Stereochemistry that produce diastereomers

As the number of stereocenters in a molecule increases, the number of possible diastereomers increases. A molecule with four dissimilar stereocenters, for example, can exist as one of sixteen stereoisomers. Of these sixteen stereoisomers there are four pairs of enantiomers, and the remaining four pairs are diastereomers. Molecules with configurational diastereomers also arise from many systems other than those with stereocenters. One of the most common examples is a double bond that is substituted in such a way that diastereomers exist. Any combination of two or more molecular features that give rise to stereoisomers will always produce diastereomers, whereas sources of chirality are needed to produce enantiomers. Because stereochemistry can have a high impact on molecular properties, diastereomers generally have easily discernable differences in their physical and chemical behaviors. Some molecules possess greater than or equal to two tetrahedral stereocenters and are nonetheless achiral. These are called meso stereoisomers. These occur when the internal symmetry of the molecule makes it superimposable on its mirror image. 

The desilylacetylated qrcloadducts, produced from the reactions of trimethylsilyl-diazomethane with 3-crotonoyl-2-oxazolidinone or 3-crotonoyl-4,4-dimethyl-2-oxa-zolidinone, were transformed to methyl traws-l-acetyl-4-methyl-l-pyrazoline-5-car-boxylate through the reactions with dimethoxymagnesium at -20 °C. When the optical rotations and chiral HPLC data were compared between these two esters, it was found that these two products had opposite absolute stereochemistry (Scheme 7.39). The absolute configuration was identified on the basis of the X-ray-determined structure of the major diastereomer of cycloadduct derived from the reaction of trimethylsilyldiazomethane to (S)-3-crotonoyl-4-methyl-2-oxazolidi-none. 

That only the wrong C-16 diastereomer seemed to be produced in this reaction was then demonstrated by the Kutney group, who prepared a series of binary indole-indoline alkaloids using the chloroindolenine approach. The apparent simplicity of this coupling reaction and the rapidity in assembling such binary alkaloids prompted an extensive study of reaction conditions (28), with the desire to find a procedure suitable for generation of the C-16 (S) isomer, required for anticancer activity. Despite the intensive effort involved in this in-depth study, no success could be realized, and it was therefore widely accepted that .. . it is very unlikely that any natural dimer could be obtained in this way (7). At this point it may be noted, however, that we were able to show subsequently that the desired C-16 -C-14 PARF relative stereochemistry can be obtained as a preferential result, albeit only in very low yield [3.6% PARF versus 2.1% PREF (priority reflective)], when the chloroindolenine reaction with vindoline is initiated with silver tetrafluoroborate (13). 

The only concern is die cis stereochemistry of die cycloadduct O. If die planar azomethine ylide adopts the least sterically hindered W geometry, then the cis isomer will be produced as a pair of enantiomers. The use of d.v-stilbenc as the dipolarophile to obtain die all-cis geometry in one step would require that only die endo transition state produces product. Although endo transitions are favored in 1,3 dipolar cycloadditions, mixtures of diastereomers from the exo and endo transition states are usually formed. Catalytic hydrogenation has a higher facial selectivity and is much more likely to give a single diastereomer. 

P-Keto esters and -keto amides, each substituted between the two carbonyl units with a 2-[2-(tri-methylsilyl)methyl] group, also undergo Lewis acid catalyzed, chelation-controlled cyclization. When titanium tetrachloride is used, only the product possessing a cis relationship between the hydroxy and ester (or amide) groups is product yields range from 65 to 88% (Table 8). While loss of stereochemistry in the product and equilibration of diastereomers could have occurred via a Lewis acid promoted retro aldol-aldol sequence, none was observed. Consequently, it is assumed that the reactions occur under kinetic, rather than thermodynamic, control. In contrast to the titanium tetrachloride promoted process, fluoride-induced cyclization produces a 2 1 mixture of diastereomeric products, and the nonchelating Lewis acid BF3-OEt2 leads to a 1 4.8 mixture of diastereomers. 

The stereoselection in the cyclization of each diastereomer was examined independently. The stereochemical outcome of the cyclization should be predictable based on our assumption regarding the relationship between enolate stereochemistry and cyclopropane stereochemistry, the principles of asymmetric, intermolecular alkylation of optically active amides (9-13) and the assumption that the mechanism of cyclopropane formation involves a straightforward back-side, %2 reaction. In the case of the major diastereomer, the natural tendency of the enolate to produce the cis-cyclopropane will oppose the facial preference for the alkylation of the chiral enolate. Consequently, poorer stereochemical control would be ejected in the ring closure. In the minor diastereomer these two farces are working in tandem, and high degrees of stereocontrol should result. 

Styrene (and derivatives) also possesses the rare monomer quality that the neat material, without initiator, may be spontaneously polymerized by simply heating to 80-100°C for 24-48 hr. It is thought that this occurs via the initial Diels-Alder dimerization of styrene to the two diasteomers A and B [14]. The two diastereomers appear to have an extremely labile hydrogen, which is both doubly allylic and tertiary. However, only dimer A has the correct stereochemistry (an axial phenyl), which enables the excess styrene to abstract a hydrogen atom from it, producing two radical species (Eq. 23.5). 

P, P] By using antimony(V) chloride and tin(II) triflate, monothioketene acetals can be induced to add to a,/J-unsaturated thioesters (76). Interestingly, neither antimony(V) chloride nor tin(II) chloride by themselves effectively promote the reaction, implying that the combination of Lewis acids produces a new species. The stereochemistry of these additions is summarized in Scheme 44 and Table 16. For all cases studied, the reaction uniformly provides the anti diastereomers with good selectivity. The selectivity observed is somewhat lower than the optimized results observed with thioketene acetals and enones (vide supra). 

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