Diasteromeric excess

In their work concerning the anions derived from chiral phosphonamidates, Denmark and Chen [68] obtain a high diasteromeric excess of 88% for the compound 108 resulting from the addition of the carbanion of phenylsulfenyl phos-phonamidate 107 to 4-terf-butylcyclohexanone (Scheme 31). 

The interaction of the chiral quaterpyridine 75, bearing fused chiral groups in the 5,6- and 5", 6" -positions, with copper(I) has been investigated. The respective ligand isomers were demonstrated to form structurally characterised P or M dinuclear double helicates in approximately 99% diasteromeric excess upon coordination to the above ion. ... 

Schultz and Harrington [37] added Grignards to a-amido enone (see Table 4, entry 6). They obtained high diasteromeric excesses with a variety of nucleophiles for example, with methyl Grignard reagent, the d.e. ranged from 85 to 90%. 

Dichloro-5,6-dicyano-1,4-benzoquinone % Diasteromeric excess Diethylamine HN(CH2CH3)2... 

Combination of FucA and fuculose isomerase for the synthesis of interesting L-fucose analogs having tails with increased hydrophobicity and reactivity (Fessner et al. 2000). Homologues and unsaturated analogs of L-lactaldehyde were well tolerated by FucA with high diasteromeric excess (> 95%). 

Figure 2.7 Energy profile of resolution or asymmetric synthesis. In both cases the diasteromeric transition states leading to different enantiomers have different energy and this difference A A G1- governs the enantiomeric excess of products.

The mixture of the crude product hydrazone 138 and the excess of hydrazone 129 was used directly in an acidic acetalization reaction utilizing aqueous 3n HCl in a biphasic system and gave a diastereomeric mixture of sordidin (126) and 7-epi-sordidin (7-epi-126) in 84% yield over two steps in a 1.5 1 ratio. Gratifyingly, we succeeded in separating the desired sordidin epimers by preparative gas chromatography. As a result, both could be obtained in diasteromerically pure form (sordidin de > 99% 7-epi-sordidin de > 97%) and with a high enantiomeric excess for each epimer (ee > 98%). 

The oxidation of 2a with elemental sulfur and tellurium quantitatively furnish the three-membered heterocycles 15 and 16, respectively. The latter do not yield l,3-dichalcogen-2,4-phosphasila-cyclobutanes on treatment with excess elemental sulfur or tellurium. The reaction of 6 and 7 with elemental tellurium furnished a 1 1 mixture of the diasteromeres 17a,b and 17c,d (/-butyl/Si(/Pr)3 m-and /rans-forms), respectively, which were characterized by means of NMR spectroscopy ( P, Si, Te) [12, 19]. 

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