Introduction
CBS Oxazaborolidines 1a-c are well-established catalysts, which mediate the enantio- and diastereoselective reduction of highly functionalized ketones.
[1]
[2]
Recently, the substrate scope has been extended to include oxime ethers
[3]
and ketimines.
[4]
Accordingly, these reagents have been extensively applied in natural product synthesis, for instance in the recently published total syntheses of (-)-laulimalide,
[5]
brevetoxin-B,
[6]
(+)-tanikolide,
[7]
and bistramide A.
[8]
Additionally, oxazaborolidines 1 were used in the kinetic resolution
[9]
and the dynamic kinetic resolution of atropoisomeric lactones.
[9b]
[10]
More recently, the protonated oxazaborolidinium salts 2a and 2b were used as chiral Lewis acids
[11]
in catalytic enantioselective Diels-Alder reactions,
[12]
the cyanosilylation of aldehydes,
[13]
Michael additions,
[14]
and a vinylogous Mukaiyama aldol reaction.
[15]
Compound 1a is typically generated in situ by reaction of (S)- or (R)-2,2-diphenylhydroxymethylpyrrolidine with BH3·THF,
[2]
while oxazaborolidines 1b,c may be prepared using the corresponding boronic acids.
[1c]
[16]
In a similar fashion, reaction of the same chiral pyrrolidine substrate with o-tolylboroxine furnishes o-tolyl-CBS-oxazaborolidine,
[16]
which, when protonated with triflic acid or triflimide, affords the oxazaborolidinium salts 2a and 2b, respectively.
[12b]
[d]
The facile preparation and modification of the oxazaborolidines, along with their high reactivity and selectivity, render members of this class of molecules as important candidates for future applications in asymmetric catalysis.
