Hexopyranose structure

Conformational Preferences for Solvated Hydroxymethyl Groups in Hexopyranose Structures... 

Besides glycosidases recognizing hexopyranose structures recently there were reports on the chemoenzymatic use of an a-L-arabinofuranosidase which could be also used to synthesize /J-D-galacto-furanosides [88]. 

All derivatives used were prepared by essentially standard literature procedures and had physical constants in accord with previously reported values. Furthermore, the P.M.R. spectra were in each case consistent with the assigned structures. All solutions were concentrated under reduced pressure and m.p. s are uncorrected. (I) 2-Deoxy-D-arafczno-hexopyranose was a commercial sample from Pfanstiehl Lab. Inc., Waukegan, Illinois and was used without further purification. (II) 3, 4, 6-Tri-O-acetyl-D-glucal (1) was a commercial sample from Aldrich Chem. Co., Milwaukee, Wisconsin and was purified by distillation and recrystallized three times from aqueous ethanol. (Ill) 1, 3, 4, 6-tetra-0-acetyl-2-deoxy-a-D-arahino-hexopyranose (4) was prepared by the method of Bonner (11) while the corresponding / -anomer (5) was synthesized following the procedure of Overend, Stacey, and Stanek (47). (IV) 5, 6-Dideoxy-1, 2-0-isopropylidene-a-D-xj/io-hex-5-enofuranose (20) was provided by A. Rosenthal and G. Khan of this Department. 

Chemical shifts and chemical-shift differences between pairs of ano-mers in the monodeoxymonofluoro-D-hexopyranose series were observed to be strongly dependent on the stereochemistry of electronegative substituents in the molecule. In consequence, no a priori distinction between anomers can be made on the sole basis of their respective fluorine shifts. The use of fluorine shifts as a probe for structural and conformational... 

The geometrical parameters of 1 were based on the standard structures of hexopyranoses ( ). The glycosidic linkage is described by the two torsion angles... 

The crystal structure of the hydrochloride and hydrobromide of isoquinocycline A was elucidated by the method of single, isomor-phous replacement.67 The glycosyl group, which constitutes about 20% of the molecule, is that of a branched-chain, eight-carbon sugar, namely, 2,6-dideoxy-4-(L-glycero-l-hydroxyethyl)-a-L-xylo-hexopyranose-IC (17). 

SDMA in toluene for 20 min at 0° under an argon atmosphere gave the 5-C-(phenylphosphinyl) compound 124, which, on acid hydrolysis, afforded the 5-C-(phosphinyl)hexopyranose 125. This was treated with acetic anhydride-pyridine, to give the peracetates (126), from which crystalline l,2,4-tri-0-acetyl-3,6-di-0-benzyl-5-deoxy-5-C-[(S)-phenyl-phosphinyl]-/ -D-glucopyranose-4C1 (127) was isolated in 2% overall yield from 123 none of the other diastereoisomers of 127 were obtained. Structure 127 was established by 400-MHz, -n.m.r. spectroscopy (see Section 11,5). 

The analysis of crystallographic results shows the important role played by the anomeric center in the structure of sugars. In Table I51"59 are reported the C-C and C-0 bond-lengths in some hexopyranoses in both anomeric forms, and in some disaccharides. 

It may be that all three of the above reactions occur simultaneously when possible, and the latter two become apparent only when the first is sterically impossible. Changes in structure which favor ionization should increase the contribution of the third reaction mechanism, a prediction which has some support in fact.8 That l,6-anhydro-/3-D-hexopyranoses... 

Armstrong and Sutherlin applied a combinatorial concept, by keeping one sugar unit constant while varying the structure of others to include all possible stereomers. This is exemplified by the synthesis of four C-linked D-glucopyranosyl a-(l— 6) hexopyranoses [141] (O Scheme 74). 

In fact, the conformation of pyranoses is dominated by two effects, not present in the cyclohexane, which appear at positions 2 and 6 of the oxane. One of them is characteristic of hexopyranoses and I propose that we call this the coplanar effect in order not to imply a particularly restrictive structure by using the name of an effect already present in methoxyethane. The other effect, present in all pyranoses, is referred to as anomeric. This name, taken from the nomenclature of sugars because it was first recognized in this family, in fact disguises its general nature since it is also present in methyl chloromethyl ether. The consequences of these effects can be modulated by cyclohexane-type interactions, but not to the point where more than a qualitative discussion is necessary. 

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