Diastereoisomerism solvent effect

For the ABBB monomer, two diastereoisomeric chiral capsules were possible, with a diastereoisomeric ratio that varies from 6 1 in cyclohexane-di2 to 1 1 in chloroform-d, with a strong solvent effect the latter was also the guest and therefore influenced the stability of the capsules both from the inside and outside of the capsule. The ABAB monomer gave rise to only one chiral racemic capsule, while for AABB two regioisomeric structures were possible, with a relative amount from 1 1 to 2 1 increasing the steric difference between A and B from n-hexyl to adamantyl residues. Moreover, the equilibrium between the regioisomeric forms could be shifted entirely on one side if two adjacent A and two B groups are covalently connected. 

Galbraith, J. A., Menzel, K. A., Ratilla, E. M. A. and Kostic, N. M. (1987) Study of stereodynamics by variable-temperature platinum-195 NMR spectroscopy. Diastereoisomerism in platinum(II) thioether complexes and solvent effects. Inorg. Chem., 26, 2073-2078. 

The stereoselectivity of the allylic hydroperoxidation also depends on se eral factors. With chiral allylic alcohols or allylic amines 200, a hydrogen is developed between ]02 and the vicinal hydroxyl or amino group. The fac differentiation results from an approach of 102 in the transition state that mi mizes 1,3-allylic strain. 201 and 202 can be obtained with a diastereoisome excess higher than 90% in CCI4. As previously indicated for the formation dioxetanes and 1,4-endoperoxides, the selectivity decreases considerably in presence of hydroxylic solvents [123]. When hydrogen bonding is no more pos ble in 203, the stereofacial differentiation is steered by steric and electronic rep sion effects at the level of the possible diastereoisomeric transition states 204 is formed selectively (Scheme 54). 

A study carried out by Kocienski and Lythgoe flrst demonstrated the trans selectivity of the Julia coupling process. The authors found the i uctive elimination could best be carried out with the acet-oxy or benzoyloxy sulfones. If the lithio sulfone derivative is used for addition to the carbonyl, the reaction can be worked up with acetic anhydride or benzoyl chloride to obtain the alkene precursor. In cases where enolization of the carbonyl is a complication, the magnesium derivative can frequently be used successfully. A modification of the reductive elimination was found to be most effective. Methanol, ethyl acetate/methanol or THF/methanol were the solvents of choice and a temperature of -20 C was effective at suppressing the undesired elimination of the acetoxy group to produce the vinyl sulfone. With these modifications of the original procedure, the ability of the reaction to produce dienes as well as rran.r-disubstituted alkenes was demonstrated, llie diastereoisomeric erythro- and threo-acetoxy sulfones could be separated and it was demonstrated that both isomers were converted to the rrans-alkene. It... 

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