Optical rotation, cellulosics

Solutions of cellulose in NH3/NH4SCN (27 73 w/w) are liquid crystalline at concentrations from 10-16% (w/w) depending on the cellulose molecular weight (64). Optical rotations of the solutions indicate the mesophase is cholesteric with a left-handed twist. The solvent does not react with cellulose. Recently, Yang (60) foimd that cellulose (D.P. 210) formed a mesophase at 3.5% (w/w) concentration at a NH3/NH4SCN of 30 70 (w/w). 

Introduction of nitrogen into the anulene ring (e.g. of 95) leads to a methano-azaanulene 107 121) with Q-symmetry which is therefore chiral (like its mono- or disubstituted derivatives)118). The low basicity of 107 (pKa 3.20) prevented its optical resolution by conventional methods (e.g. through salts with optically active acids). Excellent results were obtained, however, (as also in the case of the two isomeric carbocyclic methylesters 97 and 101 and of several derivatives of azaanulene) by chromatography on microcrystalline triacetyl cellulose in ethanol at 7 bar 1221 (see also Section 2.7.1). In many cases base line separations were accomplished to give both (optically pure) enantiomers. Enantiomeric relations were confirmed in all cases by recording the CD-spectra of both fractions. Some results of these separations are shown in Fig. 4 together with the optical rotations ([a]D in ethanol) of the enantiomers. 

Model building in the computer is used to compare the influence of remote contacts on the stereochemistry and properties of P-1,2-, p-1,3-, and P-1,4-glucans, including their optical rotation behavior, and to explore the possibility of chain folding in cellulose, chltin, and xylan derivatives. 

Total hydrolysis of the polymers gave D-glucose only. Water-soluble derivatives (ethyl or carboxymethyl ethers) of the polymers were unaffected by a-amylase, but were partially hydrolyzed by a ceUulase preparation from Acdobader xylinum. The optical rotations of several preparations of this polyglucose and of cellulose (P 1150) in tetraethylammonium hydroxide were all 0°, thereby strongly suggesting that the polyglucoses are /S-D-linked.109... 

Figure 5. Changes in optical rotation on cooling a 1% solution of cellulose sulfate (DS 0.8)...

The first successful experiments were reported by Schwab [16] Cu, Ni and Pt on quartz HI were used to dehydrogenate racemic 2-butanol 23. At low conversions, a measurable optical rotation of the reaction solution indicated that one enantiomer of 23 had reacted preferentially (eeright-handed quartz gave the opposite optical rotation it was deduced that the chiral arrangement of the crystal was indeed responsible for this kinetic resolution (for a review see [8]). Later, natural fibres like silk fibroin H5 (Akabori [21]), polysaccharides H8 (Balandin [23]) and cellulose H12 (Harada [29]) were employed as chiral carriers or as protective polymer for several metals. With the exception of Pd/silk fibroin HS, where ee s up to 66% were reported, the optical yields observed for catalysts from natural or synthetic (H8, Hll. H13) chiral supports were very low and it was later found that the results observed with HS were not reproducible [4],... 

In their study of the optical rotations of the starch and cellulose oligosaccharides, Freudenberg and Kuhn developed the equation64... 

Free Glycol Groups in Methylated Cellulose Calculated from the Optical Rotation... 

This work was followed by the detection of an aldoheptose in Escherichia coli products and the subsequent isolation of jj-glycero-i>-manno-heptoee by Weidel, again in connection with phage-receptor studies. The sugar was obtained from the lipocarbohydrate fraction of Escherichia coli B cell-membranes and was isolated, after acid hydrolysis, on a cellulose-powder column. In this case, sufficient of the product was obtained to show that the diethyl dithioacetal and its hexaacetate had equal but opposite optical rotations, as compared with those of the same derivatives prepared from authentic D-glyc o-n-manno-heptose. 

Cholesteric lyotropic mesophases of cellulose in dimethylacetamide-LiCl solutions have been observed by Ciferri and coworkers (9-11). While cellulose/TFA-CH2Q2 mesophases have positive optical rotations, the cellulose/ LiCl/DMAC mesophases have negative rotations. 

As previously considered in the case of the oligosaccharides, the molecular rotation of cellulose is an additive function of the optical rotations contributed by the constituent basic units. When the number of intermediate basic units become infinitely large, the m value in equation... 

Lacroix et al. compared an HPLC with a H NMR method for quantification of the R-enantiomer in S-timolol maleate, a -blocking substance (Figure 6-7) [17]. For satisfactory resolution, the HPLC required the expensive cellulose tris-3,5-dimethylphenylcarbamate column (Chiracel OD-H), a mobile phase of 0.2% dimethylamine and 4% isopropano in hexane and H NMR (400-MHz) R-TFAE as a CSA in CDCIj. The resonance of the tert-butyl group between 1.0 and 1.1 ppm was used to measure the ee. With both methods, 0.2- 4.0% of the R-enantiomer in S-timolol could be determined precisely. Additionally, both methods (HPLC and NMR) were found to be superior to specific rotation, which is used in the USP XXIII for determination of optical purity. This superiority of NMR and HPLC methods over optical rotation has often been reported [2],... 

The Kramers-Kronig [15] transformation relates the optical rotation (ORD) to the circular dichroism (CD). This spectroscopic method is widely used for the determination of the pitch as well. Figure 5 shows examples for left- and right-handed helicoidal structures of cellulosics as they appear in ORD and CD measurements. 

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