Transition diastereotopic

The reaction is believed to proceed via a six-membered cyclic transition state, analogously to the carbonyl addition of enolates, but the energy differences between boat- and chair-like arrangements are lower for x-sulfinyl carbanions69. Tor tert-butyl sulfoxides only anti- and, vn-products are obtained, arising from the approach onto the same diastereotopic face of the anion, but with different relative topicity. The exchange of lithium by zinc causes an increase of the anft-produci, but attempts to titanate the anion failed (see Table 3)69. 

Six-Membered Ring (endo-Cyclic). All the previous discussion of stereo-attack is based on steric hindrance, but in the case of a six-membered ring (endo-cyclic) enolate, the direction is affected simultaneously by stereo-electronic effects (Scheme 2-4).10 In the transition state, the attacking electrophiles must obey the principle of maximum overlap of participating orbitals by perpendicularly approaching the plane of atoms that constitute the enolate functional group. Electrophile attacks take place on the two diastereotopic... 

A much more efficient procedure consists in the deprotonation of prochiral substrates 4 by chiral base 5 (equation 2). The removal of the enantiotopic protons in 4 proceeds through diastereotopic transition states having different energies AG and thus yielding the diastereomeric carbanions 6 and epi-6 in unequal amounts (equation 2). 

The unsymmetrical structure of (—)-sparteine creates an additional stereogenic centre at the lithium cation. As a consequence, two diastereotopic transition states for the interaction of isopropyllithium/)—)-sparteine (11) have to be taken into consideration. 

As a theoretical model, diastereotopically distinct isotopic substitution in 1,3-butadiene was utilized by Bach and coworkersl9b to provide an independent test of the ability of high-level ab initio calculations to accurately predict a transition structure for epoxidation. The calculated KIE for deuterium substitution at the a-carbon in the TS for epoxidation of 1,3-butadiene (Figure 24) (Ha) is 0.99, in excellent agreement with the experimental value for an aryl-substituted styrene. The KIE for diastereotopically distinct hydrogen (Ht,) on the /J-carbon cis to the vinyl substituent is 0.80, while that for Hc is predicted to be 0.82. The calculated KIE is 0.66 for this transition structure with Hc, reflecting the extensive... 

In the second step, achiral 9-borabicyclo[3.3.1]nonane (9-BBN) adds to the less hindered diastereotopic face of a-pinene to yield the chiral reducing agent Alpine-Borane. Aldehydes are rapidly reduced to alcohols. The reaction with deuterio-Alpine-Borane, which yields (R)-a-d-henzy alcohol in 98% enantiomeric excess ( ) reveals a very high degree of selectivity of the enantiotopic faces of the aldehyde group in a crowded transition state ... 

Just as there are enantiotopic and diastereotopic atoms and groups, so we may distinguish enantiotopic and diastereotopic faces in trigonal molecules. Again we have three cases (1) In formaldehyde or acetone (G), attack by an achiral reagent A from either face of the molecule gives rise to the same transition state and product the two faces are thus equivalent. (2) In butanone or acetaldehyde (H), attack by an achiral A at one face gives a transition... 

Internal diastereomers of the kind depicted in Figure 3 have two kinds of interaction between the chiral centers the total interaction and that part of the total interaction which is discriminatory. The latter we call the diastereotopic interaction, and it is only this part of the interaction that causes either Kx in Figure 3 to be displaced from unity or the activation energies of the diastereomeric transition states to be different. It follows that the object in asymmetric synthesis is to maximize the diastereotopic interaction. In the absence of a clear lock-and-key compatibility for the inducing molecule and the substrate, the stereochemical criteria for maximizing the diastereotopic interaction are not always obvious. For example, it is a commonly held view that an increase in the relative steric bulk of certain groups will increase discrimination. This may only increase the nondiscriminatory interaction unless such elaboration is directed at features of the substrate which control the discrimination. 

Barbarella, G. Dembech, P. Garbesi, A. Bernard/ F. Bottom, A. Fava, A. Kinetic aridity of diastereotopic protons in sulfonium ions. A transition state conformational effed. /. Am. Chem. Soc. 1978, 100, 200-202. 

The stereochemical outcome of these electrophilic additions is consistent with a transition state in which the metal chelates the oxazolidinone carbonyl and the enolate oxygen. Reaction with an electrophile would, therefore, occur at the less hindered diastereotopic face of the (Z)-enolate, away from the shielding methyl groups of the auxiliary (Figure 24.6). Because both enantiomers of oxazolidinone 108 are equally available, the direction of the asymmetric induction can be controlled by proper choice of the absolute stereochemistry of the chiral auxiliary.106... 

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