Substituted Sugars

Note 2. To maintain integrity of carbohydrate names, it is sometimes helpful to overstep the strict order of principal group preference specified in general organic nomenclature [13,14]. For example, a carboxymethyl-substituted sugar can be named as such, rather than as an acetic acid derivative (see 2-Carb-31.2). 

Fused cyclic ethers can be derived from appropriately substituted sugars. An example is given with the stereoselective 5-exo radical cyclization of allylic... 

More successful applications of periodate oxidation to substituted sugars were made employing the sugar phosphates and phosphoric acids,63 66 the chloraloses, 62 70 the benzyl ethers of sugars,71 72 a dimethyleneoxy sugar,73... 

However, most asymmetric 1,3-dipolar cycloaddition reactions of nitrile oxides with alkenes are carried out without Lewis acids as catalysts using either chiral alkenes or chiral auxiliary compounds (with achiral alkenes). Diverse chiral alkenes are in use, such as camphor-derived chiral N-acryloylhydrazide (195), C2-symmetric l,3-diacryloyl-2,2-dimethyl-4,5-diphenylimidazolidine, chiral 3-acryloyl-2,2-dimethyl-4-phenyloxazolidine (196, 197), sugar-based ethenyl ethers (198), acrylic esters (199, 200), C-bonded vinyl-substituted sugar (201), chirally modified vinylboronic ester derived from D-( + )-mannitol (202), (l/ )-menthyl vinyl ether (203), chiral derivatives of vinylacetic acid (204), ( )-l-ethoxy-3-fluoroalkyl-3-hydroxy-4-(4-methylphenylsulfinyl)but-1 -enes (205), enantiopure Y-oxygenated-a,P-unsaturated phenyl sulfones (206), chiral (a-oxyallyl)silanes (207), and (S )-but-3-ene-1,2-diol derivatives (208). As a chiral auxiliary, diisopropyl (i ,i )-tartrate (209, 210) has been very popular. 

Fused cyclic ethers can be derived from appropriately substituted sugars. An example is given with the stereoselective 5-exo radical cyclization of allylic 2-bromo-2-deoxysugars, in the presence of 1,1,2,2-tetraphenyldisilane as the radical mediator and AIBN in refluxing ethyl acetate. The corresponding cis-fused bicyclic sugars have been prepared in moderate to good yields (Reaction 7.28) [39]. 

More complex examples exist where the 2-substituent on the furan is a polyhydroxyalkyl or glycosyl moiety <89CAR(l9l)209>. In these cases, the rearrangement products provide useful intermediates for the preparation of heterocycle-substituted sugar derivatives. Sometimes Baeyer-Vil-liger-type products are observed, such as 3-keto enoic esters instead of 3-keto enol esters <81TL1443>. 

The aqueous methanolysis of 3,4-dihydro-27f-pyrano[3,2-(r]pyridinium a-D-A -acetylneuraminoate has been studied and is believed to proceed via dissociative transition states with no intramolecular nucleophilic participation by the anomeric carboxylate group <2004JP0478>. The synthesis of the starting substituted sugar is also reported. 

Two substituted sugars, namely 2,3-0-isopropylidene-D-lyxofuranose,8to and 2,3,5-tri-O-methyl-D-lyxofuranose,l0ab undergo polymerization during slow distillation under diminished pressure the latter formed a nonreducing dimer of the trehalose type. 

Many more examples of the relation between sugar structure and taste could be given. Birch et al (10) have studied a number of free sugars, sugar derivatives, and substituted sugars. 

The application of Gabriel-Coleman synthesis to the synthesis of biopterin (30) is reliable, however, it has a serious problem in that the condensation of the pyrimidine precursor with asymmetrically substituted sugar derivatives is sometimes less regioselective or even nonregioselective. For example,... 

---