Biomimetic Organocatalytic C–C-Bond Formations
105
Scheme 36. The Stetter Reaction
Scheme 37. First attempts of an asymmetric Stetter reaction
ophilic attack of the acyl anion equivalent to the Michael acceptor 131
(Scheme 36).
First attempts of an asymmetric Stetter reaction were made 1989 in
our research group with the investigation of chiral thiazolium salts such
as 136 as precatalysts. The reaction of n-butanal (133) with chalcone
(134) in a two-phase system gave the 1,4-diketone 135 with an enanantiomeric excess of 39%, but a low yield of only 4% (Scheme 37) (Tiebes
1990; Enders 1993; Enders et al. 1993b). The catalytic activity of thiazolium as well as triazolium salts in the Stetter reaction persisted at
a rather low level. Triazolium salts have been shown to possess a catalytic activity in the non-enantioselective Stetter reaction (Stetter and
Kuhlmann 1991), but in some cases stable adducts with Michael acceptors have been observed (Enders et al. 1996a), which might be a possible
reason for their failure in catalysis.
105
Scheme 36. The Stetter Reaction
Scheme 37. First attempts of an asymmetric Stetter reaction
ophilic attack of the acyl anion equivalent to the Michael acceptor 131
(Scheme 36).
First attempts of an asymmetric Stetter reaction were made 1989 in
our research group with the investigation of chiral thiazolium salts such
as 136 as precatalysts. The reaction of n-butanal (133) with chalcone
(134) in a two-phase system gave the 1,4-diketone 135 with an enanantiomeric excess of 39%, but a low yield of only 4% (Scheme 37) (Tiebes
1990; Enders 1993; Enders et al. 1993b). The catalytic activity of thiazolium as well as triazolium salts in the Stetter reaction persisted at
a rather low level. Triazolium salts have been shown to possess a catalytic activity in the non-enantioselective Stetter reaction (Stetter and
Kuhlmann 1991), but in some cases stable adducts with Michael acceptors have been observed (Enders et al. 1996a), which might be a possible
reason for their failure in catalysis.