Boron enolates diastereofacial preferences

A related reagent (186) is prepared in three steps from (5)-atrolactic acid. The lithium enolate of (186) reacts with phenylacetaldehyde to give two aldols in a ratio of 94 6 (Scheme 12). [The full relative stereochemistry of aldols (187) and (188) was not rigorously determined, but may be deduced from the stereochemistry of (190). It is surprising that the lithium enolate of (186) has a diastereofacial preference that is opposite that of the related ketone (181).] Compound (186) has been used in a synthesis of the Prelog-Djerassi lactonic acid (191), as shown in Scheme 12. Reagents related to (181) and (186) have been used as their boron enolates for other synthetic purposes (see Chapter 1.7, this volume). 

Chiral amides (222) and (223) and imides (224) and (225) have also been studied as reagents for asymmetric aldol reactions. These reagents show excellent diastereofacial preferences as their boron and zirconium enolates, but generally show poor selectivity as their lithium enolates. The reader is referred to other chapters in this volume for a discussion of these and related reagents. 

One of the more useful chiral auxiliaries that has been developed is illustrated in equation (128). The lithium enolates of imides (229) react with aldehydes to give mainly the syn diastereomer (230), along with (231) and an anti isomer (Table 27). After purification, the major syn isomer is obtained in good yield and in good stereochemical purity. The results shown in Table 27 nicely complement the results obtained using the boron enolate. In this case, no anti products are observed and the (231) (230) ratio is >9S S in all cases. Thus, acylsultam (229), like a-alkoxy ketone (181 Scheme 11), has opposite diastereofacial preference as its lithium and boron enolates. 

---