Stereoselective displacement

R. G. Edwards, L. Hough, A. C. Richardson, and E. Tarelli, The stereoselective displacement of hydroxyl by chloride using the mesyl chloride-DMF reagent, Carbohydr. Res., 35 (1974) 111-129. 

Comparison of F and OR substituents is more complex (32). However, when the reaction takes place with inversion, only highly stereoselective displacement of the fluorine atom occurs. 

The Cl symmetry present in the boronic ester (38) derived from (S,5)-l,2-dicyclohexylethanediol allows the stereoselective displacement of each chlorine atom to yield the chiral pentenylboronic ester... 

In the ribosylation of purines, in addition to the question of regioselectivity on the purine, there is the possibility of forming epimeric products at the linking C-T of the ribose, and this is often the more difficult to control. A great deal of work has been done and many different conditions shown to be effective in specific cases, but conditions that are generally effective have not been defined. These alkylations usually employ acylated or halo-ribosides in conjunction with a mercury, silicon or sodium " derivative of the purine, and stereoselective displacements of halide can sometimes be achieved. 

These alkylations usually employ acylated or halo ribosides in conjunction with a purine derivative of mercury, silicon or sodium, and stereoselective displacements of halide can sometimes be achieved. 

Mitsunobu-type N-AIkylation of l,2,4-Dithiazolidine-3,5-dione (1). Despite its high acidity suggesting that it should be an excellent nucleophile in Mitsunobu displacement reactions of alcohols (cf. phthalimide. " ), the parent heterocycle 1 is degraded by the triphenylphosphine used in this process (see later). Similar stereoselective displacement reactions of alcohols with 1 can, however, be mediated using the readily prepared betaine reagent 7. ... 

Stereoselective All lations. Ben2ene is stereoselectively alkylated with chiral 4-valerolactone in the presence of aluminum chloride with 50% net inversion of configuration (32). The stereoselectivity is explained by the coordination of the Lewis acid with the carbonyl oxygen of the lactone, resulting in the typ displacement at the C—O bond. Partial racemi2ation of the substrate (incomplete inversion of configuration) results by internal... 

Substituted 2-haloaziridines are also known to undergo a number of reactions without ring opening. For example, displacement of chlorine in (264) with various nucleophilic reagents has been found to occur with overall inversion of stereochemistry about the aziridine ring (65JA4538). The displacements followed first order kinetics and faster rates were noted for (264 R = Me) than for (264 R = H). The observed inversion was ascribed to either ion pairing and/or stereoselectivity. 

Stereoelectronic control also plays a role in mechanistic stereoselectivity. One such case is the very fundamental 8 2 process which proceeds rigorously with inversion of configuration at carbon. Because of that intrinsic and predictable stereoselectivity, the C-C disconnective Sn2 displacement transform is very important even though it does not directly reduce the number of stereocenters, e.g. 153 => 154. 

Examples of the use of lithium aluminum hydride in stereoselective [75] (equation 9), regioselective [16] (equation 10), and product-selective [77](equation 11) displacements of fluonne are available... 

An explanation for the stereoselectivity of the reaction involves optimal overlap of the 7t-orbital of the carbonyl with the developing electron rich p-orbital on C2 during the Sj,j2 displacement of the chloride by the alkoxide (24). Thus, orbital overlap imposes conformational constraints in the transition state that leads to nonbonding interactions disfavoring transition state 15P... 

The nueleophilic displacement of 9-methoxy group of perhydropyr-ido[l,2-c][l,3]oxazin-l-ones 94 was performed by treatment with an excess of 2-(trimethylsilyloxy)but-l-ene in the presence of TiCU to give 9-(2-oxobutyl) derivatives 96 (96CJC2434). The high stereoselectivity observed in... 

Useful stereoselective syntheses of ( )- and (Z)-2-butenyl(trifluoro)silanes have been reported. The (Z)-isomer is available by hydrosilation of butadiene, the (E)-compound being prepared by displacement of chloride from ( )-l-chloro-2-butene37. 

We discovered a complementary procedure for conversion of OMen to other functional groups. The ester P-OMen bond was shown to be cleaved in a stereoselective manner reductively [85,86]. The cleavage takes place with almost complete preservation of stereochemical integrity at phosphorus. The reducing agents are usually sodium or Hthium naphthalenide, lithium biphenyUde, and Hthium 4,4 -di-fert-butylbiphenyl (LDBB). The species produced is then quenched with an alkyl hahde or methanol to afford tertiary or secondary phosphines, respectively (Scheme 5b). Overall, the displacement reaction proceeds with retention of configuration. 

Another difference between dimethylsulfonium methylide and dimethylsulfoxonium methylide concerns the stereoselectivity in formation of epoxides from cyclohexanones. Dimethylsulfonium methylide usually adds from the axial direction whereas dimethylsulfoxonium methylide favors the equatorial direction. This result may also be due to reversibility of addition in the case of the sulfoxonium methylide.92 The product from the sulfonium ylide is the result the kinetic preference for axial addition by small nucleophiles (see Part A, Section 2.4.1.2). In the case of reversible addition of the sulfoxonium ylide, product structure is determined by the rate of displacement and this may be faster for the more stable epoxide. 

Differences in chirality of substrate, and nature of solvent, have no effect on the competitive nature of the displacement of 0-alkyl and S-methyl groups in the reactions between (+)-pinacolyl alkoxide and 0-ethyl (and methyl) S-methyl methylphosphonothioates (Scheme 23). For the (R)-( + ) esters, e.g. (210), the displacements are highly stereoselective and occur with configurational inversion,but the enantiomeric esters do not display such stereoselectivity. (-)-Menthol might be considered a mirror image of (S)-pinacol, and similar reactions with the sodium salt of (-)-menthol occur highly stereoselectively... 

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