Xylose derivatives

The 1,4-addition of ZnEt2 to 2-cyclohexenone was also performed by these workers in the presence of thioether-phosphite D-xylose-derived ligands, depicted in Scheme 2.11. In all cases, the chemoselectivities in the 1,4-product were higher than 97% but the enantioselectivities were modest (<41% ee). It was noted that changing the substituent in the thioether moiety produced an elfect on both the reactivity and the enantioselectivity, as shown in Scheme 2.11. 

Scheme 10.45 Rh-catalysed hydrosilylations of ketones with D-xylose-derived thioether-phosphinite ligand.

C. Hadad, J.-P. Majoral, J. Muzart, A.-M. Caminade, and S. Bouquillon, First phosphorus D-xylose-derived glycodendrimers, Tetrahedron Lett., 50 (2009) 1902-1905. 

Alternatively, pyranosyl radicals can be generated through the reduction of 3,4,6-tri-O-benzyl glucal epoxide with Cp2TiCl2 and manganese metal.136 With the conformationally restricted 1-phenylseleno-D-xylose derivatives 151 and 152 (4Ci conformation) their reaction with Bu3SnCH2 CH=CH2 in the presence of AIBN (Scheme 51) affords the corresponding a-C-pyranosyl derivatives (153) preferentially.137... 

Pursuing the synthesis of angelmicin B, Mootoo prepared the tricyclic intermediate fragment 93 from D-xylose derivative 90 (Fig. 31).46... 

In an attempt to synthesize 2-desoxy-L-ribose, Mukherjee and Todd68 treated methyl 2,3-anhydro-jS-L-ribopyranoside (LXXII) with sodium thiomethoxide and then reduced the amorphous product. The sulfur-free sirup thus obtained was inert to the action of periodate and is therefore methyl 3-desoxy-/3-L-riboside (ayn., methyl 3-desoxy-/3-L-xyloside) (LXXIV) rather than the desired 2-desoxy derivative. The methylthio intermediate (LXXIII) was consequently a 3-methylthio-L-xylose derivative rather than a 2-methylthio-L-arabinose compound. The presence of a trace of the latter, however, is not wholly excluded since the sirupy desoxypentoside (LXXIV) gave a feeble green coloration in the Keller-Kiliani test, a reaction generally accepted as being specific for 2-desoxy-sugars. 

We have observed intramolecular reactions of septanose species under a variety of reaction conditions using a number of starting materials. Figure 10 shows the putative reactive intermediates 190-193 that were present during iodoglycosidation of the D-xylose-derived oxepine 143, epoxidation of the oxepine 142, and TMS-triflate activation of septanosyl fluorides 72 and 75, respectively. Products 194-197 were obtained in a range of yields depending on whether or not other nucleophiles were present in the reaction. All of the intermediates are electrophilic and therefore... 

D-xylose derivative has been reported to be an excellent chiral auxiliary in the Staudinger reaction for the construction of (3-lactams (Scheme 13), [61]. 

Scheme 13 Asymmetric synthesis of P-lactams by chiral auxiliary based on D-xylose derivative...

Further support for the hypothesis of conversion of D-glucose to D-xylose derivatives by way of derivatives of D-glucuronic acid comes from the observations that D-glucurono-1,4-lactone-l-C14 is equally as effective as D-glucose as a xylan precursor and that D-xylose may be isolated with 88-89% of its isotopic carbon at171 172 Cl. 

Despite the obvious possibilities for disengaging the allylic methyl group of 7 by retro-S reaction, Kim preferred to disjoin this molecule by a (Z)-selective Wittig reaction to obtain aldehyde 9 and ylide 8. Since further analysis of 9 also indicated that it could potentially be reached from the readily available 5-deoxy-D-xylose derivative 10, this was the strategy eventually settled upon. 

The high stereoselectivity observed in this cyclization reaction was attributed to the location of the carbonyl moiety in a rigid cyclic ring framework, allowing H-abstraction from the same face of the initially formed five-membered ring. The same group has further utilized the constraint imparted by the benzylidene acetal moiety on the excited state photoreactivity of the carbonyl moiety of the D-xylose derivative 52. Thus, 8-H-abstraction results in the formation of a diastereomeric mixture of cyclized product 53, which is further transformed into D-glucopyranose 54 and D-idopyranose 55 derivatives, as shown in Scheme 8.16 [17]. 

Py used a nitrone-alkene coupling in a short, asymmetric synthesis of (+ )-hyacinthacine A2 (Scheme 5.43).74 The key carbon-carbon bond-forming step in their approach involved the highly diastereoselective reductive coupling of the L-xylose-derived cyclic nitrone 66 to ethyl acrylate.74... 

In structural studies, flaxseed polysaccharides were separated into neutral and acidic fractions by use of cetyldimethylammonium bromide. Data from methylation analysis66 showed that the first fraction yielded only methylated D-xylose derivatives, with a high amount of 2,3,4-tri-O-methyl-D-xylose... 

Xylose derivatives l,2-0-Isopropylidene-5-0-(methoxy- carbonyl)-D-xylofuranose 135-136 135-136.6 -13 -15.8 / CHiOH F, C 12... 

These taxol-type compounds have already been described by Suffness and Cordell (3). The various substances extracted from the bark of T. baccata, T. watlichiana, or T. brevifolia differ only in the nature of the substituents R, Rj, and Rji R, may be a free hydroxyl group or a xylose derivative (/4), R2 a hydroxyl, acetyl (taxol), or hydroxybutyrate group (74), and R3 a phenyl (taxol) (2), tiglate (cephalomannine) 28), or hexanoate group (74). 

Enholm [12] has also prepared an enantiomerically pure soluble polymer support 82 by couphng xylose-derived chiral auxiliary 81 with 77 (Scheme 18). The chiral support was then treated with bromopropionic acid 83 to give substrate 84. Eree radical allyl transfer from allyltributyltin imder thermal conditions provided 85 in 93% yield, and basic cleavage from the resin gave (R)-(-)-2-methylpent-4-enoic acid 86 in 80% yield and 97% ee, with a 92% yield of recovered 82. Previous studies of the same process in solution had found the addition of Lewis acids to be crucial for high selectivities to be obtained. Interestingly, the addition of Lewis acids to the reaction on polymer support led to cleavage of the carbohydrate from the polymer backbone. En-... 

Claeyssens, M, De Bmyne, C K, D-Xylose-derivatives with sulfur or nitrogen in the ring powerful inhibitors of glycosidase-activities, Naturwissenschaften, 52, 515, 1965. 

Enholm et al. [172] also examined free radical allylations of carbohydrate linked bromo esters. The D-xylose-derived ester 267 was reacted with allytributyltin and ZnCl2 as a Lewis acid to give the ester 268 (Scheme 10.87). The diastereoselectivity and yield of the allylation reaction were high. 

Koell, P, Luetzen, A, D-Xylose derived oxazolidin-2-ones as chiral auxiliaries in stereoselective alkylations. Tetrahedron Asymmetry, 7, 637-640, 1996. 

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