Pentopyranose derivatives

Preparation of branched-chain 5-thio-DL-pentopyranose derivatives 532-535 included an intramolecular, aldol reaction of diacetonyl sul-... 

If a comparison is made of the coupling constants of the protons of an endocyclic methylene group with a proton on an adjacent carbon atom, for example, at C-5 of a pentopyranose derivative or in certain deoxypyranose derivatives, the two protons can be differentiated. If one of the protons is replaced by deuterium, the orientation of the proton remaining on the methylene group can readily be recognized. This principle is illustrated by the proof of structure of the enantiomers of ethanol-2-d given by Lemieux and Howard.80... 

Percentages of selected D-pentopyranose derivatives that exist as Ci conformers... 

On the other hand, pentopyranoses derivatives in solution do not exhibit the conformational effects observed with hexopyranose derivatives. In a chloroform solution of the g-D-ribo derivative, the conformation with an axial chlorine is... 

Qualitative relaxation-studies have also been reported for an extensive series of derivatives of inositols, pentopyranoses, l,6-anhydro-/3-D-hexopyranoses, furanoses, and septanoses. In all instances, the experimentally determined Ri(ns) values reflect the anticipated geometry. For the furanose derivatives especially, they provide a better means for distinguishing between epimeric pairs than the relatively ambiguous interpretation of coupling-constant data. 

In the pentose series, attack has also been observed to occur mainly from the less-hindered side of the double bond, as in the addition of trifluoro(fluoroxy)methane to di-O-acetyl-D-arabinal. Trifluoromethyl 3,4-di-0-acetyl-2-deoxy-2-fluoro-/3-D-arabinopyranoside and 3,4-di-0-acetyl-2-deoxy-2-fluoro-/J-D-arabinopyranosyl fluoride were obtained,72,74 together with a very small proportion of 3,4-di-0-acetyl-2-deoxy-2-fluoro-a-D-ribopyranosyl fluoride.74 With di-O-acetyl-D-xylal, however, the lyxose derivatives were mainly obtained.72,73 The suggestion has been made73 that the greater conformational mobility of pentopyranoses results in a preponderance of the C d) conformer of the D-lyxose derivatives,72,73 such that F-l and CFsO-l are axially disposed. For further reactions of unsaturated carbohydrates with tri-fluoro(fluoroxy)methane, see Section 11,9. 

On the other hand, in the case of pentopyranose, the 1,4-anfi-adduct was preferentially obtained (O Scheme 29), due to the preferred conformation of the intermediate oxonium cation derived from the glycal [46,51a]. 

The methodology has been extended to glycal derivatives of pentopyranose. O Table S [51] and O Table 9 [51a] show t) ical examples of C-glycosylation of arabinal and xylal, indicating the reliability of the reaction. 

Chiara et al. also reported the type [C] reaction, the cyciization between carbonyl compounds and oximes [220,221]. As illustrated in O Fig. 17, three types of substrate were s)uithesized from hexopyranoses and pentopyranoses, and the terminal O-benzylformaldoxime-ketones and the terminal O-benzylformaldoxime-a, /3-unsaturated esters were synthesized from a series of hexopyranoses, and meanwhile the terminal O-benzylformaldoxime-aldehydes were derived from a series of pentopyranoses (O Fig. 17). 

In the solid state, tri-O-acetyl-p-D-xylopyranosyl halides adopt the all-equatorial 4Ci conformation, -whereas tri-O-benzoyl-p-D-xylopyranosyl halides crystallize in the all-axial 1C4 conformation, except for tri-O-benzqyl-p-D-xylopyranosyl chloride, which adopts the (skew) conformation. The variations of bond lengths to the anomeric carbon atom are discussed with regard to the anomeric effect. By comparative studies of the conformational equilibria of acetylated and benzoylated 1,5-anhydropentitols on the one hand, and pentopyranoses exhibiting the anomeric effect on the other, data for the non-bonding interactions may be derived. Based on the principle of additivity of these factors, the conformational distribution of structurally related compound may be estimated. In connection with these results, it is noted that two benzoyl groups show a smaller 1,3-diaxial interaction than two acetyl groups the difference between these interactions is about... 

This conformational freeze is exceptional. The only other example among the tetraacetylated pentopyranoses is the -D-lyxo derivative. The calculation of the equilibrium constants from average spectra requires certain extrapolations. We cannot observe any regular effect from the nature and the polarity of the solvent. Let us now examine different types of derivatives. 

Retention Volumes of 0-Trimethylsilyl Derivatives of D-Pentopyranoses Relative to Trimethylsilyl S,3,4-Tri-0- trimethylsilyl)-p-D-xylopyranoside ... 

The introduction of substituents and the effects of solvation profoundly influence the conformational equilibria of monocyclic pyranoid sugars and their derivatives, because of the changes in steric and electronic interactions that result. The conformations of numerous polysubstituted pyranoid sugars in various solvents have been investigated, mostly by application of the n.m.r. spectroscopic method. Six aldopentopyranose tetraacetates were examined in chloroform solution near room temperature they were considered to exist almost entirely in the CJ(d) conformation, except for the jS-D-ribo derivative, which appeared to contain substantial proportions of each chair form, and the -L.-arahino derivative which was estimated to contain mostly the CJ(l) conformation. 2-Deoxy-j8-D-e/T/thro-pentopyranose triacetate was considered to exist mainly in the 1C conformation. 

K. Bock and C. Pedersen, A study of CH coupling constants in pentopyranoses and some of their derivatives, Acta Chem. Scand, B29 (1975) 258-264. 

A. Grouiller, M. L. Navarro, P. Moliere, and H. Pacheco, Synthesis of aminodideoxy-DL-pentopyranoses and their ureido derivatives, J. Carbohydr. Chem., 1 (1988) 507-524. 

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