High Throughput Synthesis High Throughput Synthesis Within Flow Reactors Within Flow Reactors

Paul Watts

CPAC, Rome, March 19th 2007

Micro ReactorsMicro Reactors• Defined as a series of interconnecting

channels formed in a planar surface• Channel dimensions of 10-300 m• Various pumping techniques

available• Hydrodynamic flow• Electroosmotic flow

• Fabricated from polymers, metals, quartz, silicon or glass

• Why glass?• Mechanically strong• Chemically resistant• Optically transparent

D

C

B

A

PET RadiosynthesisPET Radiosynthesis• Positron emission tomography (PET) is a radiotracer

imaging technique used to provide quantitative information on physiological and biochemical phenomena in vivo

• Applications in clinical research and drug discovery• Two of the most desirable radioisotopes are:

• 11C (t1/2 20.4 minutes)• 18F (t1/2 109.7 minutes)

• Syntheses must be conducted within 2-3 half-lives• Aims of miniaturisation:

• Produce the desired quantity of radiotracer (< 1 mg) at point of use

• Reduced reaction times will produce the product with enhanced specific activity

• The PET ligand will have greater sensitivity in vivo

Collaboration with NIH, Washington DC

PET ChemistryPET Chemistry• Reaction of 3-(3-pyridinyl)propionic acid

• Reaction optimised with 12CH3I (10 mM concentration) at RT

• Hydrodynamic flow (syringe pump)

• Reaction with 11CH3I• At 0.5 l/min flow rate RCY 88%

• Reaction of 18FCH2CH2OTs at 80 oC• At 0.5 l/min flow rate RCY 10%

R' XO

O

R'

NN

O

OH

CH3I or 11CH3IFCH2CH2OTs or 18FCH2CH2OTs

Lab Chip, 2004, 4, 523

0

25

50

75

100

0 2 4 6 8 10

Infusion rate (L/min)

Yie

lds

(%)

PET ChemistryPET Chemistry• Esterification reaction

• Reaction with 11CH3I (10 mM concentration) at RT• RCY 65% at 0.5 l/min flow rate

• Product isolated by preparative HPLC

N

O

O

CO2H

N

O

O

CO2*CH3

CH3I or 11CH3I

Bu4NOH, DMF

Lab Chip, 2004, 4, 523

Electroosmotic Flow (EOF)Electroosmotic Flow (EOF)

• Advantages of EOF:• No mechanical parts• Reproducible, pulse free flow • Minimal back pressure • Electrophoretic separation

• See Chem. Commun., 2003, 2886 for peptide separation

Transient positive ions

-

+

- -- -- --

--

--

- -

++

+

+

++

+

+

+

+

+ -EOF

Negative glass surface

‘Double Layer’

1818F PET ChemistryF PET Chemistry• 18F has a longer half-live

than 11C• Produced from H2

18O• For nucleophilic reactions

the fluoride needs to be separated from the water• Azeotropic distillation

• Electrophoretic separation

• Reaction

- --

-

-

-

--

-

- = 18F- or 18F-/K222

= H218O

TsOOTs 18F

OTs

J. Lab. Compd. Radiopharm., 2007, 50, in press

Electrophoresis

18F-

Stable RadiosynthesisStable Radiosynthesis• Stable isotopes routinely used in drug discovery for

drug metabolism studies (500 mg typically needed)• Amide synthesis

• Optimise reaction with ‘normal’ (cheap) unlabelled reagents

Cl R2

O

R1NH2 +

NH

R2

O

R1

Stable RadiosynthesisStable Radiosynthesis• Acetylation of aniline

• Reaction efficiency dependent of flow rate

• Reaction repeated with other derivatives

Stable RadiosynthesisStable Radiosynthesis• Once optimised substitute labelled precursor

J. Lab. Compd. Radiopharm., 2007, 50, 189-196

Electrosynthesis - Kolbe ReactionElectrosynthesis - Kolbe Reaction• Radical dimerisation (Kolbe reaction)

• Reactor diameter 1 mm• 1 mm platinum electrodes separated by 1 mm• Surface area in cell ca. 3 mm2

• Current 5 mA cm-2

OH

Oe-

-CO2

Decane (C10H22)

(Hexanoic acid)

Flow rate

(μl min-1)

Conversion

(%)

25.0 27

12.5 51

5.0 73

Reaction EfficiencyReaction Efficiency• Reaction conducted continuously for 12 hours• A base is needed to deprotonate the acid• Pyridine most successful

• Stops contamination of electrode surface

• Also works for other dimerisation reactions

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10 12 14

Time (Hrs)

% Y

ield

of

Dec

ane

Electrochemical DebrominationsElectrochemical Debrominations

Voltage

(V)

Current

(mA)

Conversion

(%)

4.4 1.1 50

5.0 1.5 86

5.6 2.5 98

• Parallel plate electrochemical reactor• Electrode area 25 mm2

• Electrodes 160 m apart• Flow rate 40 l min-1

Br

CN CN

Coupling ReactionsCoupling Reactions

Ratio of Products

R Current

(mA)

Conversion

(%)

CO2Me 0.6 100 99 1

CO2Me 1.5 77 73 27

CN 0.5 100 99 1

R

R

R

R

BrR

R

+ e-

-Br-

R = CN, CO2Me Flow Rate = 10 l min-1

Electro. Commun., 2005, 7, 918 Angew. Chem. Int. Ed., 2006, 45, 4146

Green Chem., 2007, 9, 20 Lab. Chip, 2007, 7, 141

Fine Chemical SynthesisFine Chemical Synthesis• New methodology for fine chemical synthesis

• Enhanced yields of more pure products etc• But always require purification • Generally batch work up required

Knoevenagel ReactionKnoevenagel Reaction• Solution phase Knoevenagel reaction• 1:1 Ratio of reagents (0.5 M) in MeCN• EOF

• 100 % conversion• Reaction very ‘atom efficient’

A

B

C

D

O

R R

O

Me2NH

CHOO

R R

O

• BUT product contaminated with base!!• Traditional solvent extraction needed• This clearly reduces the advantages of flow reactors

Functionally Intelligent ReactorsFunctionally Intelligent Reactors

R

R2 H

OR1

R R1

HR2R and R1 = COOCH3, COCH3, CNR2 = Ph, CH3

N

NHSi

+

• Fabricate micro reactors which enable catalysts and/or supported reagents to be spatially positioned

• Quantitative conversion to analytically pure product

Multi-Step SynthesisMulti-Step Synthesis

Aldehyde Flow Rate

(l min-1)

Conversion

(%)

Actual Yield

(g)

Yield

(%)Benzaldehyde 0.50 99.99 0.0150 g 99.4

4-Bromobenzaldehyde 0.80 99.99 0.0338 g 99.8

4-Chlorobenzaldehyde 0.65 99.99 0.0277 g 99.6

4-Cyanobenzaldehyde 0.84 99.99 0.0284 g 99.7

2-Naphthaldehyde 0.84 99.99 0.0298 g 99.8

Methyl-4-formyl benzoate 0.65 100.0 0.0253 g 99.7

4-Benzyloxybenzaldehyde 0.48 99.99 0.0219 g 99.1

Nitrothiophenecarboxaldehyde 0.75 99.99 0.0238 g 99.7

3,5-Dimethoxybenzaldehyde 0.55 99.99 0.0213 g 99.5

4-Methylbenzaldehyde 0.89 99.99 0.0284 g 99.3

N

O

O

HR

H3CO OCH3

N

O

O

R

A-15

Silica-supported piperazine

Enzymatic ReactionsEnzymatic Reactions• Enzymatic esterification

Flow reactor

Reaction Conditions:

• Acid: Hexanoic acid, octanoic acid or lauric acid • Alcohol: Methanol, Ethanol or Butanol• 1:1 ratio in hexane (0.2 M)• Room temperature

R OR'

O

OHR

O

+ R'OH

Novozyme 435 (ca. 100 mg)

Synthesis of Butyl HexanoateSynthesis of Butyl Hexanoate• Esterification reaction is equilibrium dependent• With time conversion can increase then decrease

• In flow the reaction mixture is removed so equilibrium is controlled• 96% yield

• Gain knowledge about substrate specificity• Link solution phase and catalysed reactions

Scale Out and Catalyst ScreeningScale Out and Catalyst ScreeningScale-out of reactions:

• 4 channels operating in parallel produces 4 times the product

• Larger packed reactors also feasible (5 mm diameter)

Synthesise arrays of compounds:

Flow

A1 + B2

A2 + B1

A2 + B2

A1 + B1

C1

C2

C3

C4

200 m

Flow

Amberlyst-15

99.99 %

(99.40 %)

99.99 %

(99.30 %)

99.99 %

(99.30 %)

99.99 %

(99.30 %)

99.99 %

(99.50 %)

99.99 %

(99.50 %)

99.99 %

(99.30 %)

99.99 %

(100.0 %)

99.99 %

(99.30 %)

Combinations of Acids and BasesCombinations of Acids and BasesN

O

O

H

N

O

OH3CO OCH3

S

O

OO

3

Yb3+

S

OH

OO

N

NH

Si

NSi

N N

NSi

ConclusionsConclusions• Micro reactors allow the rapid optimisation of reactions

• High-throughput combinatorial synthesis• Immobilised reagents (catalysts and enzymes) allow the

synthesis of analytically pure compounds• Micro reactors are suitable for a wide range of reactions

• Electrochemical synthesis• Catalysed reactions• Enzyme screening

• Micro reactors generate products in:• Higher purity• Higher conversion• Higher selectivity

• In situ formation of reagentsP. D. I. Fletcher et al., Tetrahedron, 2002, 58, 4735K. Jahnisch et al., Angew. Chem. Int. Ed., 2004, 43, 406H. Pennemann et al., OPRD, 2004, 8, 422P. Watts et al., Chem. Soc. Rev., 2005, 34, 235

• Dr. Charlotte Wiles• Dr. Nikzad Nikbin• Dr. Ping He• Dr. Victoria Ryabova• Dr. Vinod George• Dr. Leanne Marle• Dr. Joe Dragavon

• LioniX• Astra Zeneca• Novartis

• Mairead Kelly• Gareth Wild• Tamsila Nayyar• Julian Hooper• Linda Woodcock• Haider Al-Lawati• Ben Wahab

• EPSRC• Sanofi-Aventis• EU FP6

Research Workers and Research Workers and CollaboratorsCollaborators