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Current
Organic Chemistry
ISSN: 1385-2728
Current Organic
Chemistry
Volume 10, Number 10, July 2006
Contents
Recent Advances in Selective Biocatalysis
Guest Editor: Enzo Santaniello
Part I
Editorial Pp.
1051
Candida rugosa Lipase: A Traditional and Complex
Biocatalyst Pp. 1053-1066
P. Domínguez de María, A. R. Alcántara,
J. D. Carballeira, R.M. de la Casa, C. A. García-Burgos,
M. J. Hernáiz, J. M. Sánchez Montero and J.
V. Sinisterra
[Abstract]
Stereoselective Enzymatic Acylations (Transesterifications)
Pp. 1067-1094
Robert Chênevert, Nicholas Pelchat and Frédéric
Jacques
[Abstract]
Lipase-Catalyzed Deacylation by Alcoholysis: A
Selective, Useful Transesterification Reaction Pp.
1095-1123
Enzo Santaniello, Silvana Casati and Pierangela Ciuffreda
[Abstract]
Enzymatic Aminolysis and Ammonolysis Processes
in the Preparation of Chiral Nitrogenated Compounds
Pp. 1125-1143
Vicente Gotor Fernández and Vicente Gotor
[Abstract]
Diversity of Epoxide Hydrolase Biocatalysts
Pp. 1145-1161
M. S. Smit and M. Labuschagné
[Abstract]
Glycosyl Hydrolases and Glycosyltransferases in
the Synthesis of Oligosaccharides Pp. 1163-1193
Antonio Trincone and Assunta Giordano
[Abstract]
Abstracts
[Back to top]
Editorial
Biocatalysis represents today a well established field of
research at a crossroad between organic synthesis and biotechnology,
dealing with the application of biological systems such as
microorganisms, enzymes or catalytic antibodies to the synthesis
of organic compounds. The main advantage of biocatalysis is
related to the the fact that systems, generally defined as
biocatalysts, can be used in aqueous solutions under mild
experimental conditions of temperature and pH, typical of
biological system. This may constitute a limitation for organic
molecules, that generally are not very soluble in an aqueous
medium. On the other hand, the activity of many enzymes is
compatible with the addition of organic cosolvents and others,
such as lipases, can also work also in anhydrous organic solvents.
Additionally, well developed immobilization techniques may
protect the biocatalyst from denaturating properties of organic
solvents and cosolvents. In general, most biocatalysts can
be satisfactorily used to realize chemo-, regio- or stereoselective
transformations. This issue of Current Organic Chemistry,
entitled RECENT ADVANCES IN SELECTIVE BIOCATALYSIS is intended
to highlight the most recent applications of biocatalysis
for selective processes in organic synthesis with special
regard to protection of polyfunctional compounds or preparation
of compounds required for the synthesis of stereochemically
pure compounds, in turn endowed with biological activity.
Several reviews and books have been devoted to this topic,
that now steadily occupies a well established place in the
scientific literature.
This issue is dedicated to Professors J. Bryan Jones and Charles
J. Sih who have been pioneers in the field and, thitrty years
ago, edited with J. Rosazza of a book entitled “Applications
of biochemical systems in organic chemistry”. In these
two volumes, any organic chemist could find the most important
notions about what was, at the time, the emerging field of
biocatalysis. In line with the inspiration of this yet valuable
book, two issues of Current Organic Chemistry will be dedicated
to the topic of stereoselective biocatalysis, covering applications
of a wide range of synthetically useful reactions catalyzed
by purified enzymes or microorganisms. In Part I, aspects
of selectivity in biocatalysis will be discussed, with a special
attention to lipase-catalyzed transformations. The complexity
of Candida rugosa lipase as biocatalyst has been
critically examined (J. V. Sinisterra) and application of
lipases to stereoselective transesterification (R. Chénevert),
alcoholysis (E. Santaniello), and aminolysis (V. Gotor) reactions
have been reviewed. Diversity of epoxide hydrolases has been
discussed (M. S. Smit) and application of glycososyl hydrolases
or related transferases (A. Trincone) have also been illustrated.
Enzo Santaniello
Università degli Studi di Milano
Milano, Italy
[Back to top]
Candida rugosa Lipase: A Traditional and Complex
Biocatalyst
P. Domínguez de María, A. R. Alcántara,
J. D. Carballeira, R.M. de la Casa, C. A. García-Burgos,
M. J. Hernáiz, J. M. Sánchez Montero and J.
V. Sinisterra
Different commercial preparations from Candida rugosa
lipase lead often to an irreproducible behaviour when employed
in slightly hydrated media, even when different samples from
the same supplier are used. This conduct is triggered by several
causes, such as the different concentration of the “real”
catalyst in the different crude samples (quantity),
or the inherent problems related to heterogeneous biocatalysts
(i. e., different amount of water, diffusional problems, etc);
furthermore, for C. rugosa lipase, the diverse percentage
of different isoenzymes (quality) is another irreproducibility-inducing
factor. In this sense, for a rational understanding of all
the experimental data described for this complex biocatalyst,
topics like the description of the role of the inducer on
the fermentation, the biochemical characterisation of the
crude biocatalysts and the establishment of synthetical strategies
to overcome the irreproducibility problems must be covered.
Once these tasks are fulfilled, we will be able to understand
the relative catalytic activity of different samples from
different origins, showing that enzymes should not merely
be considered as a “white magic powder”
with synthetical utility. Different techniques to evaluate
the importance of the amount of water are discussed: 1H-NMR
for comparing the hydrolytic activity of isoenzymes, and sorption
isotherms and Thermogravimetric and Differential Thermal Analysis
(TGA/DTA) for understanding the esterification in organic
media. Furthermore, different characterisation reactions useful
for quantifying the lipase loading (heptyl oleate synthesis
and transesterification of vinyl acetate with 1-heptanol)
are proposed. Finally, a synthetical strategy based on the
acylation, via vinyl acetate, of 1-heptanol, geraniol,
nerol and cyclohexanol, for comparing the catalytical results
and the isoenzymatic profile is discussed.
[Back to top]
Stereoselective Enzymatic Acylations (Transesterifications)
Robert Chênevert, Nicholas Pelchat and Frédéric
Jacques
Hydrolytic enzymes such as lipases and esterases are
the most frequently used biocatalysts in organic synthesis.
They have as their natural function, the hydrolysis of ester
substrates in water. These enzymes are also active in organic
solvents of low polarity, and the main advantage of reactions
in hydrophobic media is the ability to carry out esterifications
instead of hydrolyses. The hydrolase-catalyzed formation of
esters from alcohols and ester acyl donors (transesterification)
is a reversible reaction. The use of activated esters as acyl
donors shifts the equilibrium constant in favor of the product.
Several activated esters have been developed, but the most
useful acyl donors are enol esters such as vinyl acetate.
After providing general background information, this review
describes recent applications of stereoselective enzymatic
acylations (transesterifications) in the preparation of chiral
non-racemic compounds. The main focus is on the potential
use of this process in the synthesis of biologically active
compounds and natural products.
[Back to top]
Lipase-Catalyzed Deacylation by Alcoholysis: A
Selective, Useful Transesterification Reaction
Enzo Santaniello, Silvana Casati and Pierangela Ciuffreda
Alcoholysis is a transesterification reaction, according
to which an ester RCOOR´
reacts with an alcohol R´´OH
with formation of another ester RCOOR´´
and liberation of the alcohol R´OH.
The deacylation may be catalyzed by a lipase in organic solvents
and constitutes a useful step in the synthesis of complex
molecules where different groups are present. The enzymatic
deprotection procedure acquires a particular significance,
considering the hydrophobic nature of the substrates and their
solubility in organic solvents. In these non-conventional
media lipases are still considerably active and a great variety
of examples of regio- and chemoselectivity of the enzymatic
deacylation have been reported. Application of resolution
or asymmetrization procedures to the preparation of stereochemically
pure products will also be examined and discussed.
[Back to top]
Enzymatic Aminolysis and Ammonolysis Processes
in the Preparation of Chiral Nitrogenated Compounds
Vicente Gotor Fernández and Vicente Gotor
Enzymes are widely recognized as valuable tools for the synthesis
of optically active compounds. Thus, lipase-catalysed acylation
or deacylation are one of the most efficient methods for the
preparation of optically active alcohols, acids and esters.
In addition, other nitrogenated organic compounds can also
be prepared through enzymatic aminolysis or ammonolysis reactions.
Amines and their derived amides are important compounds in
organic synthesis, because of the presence of theses functions
in many pharmacologically active compounds. On the other hand,
enantiopure amines are used in the fine chemical industry
as resolving agents, chiral auxiliaries and their preparation
by enzymatic acylation of amines is nowadays a well established
methodology. In addition, in the last few years, many companies,
especially from the pharmaceutical industry, use enzymatic
aminolysis and ammonolysis processes for the preparation of
enantiopure amines in a multigram scale.
[Back to top]
Diversity of Epoxide Hydrolase Biocatalysts
M. S. Smit and M. Labuschagné
Chiral epoxides and their derivatives are versatile intermediates
for the synthesis of enantiopure organic chemicals. Chiral
epoxides can be obtained through asymmetric epoxidation using
either chemical catalysts or epoxidizing enzymes such as monooxygenases
or chloroperoxidases. Alternatively hydrolytic kinetic resolution
of racemic epoxides can yield a mixture containing, in an
ideal situation, a single enantiomer of the remaining epoxide
and a single enantiomer of the formed diol. Occasionally catalysts
with complementary enantioselectivities and opposite regioselectivities
can give through enantioconvergent hydrolysis one diol enantiomer
in > 90% yield. Hydrolyses can also be performed with either
metal catalysts or biological catalysts, mainly epoxide hydrolases.
Epoxide hydrolases show great promise for the preparative
scale hydrolysis of epoxides, because they are easy-to-use
biocatalysts that accept a wide range of substrates, that
do not require co-factors and that show improved activity
when the substrate is present as a second water immiscible
phase. X-ray structures for one fungal, one bacterial and
two mammalian epoxide hydrolases are available as well as
gene sequences for more than ninety five epoxide hydrolases.
Expression systems in Escherichia coli and other
hosts have also been developed and different assays suitable
for high throughput screening have been developed and tested.
The latter developments also made possible the first attempts
at catalyst improvement through site directed mutagenesis
and directed evolution. This review will give a comparative
overview of the different types of epoxide hydrolases, which
recently gave promising results for hydrolytic kinetic or
enantioconvergent resolution of different types of epoxides
and of results obtained with various recombinant epoxide hydrolases.
[Back to top]
Glycosyl Hydrolases and Glycosyltransferases in
the Synthesis of Oligosaccharides
Antonio Trincone and Assunta Giordano
Glycobiology and related disciplines have received an
enormous interest in recent years as they shed new light on
the functional roles of carbohydrates in biological events
leading to the understanding of mechanisms of important pathologies
and to the development of new therapeutics. Although carbohydrate
can be isolated from natural sources, the synthetic strategy
plays its own role allowing access to larger quantities of
structurally defined material and entry to analogs of naturally
occurring structures. Nowadays, the bottleneck in this field
is represented by some limitations of the potential of carbohydrate
containing molecules because their complex structures make
classical chemical synthesis very difficult.
The synthesis of oligosaccharides have to face with three
main snags: (i) reactivity of the leaving group on the monosaccharide
acting as donor; (ii) regioselectivity towards a single hydroxyl
group on the acceptor molecule; (iii) stereoselectivity in
forming pure anomers of the new glycosidic product. The protecting-group
manipulations that are needed for the stereocontrol of the
products are demanding procedures thus yields and selectivity
are lowered hindering the efficient production of oligosaccharides
needed for biological testing.
A particular benefit of the renewed interest for sugar chemistry
has been the attention of scientific community to the biological
methodologies in the production of oligosaccharides, an area
which emerged in recent decades.
In the carbohydrate field different enzymes were used for
diverse purposes but enzymatic strategies for high-yield and
stereospecific construction of glycosidic bonds are based
on the action of two types of enzymes: glycosyl hydrolases
(endo- and exo-glycosidases) and glycosyltransferases. Glycosyl
hydrolases in the cells are responsible for the cleavage of
glycosidic linkages; the exo-glycosidase are involved for
glycan processing during in vivo glycoprotein synthesis.
The glycosyltransferases are instead responsible in vivo
for the synthesis of most cell-surface glycoconjugates.
This review deals with the application of different types
of glycosyl hydrolases. Elegant examples concerning the use
of genetically modified representatives of glycosyl hydrolases
(glycosynthases and thioglycoligases) will be also reported;
some recent advances on the use of glycosyltransferases are
also included.
In compiling this review we were aware of the huge amount
of excellent material published in the recent years on this
topic, thus we will limit the covering of literature to the
last decade. Attention will be devoted to the regioselectivity
towards pyranosidic templates and to the yield of reactions.
The molecular diversity obtained by the use of enzymes from
different biological sources and the biological importance
of the compounds synthesized will be focused as well.
This review will put in evidence that glycosyl hydrolases
and glycosyltransferases for the synthesis of the glycosidic
linkages will play a relevant role in the next future as on
the bench bio-catalysts for the chemists involved in
the synthesis of oligosaccharides of biological interest.
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