxy compounds is carried out in the presence of stoichiometric amounts of quaternary ammonium halides [37,56,57]. Several examples of this modified procedure, such as the regioselective mono-O-alkylation of disaccharide glycosides (Scheme 2), have been reported [58]. 

Dialkylstannylene acetals derived from a number of carbohydrate diols reacted with diacyl chlorides in the presence of a tertiary amine to give symmetrical, non-glycosidically linked disaccharides, such as compound The tungsten... 

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