Dibutylstannylene complexes

Kovac and Hodosi have applied stannylene chemistry for the synthesis of p-mannosides [50]. The use of l,2-(9-dj-stannylene acetals, first developed by 

Treatment of the less reactive secondary galacto triflate 45 with stannylene 39 also resulted in p-mannoside formation. Once again yields with DMF as a solvent were low and the long reaction time led to unwanted decomposition of starting materials. Optimum yields were obtained by changing the solvent to DMSO, which resulted in the stereoselective formation of p-mannoside 46 in a very good yield of 59% (Method 4) [52]. 

Notes and discussion. In this method a c/i-l,2-stannylene acetal, derived from mannose, nucleophilically displaces a triflate from the C-4 position of a galactopyranoside, with inversion of stereochemistry. The example reported below allows entry to a Man-(3-(l,4)-Glu derivative. It has been found that the reactivity of the electrophile and the nucleophilicity of the oxygens within the cis-1,2-stannylene acetal greatly affect the overall outcome of the reaction. The nucleophilicity of the oxygens within the stannylene complex decreases in the order equatorial anomeric equatorial non-anomeric axial anomeric. Moreover, the yield and ratio of products are found to be highly solvent dependent as the solvent polarity increases, the reactivity of the reactants and the yield of the desired Sn2 reactions increase. Thus when the reaction described herein was performed at 25 °C, in DMF, for 5 days, the disaccharide 46 formed in 40%, when it was performed at 25 °C in DMA for 3 days, 53% of the required disaccharide formed, but best yields and selectivities (59%) were obtained by performing the reaction at 25 °C, in DMSO, for 2 days. 

Round bottomed flask (1 X 50 ml) with rubber sepmm and magnetic stirrer bar 

Experimental procedure. Safety glasses should be worn during the experiment and all manipulations should be performed in a fume cupboard. 

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Srivastava, V. K. Schuerch, C., Synthesis of Beta-Deuterium Mannopyranosides and Regioselective Ortho-Alkylationof Dibutylstannylene Complexes. Tetrahedron Lett. 1979, 3269-3272. 

Stereoselective formation of the glycosidic bond can be achieved by this procedure. The dibutylstannylene complex of 3,4,6-tri-O-benzyl-D-mannose was converted into benzyl 3,4,6-tri-0-benzyl-f3-D-mannopyranoside, useful intermediate for the synthesis of 2-deoxy-2-[18F]fluoro-D-glucose [138]. 

Other Alkyl and Aryl Ethers.- 3 -0 -Allyl ethers of methyl 3-lacto-slde and two N-acylated 2-amlno-2-deoxy-lactose B-glycosldes were obtained by reaction of the dibutylstannylene complexes of these... 

Epimerization at C-2 sometimes occurs during dibutylstannylene complex formation. In particular, 1,2-cis P-per-O-acylation for formation of the stannylene acetals of free mannose, rhamnose, and lyxose is accompanied by epimerization [22]. Scheme 16 shows the epimerization process schematically. 

If the five-membered dibutylstannylene ring spans two equatorial positions, the outcome of the reaction is more complex. Benzyl 4,6-<9-benzylidene-(3-D-galacto-... 

Five-membered cyclic dibutylstannylene acetals formed on vicinal cw-axial-equatorial pairs of hydroxyl groups selectively enhance the nucleophilicity of the equatorial oxygen in 0-alkylation reactions [179, 180], On the basis of this rule Schuerch surmised that alkylation of the tin complex 103, having the anomeric oxygen locked in the equatorial position, should lead to P-mannosides [181]. This assumption was proved by treating the mannose diol 102 with Bu2SnO (—> 103) followed by in situ exposure to alkyl halides whereby P-mannopyranosides 104 were formed stereospecifically (Scheme 30) [181],... 

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