Stereoisomerism separating

Figure 11. Semi-preparative-scale stereoisomeric separation of 1.6-dioxaspiro[4.4]nonanes on Ni-CAR2 (experimental conditions cf. text) (Schurig, 1987).

These matrices have been used as chromatographic stationary phases for both enantiomeric and stereoisomeric separations [8], as artificial antibodies in immunoassay-type analyses [7], as microreactors for stereoselective synthesis [9] and as catalysts [10,11] and equilibrium-shifters [12,13] for a variety of reactions. Since MIPs can be produced for such a wide variety of molecules, these characteristics have led many researchers to investigate the possibilities of employing MIPs as the... 

Oligoenynes containing either an odd or an even number of unsaturated carbon-carbon bonds can be efficiently and selectively prepared without the need for stereoisomeric separation (Scheme 2). ... 

Zhou SS, Lin KD, Yang HY et al (2007) Stereoisomeric separation and toxicity of a new organophosphorus insecticide chloramidophos. Chem Res Toxicol 20 400-405... 

Zhou L, Lin Z, Reamer RA, Mao B, Ge Z. Stereoisomeric separation of pharmaceutical compounds using CE with a chiral crown ether. Electrophoresis 2007 28 2658-2666. 

Kwon C, Jung S. Stereoisomeric separation of some flava-nones using highly succinate-substituted a-cyclosophoro-octadecaoses as chiral additives in capillary electrophoresis. Carbohydr. Res. 2011 346 133-139. 

In addition to constitution and configuration, there is a third important level of structure, that of conformation. Conformations are discrete molecular arrangements that differ in spatial arrangement as a result of facile rotations about single bonds. Usually, conformers are in thermal equilibrium and cannot be separated. The subject of conformational interconversion will be discussed in detail in Chapter 3. A special case of stereoisomerism arises when rotation about single bonds is sufficiently restricted by steric or other factors that- the different conformations can be separated. The term atropisomer is applied to stereoisomers that result fk m restricted bond rotation. ... 

P -Bonding is obviously also the initiating step in the complex photoisomerization sequence of the stereoisomeric 1,5-dien-3-ones (162) and (163) in ethanol. After low conversions of the starting dienones, an isomer containing an analogous chromophoric system [(164) and (165), respectively] was found to build up temporarily in each case. On longer photolysis times, both compound pairs (162)/(164) and (163)/(165), are consumed, and the mixtures of the four diastereomers (166)-(169) were isolated from both runs. According to separate irradiation experiments with each of these products, (166) and (167) on one hand, and (168) and (169) on the other, are... 

Figure 3.7 shows some early examples of this type of analysis (39), illustrating the GC determination of the stereoisomeric composition of lactones in (a) a fruit drink (where the ratio is racemic, and the lactone is added artificially) and (b) a yoghurt, where the non-racemic ratio indicates no adulteration. Technically, this separation was enabled on a short 10 m slightly polar primary column coupled to a chiral selective cyclodextrin secondary column. Both columns were independently temperature controlled and the transfer cut performed by using a Deans switch, with a backflush of the primary column following the heart-cut. 

A. Mosandl, U. Hener, U. Hagenauer-Hener and A. Kuster mann, Stereoisomeric flavor compounds. 33. Multidimensional gas chromatography dkect enantiomer separation of -y-lactones from fr uits, foods and beverages , 7. Agric. Food Chem. 38 767-771 (1990). 

H.-G. Schmarx, A. Mosandl and K. Grob, Stereoisomeric flavour compounds. XXXVIII dkect chir ospecific analysis of y-lactones using on-line coupled EC-GC with a chkal separation column , Chromatographia 29 125-130 (1990). 

As we have seen previously (Section 7.1), within two decades of Schraube and Schmidt s discovery (1894) of isomeric diazoates, (Z)-and (ii)-isomers were found for all major stable addition products of arenediazonium ions with nucleophiles with the exception of triazenes. However, in the 1970s Wiberg and Pracht (1972), and also Fanghanel et al. (1975 a, 1975 b), discovered examples of stereoisomeric triazenes. They showed that 3,3-di-(trimethylsilyl)-l-phenyltriazenes (13.1, 13.2) and l-aryl-3-[3 -methylbenzothiazolinylidene(2 )]triazenes (13.3, 13.4) exist in two isomeric forms that can be separated and characterized on the basis of their chemical and UV spectral properties as (Z)- and ( -isomers. 

Direct hydrogenation of key intermediate 248 over the Adams catalyst and subsequent lithium aluminum hydride reduction yielded the two stereoisomeric alcohols 256 and 257, which were separately transformed to ( )-corynantheal (258) and ( )-3-epicorynantheal (259), respectively, by Moffatt oxidation, followed by Wittig reaction with methyltriphenylphosphonium bromide and, finally, by demasking the aldehyde function (151, 152). 

Tetracyclic keto ester 467, prepared earlier (253), was treated with the anion of diethyl methoxycarbonylmethylphosphonate in dimethylformamide. The reaction supplied the unsaturated ester 492, which was catalytically hydrogenated to diester 493. Dieckmann condensation of 493 yielded two nonenolizable keto esters (494 and 495), which could be separated by fractional crystallization. Sodium borohydride reduction of 18a-methoxyyohimbinone (494) gave two alcohols (496 and 497) in a ratio of about 10 1 at the same time, reduction of 180-methoxyyohimbinone (495) furnished another two stereoisomeric alcohols (498 and 499) in approximately equal amounts. Demethylation of the four stereoisomers (496-499) resulted in the corresponding 18-hydroxyyohimbines (500-503)... 

Dienes reacted considerably faster than the (Z)-isomers, and this feature was utilized to separate the (Z)-isomer from the stereoisomeric mixture. When 6-phenyl-1,3-hexadiene (EIZ = 50/50) was treated with 0.6 molar amounts of PPh3 and CF3S03H at 0 °C for 6 h, the ( )-phosphonium salt (53%) was formed. The unreacted (2)-1,3-diene could be separated by ether extraction in 41% yield. This procedure provides easy access to (Z)-1,3-dienes.22... 

Fig. 2.7.2. (—)-LC-ESI-MS chromatograms of a standard APG solution, (a) Total ion current trace fromm/2 200 to 600. (b)XIC of Cg-, C10-, and Ci2-monoglucoside, and (c) XIC of Cg-, C10-, and Ci2-diglucoside. Peak numbering as in Table 2.7.1 indices a and b denote different stereoisomeric forms. (Separation on a RP-Cg column with a water/acetonitrile gradient) (Reprinted from [1],...

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