of side reactions make four steps of downstream processing (deodorization,
bleaching, drying, filtration) redundant.
Using the same technology, 6-O-acyl derivatives of alkyl glucopyranosides,
which are used in cosmetics as fully biodegradable nonionic surfactants [158],
were synthesized from fatty acids and the corresponding 1-O-alkyl
glucopyranosides under catalysis of thermostable Candida antarctica lipase B in
the absence of solvents [151] (Scheme 3.1, bottom).
Acyl Transfer
Trans- or interesterifications, which do not form water during the course of the
reaction, are usually easier to perform (Schemes 2.1 and 3.2) [159–161]. Furthermore, the water content of the reaction medium (more accurately the ‘water
activity’, a W ), which is a crucial parameter for retaining the enzyme’s activity,
remains constant. As a consequence, it has only to be adjusted at the beginning of
the reaction, but not constantly monitored. Any trace of chemically available ‘bulk’
water, which may be present in the reaction medium, is quickly consumed at the
expense of acyl donor, which is usually used in excess. The structural water, which
is required to retain the enzyme’s activity is chemically ‘not available’ because it is
too tightly bound onto the enzyme’s surface to be removed.
In contrast to hydrolytic reactions, where the nucleophile (water) is always in
excess (55 mol/L), the concentration of the ‘foreign’ nucleophile in acyl transfer
reactions (such as another alcohol) is always limited. As a result, trans- and
interesterification reactions involving non-activated esters as acyl donors are generally reversible in contrast to the irreversible nature of a hydrolytic reaction. This
leads to a slow reaction rate and can cause a severe depletion of the selectivity of the
reaction for kinetic reasons (Fig. 2.6).
In order to avoid the undesired depletion of the optical purity of (predominantly)
the remaining substrate during an enzymatic resolution under reversible reaction
conditions, two tricks can be applied to shift the equilibrium of the reaction.
• Use of a large excess of acyl donor may impede enzyme activity.
• A better solution, however, is the use of special acyl donors which ensure a more
or less irreversible type of reaction.
The reversibility of transesterification reactions is caused by the comparable
nucleophilicity of the incoming nucleophile (Nu
1 ) and the leaving group of the acyl
donor (Nu
2 ), both of which compete for the acyl-enzyme intermediate in the
forward and the reverse reaction (Scheme 3.2). If the nucleophilicity of the leaving
group Nu
2 is decreased by the introduction of electron-withdrawing substituents,
the reaction is shifted to the right, i.e., towards completion. This concept has been
verified by the introduction of ‘activated’ esters [162], such as 2-haloethyl,
cyanomethyl and oxime esters (Scheme 3.3). Although acyl transfer using activated
esters is still reversible in principle, the equilibrium of the reaction is shifted so far
to the product side that for preparative purposes it can be regarded as quasiirreversible [163].
3.1 Enzymes in Organic Solvents
327
bleaching, drying, filtration) redundant.
Using the same technology, 6-O-acyl derivatives of alkyl glucopyranosides,
which are used in cosmetics as fully biodegradable nonionic surfactants [158],
were synthesized from fatty acids and the corresponding 1-O-alkyl
glucopyranosides under catalysis of thermostable Candida antarctica lipase B in
the absence of solvents [151] (Scheme 3.1, bottom).
Acyl Transfer
Trans- or interesterifications, which do not form water during the course of the
reaction, are usually easier to perform (Schemes 2.1 and 3.2) [159–161]. Furthermore, the water content of the reaction medium (more accurately the ‘water
activity’, a W ), which is a crucial parameter for retaining the enzyme’s activity,
remains constant. As a consequence, it has only to be adjusted at the beginning of
the reaction, but not constantly monitored. Any trace of chemically available ‘bulk’
water, which may be present in the reaction medium, is quickly consumed at the
expense of acyl donor, which is usually used in excess. The structural water, which
is required to retain the enzyme’s activity is chemically ‘not available’ because it is
too tightly bound onto the enzyme’s surface to be removed.
In contrast to hydrolytic reactions, where the nucleophile (water) is always in
excess (55 mol/L), the concentration of the ‘foreign’ nucleophile in acyl transfer
reactions (such as another alcohol) is always limited. As a result, trans- and
interesterification reactions involving non-activated esters as acyl donors are generally reversible in contrast to the irreversible nature of a hydrolytic reaction. This
leads to a slow reaction rate and can cause a severe depletion of the selectivity of the
reaction for kinetic reasons (Fig. 2.6).
In order to avoid the undesired depletion of the optical purity of (predominantly)
the remaining substrate during an enzymatic resolution under reversible reaction
conditions, two tricks can be applied to shift the equilibrium of the reaction.
• Use of a large excess of acyl donor may impede enzyme activity.
• A better solution, however, is the use of special acyl donors which ensure a more
or less irreversible type of reaction.
The reversibility of transesterification reactions is caused by the comparable
nucleophilicity of the incoming nucleophile (Nu
1 ) and the leaving group of the acyl
donor (Nu
2 ), both of which compete for the acyl-enzyme intermediate in the
forward and the reverse reaction (Scheme 3.2). If the nucleophilicity of the leaving
group Nu
2 is decreased by the introduction of electron-withdrawing substituents,
the reaction is shifted to the right, i.e., towards completion. This concept has been
verified by the introduction of ‘activated’ esters [162], such as 2-haloethyl,
cyanomethyl and oxime esters (Scheme 3.3). Although acyl transfer using activated
esters is still reversible in principle, the equilibrium of the reaction is shifted so far
to the product side that for preparative purposes it can be regarded as quasiirreversible [163].
3.1 Enzymes in Organic Solvents
327