Prostereogenic enolates 1,4-additions

The big difference between the extent of asymmetric induction on the addition to a prostereogenic carbonyl group of simple carbanions a to a chiral sulfoxide on the one hand and enolates of sulfinyl esters on the other, can be attributed to the capacity of the ester function to chelate magnesium in the transition states and intermediates. The results already described for the addition of chiral thioacetal monosulfoxide to aldehydes (see Section 1.3.6.5.) underscore the importance of other functions, e.g., sulfide, for the extent of asymmetric induction. 

In contrast to the usual anti selectivity a remarkably high syn selectivity is observed in the addition of thioester enolates to 2-alkylidenealkanones297. The syn selectivity is probably due to a stereoselective internal autoprotonation of the resulting enolates by the dithioester a-pro-tons298 in these cases where the prostereogenic centers reside exclusively in the enone part (see also Section D.2.I.). 

N, P ] and [P, P ] Aldehydes with an a-stereocenter exhibit unusually high diastereofacial preferences for the addition of silyl enol ethers and ketene acetals with Lewis acid assistance (81). Heathcock and Uehling found good levels of facial discrimination in the addition of silyl enol ethers to chiral enones (Scheme 38, Table 11) (82). With the more substituted silyl enol ether, only one diastereomeric addition product is obtained (Eq. [1], Scheme 38). Use of a prostereogenic silyl enol ether allows control over the relative... 

In this event, the addition of 2.1 equivalents of NaHMDS in THF at —78°C converted 17 into a prostereogenic cyclic enolate which then attacked the tartrate-derived dielectrophile 18 with a high level of facial selectivity to generate an intermediate of type 19. Subsequent cyclization was then biased to provide the desired pseudo meso product 20, a compound that was ultimately obtained in 92% yield. In contrast, when the conditions were changed to prevent chelation by using LiHMDS instead of NaHMDS in a solvent mixture of THF and HMPA (hexamethyl-phosphoramide), the C2-symmetric variant of this product (27, see column figures) was formed in 58 % yield. This alternate outcome presumably reflects the preliminary formation of an acyclic lithium dienolate which led to a monoalkylated product corresponding to 26. ... 

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