Acyclic carbohydrates

Like the polymethylene hydrocarbons, acyclic carbohydrates and alditols tend to adopt the planar, zigzag conformation wherein all of the carbon atoms lie in the same plane. " However, non-bonded interactions between parallel hydroxyl groups in the 1,3 positions (sy/i-axial interaction) are the most highly destabilizing, and, in xylitol and ribitol, this... 

The successful selective modification of the cycloamyloses must overcome rather severe synthetic difficulties derived primarily from the multiplicity of potentially reactive cycloamylose hydroxyl groups. Utilizing techniques developed for the modification of acyclic carbohydrates, the cycloamyloses... 

Monosaccharides have many structural variations that correspond to local minima that must be considered. Acyclic carbohydrates can rotate at each carbon, and each of the three staggered conformers is likely to correspond to a local minimum. The shapes of sugar rings also often vary. Furanose rings usually have two major local minima and a path of interconversion. Experimental evidence shows a clear preference for only one chair form for some pyranose rings, but others could exist in several conformers. For exanqple, the and conformers must all be considered as possible structures for L-iduronate, as discussed by Ragazzi et al. in this book. 

Acyclic Carbohydrate Derivatives iV -Dimethyl-and iV -Dihexyh laramide Model Compounds for Synthetic Pofy(hexamethylene i laramide)... 

The conformers in Figure 2 are named descriptively and given a rotational label using terminology appropriate for acyclic carbohydrate conformers [(5, (pp 182-203)]. 

Compounds 1-10. c-x " c-x represent 05 and Cl of cyclic and acyclic carbohydrate, respectively, in which the carbohydrate moiety may be any mono or oligosaccharide, a or ano-mer (cyclic forms) C Hjn+, is a saturated aliphatic chain, while alkyl maybe either a saturated or an unsaturated aliphatic chain... 

Acyclic carbohydrates with one alkyl chain are either derived from cyclitols or aldonic acids. Acyclic cyclitol derivatives 3 (X=0, S, NH, N(CH3), NHCO, N(CH3)C0 R=Q-Ci6 sug=Glc-ol, Man-ol) form thermotropic liquid crystals... 

Acyclic carbohydrates with two alkyl chains have been studied in detail in the case of dialkyldithioacetals 9. It is one of the more complete series whose results have been cross-checked and compiled [43]. Their liquid crystalline properties were compared with those of dodecyl a- and -D-glucofuranosides [50] depending on their structures, they form either hexagonal disordered columnar phases or a new rectangular columnar phase [77]. 

The stereoselective normal electron demand Diels-Alder reaction of chiral 13-diaza-13-butadienes 42, derived from acyclic carbohydrates, with diethyl azodicarboxylate 2 yields the corresponding functionalized l,23,6-tetrahydro-133,4-tetrazines 43. The observed stereoselectivity is markedly dependent on the relative stereochemistry at C-1 3 - Reactions proceed slowly in benzene solution at room temperature, but are greatly accelerated by microwave irradiation <99JOC6297>. 

O. Kjplberg and K. Neumann, Synthesis of acyclic carbohydrate isopropylidene mixed acetals using 2,3-dichioro-5,6-dicyano-p-benzoquinone as a catalyst, Acta Chem. Scand 48 80(1994)... 

J. Marco-Contelles, C. Pozuelo, M. L. Jimeno, L. Martinez, and A. M. Grau, 6-exo Free radical cyclization of acyclic carbohydrate intermediates A new synthetic route to enantiomerically pure polyhydroxylated cyclohexane derivatives, J. Org. Chem. 57 2625 (1992). 

Compounds 79 [39] and 80 [40] are examples of products obtained from acyclic carbohydrate alkenes that exemplify types of enantiomerically pure cyclohexane deriva-... 

Finally, the mass spectra of acyclic carbohydrate derivatives will be considered in many aspects, these differ essentially from those of the cyclic forms. 

Acyclic carbohydrate derivatives containing unsaturation, with some exceptions, exhibit much of the chemistry already described for cyclic analogues, and their usefulness should not be overlooked. Unsaturated derivatives of alditols can be made, for example, by standard eliminations from alditol v/r-disulfonates or epoxides or by /f-eliminations from O-substituted 1-deoxy-l-nitroalditols and 1-deoxy-l-sulfonylalditols. Additions of hydrogen and ammonia to the unsaturated nitroalditols made in this way offer, respectively, routes to 2-deoxy- and 2-amino-2-deoxyaldoses.255... 

In this Chapter, only anhydrides of aldoses and ketoses are included. Anhydrides of acyclic carbohydrates, for example, alditols, aldonic acids, and intermolecular anhydrides are not discussed. 

Marco-Contelles, J. et al. (5-Exo Free Radical Cyclizalion of Acyclic Carbohydrate Intermediates A New Synthetic Route to Enantiomerically Pure Polyhydroxylated Cyclohexane Derivatives. 3.1 1992 [144]... 

The interpretation of the mass spectra of oligosaccharide derivatives follows the general principles that have been elaborated for simple cyclic and acyclic carbohydrates (66MI1 74MI1). 

Most of the key transformations described here involve carbon-carbon bond formation. Carbohydrate functional group manipulations are a long-standing research area in carbohydrate chemistry often reviewed in specialized books and series. We hope to provide the reader with a set of different chemical modifications of carbohydrate ring systems or acyclic carbohydrates available to devise new synthetic routes to biologically relevant targets. 

A non-specific bacterial acid phosphatase from Shigella flexneri (PhoN-Sf) has been screened for regioselective phosphorylation of primary alcohol(s) of more than 20 different cyclic and acyclic monosaccharides using pyrophosphate as the phosphate donor (O Scheme 61) [368]. These studies have shown that PhoN-Sf is capable of phosphorylating a range of hexoses (D-glucose epimers, glycosides, and C-2 derivatives), pentoses, heptoses, ketoses, and acyclic carbohydrates. 

Cyclic sulfates can be prepared from cyclic but not acyclic carbohydrate units by reaction with sulfuryl chloride in pyridine [98] or phenyl chlorosulfate and sodium hydride [99]. The reactivity of cyclic sulfates is quite similar to that of epoxides. Thus, they can be used to form fluoro or azido derivatives of polysaccharides by highly regiospecific approaches, usually, but not always, leading to trans-diaxial products from which the sulfate can be easily removed by mild, acid-catalyzed hydrolysis [100,101]. 

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