Ribitol, conformation

Ribisco activase 707 Ribitol, conformation 44 Ribitolteichoic acids 431s Riboflavin 721,766, 783s... 

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... 

As has already been pointed out (see p. 59), the carbon chain in ribitol17 is nonplanar, and the molecule adopts a sickle conformation.18 This conformation is just one of the conformational changes that can be directly attributed to interaction (repulsion) between nonbonded atoms, Were ribitol planar, the contact between 0-2 and 0-4 would be close, as shown in Fig. 3. The C-O bonds would be... 

Fig. 3. —Distance Between 0-2 and 0-4 in the Planar, Zigzag Conformation of Ribitol.

Meso configurations can have enantiomeric conformers in crystals. Ribitol and xylitol have meso configurations and are optically inactive in solution. In the crystalline state, they have the bent-chain enantiomorphic conformations. Xylitol crystallizes in space group P212121, so each crystal contains either only the optically active left-handed or the optically active right-handed conformers. There are, of course, an equal number of left- and right-handed crystals in any one batch, which on dissolution give no optical activity.1... 

Ribitol, on the other hand, crystallizes in space group P21/c, with left- and right-handed conformers in the same crystal, related by a crystallographic center of symmetry. The choice of these two alternatives depends upon which allows the greatest hydrogen-bonding energy. As yet, theory cannot make this prediction. 

Molybdenum(VI) and tungsten(VI) are found to form only one of two possible isomeric ery-r/zro-type dimetallates analogously to 14 (O Fig. 2) with fourfoldly deprotonated ribitol. One of the terminal hydroxyl functions is not part of the coordination site. Even the formed isomer proves to have low stability due to unfavorable steric interaction between the terminal carbon atoms of the sickle conformer [25,26,37]. 

The configurations of a series of 2- and 5-substituted derivatives of 1,4 3,6-dianhydro-hexitols (4) have been established from their C n.m.r. spectra. Crystal structure measurements have shown thatl,5-anhydro-2,3,4-tri-0-benzoyl-xylitol and -ribitol both adopt the expected Ci(d) conformation, whereas l,2,3,4-tetra-0-benzoyl-/3-D-xylopyranoside adopts the alternative C4 form, emphasizing the importance of the anomeric effect in the latter case. Unsaturated alditol derivatives are mentioned in Chapter 12. 

The key-step in the synthesis of the 4-thio-oc,P-D-ery/Aro-pentofuranose derivative 31 from L-arabinose was the well-established ring-closure 29 ->30 (see Vol. 29, Chapter 11, Ref. 14). A range of other 2-deoxy-D-ery/Aro-pentose dibenzyl-dithioacetals have been prepared and similarly cyclized to 2-deoxy-4-thio-D-eryt/jro-pentofuranose derivatives. A synthesis of 4-thio-l,4-anhydro-L-ribitol (32) from D-arabinitoI was based on the episulfonium ion rearrangement shown in Scheme The conformation of 4-thio-L-lyxono-1,4-lactone is referred to in Chapter 21. 

The regioselective complexation of D-mannitol, -galactitol, -xylitol, -ribitol and -erythritol with bis(phosphlne)platinum(II) carbonate complexes has been studied, and conformational analyses of the products by H n.m.r have been performed. Also a study on the preferred conformations of hex-l,S-dien-3,4-diols, which includes reference to hydroxylation reactions, suggesting an approach to hexitol and higher alditol syntheses has been reported. ... 

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