Stereoselectivity pseudo-equatorial positions

In each case, stereoselectivity can be explained by assuming that the reaction proceeds through a chair-like transition state, in which there are interactions between the two ends of the C-Li and C=C bonds, and in which the substituents all occupy pseudo-equatorial positions (structures 338, 339 and 340). The same transition state model suffices to explain the stereoselectivities of the furan and pyrrolidine forming reactions above. Stereoselectivity in the cyclisations of the selenide-derived tertiary organolithiums would arise from a conformation with a precedented pseudo-axial phenyl ring. 

As you can see, reduction of 2-substituted cyclopentanones may not be very stereoselective. The substituent probably occupies a pseudo equatorial position and the two faces of the ketone are very similar. 

The observed regioselectivities and stereoselectivities of the intramolecular addition of a carbon-lithium bond to an unactivated alkene (Table 1) could be rationalized by recourse to a transition-state model that resembles a cyclohexane chair in which substituents preferentially occupy pseudo-equatorial positions (structures 26-28). The same model is proposed to explain the modest selectivities in the analogous radical cyclizations (Scheme 8). 

The reaction of 45ZnBr with isobutyraldehyde produces stereoselectively the syn-homopropargylic alcohol 46 via a six-membered ring transition state [34]. The diastereoselectivity of this reaction was also studied by reaction of the substituted silylated enyne 47 (Scheme 7-36) [35]. The stereochemical outcome of this carbocyclization was interpreted by a chair-like transition state in which the R group preferentially occupies a pseudo-equatorial position. 

These intramolecular C-4-alkenyl nitrone cycloaddition reactions afford m-fused products with complete endo stereoselection. The induced stereoselection is also high and may be rationalized by the assumption that the transition structures with the bulkier substituent(s) at the stereocentcr(s) in a pseudo-equatorial position are favored over those with these groups in the pseudo-axial position. In conformationally mobile acyclic systems this generally results in a trans arrangement between the preexisting stereocenter and the newly formed stereocenter. This tendency seems compelling for x-substituted nitrones. In cyclic systems steric factors dictate the stereochemical outcome of the reaction. 

For the formation of six-membered rings, the chair-Uke transition state 28 (Figure 4.19) generally explains the stereoselectivity observed when hept-6-en-l-yl radicals undergo 6-exo-cyclization. Both the substituent(s) and alkene prefer to adopt pseudo-equatorial positions, therefore 1-, 3-, and 5-substitutedheptenyl radicals give predominantly trans-disubstituted cyclohexyl products, while 2- and 4-substituted heptenyl radicals give predominantly ds-disubstituted cyclohexyl products. 

Mannuronic acid donors also proved very suitable for the stereoselective construction of 1,2-cis-mannosyl linkages, and these donors provide anomeric triflates upon activation with a triflate-based electrophile [10]. Interestingly, the anomeric triflates generated from mannuronic acid donors preferentially reside in a flipped C4 Chair conformation, placing the anomeric triflate in an unfavorable equatorial position (as in structure 21, generated from either 19 or 20, Scheme 4.3) [11]. In addition, this conformer places three of the five ring substituents in sterically unfavorable axial positions. A rationale for this unusual behavior has been forwarded, based on the influence of the orientation of the substituents on the pyranosyl core on the stability of a pyranosyl oxocarbenium ion [12]. In a half-chair oxocar-benium ion, the O-substituents at C-3 and C-4 prefer to take up a pseudo-axial position, which is less electron-withdrawing than the alternative pseudo-equatorial... 

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