Preface
VII
known Stetter reaction with diverse modes of action is provided by
K. Zeitler.
Complementary to the great variety of Lewis basic organocatalysts,
Brønsted acids gain increasing importance and significantly expand
the scope of organocatalytic processes. M. Rueping and E. Sugiono
demonstrate the versatility of chiral BINOL-phosphate-based Brønsted
acids in the asymmetric hydrogenation of quinolines, benzoxazines and
pyridines and they highlight the potential of this method in alkaloid
total synthesis. The first radical reactions catalyzed by a chiral organic
additive are realized in a piperidone and dihydroquinolinone synthesis
by T. Bach and coworkers. An organocatalyst derived from Kemp’s acid
interacts with the secondary lactam or amide functionality of the substrates through double hydrogen bond contacts. This is an addition to
the covalent involvement of amine organocatalysts, and allows for optimizing the stereochemical outcome by using nonpolar solvents, such as
trifluorotoluene. Hydrogen bonding networks in chiral thiourea catalysis are also elegantly used by A. Berkessel in the kinetic resolution of
oxazolones and oxazinones.
Various aspects of organocatalysis with larger molecules are also covered in this book. Possible benefits from immobilization approaches for
organic catalysts are pointed out by M. Benaglia. Apart from catalyst
recycling or simplified workup procedures, catalyst immobilization can
be additionally advantageous in terms of catalyst development and optimization. The use of soluble supports, such as polyethylene glycol, often
allows the direct transfer and application of already optimized reaction
conditions.
In the field of enzyme catalysis, some of the major drawbacks, such
as narrow substrate scope, or low selectivity and thermostability, can be
successfully addressed by using directed evolution. In contrast to a rational design which uses site-specific mutagenesis, these studies utilize
genomic technologies like saturation mutagenesis and gene shuffling
to create powerful, tailor-made proteins as large molecule organocatalysts. An even more effective strategy to enhance enzyme catalysis is the
symbiosis of rational design and randomization, as applied in CASTing
(combinatorial active-site saturation test) in combination with iterative
saturation mutagenesis which was introduced by M.T. Reetz.
VII
known Stetter reaction with diverse modes of action is provided by
K. Zeitler.
Complementary to the great variety of Lewis basic organocatalysts,
Brønsted acids gain increasing importance and significantly expand
the scope of organocatalytic processes. M. Rueping and E. Sugiono
demonstrate the versatility of chiral BINOL-phosphate-based Brønsted
acids in the asymmetric hydrogenation of quinolines, benzoxazines and
pyridines and they highlight the potential of this method in alkaloid
total synthesis. The first radical reactions catalyzed by a chiral organic
additive are realized in a piperidone and dihydroquinolinone synthesis
by T. Bach and coworkers. An organocatalyst derived from Kemp’s acid
interacts with the secondary lactam or amide functionality of the substrates through double hydrogen bond contacts. This is an addition to
the covalent involvement of amine organocatalysts, and allows for optimizing the stereochemical outcome by using nonpolar solvents, such as
trifluorotoluene. Hydrogen bonding networks in chiral thiourea catalysis are also elegantly used by A. Berkessel in the kinetic resolution of
oxazolones and oxazinones.
Various aspects of organocatalysis with larger molecules are also covered in this book. Possible benefits from immobilization approaches for
organic catalysts are pointed out by M. Benaglia. Apart from catalyst
recycling or simplified workup procedures, catalyst immobilization can
be additionally advantageous in terms of catalyst development and optimization. The use of soluble supports, such as polyethylene glycol, often
allows the direct transfer and application of already optimized reaction
conditions.
In the field of enzyme catalysis, some of the major drawbacks, such
as narrow substrate scope, or low selectivity and thermostability, can be
successfully addressed by using directed evolution. In contrast to a rational design which uses site-specific mutagenesis, these studies utilize
genomic technologies like saturation mutagenesis and gene shuffling
to create powerful, tailor-made proteins as large molecule organocatalysts. An even more effective strategy to enhance enzyme catalysis is the
symbiosis of rational design and randomization, as applied in CASTing
(combinatorial active-site saturation test) in combination with iterative
saturation mutagenesis which was introduced by M.T. Reetz.