Aldol reactions external chiral reagents

External chiral reagents for the aldol reaction of ketones 262... 

The (0)-reagent (91) is formed almost exclusively from S-3-(3-ethyl)pentyl propanethioate (92) and borolanyl triflate (90). Despite its apparent steric demand, (91) still retains a high degree of reactivity towards aldehydes (in Scheme 21 the interaction between R and Me is not exceedingly large). Summarized in Table 6 are the results obtained from aldol reactions of representative aldehydes with (91 Scheme 38). All reactions proceed smoothly at -78 °C and the major products have the 2,3-anti stereochemistry (antiisyn > 30 1). With (2 ,55)-(90) the aldehydes examined provide, in most cases, the (2R) aldol products with more than 98% ee. It is important to note that the external chiral moiety can be recovered as its 2,2-dimethylaminoethanol complex, and that the aldol products are equipped with a versatile thioate functionality for further synthetic transformation. 

In contrast to all known chiral boron enolates, the external reagent (95) derived from ethanethioate (96) and 2,S-dimethylborolanyl triflate (25,S5)-(90) effects asymmetric induction in the aldol reaction to provide P-hydroxy Aioesters of approximately 90% ee for most aldehydes (Scheme 41 Table 8). ... 

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