Biomimetic Organocatalytic C–C-Bond Formations
91
Further contributions to the research on the asymmetric benzoin condensation were made by Leeper et al. using novel chiral, bicyclic thiazolium salts, which led to enantiomeric excesses up to 21% and yields
up to 50% (Knight and Leeper 1997). Another thiazolium catalyst containing a norbonane backbone gave benzoin in quantitative yields with
an enantiomeric excess of 26% (Gerhards and Leeper 1997). In 1998,
Leeper et al. reported novel chiral, bicyclic triazolium salts that produced aromatic acyloins with varying enantioselectivities (20%–83%
ee) (Knight and Leeper 1998).
Inspired by these results, a new chiral, bicyclic triazolium salt was
developed in our research group in 2002 on the basis of (S)-tert-leucin.
Utilizing a modified three-step procedure of Knight and Leeper the
oxazolidin-2-one 99 was methylated with Meerwein’s reagent to form
the corresponding imino ether 100. This was treated with phenylhydrazine to yield the corresponding hydrazone 101 and cyclized with
trimethyl orthoformate in methanol to give the triazolium salt 102 in
50% yield as a crystalline solid (Scheme 26).
Scheme 26. Synthesis of the triazolium salt 102
91
Further contributions to the research on the asymmetric benzoin condensation were made by Leeper et al. using novel chiral, bicyclic thiazolium salts, which led to enantiomeric excesses up to 21% and yields
up to 50% (Knight and Leeper 1997). Another thiazolium catalyst containing a norbonane backbone gave benzoin in quantitative yields with
an enantiomeric excess of 26% (Gerhards and Leeper 1997). In 1998,
Leeper et al. reported novel chiral, bicyclic triazolium salts that produced aromatic acyloins with varying enantioselectivities (20%–83%
ee) (Knight and Leeper 1998).
Inspired by these results, a new chiral, bicyclic triazolium salt was
developed in our research group in 2002 on the basis of (S)-tert-leucin.
Utilizing a modified three-step procedure of Knight and Leeper the
oxazolidin-2-one 99 was methylated with Meerwein’s reagent to form
the corresponding imino ether 100. This was treated with phenylhydrazine to yield the corresponding hydrazone 101 and cyclized with
trimethyl orthoformate in methanol to give the triazolium salt 102 in
50% yield as a crystalline solid (Scheme 26).
Scheme 26. Synthesis of the triazolium salt 102