Ethers from carbohydrates

C1O3/ACOH, 25°, 50% yield, [- ROCOPh (- ROH + PhC02H)]. This method was used to remove benzyl ethers from carbohydrates that contain functional groups sensitive to catalytic hydiogenation or dissolving metals. Esters are stable, but glycosides or acetals are cleaved. 

Considerable developments are taking place in this area as evidenced by the wide range of syntheses reported below. An important review of progress to date Carbohydrate derivatives in the asymmetric synthesis of natural products has been provided by Fraser-Reid and Anderson. A further review covers the preparation of asymmetric crown ethers from carbohydrates. Highly abbreviated Schemes are used in this report because space does not permit the reproduction of the lengthy routes used in many of the syntheses. 

An interesting example from carbohydrate chemistry is the boron trifluoride-diethyl ether complex catalyzed nucleophilic addition of silyl enol ethers to chiral imines (from n-glyceralde-hyde or D-serinal)22. This reaction yields unsaturated y-butyrolactones with predominantly the D-arabino configuration (and almost complete Cram-type erythro selectivity). 

Closely related to the synthetic work reported in the previous section is the incorporation (131) of a 2,5-anhydro-3,4Hdi-0-methyl-D-mannitol residue (Figure 15) into the 18-crown-6 derivative d-91. Other derivatives of D-mannitol that have been built into crown ether receptors include l,4 3,6-dianhydro-D-maiuiitol (132), l,3 4,6-di-0-methylene-D-marmitol (13 134), and 1,3 4,6-di-O-benzylidene-D-mannitol (134). Examples of chiral crown compounds containing these residues include dd-92, dd-93, d-94, and d-95. Although not derived from carbohydrates—but rather (135) from the terpene, (-t-)-pulegone—... 

Protection of hydroxyl groups Benzyl or allyl ethers of carbohydrates are obtained in satisfactory yield by reaction with 1 or 2 in cyclohexane in the presence of triflic add (20°). Trichloroacetamide precipitates and the desired ethers are obtained from the filtrate after neutralization. 

Many unidentified peaks are present in all chromatograms of the various methyl ether and methyl ester fractions. Certain peaks may be attributed to the presence of partially or fully methylated carbohydrates or their derivatives. Based upon predicted retention times, considerations of molecular weight, and polarity of such compounds, and preliminary investigations of model compounds, it seems highly improbable that all of the unidentified compounds are derived from carbohydrates. Hence, identification of other compounds in these chromatograms can be pertinent not only to a more precise description of products formed in the various reactions but also to basic information concerning lignin chemistry. 

Methyl 4-0-(4-methoxybenzyl)2,3-di-0-methyl-oc-D-glucopyranoside has been prepared [371] by Liptak s procedure (see Sect. 2.6). This lithium aluminium hydride — aluminium trichloride method was also used in the synthesis of 4-hydroxy-3-methoxy-benzyl [372], 4-hydroxy-3,5-dimethoxybenzyl [372], and 1-phenylethyl [373] ethers from the 4,6-acetals derived from vanillin, syringealdehyde, and acetophenone. Various vinylbenzyl ethers were prepared by the reaction of carbohydrates with vinylbenzyl chloride, and copolymerized with styrene [374]. 

Pyranones, furanones, and related compounds (6, 7). Structurally these substances are generally cyclic ethers, mainly furanoid compounds. They are found in condensates from carbohydrates that have been subjected to browning reactions. 

J. Marco-Contelles, C. Destabel, P. Gallego, J. L. Chiara, and M. Bemabe, A new synthetic approach to the carbocyclic core of cyclopentane-type glycosidase inhibitors Asymmetric synthesis of amino-cyclopentitols via free radical cycloisomerization of enantiomerically pure alkyne-tethered oxime ethers derived from carbohydrates, J. Org. Chem., 61 (1996) 1354-1362. 

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... 

Saccharin was discovered in 1879 by Remsen and Fahlberg. It is only slightly soluble in cold water, but is soluble in hot water, acetone, alcohol or ether. From acetone it crystallizes in beautiful crystals. It is a valuable medicinal substance as it can be used for its sweetening effect in food, by persons who have the disease known as diabetes and who are unable to use cane sugar, and only a minimum of any carbohydrate food. It does not possess any nutritive value, however. It is also used as a food preservative, but its use is restricted or prohibited by most pure food laws. It is interesting that only the or//fo-sulph-amine benzoic acid yields such a sulphinid anhydride. The para compound, on heating, yields other products. 

Stereoselective Formation of p-Lactams from Carbohydrate Vinyl Ethers (Scheme 10.13, Ref. [38])... 

Spectra of fulvic acids must be interpreted carefully purification of this fraction leads to losses that can reach 50%. Comparison of spectra of different humic fractions of the same sample, such as a sediment from the Oman Sea (Fig. 6) illustrates several differences. Oxygenated functional groups are more important in fulvic and humic acids than in stable residues. Particularly important are the absorption bands at 3400 cm (OH from alcohols, acids, etc.), 1710 cm (C==0 from quinones, ketones, carboxylic acids), 1250 cm (C—O from alcohols, esters, ethers) and 1050 cm (C—O from carbohydrates). Absorption at 1050 cm is nearly absent in stable residues. Aliphatic content increases from fulvic acids to humic acids and stable residues (bands between 2870 and 2960 cm ) and the shape of the aliphatic bands (2900-2950 1450 1375) indicates that fulvic acids contain mainly CH groups. 

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