CHIRAL RECOGNITION OF ETHERS BY NMR SPECTROSCOPY 59 OCH 6454440 30 42458454450 3.36 378 374 solutes like 3.3,3-trichloromethyloxirane. In another tion (CP)and magic angle spinning (MAS)techni in no longer out of sc Some approaches led to su pers soct al packing forces are differe rent for nd the CSA which pr it is a al or he one 1 ould be dist hed fron ereoisomers by their c mical shift ten 、which xation inc 42g 60 64 130 07 0.72 t+0.0g +009 +027 Chirality DOI 10.1002/chinsolutes like 3,3,3-trichloromethyloxirane.111 In another report, the authors investigated partially deuterated ethers, oxiranes, alcohols, esters, tosylates, chlorides, bro￾mides, and hydrocarbons; they found excellent signal dis￾persions in 1 H-decoupled 2 H NMR spectra.112 In both techniques, using cyclodextrins and lyotropic liquid crystals, it is not a specific interaction of functional groups of the solute and the CSA which produces signal dispersions. Rather, it is a differential orientation of both enantiomers in a chiral spatial environment, CD cones and partially ordered PBLG molecule strands, respectively. So, we propose to call such auxiliaries ‘‘chiral hosting agents (CHA)’’ rather than CSA. Some interesting approaches using solid state NMR spectroscopy should be mentioned. During the last two decades, enormous progress has been made in high-reso￾lution solid state NMR by introduction of cross-polariza￾tion (CP) and magic angle spinning (MAS) techni￾ques.113–116 So, chiral discrimination by solid state NMR in no longer out of scope. Some approaches led to suc￾cessful enantiodifferentiation, e.g., by isotropic chemical shift comparison exploiting the fact that conformational structures and crystal packing forces are different for solid material containing racemate or pure enantiomers, respectively.113 Another technique is the application of the ODESSA (one-dimensional exchange spectroscopy by sideband alternation) pulse sequence117; some race￾mic oxazaphosphorinanes could be distinguished from their enantiomers.118 Finally, it is possible to differentiate stereoisomers by their chemical shift tensors, which can be extracted from the side bands in their solid-state NMR spectra. Using a compound with two stereocenters and, hence, four stereoisomers, Harper et al. demonstrated that a comparison of experimental shift tensors of all iso￾Scheme 23. Structure of the racemic cryptophane A (59; D3 symmetry) and sections of its 1 H NMR spectra; free 59 (bottom) and in the presence of an equimolar amount of the dirhodium complex 52 (top). Signal dispersions are Dm 5 7 Hz for H-7a, Dm 5 4 Hz for H-7b and Dm 5 6 Hz for OCH3. 99 TABLE 4. 13C complexation induced shifts (Dd) of 2-butyl phenyl ether (60) and its para-substituted derivatives 61 (X5F), 62 (X5Cl), 63 (X 5 Br), 64 (X5I), and 65 (X 5 NO2) 100 D( 13C) X5 60 H 61 F 62 Cl 63 Br 64 l 65 NO2 2 10.65 11.30 10.77 10.72 10.49 10.16 10 10.03 20.15 20.07 20.03 10.14 10.07 20 /60 10.42 10.84 10.46 10.44 10.31 10.11 30 /50 10.05 20.03 20.01 10.01 20.05 10.10 40 10.04 10.27 10.27 10.28 10.53 10.10 CHIRAL RECOGNITION OF ETHERS BY NMR SPECTROSCOPY 63 Chirality DOI 10.1002/chir