Sugar diols

Homopolycarbonates based on 1 and 2 have been prepared by several groups. The interfacial polycondensation typical for the synthesis of aromatic polycarbonates is not useful with alditols, including 1, because they are water-soluble and less acidic than diphenols. The 1-based homopolycarbonate was prepared by phosgena-tion of the sugar diol, with phosgene or diphosgene in pyridine-containing solvent mixtures at low temperatures. The polycondensation of the isosorbide bischloro-formate in pyridine is an alternative approach. 

Kricheldorf [17] studied liquid-crystalline cholesteric copoly(ester-imide)s based on 1 or 2. The comonomers to obtain these chiral thermotropic polymers were N-(4-carboxyphenyl)trimellitimide, 4-aminobenzoic trimellitimide, 4-aminocinnamic acid trimellitimide, adipic acid, 1,6-hexanediol, and 1,6-bis(4-carboxyphenoxyl) hexane. Apparently the poly (ester imide) chains are so stiff that the twisting power of the sugar diol has little effect. 

Since the formation of cyclic pyruvate acetals from alkyl pyruvates and diols under classical conditions (i.e. catalysis by acids) is expected to be unfavoured due to the necessity of the intermediate formation of a destabilized carbocation, early attempts to prepare 1-carboxyethylidene sugars used indirect procedures. Later, it was shown that under carefully controlled conditions the direct acetalation of a sugar diol with methyl pyruvate can also be used for preparative purposes. 

The direct pyruvylation of sugar diols also allows the selective synthesis of both diastereomers of 4,6-acetals [39c]. Thus, in acetonitrile as the solvent, the thermodynamically favoured diastereomers are formed, whereas in methyl pyruvate and with shorter reaction times, the kinetically favoured acetals are obtained. 

Sugars, diol-containing compounds Neutral Boronic acid carrier Acid Covalently bound complexes Protonation of carrier [94,95]... 

The di-O-tosylates (prepared by action of tosyl chloride in pyridine) are reduced with zinc (Nal/Zn route e Tipson-Cohen reaction) [13]. Cyclic ortho-esters (prepared by reaction of the diol with ethyl orthoformate) are transformed into olefins by simple heating in the presence of acids (Eastwood reaction, route b) [14]. Cyclic thiocarbonates (obtained by reaction of a diol with thiophosgene or (V,(V -thiocarbonyl-di-imidazole) are reduced to olefin with trimethyl phosphite (Corey-Winter method, route c) [15]. Finally, reduction of vicinal di-xanthates with tri- -butyltin hydride according to the Barton procedure [16] affords olefins via a reductive elimination process route a). The Corey-Winter, Garegg, and Tipson-Cohen methods are most commonly applied for deoxygenation of sugar diols. 

In Fig. 8.42 the structures of both sugar lactones and sugar diols used for the synthesis of sugar orthoesters are given. 

Kricheldorf, H.R. (1997) Sugar diols as building blocks of polycondensates. Journal ofMacromolecular Science, Reviews in Macromolecular Chemistry and Physics, C37 (4), 599-631. 

Cholesteric polyesters were prepared from silylated derivatives of 2,3-di-(9-isopropylidene-D-threitol, DAS, or DAM with dicarboxylic acid dichlorides by polycondensation in solution [34]. Trifluoroacetic acid-water allowed an easy cleavage of the isopropylidene group without hydrolysis of the polyester. All these polyesters formed a broad cholesteric phase, and the polymers containing 5 or 10 mol per cent sugar diol displayed a blue Grandjean texture. 

Kricheldorf H.R., Sugar diols as building blocks of poly condensates, J. Macromol. ScL, Rev. Macromol. Chem. Phys., C37, 1997,599-631. 

Schwarz G., Kricheldorf H.R., New polymer synthesis. LXXXlll. Synthesis of chiral and cholesteric polyesters from sUylated sugar diols , J. Polym. Sci. Part A Polym. Chem., 34, 1996,603-611. 

The poorer enantioselectivities observed in the Sharpless epoxid-ation of allylic alcohols following the use of 1 1 complexes of various sugar diols compared with diisopropyl tartrate has been ascribed to the observed formation of tricyclic dimers between the former and titanium tetraisopropoxide, whereas the latter gives a monocyclic 2 1 complex. 

Figure 2.20 Structures of sugar diols (a) D-isosorbide, (b) isomannide, (c) methyl 4,6-0-benzylidene-a-D-glucop5Tanoside, (d) methyl 2,6-di-O-pivaloyl-a-D-glucopyranoside, (e) l,2 5,6-di-0-isopropylidene-D-sorbitol, and (0 2,3,1, 3, 4, 6 -hexa-0-acetylsucrose.

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