Selective displacement

It is possible to prepare 1-acetoxy-4-chloro-2-alkenes from conjugated dienes with high selectivity. In the presence of stoichiometric amounts of LiOAc and LiCl, l-acetoxy-4-chloro-2-hutene (358) is obtained from butadiene[307], and cw-l-acetoxy-4-chloro-2-cyclohexene (360) is obtained from 1.3-cyclohexa-diene with 99% selectivity[308]. Neither the 1.4-dichloride nor 1.4-diacetate is formed. Good stereocontrol is also observed with acyclic diene.s[309]. The chloride and acetoxy groups have different reactivities. The Pd-catalyzed selective displacement of the chloride in 358 with diethylamine gives 359 without attacking allylic acetate, and the chloride in 360 is displaced with malonate with retention of the stereochemistry to give 361, while the uncatalyzed reaction affords the inversion product 362. 

The apphcation of bimolecular, nucleophilic substitution (S ) reactions to sucrose sulfonates has led to a number of deoxhalogeno derivatives. Selective displacement reactions of tosyl (79,85), mesyl (86), and tripsyl (84,87) derivatives of sucrose with different nucleophiles have been reported. The order of reactivity of the sulfonate groups in sucrose toward reaction has been found to be 6 > 6 > 4 > 1. ... 

Thus, a 2-D separation can be seen as 1-D displacement operating in two dimensions. The 2-D TLC separation is of no interest if selection of the two mobile phases is not appropriate. With this in mind, displacement in either direction can be either selective or non-selective. A combination of two selective displacements in 2-D TLC will lead to the application of different separating mechanisms in each direction. As an extreme, if the solvent combinations are the same (5ti = 5t2 5vi = va) or very similar (5ti 5vi 5ya), the compounds to be separated will be poorly resolved or even unresolved, and as a result a diagonal pattern will be obtained. In such circumstances, a slight increase in resolution might occur, because of an increase by a factor of V2 in the distance of migration of the zone (4). 

Attempted selective displacement (96) of the primary tosylate function in 34 with sodium iodide in refluxing 2-butanone led to the 6-deoxy-6-iodo derivative 35 in 32% yield only, while the di-iodo derivative 36 was formed in 45% yield. These results are to be compared with those reported by Owen and Ragg (85) who observed no reaction with either potassium thiolacetate or potassium thiocyanate in the corresponding / -series. 

The intermediate values correspond to the approximation of the quasiequilibrium conditions of sharpening of zone boundaries. At the same time, selective displacement in equilibrium sorption dynamics conception is carried out with the... 

The selective displacement of the 4-chlorine in 2,4-dichloro compounds can be capricious but a much better result was obtained starting with 4-chloro-2-methylthiopyrimidine 60. A displacement of chlorine was followed by oxidation of the methylthio to sulfone and displacement of sulfone <06BMCL5633>... 

A third route to nonracemic a-alkoxy and a-hydroxy stannaries employs the chiral acetal 73 prepared from (f ,f )-2,4-pentanediol (Scheme 30)66. Addition of various Grignard reagents to this acetal in the presence of TiCLt results in selective displacement yielding (S )-a-alkoxy stannanes. The corresponding a-hydroxy derivatives can be obtained after oxidation and mild base treatment. Organocuprates can also be employed to cleave this acetal but with somewhat lower selectivity67. 

FIGURE 11.7 Selective displacement of AVP on DEAE Sepharose Fast Elow. Column dimensions were 10 X 290mm load was 6.5 mL AVP feedstock/mL resin. The flow rate was 1 mL/min. (Reprinted with permission from Elsevier from Barnthouse, K.A. et ah, J. BiotechnoL, 66, 125, 1998. Copyright.)... 

Thus, this modification is now called the Gerster-Hayakawa synthesis. In order to obtain the oxo-analog of the Gerster intermediate, Hayakawa and co-workers commenced the synthesis (Scheme 4.4) with selective displacement of the fluorine ortho to the nitro... 

In the case of 5,7-difluoro-4(377)-quinazolinone 194, selective displacement of the 5-fluorine was achieved at room temperature, and this was followed by displacement of the 7-fluorine by a different alcohol at reflux in THE <2005BML5446>. 

The analyte must be efficiently recovered. The usual mechanism for solvent desorption is selective displacement of the analyte. Selective displacement occurs as a more polar solvent displaces a less polar one on charcoal, just as a more active ion displaces a less active one on ion exchange resins. CS2 is frequently used to recover substances from charcoal, but simple alcohols cannot be displaced from charcoal by CS2, and it is necessary to add l%-5% of another alcohol to the CS2 to facilitate desorption. Frequently, low recoveries can be increased by increasing the quantity of solvent, if analytical sensitivity permits. Prospective solvents may be chosen based on polarity or solubility of the analyte. 

A recent report by Yoneda et al. describes the selective displacement of formyl hydrogen with fluorine in aliphatic aldehydes by electrolysis in base nHF (base=pyridine, Et3N n=3-6) systems, both neat and in acetonitrile and sulpholane solutions [32],e.g. 

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