Enolization stereoselectivity

Another example is the asymmetric synthesis of ( )-pinidine 208 and its isomers. These syntheses are achieved via asymmetric enolization, stereoselective cyclopropanation, and oxidative ring cleavage of the resulting cyclopropanol system (Scheme 5-68).123... 

In studies not yet published (66), the A/-acyl-oxazolidine-2-one 62 has been found to exhibit exceptionally high levels of (Z)-enolization stereoselection with either amide bases (LDA, THF, -78°C) or boryl triflates [(n-C4H9)2BOTf, CH2CI2, -78°C] in the presence of diiso-propylethylamine (DPEA). Upon aldol condensation, the enolates 63a and 63b afford the aldolates 64 (Scheme 11), which react readily with nucleophiles at the carbonyl function (Table 22). As discussed earlier, the large preference for (Z)-enolate formation in this system can be attributed to allylic strain considerations (37)... 

In the presence of water-free late transition metalphosphine cation complexes as Lewis acids, glyoxylatetosylamine imine reacted with silicon enolates stereoselectivity [23-26]. It was proposed that imine coordinated to the metal such as Ag(I), Pd(II), and Cu(I) in a bidentate manner [23]. The copper-based catalyst was the most effective, and the desired product was obtained in high yields with high enantioselectivities [(Eq. (4)]. 

The above described total synthesis features the first enantiodivergent approach to (+)- and (—)-scopadulcic acid A. The central transformations are the stereoselective carbonyl group reduction with (S)-Alpine Borane , the use of enolization stereoselection to dictate which enantiomer is produced, and the palladium-catalyzed bis-Heck cyclization which occurs with complete stereo- and regiocontrol to establish the scopadulan scaffold. 

An ingenious way of getting a syn ester aldol product is to do the more reliable ketone syn aldol with a bulky group (to ensure the ds-enolale is formed) and then to oxidize off the bulky group. Here s what we mean. The starting material is very like the f-butyl ketone that you saw enolize stereoselectively above only the ds-enolate can form. The enolate reacts highly syn selectively with... 

The reagent hydroxylation of lithium enolates Stereoselectivity (substrate control)... 

Both diastereomers 5 and 6 have exclusively E configuration. This requires the branched side chain of the tertiary allylic alcohol moiety of 4 to occupy the equatorial position. The diastereomeric ratio derives from the enolization stereoselectivity. For a less selective example see also ref 30. 

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