Diastereotopic face discrimination

The success of this so-called second-order stereoselectivity hydrogenation depends on the fact that (a) the rate of racemization has to be fast with respect to the rate of hydrogenation (b) the diastereotopic face discrimination by the chiral... 

In contrast to the multiplicity of examples of diastereotopic face discriminations in combination with other enzyme specificity discussed later, there are relatively few alcohol dehydrogenase catalyzed reductions that involve diastereotopic face selectivity alone. However, whenever a carbonyl group is present in a single chiral stereoisomer, diastereotopically face-selective reductions are possible. The reductions of the L-homocysteine derivative (47) (48), and of the (-)-oxocineole (15,4/ )-(49) (50), are two... 

Simple allyl alkali metal compounds have only a small capability for discriminating between diastereotopic faces of carbonyl compounds. Although a matter of simple diastereoselectivity, this can be concluded from the reaction of conformationally locked 4-/erf-butylcyclohexanone... 

Diallylzinc discriminates between diastereotopic faces of alkyl cyclohexanones better than diallylmagnesium and slightly better than diallylcadmium8. 

The anion crystallizes as a dimer with bonding occurring via a Li20, four-membered ring. Two further coordination sites on each Li4 arc occupied by the TMF.DA N-atoms. This result is fully consistent with the four-center chelate structure which was proposed before for a-lithio sulfoxides 39- 40 and believed to be responsible for the planar configuration of the anionic carbon atom, This chelation discriminates between the two diastereotopic faces and for this reason a-sulfinyl carbanions react with electrophiles in a highly stereoselective manner (see the following section). 

Aspinall, I H, Cowley, P M, Stoodley, R J, Enhanced discrimination by aza dienophiles over their olefinic counterparts for the diastereotopic faces of methyl ( , )-5-(2, 3. 4, 6 -tetra-0-acetyl-p-D-glucopyranosyloxy)penta-2,4-dienoate. Tetrahedron Lett., 35, 3397-3400, 1994. 

Discrimination of diastereotopic groups and faces in intramolecular 13-DC of nitrile oxides and Af-benzyl nitrones were studied. For example nitrile oxides 38 afforded isoxazolines 39 with complete diastereotopic group selection (dr >99 1) and a high degree of n-face discrimination (dr from 10 1 to 26 1) <01JOC3834>. 

Thus the chiral BINOL-Ti catalyst effects efficient chiral recognition of the enantio-face of the aldehyde and discrimination between the diastereotopic protons of the ene component in a truly catalytic fashion. 

In the hydroformylation of chiral terminal alkenes only the branched products, not the linear products, give information about the discrimination of the diastereotopic alkene planes. The branched product, however, is normally the minor product, thus incomplete information is gained from experiments of this type. In the rhodium-catalyzed deuterioformylation of racemic 3-methyl-1 -pentene the complete extent of attack on each diastereoface of the olefin can be evaluated from the diastereomeric composition of both regioisomers obtained as products148. An overall preference for attack on the Si-face of the S-enantiomer, and on the Re-face of the S-enantiomer is found. Quantitative evaluations show that the relative abundances of anti- and y M-2,3-dimcthylpentanals alone (obtained from simple hydroformylation) do not even qualitatively reflect the extent of the overall discrimination of the tw o diastereotopic planes of either enantiomer148. 

Discrimination of Enantiotopic or Diastereotopic Carbonyi n-Faces 31S Tab. 7.3. Preparation of optically active allenecarboxylates with the anion of (S)-31b. 

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