Stereoinduction 1,5-, boron aldol reactions

Hydrogen bonding and steric effects have been investigated in a theoretical study of the origin of the diastereoselectivity in the remote 1,5-stereoinduction of boron aldol (g) reactions of /3-alkoxy methyl ketones 125 high levels of 1,5-anti-stereocontrol have been achieved in such reactions of tf-methyl-a-alkoxy methyl ketones, giving both Felkin and anti-Felkin products.126 (g)... 

As previously mentioned, certain methyl ketone aldol reactions enable the stereocontrolled introduction of hydroxyl groups in a, 5-anti relationship (Scheme 9-7) [9], and this was now utilized twice in the synthesis. Hence, methyl ketones 48 and 98 were converted to their respective Ipc boron enolates and reacted with aldehydes 97 and 99 to give almost exclusively the, 5-anti aldol adducts 100 and 101, respectively (Scheme 9-34). In the case of methyl ketone 48, the j -silyl ether leads to reduced stereoinduction however, this could be boosted to >97%ds with the use of chiral ligands. In both examples, the y9-stereocenter of the aldehyde had a moderate reinforcing effect (1,3-syn), thus leading to triply matched aldol reactions. Adducts 100 and 101 were then elaborated to the spiro-acetal containing aldehyde 102 and ketone 103, respectively. 

Aldol Reactions. The title reagent and its various congeners continue to find application in Mukaiyama-t3fpe aldol reactions as versatile nucleophilic propionate synthons with predictable behavior.For example, the ( )-isomer of the reagent produced the expected major isomer in a boron trifiuoride etherate mediated addition to a complex aldehyde en route to a C19-C35 subunit of swinholide A (eq 14). Remote stereoinduction in Mukaiyama aldol reactions of the reagent with (2-sulfinylphenyl)acetaldehydes was recently observed anti aldol adducts were obtained preferentially regardless of the geometry of the silyl ketene thioacetal employed. ... 

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