Pentopyranoses

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. 

H- and F-N.m.r. Data for 3-Deoxy-3-lluoro- and 4-Deoxy-4-fluoro-pentopyranoses and -pentopyranosides... 

Scheme 27 Synthesis of thio-D-pentopyranoses from 5-bromo-D-pentono-1,4-lactones...

Das in der Peptidchemie bewahrte Kopplungsreagenz 1-Triphenylphosphonionooxy-benzo-triazol-hexafluorophosphat liefert mit einem Hexulosonsaure-Derivat zunaebst das Amid, das ohne weitere Reinigung thermisch zu l-C-(2-Benzimidazolyl)-l,2 3,4-di-O-isopropyli-den-fi-D-arabino-pentopyranose (75% Schmp. 210°) cyclisiert230. 

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

Because of the absence of a chiral center at C-5, such pentopyranoses as 7 were judged to be the most amenable to preparation and study. In... 

In the pentopyranose series, population of both chair conformers to a significant extent is the rule rather than the exception. 

Schuchmann MN, von Sonntag C (1988) The rapid hydration of the acetyl radical. A pulse radiolysis study of acetaldehyde in aqueous solution. J Am Chem Soc 110 5698-5701 Schuchmann MN, von Sonntag C, Tsay YH, Kruger C (1981) Crystal structure and the radiation-induced free radical chain- reaction of 2-deoxy-p-D-erythro-pentopyranose in the solid state. Z Naturforsch 36b 726-731... 

P. Koll and S. Deyhim, Versuche zur Darstellung von (R,S)-2,7-Dioxabicyclo[2.2.1]heptan (= l,5-Anhydro-2,3-didesoxy-/i-D-g7ycero-pentofuranose = l,4-Anhydro-2,3-didesoxy-u-D-gTycero-pentopyranose), Chem. Ber., Ill (1978) 2913-2918. 

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... 

Addition of 0,0-Dialkyl Phosphorodithioates to Acetylated 1,2-Unsaturated Hexo- and Pentopyranoses General Procedure14 ... 

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

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. 

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