98
D. Enders, M.R.M. Hüttl, O. Niemeier
Scheme 31. Preparation of triazolium salts 123 derived from pyroglutamic acid
sis of 123 also gave access to the chiral tetracyclic triazolium salt 127,
which was obtained as a solid in 59% yield.
The asymmetric intramolecular benzoin condensation with model
substrate 106 (R = Me) and the chiral triazolium salt 119 as precatalyst gave rise to the desired acyloin 107 in good yields by utilizing
toluene as solvent and diazabicycloundecane (DBU) or KOt-Bu as the
base (Scheme 33). Unfortunately only moderate enantiomeric excess
(37%–48%) could be achieved, even at 5°C. The use of 10 mol% of
the TBS-substituted catalyst 123 and stoichiometric amounts of DBU
in toluene at room temperature enabled the methyl-substituted acyloin
106 to be obtained in high yield (92%) but with only moderate enantioselectivity (61%). Application of the TIPS substituted catalyst 123 under the same conditions resulted in an increased enantiomeric excess of
77%, which could be further improved to 84% with almost unchanged
yields by performing the reaction at 5°C. The reactions with the tetracyclic catalyst 127 were conducted in tetrahydrofuran for better solubility. Furthermore, DBU was found to cause side-reactions that could
be suppressed when KOt-Bu was used in substoichiometric amounts (9
D. Enders, M.R.M. Hüttl, O. Niemeier
Scheme 31. Preparation of triazolium salts 123 derived from pyroglutamic acid
sis of 123 also gave access to the chiral tetracyclic triazolium salt 127,
which was obtained as a solid in 59% yield.
The asymmetric intramolecular benzoin condensation with model
substrate 106 (R = Me) and the chiral triazolium salt 119 as precatalyst gave rise to the desired acyloin 107 in good yields by utilizing
toluene as solvent and diazabicycloundecane (DBU) or KOt-Bu as the
base (Scheme 33). Unfortunately only moderate enantiomeric excess
(37%–48%) could be achieved, even at 5°C. The use of 10 mol% of
the TBS-substituted catalyst 123 and stoichiometric amounts of DBU
in toluene at room temperature enabled the methyl-substituted acyloin
106 to be obtained in high yield (92%) but with only moderate enantioselectivity (61%). Application of the TIPS substituted catalyst 123 under the same conditions resulted in an increased enantiomeric excess of
77%, which could be further improved to 84% with almost unchanged
yields by performing the reaction at 5°C. The reactions with the tetracyclic catalyst 127 were conducted in tetrahydrofuran for better solubility. Furthermore, DBU was found to cause side-reactions that could
be suppressed when KOt-Bu was used in substoichiometric amounts (9