5- vinyl pyranoses

In this regard, zirconium-butene complex ( Cp2Zr ) [74] was widely used to promote the carbocyclization of vinyl glycosides [75], Thus, sequential treatment of 5-vinyl pyranose 91 with Cp2Zr-(n-Bu)2 (prepared in situ fromzirconocene dichloride and 2 equivalents of butyl lithium) and boron trifluoride etherate in THF afforded vinyl cyclopentanol 92 as the major product in a 75% yield (Scheme 3.36) [76]. 

The reaction was extended to other 5-vinyl pyranoses and the major product has always a cM-configuration between the newly formed stereocenters and a trani-configuration between the vinyl and the adjacent substituent. Considering these results, the proposed mechanism for the CpZr -mediated carbocyclization is depicted in Scheme 3.37. Ligand exchange with the vinyl followed by reductive elimination of the alkoxy group produces aZ-allylic zirconacycle, which immediately cyclizes to the cyclopentane 92 (Scheme 3.37). 

As an example of the use of this methodology in total synthesis, the zirconocene-promoted ring contraction was the key step in the route to (+)-epiafricanol (100) reported by Paquette et al. [78]. Vinyl pyranose 95, obtained from D-glucose in 12 steps, was treated with Cp2Zr in toluene at -78°C and then with boron trifluoride etherate at room temperature, affording vinyl cyclopentane 96 in a 63% yield. Conversion of 96 to ketone 97 with periodinane as the oxidant in the presence of sodium bicarbonate and addition of the lithium derivative to 4,4-dimethyl-5-iodo-l-pentene 98 gave rise to 99. Cyclization of 99 by RCM followed by cyclopropanation finally afforded the desired product 100 (Scheme 3.39). 

Hydrogenation of exocyclic, pyranoid vinyl ethers could afford a mixture of both possible 6-deoxy-D and L-hexoses. Our observations show that the proportion of each isomer is dependent upon the catalyst and the substituents on the vinyl ether. Thus, treatment of a methanol solution of l,2,3,4-tetra-0-acetyl-6-deoxy-/ -D-xylo-hex-5-eno-pyranose (12) with hydrogen in the presence of a palladium catalyst afforded a mixture which was shown by gas chromatography to contain 96% of the 6-deoxy-D-gluco isomer (11) and 4% of the 6-deoxy-L-ido isomer (13). In this... 

This enzyme oxidizes a- and P-anomers of D-glucose to the same extent and shows excellent stability and sensitivity about twice that for the methods with immobilized glucose oxidase. In this approach, pyranose oxidase is immobilized on tresylate-poly(vinylalcohol) beads and packed into a stainless column and peroxidase is immobilized on tresylate-hydrophylic vinyl polymer beads and packed into a transparent FITE tube that is used as the CL flow cell. The H202... 

Starch thiols can have the SH group directly on the pyranose ring or in a side chain. There are several methods of synthesizing starch thiols of the first type. One of them is based on the pyrolysis of starch xanthates (32), but the reaction proceeds in two parallel routes one producing thionates (35) and the other producing thiols (37).2675,2677 Reduction of starch xanthates with NaBPU in alkali is another approach to thiols. Thiols prepared in this manner were subjected to graft polymerization with vinyl polymers. Nucleophilic substitution of the chlorine... 

The functionalization of an exocyclic carbon atom of the pyranose or furanose ring using vinyl sulfone-modified carbohydrate has not been explored in spite of the efficient use of vinyl sulfone-modified pyranose derivative as a Michael acceptor for the synthesis of maytansinol [1]. 

The Michael-type addition reaction of carbon nucleophiles to vinyl sulfone-modified carbohydrates should be considered as an efficient route for the synthesis of branched-chain sugars because almost all carbohydrates in pyranose and furanose form could be converted to their vinyl sulfone derivatives very easily [5, 12-14]. Moreover, the product of the reaction carrying sulfone functionality has the potential to undergo a wide variety of transformations [15]. For a review on desulfonylation reaction, see Ref. [16]. 

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