Anti aldol reaction, dibutylboron triflate

The Masamune aldol condensation, in common with the Evans aldol condensation, involves a boron enolate of an ester containing a norephedrine derived chiral auxiliary however, unlike the latter, the Masamune aldol delivers a 3-hydroxy-2-methyl carbonyl moiety with the anti-stereochemistry. Crucial to the success of this reaction is the use of dicyclohexylboron triflate to generate the boron enolate. Note in the Evans aldol condensation, dibutylboron triflate is utilized. 

SCHEME 8.9. The anti aldol reaction using excess amounts of dibutylboron triflate. 

Oppolzer et al. used (2M)-bornanc-10, 2-sultam as an effective chiral auxiliary to achieve a highly enantioselective syn-aldol reaction17 (Scheme 2.1p). Treatment of A-propionylsultam (46) with dibutylboron triflate and Hunig s base at -5°C in CH2CI2 followed by addition of aldehydes at -78° C provided, after a simple crystallization, the pure vyn-aldols 47a. It is noteworthy that no anti-aldol product was observed in the aldol reactions with any of the aldehydes. From the1 nuclear magnetic resonance (NMR) study, it was confirmed that the boron... 

High selectivity for either the anti or the syn aldol product can be obtained by using an appropriate thioester and a boron-mediated aldol reaction. The trans-enolate and hence the anti aldol product can be obtained from the 5-tert-butyl ester and dibutylboron triflate (1.68). The trans-enolute (note the convention that the O-metal substituent takes a higher priority than the SR substituent) is favomed owing to the steric interaction of the bulky tert-butyl group with the ligands on the boron... 

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See also in sourсe #XX -- [ ]

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