Dibutylstannylene acetals terminal diols

The reactions of tributylstannyl ethers and dibutylstannylene acetals of diols or polyols containing one hydroxyl group that is at the end of a chain (terminal diols) are listed in Table IX. Some reactions on compounds of this type, primarily unprotected glycosides, have been discussed earlier, but for those examples, the terminal hydroxyl group was not critical in... 

It has been observed that reactions of dibutylstannylene acetals of terminal 1,3-diols, or polyols having primary hydroxyl groups free, with t-butyldimethylsilyl chloride yield primary silyl ethers, even for such compounds as methyl a-D-mannopyranoside, where the expectation (see Table VI) for most reactions is for substitution to occur on a secondary oxygen atom.85 88 These observations probably arise from a very large kinetic preference for reaction with primary oxygen atoms, that is, ki2 and k]2 > k2 and K22, in Fig. 12, rather than a rearrangement from initially formed secondary silyl ethers. 

Reactions of Dibutylstannylene Acetals and Tributyltin Ethers of Terminal 1,2- or 1,3-Diols... 

From a synthetic point of view, the most important observation is that dibutylstannylene acetals and tributylstannyl ethers can be used to effect terminal substitution of diols in excellent yields, often better than can be obtained by direct reaction at low temperatures, even for benzoylation or p-toluenesulfonylation, where direct reaction is reasonably effective. Terminal O-alkylation, which cannot be performed directly, is routine through choice of the appropriate conditions, as outlined in the sections following. These types of reactions are considered first, followed by reactions where the nonterminal oxygen atom is favored. 

Both tributylstannyl ethers and dibutylstannylene acetals of terminal triols yield in most cases the product of reaction with 1,2-diols in preference to other hydroxyl groups, as shown in Figs. 47 and 48. Figure 47 also illustrates the tendency of t-butylchlorodimethylsilane to react with terminal 1,3-diols in preference to terminal 1,2-diols.87 Figure 48 shows that the preference for reaction at terminal oxygen atoms is considerably stronger than the preference for reaction next to unsubstituted centers, discussed in the previous section. As the numerous examples in Table IX demonstrate, this selectivity is maintained over a wide range of structural features. 

The two sets of conditions most employed for benzylation of dibutylstannylene acetals are benzyl bromide with cesium fluoride in DMF at room temperature and benzyl bromide with tetrabutylammonium iodide or bromide in toluene or benzene at elevated temperatures. Although there are no examples with careful analysis of the product mixtures where the same substrate has been allowed to react under both sets of conditions with terminal 1,2-diols, examination of Table IX suggests that the latter condi-... 

Dibutylstannylene acetals and tributylstannyl ethers of terminal 1,3-diols also react preferentially with most electrophiles on the terminal oxygen atom. The terminal 1,3-diol can be acyclic, or the secondary oxygen atom can be on a ring adjacent to an hydroxymethyl group, such as 0-4 and O-6 of aldohexopyranoses or 0-3 and 0-5 of aldopentofuranoses. Figures 50 to 53 show some examples. 

Fig. 50.—Reactions of the dibutylstannylene acetal of an acyclic terminal 1,3-diol.12...

Fig. 52.—Benzylation of the dibutylstannylene acetal of a terminal 1,3-diol from a /3-d-gulopyranoside derivative.318...

Fig. 53.—p-Toluenesulfonylation of the dibutylstannylene acetal of the terminal 1,3-diol in phenyl 2,3-dideoxy-l-thio-a-D-ffireo-2-enopyranoside.10... 

As noted above, the prodncts of acylation reactions rearrange in reactions of stannylene acetals derived from terminal 1,2-diols, bnt do not rearrange at room temperatnre for those derived from trans-diols on pyranose rings. In the absence of added nucleophiles, the dibutylstannylene acetal of methyl 4,6-0-benzylidene-Q -D-mannopyranoside yields an 85 15 ratio of the axial benzoate over the eqnatorial benzoate, only consistent with kinetic acylation on the dicoordinate oxygen atom. °... 

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