Diastereoisomer interconversion

Diastereoisomers are stereoisomers which do NOT have a mirror image of one another. Figure 11.20 shows the diastereoisomers of 2-butene (alkenes such as this are sometimes called geometric isomers and are a consequence of the prohibition of rotation about double bonds). If a vertical mirror was placed between the two structures in Fig. 11.20 they would not reflect onto one another. If the functionality is on the same side then the isomer is the cis-form, if on the opposite side then it is the trans- form. The chemical properties are very similar because the functional groups are identical. However, as they have different shapes their physical properties are different. Interconversion requires breaking and remaking bonds so these isomers are also stable under normal conditions. 

In the equilibration studies previously cited, two mechanisms for the interconversion of aldol diastereoisomers are possible, the most obvious being via the retroaldol process (28). In some instances, however, base-catalyzed equilibration via the aldolate enolate 18 is certainly possible (eq. [11a]), and such enolates are well documented as useful intermediates in synthesis (19). For example, Frdter has demonstrated that aldolate enolate 18a may be generated from... 

Diastereoselectivity denotes the preferential formation of one out of several diastereoisomers. Selected examples are presented in the following text for anodic and cathodic C, C bond formations and for functional group interconversions. 

Whereas racemization is the complete loss of optical activity with time, epimerization is the reversible interconversion of diastereoisomers to an equilibrium mixture which is not necessarily optically inactive. Diastereoisomers arise from the combination of the two chiral centers in 9, namely the metal centered, R and S, and the resolved (S) optically active ligand center. The diastereoisomers (RS) and (SS) differ in their properties. 

Grigg and co-workers (310) recently examined the 1,3-APT reaction of various aldoximes (270) (R or R = H) with divinyl ketone (Scheme 1.56). While ketoximes 270 (R = R) form a mixture of adducts, 271 and 272 via nitrone 273, the aldoximes selectively afford 272 (as a mixture of endo and exo diastereoisomers). Under the thermal reaction conditions, the oxime starting materials can undergo ( /Z) isomerization, while the nitrone intermediate was expected to be unaffected and the isolated cycloadducts showed no interconversion via cycloreversion. Thus, the increasing selectivity for endo-212 [via ( )-273, R = H] over exo-212 [via (Z)-273, R = H] with the increasing size of the aldoxime substituent was attributed primarily to the inhibition of oxime isomerization by steric clash between R or R and the oxime OH. In contrast, Lewis acid catalysis, in particular by hafnium (iv) chloride, of the cycloaddition of various aldoximes with this dipolarophile gave exo-271 exclusively (216). 

For a complete quantitative description of the solvent effects on the properties of the distinct diastereoisomers of dendrimers 5 (G = 1) and 6 (G = 1), a multiparameter treatment was used. The reason for using such a treatment is the observation that solute/solvent interactions, responsible for the solvent influence on a given process—such as equilibria, interconversion rates, spectroscopic absorptions, etc.—are caused by a multitude of nonspecific (ion/dipole, dipole/dipole, dipole/induced dipole, instantaneous dipole/induced dipole) and specific (hydrogen bonding, electron pair donor/acceptor, and chaige transfer interactions) intermolecular forces between the solute and solvent molecules. It is then possible to develop individual empirical parameters for each of these distinct and independent interaction mechanisms and combine them into a multiparameter equation such as Eq. 2, "... 

The relative proportions of the separated diastereoisomers 40a and 40b (Ri = N02, R2 = CH3), the only bis-TB derivatives prepared via both synthetic pathways (Scheme 8), were not identical in each of the methods. The step-by-step synthetic approach (01JOC1607) afforded a 4 1 mixture of isomers 40a and 40b. However, in the case of the simultaneous strategy (04EJ01097), a 1 1 mixture of isomers 40a and 40b was formed under reaction conditions nearly identical to those of the cyclization method. Only the anti diastereoisomer 40b was isolated when the reaction temperature was reduced from 90 to 50°C. This difference in stereoselectivity during the formation of 40 (R3 = N02, R2 = CH3) via similar reactions can only be explained by stereoinduction of by-products, because in the reaction mixture the diastereoisomers 40a and 40b are in thermodynamic equilibrium due to the ready interconversion (racemization) of the methanodiazocine bridges in acidic media. 

Further studies described the interconversion of stegane diastereoisomers via thermal (for the biaryl axis) or acid/base-induced (for the a-lactone position) processes, Fig. (18) [110,111,112], These isomerization possibilities were largely exploited in the different total syntheses discussed in the following pages. 

The (j,Y-bidentate chromium(m).ATP complex (74 M = Cr +) has been separated into four diastereoisomers by reverse-phase h.p.l.c. The six-membered rings of the A and A forms are puckered, with the adenylate able to adopt an axial conformation (which is stabilized by transannular hydrogen bonding) or an equatorial conformation, in each. CD spectroscopy was used to identify the screw sense of the resolved isomers, and the interconversion rates measured, the axial-equatorial interconversion rates for the A and A isomers being more rapid than the A-A isomerization. Both (74 M = Cr +) and the corresponding... 

When X = Cl diastereomers could be separated on Chirex 3020AL and are stable. A remarkable difference in activity was observed for the two atropisomers (08BMCL3344). When X = F, the rate constant for the interconversion (k = 5.07 X10 s ) of these two atropisomers was determined by proton NMR analysis of a diastereoisomer-enriched sample in aqueous solution at 25 °C, and the corresponding barrier to rotation (97.4 kj mol ) was calculated using the E5ning equation. The diastereoisomer half-life at physiological temperature (37 °C) in aqueous media was estimated to be about 46 min (05CHI559). 

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