3 Enzymes in Organic Synthesis

8/3/2019 3 Enzymes in Organic Synthesis

1/15

3. Enzymes in organic synthesis

Literature: F. Theil, Enzyme in der Organischen Synthese, Spektrum, 1997,

Heidelberg.

Enzymes in brief

biocatalysts

proteins (or RNA chains: ribozymes)

complex three-dimensional structures (stereochemistry important)

key/lock vs induced-fit model

use in organic synthesis: usually isolated enzymes or specific enzymes in livingcells

use in biotechnology/brewery: living organisms, involvement of multiple enzymes

3. Enzymes in organic synthesis Organisch-Chemisches Praktikum II, 2011


2/15

Classes of enzymes

class name function importance in

organic synthesis

1 oxidoreductases oxidations and reductions high

2 transferases transfer of functional groups low

3 hydrolases hydrolysis and condensation very high

reactions

4 lyases addition of small molecules to medium

double bonds and elimination

reactions

5 isomerases isomerization reactions low

6 ligases formation of covalent bonds low



3/15

Characteristics of the use of enzymes in organic synthesis

Advantages

significant enhancements of reaction rates possible mild reaction conditions: pH 7, aqueous solution

highly specific reactions (no/little byproducts)

high regio- and stereoselectivity

environmentally friendly

Disadvantages

often low substrate concentrations

often inhibition by substrates or products

only one enantiomer available

often relatively high sensitivity

activity often depends on origin, often not well-defined

often expensive



4/15

Oxidoreductases I: dehydrogenases

hydrogenation and dehydrogenation reactions

cofactors needed

O

OH

N

HO

O P O P O

H2NO

O O

OO O

HO OR

NN

N

N

NH2

O

OH

N

HO

O P O P O

H2NO

O O

OO O

HO OR

NN

N

NNH2

H

H

R = H nicotinamide adenine dinucleotide (NAD+)

PO32 nicotinamide adenine dinucleotide phosphate (NADP+)

R = H 1,4-dihydronicotinamide adenine dinucleotide (NADH)

PO32 1,4-nicotinamide adenine dinucleotide phosphate (NADPH)



5/15

Regeneration of NADH and NADPH

from formic acid:

from glucose:

from alcohols:

HCOOH CO2 + H+

NAD+ NADH

f ormate dehydrogenase

NADP+ NADPH

OHO

HOOH

OH

OH

OHO

HOOH

OH

Oglucose dehydrogenase

alcohol dehydrogenase

OH O

+ H+

+ H+

NADP+ NADPH



6/15

Stereoselective hydrogenation of ketones

Prelog rule: for most dehydrogenases preferred attack from the Reface

use of second enzyme for cofactor regeneration

alternative: use of whole cells, which provide cofactor regeneration system

most popular: bakers yeast

RR'

O1

23

Re

Si



7/15

Characteristics of the use of bakers yeast in organic synthesis

Advantages

easy availability regeneration of cofactor by other enzymes present in the cells (use of sucrose as

co-substrate)

broad substrate scope

Disadvantages

complexity: contains different dehydrogenases with different activities

contains additional enzymes (e.g., hydrolases), which can catalyze side reactions

much biomass relative to substrate: tedious work-up

low substrate concentration: low productivity



8/15

Catalysis by bakers yeast: steric effects

reduction of-ketoesters:[1]

R1 OR2

O O

R1 OR2

OH O

R1 OR2

OH O

R1 R2 ee(%) R1 R2 ee(%)

Et Et 40 CH3 H 95

Pr H 100 CH3 Et 95

Bu H 100 Et C8H17 99Ph Et 100 Pr C8H17 71

[1] C. J. Sih, C.-S. Chen, Angew. Chem. 1984, 96, 556.



9/15

Oxidation reactions catalyzed by dehydrogenases

biphasic system:

n-hexane

water

NH4+

NAD+ NADH

horse liver

OH

OHO

O

alcohol dehydrogenase

glutamate dehydrogenase

OOCCOOH

NH2

OOCCOOH

+ H2O

O



10/15

Oxidoreductases II

monooxygenases: transfer of 1 oxygen atom

epoxidation reactions

Baeyer-Villiger oxidations

dioxygenases: transfer of 2 oxygen atoms

bishydroxylation of arenes

O2

Pseudomonas putida

R

OH

OH

R

peroxidases: oxidation by hydrogen peroxide or organic peroxides

chlorperoxidaseS

CH3+ H2O2

SCH3

O

H2O91% ee

98% yield



11/15

Hydrolases I: lipases

natural substrates: triglycerides

> 30 commercially available lipases (from fungi, bacteria, plants, mammals)

highly specific reactions (no/little byproducts) relatively high thermal stability

good activity in water and in many organic solvents, no cofactors needed

broad substrate scope (induced fit)

mechanism:

N

HN

OO

H

O

NH

N

OHO

O

O

R OR'

N

HN

OO

O

R O+ RCOOR' R'OH

aspartate

histidine

serine acyl enzyme

catalytic

triad



12/15

Ester hydrolysis catalyzed by lipases I

pH 6 8 (addition of/titration with NaOH solution)

alcohol component variable

carboxyl component less variable use for stereoselective transformations

for best results optimization necessary: lipase

reaction conditions

alcohol/carboxyl component

asymmetrization reactions:

AcO OAc

OO

AcO OH

OO

PFL

pH 798% ee, 98% yield

Pseudomonas f luorescens/cepacia

lipase



13/15

Ester hydrolysis catalyzed by lipases II: kinetic resolution of racemic mixtures

product (alcohol/carboxylic acid): highest eevalues for low conversions

substrate (ester): highest eevalues for high conversions

example: P. Kalaritis et al., J. Org. Chem. 1990, 55, 812.

COOEt

PFL

pH 7 - 8

X

COOEt

X

+ COOEt

X

+ COOH

X

X conversion (%) ee(ester, %) ee(acid, %)

F 50 84

60 99.9 69

OH 50 79 95

Br 50 73 69



14/15

Esterification catalyzed by lipases

works well for linear aliphatic carboxylic acids

acylation reagents: free carboxylic acids (H2O needs to be removed from equilibrium)

esteractivated esters, e.g. RCOOCH2CF3

use of various organic solvents possible (C6H14, Et2O, or CHCl3)

good for kinetic resolutions of racemic mixtures of alcohols

good for regioselective acylations in carbohydrate chemistry:

lipases also catalyze formation of amides

OHO

HO

OH

OH

OH

ON

O

lipase

OHO

HO

OH

OH

O

O

65%



15/15

Hydrolases II: glycosidases

hydrolysis of glycosidic bonds

glycosidation: reaction of sugar with alcohol thermodyanmically unfavorable

instead: transglycosdiation

OHO

HOOH

OR

OH

H2O

glycosidase

OHO

HOOH

OH + ROH

OH

-galactosidase

O

OH

HO

O

OH

OH

NO2

O

OH

HO

OCH3

OH

OH

+O

OH

O

OCH3

OH

OHO

OH

HOOH

OH


Documents

3 Enzymes in Organic Synthesis