Stannylene derivatives

The formal tin counterpart of the dioxolanes formed from vicinal diols by reaction with ketones are stannylenes, formed by reaction with dialkyltin oxides. The commonest reagent is dibutyltin oxide, Bu2SnO, which converts diols to stannylenes when refluxed in an aprotic solvent such as benzene, with continuous distillation out of the water generated. The partly ionic nature of 

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A mixture of the diol (2 mmol) and dibutyltin oxide (545 mg 2.2 mmol) in benzene was refluxed overnight with azeotropic removal of water by a Dean-Stark condenser Evaporation of (he solvent at 100 0 then gave the crude stannylene derivative, which was used without further purification. Btominoiyses were carried out either on solutions of the stauaykne derivatives in benzene (5 mL) in the presence of 4-A molecular sieves (2 g)... 

A mixture of22 (200 mg, 0.33 mmol) and Bu2SnO (90 mg, 1.1 Eq) was refluxed in methanol (10 mL) for 1 h, evaporated, benzene added, and the solution evaporated again to give a syrup. The stannylene derivative was dissolved in 5 mL of dichloromethane, containing... 

S. David, A. Thiefihy, and A. Veyribes, A mild procedure for the icgiospedfic benzylation and allylatian of poly-hydroxy-compounds via their stannylene derivatives in non-poiar solvents, J. Chan. Soc. Perkin Tram. 3 1796 (1981). 

Monoetherification of polyols.12 Monobenzylation and monoallylation of polyols can be conducted conveniently under mild conditions by conversion to the stannylene derivative (dimeric) by di-n-butyltin oxide (5, 189 9, 141). The stannylene is then treated with benzyl bromide or allyl bromide and tetra-n-butylammonium iodide (1 equiv.) in benzene. The same conditions can be used to prepare monomethoxymethyl ethers. Quaternary ammonium bromides are less efficient catalysts than the iodides. These salts also accelerate reaction of stannylenes with acid anhydrides. The mechanism for this activation is not clear it may involve coordination of I" to tin. 

Substitution of a six-membered stannylene derivative involving the primary position always occurs at that position. Various 6-0-benzyl derivatives of d-gluco and d-galacto configuration have been prepared by this method in good yields [54, 135]. Treatment of allyl 2-acetamido-3-0-benzyl-2-deoxy-4,6-0-dibutylstannylene-a-D-galactopyranoside with benzyl bromide in the presence of tetrabutylammonium iodide furnished 75 % of the 6-benzyl ether [141]. 

Five-membered stannylene derivatives of furanosides exhibit almost no regio-selectivity. A 1 1 mixture of 2 - and 3 -0-benzyl derivatives has been obtained from uridine [128]. Similarly, methyl 5-0-benzoyl-P-D-ribofuranoside gave a 2 3 mixture of 2-benzyl and 3-benzyl ethers in 70 % yield [147], a slightly better result (ratio of 1 3) was obtained-for the parent methyl P-D-ribofuranoside [147a], The reversed ratio (4 1) was found for the a-anomer, methyl 2-0-benzyl-oc-D-ribofuranoside being the major product [147a]. 

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... 

Scheme 4.165 Dimeric structure of stannylene derivatives of 1,2-dfols...

There are fire risks associated with the use of sodium hydride and alternative, innocuous bases would be welcome for large scale work One possibility is a mixture of barium oxide and barium hydroxide octahydrate in DMF. In the example shown in Scheme 4 191,355 the reaction is quite slow (7 days) and the yield only 55% but the method deserves further scrutiny. Another metal alkox-ide-based procedure for the regioselective protection of an equatorial alcohol proceeds via the stannylene derivative (see section 4,3.3)356... 

O-AJkylation of stannylene derivatives of 1,2-diols can be used to introduce members of the trityl ether family onto a secondary alcohol — an otherwise difficult task. In the example shown in Scheme 4.212, a very labile bis- 4-meth-oxyphenyl)(phenyl)methyl ether was installed on a Shikimic Acid derivative.400... 

Danishefsky, S J, Hungate, R, Schulte, G, Total s3fnthesis of octosyl acid A. Intramolecular Wilhamson reaction via a cyclic stannylene derivative, J. Am. Chem. Soc., 110, 7434-7440, 1988. 

The tin-mediated strategy was applied to my o- and chiro-inositol. Thus, treatment of myo-inositol with 5 equiv of dibutyltin oxide and allyl bromide in the presence of tetrabutylammonium bromide furnished two major tetraallylated products 119 and 120. Parallel results were obtained when crotyl bromide was used. When 3 equiv of the tin oxide were used, 1,3,4-triallyl ether 121 was a major product (Scheme 3-18).57 Five membered cyclic stannylene derivatives like 123 may be formed predominantly at the 1,2-cis-diol site and then the more reactive alkoxide reacts with an electrophile, as is shown in Scheme 3-18.58... 

X-ray crystallography and the data show that the complex is not a stannylene derivative, rather the py ligand allows the tin atom to achieve a co-ordination number of four and a valence-shell octet of electrons. ... 

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