Dibutylstannylene acetals regioselectivity

Glen, A. Leigh, D. A. Martin, R. P. Smart, J. P. Trascello, A. M., The Regioselective Tert-Butyldimethylsilylation of the 6 -Hydroxyl Group of Lactose Derivatives Via Their Dibutylstannylene Acetals. Carbohydr. Res. 1993,248, 365-369. 

In related work, carbohydrate 1,2-diol dibutylstannylene acetals have been regiospecifically oxidized by IV-bromosuccinimide to give a-hydroxy-ketones. The reaction is illustrated for the preparation of 3-deoxy-l,2-O-isopropylidene-a-D-eryt/zro-hexofuranos-5-ulose (223) in Scheme 24. The endocyclic diol is also regioselectively oxidized to one product (224) in 44% yield, although 55% of the starting compound was recovered. (Scheme 24).439a... 

Dibutylstannylene acetals have yielded propenylidene acetals on reaction with tetrakis(triphenylphosphinepalladium and acrolein diacetate, in some cases with excellent regioselectivity.72 For instance,D-glucal, methyl a-D-glucopyranoside, methyl a-D-mannopyranoside, and methyl /J-d-glucopyranoside yield the 4,6-O-propenylidene acetals in 89, 85, 83, and 80% yields, respectively, as mixtures of diastereomers at the acetal center. Galactose derivatives give mixtures of 3,4- and 4,6-O-propenylidene acetals.72... 

Fig. 22.—Methylation of a dibutylstannylene acetal under conditions that give improved regioselectivity.110...

Fig. 23.—Reversal in the regioselectivity of benzylation of a dibutylstannylene acetal of a trans-d o in the presence of cesium fluoride.91...

Other partially protected carbohydrates also undergo very regioselective oxidation. Noteworthy is the oxidation of isopropylideneglucofuranose 29 to 5-ketofuranose 30 (O Scheme 10) [109]. For oxidation of the axial hydroxy group in cis-1,2 diols, the dibutylstannylene acetal method is often employed. Oxidation of methyl fucoside 31 with this procedure gives ketone 32 in good yield (O Scheme 10) [111]. 

The first reactions using both dibutylstannylene acetals and tributyltin ethers as intermediates were performed in 1974 by Wagner et al., who formed benzoates and p-toluenesulfonates from dibutylstannylene acetals of nucleosides with good, but often opposite, regioselectivity and also formed phosphates from bis(tributyltin) ethers (Scheme 5.1.1). Shortly afterwards, David reacted dibutylstannylene acetals of diols with bromine to give mono-keto products highly selectively. ... 

The above kinetically controlled regioselectivity is also reflected in the selective monosilylation of butane-1,2,4-triol. In this case, the sole product obtained is 4-r-butyldimethylsiloxy-butane-l,2-diol with 99% yield (eq 28). The authors rationalized that, with the bulky TBDMSCl, the dibutylstannylene acetal rapidly migrates between the 1,2-diol and 1,3-diols and affords the product of kinetic control. This sequence of reactions was also used to selectively block the equatorial alcohol of a cw-diol on the pyranoside ring... 

Silyl Ethers. - Regioselective silylation of lactose derivatives using dibutylstannylene acetals has afforded the 6 -0-silyl ethers in contrast to alkylations which give the 3 -0-ethers under the same conditions. Some Tbdms ethers have been cleaved by absorption onto alumina and then irradiation in a microwave oven. The silylation of 2 -deoxynucleosides is discussed in Chapter 20. 

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