Stereoselectivity in addition

Table 8.2. Stereoselectivity in Addition of Organometallic Reagents to Some Chiral Aldelqrdes and Ketones"...

In this context, pyranoid sugar enolones of type 8, or 9, i. e. those that carry chiral centers on either side of the enolone structural element, are even more powerful building blocks stereoselectivities in addition reactions. They are accessible in various substituted forms from the respective hydroxyglycal esters in another, preparatively delightful reaction channel, a chlorination - hydrolysis elimination sequence (27, 28). 

This value is much larger than that observed in the previously reported deracemizations of [Cr(ox)3]3 and Cr(acac)3. In this reaction, the basic condition is necessary, and the addition of Hacac increases the enantiomer excess, for which the reason will be discussed below. The reaction mechanism shown in Scheme 17 was proposed. In the mechanism, the 3MLCT excited A- [Ru(( — )-men-bpy)3]2+ is oxidatively quenched by Co(acac)3 to form an exciplex with Co(acac)3 followed by electron transfer to Co(acac)3 from A- [Ru(( — )-menbpy)3]2 +, which leads to the formation of a successor complex, [A-Ruin(( — )-menbpy)33 + Con(acac)3 ]. This successor complex dissociates to A-[Rum(( — )-menbpy)3]3+, Co(acac)2, and acac. If the reducing reagent is absent or the reducing reagent does not effectively reduce the ruthenium(III) complex, Co(acac)2 reduces A-[Rum(( — )-menbpy)3]3+ to A-[Run(( — )-menbpy)3]2+ concomitantly with the formation of Co(acac)3. As discussed in Sec. II.A., the photoreduction of Co-(acac)3 occurs stereoselectively. In addition, the oxidation of Co(acac)2 to Co-(acac)3 occurs stereoselectively, because Co(acac)2 reacts with the chiral ruthen-... 

Deprotonation of esters with lithium dialkylamides gives rise to ( )-enolates. However, with normal alkyl propionates there is little or no stereoselectivity in additions to aldehydes (equation 65). It was found by Meyers and Reider that certain esters that contain additional ether oxygens in the alcohol moiety give reasonably high anti selectivity (equation 66). Unfortunately, this high selectivity is not general, as is shown by the example in equation (67). ... 

The stereoselectivity in addition and abstraction reactions of cyclopentyl radicals has been reviewed recently3. It has been concluded that /f-substituents at the radical, as well as the alkene substituents, have a large influence on the selectivity, however only small solvent effects have been found. 

Isopropylidene-protected ct.v-diols, which are frequently utilized in carbohydrate chemistry, are found to have a large influence on the stereoselectivity in addition reactions when they are located adjacent to the radical center35,75. The selectivity seems to be somewhat lower when the radical is located at the anomeric center compared to other ring positions. 

Cocyclodimerizations of norbornene with butadiene or acetylene dicarboxylates, like many other examples, reveal an exo stereoselectivity in addition reactions25. 

Table 7.5. Stereoselectivity in Addition of Organometaiiic Reagents to Some Chirai... 

In the preceding reaction, stereoisomer A forms stereoisomer B but does not form D, so the reaction is stereoselective in addition to being stereospecific. All stereospecific reactions, therefore, are also stereoselective. All stereoselective reactions are not stereospecific, however, because there are stereoselective reactions in which the reactant does not have a carbon-carbon double bond or an asymmetric carbon, so it caimot exist as stereoisomers. 

Stereoselectivity in addition reactions of borane with bicyclic alkenes is similar to that observed for reduction of bicyclic ketones with hydride reducing agents. Boranes add to norbornene derivatives from the exo face (27 gave 85% exo attack upon reaction with diborane). In 27-30, all values are percentage yields of the organoborane products obtained by reaction from that face. Interestingly, only 80% exo attack was observed when 27 reacted with disiamylborane, and the size of the borane appears to have little effect on selectivity. 

Stereoselectivity in addition of organometallic reagents to some chiral aldehydes and ketones... 

P450s are able to catalyze numerous different reaction types and can oxidize a wide range of molecules. Many of these compounds occur in nature and can be important precursors. Thereby, P450s often exhibit high regio-, chemo-, and/or stereoselectivity. In addition, enzyme engineering can be applied to further... 

Compared to 1° and 2° alcohols, 3° alcohol derived allylic esters have received comparatively litde attention as substrates in the Ireland-Claisen rearrangement. This may to some extent reflect difficulty in the synthesis of the requisite i" ester. In addition, stereocontrol in the formation of the resultant alkene may be problematic. The difference in size of the two allyhc substituents is in general large for 2" allyhc stereocenters since one substituent is always H (e.g., R = H, H, Scheme 4.101). However, for 3° aUyhc esters, a smaller size difference will lead to low alkene stereoselectivity. In addition, if there is a lack of differentiation between the two chair transition states, the C2 and C3 stereocenters will be formed in both antipodal forms. 

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