Stannylene acetals

Bu2SnO, benzene BnBr, DMF, heat, 80% yield. This method has also been used to protect selectively the anomeric hydroxyl in a carbohydrate derivative. The replacement of Bu2SnO with Bu2Sn(OMe)2 improves the process procedurally. The use of stannylene acetals for the regioselective manipulation of hydroxyl groups has been reviewed. ... 

Monoalkylation of a vic-glycol.1 Selective monoalkylation or monoacylation of the vic-glycol group of dimethyl L-tartrate is possible by conversion to the O-stannylene acetal (1) by reaction with dibutyltin oxide. The acetal is converted selectively to a mono derivative (3) by reaction with an alkyl halide or acyl chloride (excess) and CsF (about 2 equiv.). KF or Bu4NF are less effective than CsF. 

Srivastava and Schuerch [255] and Desinges et al. [256] demonstrated the potential utility of glycosyl 1,2-O-stannylene acetals, and Hodosi and Kovac [257] established this method involving alkylation of a 1,2-O-stannylene acetal with a triflate derivative as an aglycon (Scheme 5.97). This glycosyl donor was prepared by the... 

O-stannylene acetals,84 glycals and 1,2-anhydrosugars,85 and to selective de-O-benzylation of position-2 with TIBAL, DIBAL-H57 or Lewis acid catalysts,86 or to the one-pot access to 3-O-benzyl-4,6-0-benzylidene glucosides by tandem catalysis recently reported.87 The 1,2-lactones recently reported by Linker and co-workers35 are also synthons which provide... 

Hodosi and Kovac reported the highly efficient P-mannosylation, which makes use of mannose-derived 1,2-O-stannylene acetal 15 in combination with an aglycon-derived triflate (Scheme 7.13).58 A remarkable feature of this method is its extreme simplicity. Even totally unprotected mannose can be used as precursor of 15. Undoubtedly, this method can be seen as one of the most facile methods for the stereoselective synthesis of P-mannosides. 

Scheme 7.13 (3-Mannosylation using mannose-derived 1,2-O-stannylene acetal in combination with an aglycon-derived triflate.

Hodosi, G. Kovac, P., Manipulation of free carbohydrates via stannylene acetals. Preparation of beta-per-O-acyl derivatives of D-mannose, L-rhamnose, 6-0-trityl-D-talose, andD-lyxose. Carbohydr. Res. 1997,303, 239-243. 

Selectivity can be an overriding commodity in cases where reactivity is dictated by logic and accepted concepts. Such is the case with stannylene acetals of diols and trialkyl-stannyl ethers of alcohols. Enhanced nucleophilicity of oxygen attached to tin and well-documented stereoelectrorric effects associated with methine carbon atoms of trialkyhin ethers lead to remarkably selective reactions of (7-substitution and oxidation in polyhydroxy compounds. 

Selective O-Substitution and Oxidation Using Stannylene Acetals and Stannyl Ethers... 

Organotin complex 59 has been used in the kinetic resolution of 1,2-diols [55]. This catalyst is postulated to react with the diol to form a stannylene acetal that is subsequently benzoylated. Utilization of Na2C03 as a solid base and addition of controlled amounts of water allow for resolution of certain diols with modest to good selectivities (/crc[=3.2-22.4, when enantiomeric excesses are corrected for the ee of catalyst, see Scheme 14) [56]. 

X. Kong and T. B. Grindley, An improved method for the regioselective oxidation of stannylene acetals and dimerization of the a-hydroxyketone products, J. Carbohydr. Chem., 12 (1993) 557-571. 

The use of performed dibutyltin dimethoxide is advantageous12-14 reactions of diols are complete in 5-15 min in benzene at reflux. This technique avoids the problems with working in methanol just mentioned, and gives better yields. The preparation of stannylene acetals is also accelerated by microwave irradiation,15 and selective reactions can be performed, albeit in low yields, on some diols and polyols by microwave irradiation in the presence of a catalytic amount of dibutyltin oxide.16,17... 

The third important aspect of these structures is related to the geometries of the dimers or terminal units of oligomers of stannylene acetals (Figs. 5, 6a, and 6b).40,59 60 The tin atoms adopt a distorted trigonal bipyramidal geometry with the alkyl groups in equatorial orientations. The tricoordinate... 

Only when a considerable number of large substituents are present do stannylene acetals exist to any extent as the monomer 2,2-dibutyl-4,4,5,5-tetraethyl-l,3,2-dioxastannolane is present almost entirely as the dimer at room temperature in chloroform at 20°C, but the monomer-dimer equilibrium gradually shifts to favor monomer above about 80°C. 2,2-Di-r-butyl-4,4,5,5-tetramethyl-l,3,2-dioxastannolane is a monomer at room temperature.41... 

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... 

Simple 2,2-dibutyl-l,3,2-dioxastannolanes form solid complexes of monomer units with certain nucleophiles, such as pyridine and dimethyl sulfoxide, that have 1 1 stoichiometry and pentacoordinate tin atoms.62 Such complexes are less stable for more-substituted stannylene acetals, such as those derived from carbohydrates.62 Unfortunately, the precise structures of these complexes have not yet been defined. Addition of nucleophiles to solutions of stannylene acetals in nonpolar solvents has been found to markedly increase the rates of reaction with electrophiles,63 and transient complexes of this type are likely intermediates. Similar rate enhancements were observed in reactions of tributylstannyl ethers.57 Tetrabu-tylammonium iodide was the nucleophile used first,57 but a wide variety of nucleophiles has been used subsequently tetraalkylammonium halides, jV-methylimidazole,18 and cesium fluoride64,65 have been used the most. Such nucleophilic solvents as N,N-dimethylformamide and ethers probably also act as added nucleophiles. As well as increasing the rates of reaction, in certain cases the added nucleophiles reverse the regioselectivity from that observed in nonpolar solvents.18,19... 

Alkylation requires more vigorous conditions. These reactions were originally performed on the stannylene acetal with the alkylating reagent in DMF at elevated temperatures (45°C for methyl iodide or 100°C for benzyl bromide)66 or on the tributylstannyl ether in neat benzyl bromide or allyl bromide at 80-90°C.67 It was then discovered that the presence of added nucleophiles markedly accelerates the reactions, so that alkylation of both tributylstannyl ethers and dibutylstannylene acetals in benzene, which is very slow at reflux with benzyl bromide alone, occurs at a reasonable speed at reflux in the presence of added tetrabutylammonium halides.57,63 Many other nucleophiles are also effective, including A-methylimidazole68 and... 

Hodosi and Kovac observed that, when free sugars are treated with excess dibutyltin oxide in methanol for extended periods of time at temperatures above 60°C, equilibration of the configuration at C-2 occurs.73 This observation led to the efficient formation of 6-O-trityl-D-talose from 6-0-trityl-D-galactose, but also indicates the need for care in the formation of stannylene acetals from free sugars.73... 

One of the most valuable properties of stannylene acetals derived from 1,2- or 1,3-diols is that in reaction with electrophiles, monosubstituted products are obtained much faster than disubstituted products. This selectivity has been used to advantage many times to break the symmetry present in simple symmetrical diols or polyols, as shown in Table III.2,12,65,69,93 100 Some examples are shown in Figs. 15 to 19 (page 48). In later work, Mash and co-workers have shown that l,n-diols can be converted to mono-O-benzyl derivatives in reasonable yields.101... 

For benzylation, the greatest preference for reaction next to the axial substituent (about 9 1) has been obtained by reaction in benzyl bromide as solvent for both tributylstannyl ethers109 and for dibutylstannylene acetals,110 with still greater preferences using larger alkyl groups in the dialkyl-stannylene acetals, such as hexyl or neopentyl groups.110 Methylation reac-... 

An interesting development is the observation that alkylation of dibutyl-stannylene acetals of this type in the presence of cesium fluoride in DMF gives different regiochemistry than that obtained under all other conditions. For instance, as shown in Fig. 23, the reaction of the dibutylstannylene acetal of methyl 4,6-O-benzylidene-a-D-glucopyranoside with benzyl bromide yields ratios of 0-2 to 0-3 products of 74 8, 41 15, and 46 19, in neat benzyl bromide,110 in toluene containing tetrabutylammonium iodide,91 and in DMF,91 respectively, but 25 52 in DMF containing cesium fluoride.91 Similar reversals are obtained in allylation reactions on the same substrate.91109... 

Selective O-Substitulion and Oxidation Using Stannylene Acetals and Stan-nyl Ethers. David, S. In Preparative Carbohydrate Chemistry, Hanessian, S., Ed. Dekker New York. 1997 p 69. 

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