Carbohydrates, 2,3-anhydro

A simple and efficient method for rearranging 2,3-anhydro carbohydrates to unsaturated sugars under mild conditions has been reported (Scheme 127).191 High stereoselectivity provides a method for the preparation of unsaturated carbohydrate derivatives. 

D-galactose-4-sulfate—3,6-anhydro-D-galactose-2-sulfate (see Carbohydrates). 

Etherification. Carbohydrates are involved in ether formation, both intramoleculady and intermoleculady (1,13). The cycHc ether, 1,4-sorbitan, an 1,4-anhydroalditol, has already been mentioned. 3,6-Anhydro-a-D-galactopyranosyl units are principal monomer units of the carrageenans. Methyl, ethyl, carboxymethyl, hydroxyethyl, and hydroxypropyl ethers of cellulose (qv) are all commercial materials. The principal starch ethers are the hydroxyethyl and hydroxypropylethers (see Cellulose ethers Starch). 

The widespread occurrence of long-range couplings in both furanose and pyranose derivatives explains why so many of the P.M.R. spectra of carbohydrate derivatives are apparently poorly resolved, even when the resolution of the spectrometer is above reproach. For example, the Hi resonance of the 1,6-anhydro-D-glucose derivative (12) is coupled to all of the other six ring protons. A further example of the line-broadening effect follows a consideration of the spectrum of 5,6-dideoxy-5,6-epithio-l,2-0-isopropylidene-/ -L-idofuranose for which the half-height... 

Note. The treatment of anhydro (see 2-Carb-26), dehydro (see 2-Carb-17.3) and deoxy as detachable prefixes follows long-standing practice in carbohydrate chemistry, but is in conflict with [14] (p. 12). 

This section presents selected examples of the use of zeolites and related porous materials for transformations of carbohydrates, that fall beyond the scope of the previous paragraphs. They include the use of zeolites in click chemistry and in a variety of reactions, including the synthesis of anhydro sugars, cyclization, elimination, and addition reactions to the carbonyl group. 

G. V. M. Sharma, K. C. V. Ramanaiah, and K. Krishnudu, Clay montmorillonite in carbohydrates use of claysil as an efficient heterogenous catalyst for the intramolecular Ferrier reactions leading to 1,6-anhydro rare saccharides, Tetrahedron Asymm., 5 (1994) 1905-1908. 

Carbohydrates.—The acid (87), an intermediate in the synthesis of oxaprostaglandin derivatives from D-ribofuranose sugars,84 is obtained from the aldehyde (88) and the sodium salt of (79). The condensation of 2,5-anhydro-D-allose derivatives with (89) gave the expected products (90).85 Similarly, 1,4-furanoses (91) afford (92).86 These unsaturated halides are useful intermediates for further modifications. 

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—... 

The particular wood species we chose for this study is aspen (Populus tremuloides), which is plentiful in Canada and in the northern U.S.A. The chemical composition we found to be glucan 53.4%, xylan 14.9%, total carbohydrate 79.0%, lignin 17.1% and extractives 3.8%. We would expect total fermentable sugars of about 56% in this sample of aspen in anhydro form (Timell has reported about 60% in another sample (15)) which upon hydrolysis would yield about 1,250 lb wood sugars per ton of wood (dry basis), from the stoichiometry. Theoretical conversion of this sugar to ethanol would yield 640 lb or 81.1 gallons of anhydrous... 

E. Bozo, A. Medgyes, S. Boros, and J. Kuszmann, Synthesis of 4-substituted phenyl 2,5-anhydro-1,6-dithio-a-gluco- and -a-guloseptanosides possessing antithrombotic activity, Carbohydr. Res., 329 (2000) 25-40. 

For the properties of the methyl derivatives of 3,6-anhydro-L-galactose see Advances in Carbohydrate Chemistry, 2, 77 (1946).)... 

Formation of l,2 5,6-di-0-isopropylidene-a-D-n7i( -hexo-furanos-3-ulose (Fig. 2.13, 2.4.2.1 Table 2.3) was one of the first carbohydrate oxidations effected by stoich. RuOyCCl [310, 311]. The reagent was also used for oxidation of 1,6-anhydro-3,4-0-isopropylidene-P-D-galactopyranose to the -P-D-/yxo-hexopyranos-2--ulose, part of a route leading to 1,6-anhydro-P-D-talopyranose (Fig. 2.15) [321]. 

A third strategy was applied to the synthesis of a neighboring family of amphiphilic carbohydrates. The starting material was 5,6-anhydro-l,2-0-iso-propylidene-a-D-glucofuranose, easily obtained in a few steps from o-glucose. The latter was reacted with M-benzyl-n-hexadecylamine to afford regioselec-tively the 6-alkylamino-6-deoxy compound 35 [ 119]. 

Certain carbohydrates are readily converted by acid or heat into anhydro derivatives, and this reaction may complicate both identifi-... 

The reaction of carbohydrates in alkaline or acidic aqueous solutions results in a myriad of products, many of which have been recognized for well over a century. The number of identified products has greatly increased in recent years, owing to the development of sophisticated techniques for separation and identification. With the exception of anhydro sugars and oligosaccharides, found as concentration-dependent, equilibrium constituents (reversion products) in acidic solutions, all of the products result from reactions of intermediates present in the Lobry de Bruyn-Alberda van Ekenstein transformation. 

---