Pseudo-equatorial position

As a consequence of the pericyclic reaction path, the addition of a-stereogenic allylmctals to carbonyl compounds is accompanied by an effective 1,3-chirality transfer in the allylic moiety combined with 1,4-chira induction at the prostereogenic carbonyl group3032. The scheme also demonstrates the importance of the orientation of the substituent X in the six-membered transition state. By changing from a pseudo-axial to a pseudo-equatorial position, the cation-induced sy/i-attack addresses opposite faces of both prostereogenic moieties, leading to a Z-and an -isomer, these being enantiomeric in respect to the chiral moiety. 

The first total synthesis of (T)-arteannuin M was accessed using a tandem oxy-Cope/ene reaction.87 Divinylcyclohexanol 138 was heated in toluene and l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) to give compound 140 as a single diastereomer in 55-60% yield (Scheme 28). The diastereoselectivity is controlled by the pseudo-equatorial position of the alkyl group in the transition state 139. 

Yttrium-catalyzed enyne cyclization/hydrosilylation was proposed to occur via cr-bond metathesis of the Y-G bond of pre-catalyst Cp 2YMe(THF) with the Si-H bond of the silane to form the yttrium hydride complex Ig (Scheme 8). Hydrometallation of the C=G bond of the enyne coupled with complexation of the pendant G=G bond could form the alkenylyttrium alkyl complex Ilg. Subsequent / -migratory insertion of the alkene moiety into the Y-C bond of Ilg could form cyclopentylmethyl complex Illg. Silylation of the resulting Y-C bond via cr-bond metathesis could release the silylated cycloalkane and regenerate the active yttrium hydride catalyst. Predominant formation of the /ra //j--cyclopentane presumably results from preferential orientation of the allylic substituent in a pseudo-equatorial position in a chairlike transition state for intramolecular carbometallation (Ilg —IHg). 

The phenyl or butyl substituent preferentially occupies a pseudo-equatorial position in the chair-like transition state which accounts for the observed diastereoselectivity. Since this extra stereogenic centre is quite far from the reacting centres diastereoselection is no longer complete. 

Ether was a better solvent in cases where the propargylic position was unsubstituted (methylene group) since, in THF, a competitive [1,2] -Wittig rearrangement took place and led to diminished yields. When applied to the secondary homoallylic propargyl ethers 388, the zinc-ene-allene cyclization afforded the c -2,3,5-trisubstituted tetrahydrofurans 389 with moderate to satisfactory levels of diastereoselectivity, which could be rationalized by the preferential pseudo-equatorial positioning of the homoallylic substituent in the cyclic... 

Et (or Ec) = R in pseudo-equatorial position trans (or cis) with the respect to the fifth, non-cyclic ligand,... 

Only one of the two isomeric 1-hydroxy-3-oxoindolizidines (96) can be benzoylated with benzoyl chloride. This difference in reactivity is explained by the assumption that the hydroxy group in the reactive isomer occupies a more accessible pseudo equatorial position and that it occupies a pseudo axial position in the inactive isomer. 

In 3,7,10-trimethylsilatranes the -carbons are on the flaps of their ring envelopes and the methyl groups occupy a pseudo-equatorial position in the five-membered rings149,166,167. Therefore, in contrast with the symmetric isomer 40 of 1-phenyl-3,7,10-trimethylsilatrane166, the asymmetric isomer 41 of l-(p-tolyl)-3,7,10-trimethylsilatrane149... 

The proposed transition state for the IMSC condensation leading to these five-membered ring heterocycles is depicted in Scheme 13.81. In this envelopelike conformation 219 the substituents preferentially occupy pseudo-equatorial positions. 

The heterocyclic ring assumes the familiar half-chair conformation as in somewhat related but simple systems.lb,2,6 The relative configurations of the substituents at P(l) and P(l) are quite different. The phenyl group assumes a pseudo axial position in the unprimed part and a pseudo equatorial position in the primed part. The considerable deviation of the dihedral angle of P(1)-C(9)-C(9) -P(1) from 180° to 143.6° appears unique and we have no explanation at this time. These data on the crystalline meso-3 appear to be the first recorded on a bisphosphinolinium salt. 

An X-ray diffraction analysis of 4-(3,5-dimethylphenyl)tetrahydrothiopyran-4-ol 47 indicates that the aryl group occupies a pseudo-equatorial position in the chair conformer of the thiopyran ring. Some flattening at the sulfur end of the ring is indicated (Figure 4) <2005PS(180)53>. 

For the pronounced endo selectivity in the reaction between aromatic aldehydes and 2,3-dihydrofuran, we can consider the two biradical conformers 10 and 11 to be responsible, with the alkyloxy substitutent localized in a pseudo-equatorial position and 10 being more populated because of fewer steric interactions (Scheme 3.17). When a methyl group is present, the increasing gauche... 

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. 

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