e-NEWSLETTER
August/September 2004

Highlights from Chiral Europe 2004
Hilton Hotel, Mainz, Germany, June 14th - 16th 2004
organised by Scientific Update

This latest meeting in the long running series was attended by 78 people, with 16 nationalities represented.  There were 17 presentations including a Keynote presentation by Professor Dieter Seebach of ETH-Zürich, and the Degussa Fine Chemicals Prize Lecture by Professor Benjamin List of the Max Planck Institute.  The conference was sponsored by Degussa. 

Copies of the full conference proceedings can be purchased from Scientific Update upon request.

 30 Years of Amino Acid and Peptide Chemistry – Summary and Outlook
Professor D. Seebach, ETH-Zürich, Switzerland

This keynote presentation was concerned with Professor Seebach’s work on β– and γ- amino acids and “proteins” synthesized from these building blocks.  Insertion of one or two CH₂ groups in each and every amino acid residue of peptides leads to ß- and γ-pep­tides of most exceptional properties. They form secondary structures in solution, such as heli­ces, turns, and pleated sheets, with as few as two residues, they are entirely stable to the action of peptidases and proteases, they are metabolically stable in mammals, insects and plants, and they are only slowly biodegraded. Still, these small peptides can mimic a-peptidic hormones and other ligands in their interactions with proteins (receptors). These features make the peptides built from homologated proteinogenic amino acids promising candidates for diagnostic and pharmaceutical applications.

In the lecture a review was given of the results obtained since the beginning, eight years ago, of Professor Seebach's journey into the worlds of β- and γ-peptides. Results of the newest investigations were described, with an emphasis on biomedical aspects. They involved

For a current list of references to Professor Seebach?fs papers on β - and γ-amino acids and ?-peptides see
http://infosee.ethz.ch/seebach/seebach.html.
An extensive review article entitled ??A`- and ?A?L?|Peptides Built of Homologated Proteinogenic Amino Acids and Other Building Blocks?g has been published in the journal Chemistry & Biodiversity (2004, 1(8), 1111-1239). Another review-type article has recently appeared in Tetrahedron (August issue ?| 2004, 60(35), 7455-7506).  

New Applications of Ylide Technology in Asymmetric Synthesis
Professor V. Aggarwal, University of Bristol, UK

The majority of Professor Aggarwal’s presentation concerned his work on the use of sulphur ylides to form three membered rings such as epoxides.  The catalytic cycle for the reaction is shown below.

Chiral epoxides can be produced by this process if R₂S is a chiral, and typically cyclic, sulphide.  The reaction works well for most aromatic and heteroaromatic aldehydes, with the exception of pyridine aldehydes.  Disubstituted aliphatic aldehydes of general formula RR’CHCHO are also good substrates but other aliphatic aldehydes and ketones are poor substrates.  A similar pattern is found for aldehydes that are used as hydrazone precursors.  In cases where the yield and / or ee from the catalytic reaction are low stoichiometric amounts of the chiral sulphide can be used to overcome this problem.

The reaction works equally well if imines are used in place of aldehydes, giving aziridines, and cyclopropanes can be formed if an a,b-unsaturated ester is used as substrate instead of an aldehyde.  Some applications of the methodology in synthesis were described including the syntheses of  CDP 840 and SK&F 104353 (see Scheme 2), as well as some mevalinic acid analogues and a precursor to (+)-LY354740 (see Scheme 1).

In the final part of the presentation some very recent work that has only recently started, on the reactions of sulphur ylides with  boranes was described.  This origin of this work is in the polymerisation of organoboranes, which is catalysed by sulphur ylides. 

This reaction occurs rapidly with n-alkyl boranes, but is significantly slower of the product contains a branched alkyl group and is therefore more sterically hindered than the starting material.  This then becomes a synthetically useful reaction and if a chiral sulphide is used the reaction is enantiospecific.

Chemoenzymatic Synthesis of Pharmaceutically Important Diols
Dr.M. Müller, Research Centre, Jülich, Germany

In this lecture three different approaches to the preparation of various 1,3-diols were described.  The first examples concerned the use of metabolic diversity to produce in 2 steps all 4 possible isomers of statin side chain precursor diols (see below).

In the second part of the lecture the use of enzymatic diversity was described in the synthesis of some cross benzoin type products (see below, BAL = benzaldehyde lyase, BFD = benzoylformate decarboxylase).  Ortho-substituted benzaldehydes are not accepted as substrates in this case.

In the final part of the lecture the use of biosynthetic diversity to produce chorismate and isochorismate derivatives, produced in the shikimate biosynthetic pathway, for use as chemical building blocks and subsequent downstream chemistry was described.  The biosynthetic work has been published (Angew. Chem., 2001, 113, 578, and Chembiochem, 2003, 4, 775).   One example of some of the downstream chemistry from 2,3-trans-dihydroxybenzoic acid is shown below.

Scalable Methods for Functionalised Chiral Alcohols
Dr B. Bosch, Bayer Chemicals AG, Germany

Dr Bosch presented work on three approaches to the production of chiral alcohols that have been developed at Bayer.  These include improvements to the Juliá-Colonna epoxidation of enones, the development of proprietary ligands for asymmetric hydrogenation of keto-esters and improvements to the preparation of asymmetric transfer hydrogenation catalysts.

In the Juliá-Colonna epoxidation enones are converted to chiral α,β–epoxyketones with a peroxide catalysed by a polyamino acid, typically polyleucine, under biphasic conditions (toluene – water).  Bayer have improved the preparation of the polyleucine catalyst, by converting (D)- or (L)-leucine in to an N-carboxyanhydride by reaction with phosgene, followed by 1,3-diaminopropane promoted polymerisation in toluene.  In a modification to the original epoxidation conditions, which require long reaction times and the gel like nature of the catalyst makes stirring and scale up difficult, a phase transfer catalyst is added giving so-called triphasic/PTC conditions.  This gives a much faster reaction and is much more amenable to scale up.

In the second part of the talk, Dr Bosch outlined developments that have been made to the synthesis of the Cl-MeO-BIPHEP ligands.  In the first synthesis biaryl coupling was carried out on a phosphine oxide and the resulting bis-phosphine oxide was resolved.  Each enantiomer then had to be reduced back to the phosphine oxidation state.  In the second-generation synthesis a different strategy is used as shown below.

The resolved bis-phenols can also be converted to monodentate phosphite type ligands by reaction with RPCl₂.  An example of the use of the bis-phosphine ligands was presented which involved a dynamic kinetic resolution (see below).  Interestingly attempts to remove ruthenium from the product required 3 different techniques – filtration over silica, use of activated charcoal and treatment with trithiocyanuric acid, which reduced ruthenium levels to 150 ppm.

The final section of the presentation focused on the transfer hydrogenation of aromatic ketones using the catalyst systems developed by Professor Noyori.  A significant improvement to the ligand synthesis is to carry out the tosylation in a biphasic system using sodium hydroxide as base in place of triethylamine.  This gives a much-improved selectivity for monotosylated amine over the bis-tosyl side product.

The scope of the system has been expanded and contrary to Professor Noyori’s findings, good enantioselectivity is observed with relatively electron-rich substrates.  Perhaps surprisingly reduction of a β–keto ester (ArCOCH₂CO₂Me) proceeds faster (12 hr, compared to 18 hr) at larger scale (500 X scale up) using triethylamine/formic acid as the reductant.  Efficient removal of CO₂ is crucial to the success of the reaction which gives a 97% yield of >97% ee product.

Novel Stationary Phases for Analytical and Preparative Separation of Drug Enantiomers.
Dr E. Francotte, Novartis Pharma AG, Switzerland

Dr Francotte presented work carried out at Novartis on developing new chiral stationary phases for both analytical and preparative scale separations.  A survey of the analytical separation of ~1000 racemic compounds showed that ~905 of the mixtures could be separated by just 4 stationary phases (Chiralcel OD, Chiralcel OJ, Chiralpak AD and Chiralpak AS).  In preparative separations productivity becomes a key parameter and there is often a trade off between productivity and selectivity (α).  Work at Novartis centered on polysaccharide based stationary phases and they found that the mobile phase plays a crucial role in the chiral recognition process, which is the basis of enantioselective separation.

Apparently small changes in the mobile phase can have a big effect on the selectivity.  So for example the selectivity of the separation of N-Cbz-3-piperidinol enantiomers (Chiralcel OD stationary phase) improves from 1.07 when the solvent system is hexane/isopropanol (95:5) to 1.21 when the solvent system is changed to hexane ethanol (98:2).  Perhaps even more surprisingly the elution order of the enantiomers of 4-alkoxy-3-piperidinol is inverted when the solvent ratio of a hexane/ethanol/methanol system is changed from 95:2.5:2.5 to 95.5:3:1.5, with an improvement in selectivity also being observed (1.07 to 1.21).

Polysaccharide based stationary phases generally give good separations but they are limited by their solubility in some of the organic solvents that could be used to improve the solubility of the racemate.  So Novartis have followed up on Professor Okamoto’s work on immobilising  polysaccharide derivatives on silica.  This is achieved by a radical process with the radicals either being generated photochemically or thermally (using a radical initiator such as di-tert-butyl peroxide).  Solvents such as chloroform can be used with the immobilised systems, which can give real benefits in solubility and selectivity.  Immobilised polysaccharide derivatives such as paramethylbenzoyl cellulose, various carbamates of cellulose  and amylose were described.  The optimisation of chiral separations using solvent systems such as heptane/chloroform and hexane/ethanol/dichloromethane were presented as well as some preparative examples using SMB.

Asymmetric Catalysis using Amino-Acid-Derived Ligands
Professor C. Bolm, RWTH Aachen, Germany

This lecture consisted of three sections – C-C bond forming reactions, oxidations and the synthesis of sila-tert-leucine derivatives.  Two variations on the enantioselective addition of aryl zinc reagents to aldehydes (catalysed by chiral ferrocenyl ligands) were presented – direct addition of diphenyl zinc and in situ generation of the diaryl zinc reagent by metal exchange between diethyl zinc and an arylboronic acid.  A study of different ligands and metallocenes showed cyrhetrene to be a particularly effective catalyst (at 10 mol%), giving ee’s consistently >95% for additions to aromatic aldehydes but lower ee’s (~78%) for additions to aliphatic aldehydes.  Reducing the catalyst loading to 2 mol% resulted in slightly lower ee’s (2-15% lower) depending on the substrate.

The oxidation section concerned the enantioselective oxidation of sulphides to sulphoxides with hydrogen peroxide using Fe(acac)3 and a chiral ligand such as the Schiff’s base formed from 3,5-diiodosalicaldhyde and tert-leucinol.  This produces sulphoxides in low yields (<44%) and with moderate to good enantioselectivity depending on the substrate.  In general the larger the difference in size of the two groups attached to sulphur, the higher the ee of the sulphoxide as with other sulphur oxidation systems.  The use of benzoic acid derivatives as additives was investigated and the use of 1 mol% of p-anisic acid was found to improve sulphoxide yields to 50-70% and ee’s to >70% and in some cases >90% even for some of the most difficult substrates.  So for example oxidation of phenyl benzyl sulphide gives the corresponding sulphoxide in 73% yield and 79% ee under these conditions.

The synthesis of sila-tert-leucine was carried out by reacting ethyl 2-trimethylsilyldiazoacetate with Boc-amide (tert-BuOCONH₂) and subsequent deprotection.  Varying the amide component of the reaction leads to dipeptides containing an α–silyl-α-amino acid.  Reaction of the benzyl TMS diazoacetate with epoxides yields the benzyl 1-hydroxy-1-trimethylsilylacetate, which can be converted to α–silyl-α-hydroxyacetic acid.  Enantiomerically pure α–silyl-α-hydroxyacetic acid is being investigated as an enantioselective catalyst for borane promoted aldol reactions as shown below.

In his introduction Dr Lygo described the background to his work using cinchona alkaloid derived salts as chiral phase transfer reagents.  This class of compounds has been used as chiral PTC reagents in the alkylation of glycine imines and the epoxidation of chalcones (Weitz-Scheffer epoxidation).  The alkylation of glycine imine with benzyl bromide for example goes in >85% yield and 91% ee and the synthesis of 2-amino-4-bromopentenoic acid ( a useful building block for the synthesis of aspartic acid derivatives and kynurenine derivatives) demonstrated the use of this methodology in synthesis.  A variety of potential catalysts were screened robotically for this alkylation and some of the results are shown below.  The low ee obtained with the cinchonine derived catalyst is attributed to its low solubility in the reaction medium.

Oxidoreductases – Catalysed Syntheses of Chiral Building Blocks
Dr H. Gröger, Degussa AG, Germany

The attraction of oxidoreductases as biocatalysts is that potentially they can convert a prochiral substrate in to 100% of the desired enantiomer, rather than converting 50% of a racemate in to the desired compound.  Dr Gröger concentrated on the use of oxidoreductases in the synthesis of (L)-amino acids and optically active alcohols, although they can also be used to prepare chiral α-hydroxycarboxylic acids and (D)-amino acids.

These enzymes can be used as biocatalysts for the synthesis of unnatural amino acids such as (S)-tert-leucine and (S)-neopentylglycine by reductive amination of the appropriate α–keto acid.  The reductive amination requires two enzymes and a co-factor – leucine dehydrogenase to effect the amination, NADH to carryout the reduction and formate dehydrogenase (FDH) to recycle NAD back to NADH.  The instability of the original wild type FDH was a concern, but this problem was solved using rational protein engineering.  Site-directed mutagenesis led to 2 amino acid changes, which significantly enhanced the half-life of the enzyme.  The second-generation process which is in development is to use a whole cell to carry out the biotransformation, with a designer bug giving conversion >95% and enantioselectivity >99%.

In the second part of the presentation Dr Gröger described work on finding organisms that will reduce ketones to alcohols enantiospecifically.  A variety of organisms, which came from a strain collection and soil samples, were screened for activity.  A new full- length gene was identified and isolated and was expressed in E. coli to allow high cell density fermentation to be carried out.  Space-time yields for the bio-reduction in a membrane reactor were ~100g/L/day, but suitable reaction media had to be identified to allow substrate concentrations to be run at economic levels.  Selected examples of alcohols, which have been produced by this method are shown below.  In all cases conversion is >95% and ee >99%.

Degussa Fine Chemicals Prize Lecture

The first Degussa Fine Chemicals Prize was awarded to Professor Benjamin List who delivered a lecture on the use of organocatalysis, initially reviewing earlier work in the area and the question of asymmetric amplification and the various mechanisms / transition states that have been proposed to explain the enantioselectivity observed in proline catalysed reactions.  Professor List’s work in this area started by examining the intermolecular version of the Hajos-Parrish-Eder-Sauer-Wiechert reaction.  This intermolecular aldol reaction, whilst being enantioselective shows no non-linear effects, unlike the intramolecular version, which led Professor List to re-investigate the intramolecular reaction.  It has now been shown, with the benefit of modern analytical techniques, that there is no non-linear effect in the intramolecular reaction either.

Proline catalyses enantioselective versions of traditional transformations such as aldol reactions, three component Mannich reactions as shown below (typical yields 35-92%, ee’s 70-99%), α-alkylation, α-amination (by reaction with azodicarboxylates) and α-hydroxylation (by reaction with nitrosobenzene) of aldehydes (see below), Michael reactions and many of these reactions can be run inter or intramolecularly.

Professor Börner described the synthesis of a number of analogues of the DuPHOS ligand system.  The first ligand described was BASPHOS, which shows good activity for the asymmetric hydrogenation of acetamidocinnamic acids and their esters, but is not such a good catalyst for the reduction of itaconic acid and esters, with poor conversion being the main problem.  The next stage in the development of these ligands was the capping of the hydroxyl groups as ethers (typically benzyl ethers or tert-butyl ethers) to produce the RoPHOS family of ligands, which show comparable activity to DuPHOS depending on the substrate (DuPHOS is a slightly better ligand for acetamidocinnamate substrates while RoPHOS is slightly better for itaconate reductions).

The next generation of catalysts are the catASium® M ligands where the phenyl linker of DuPHOS has been replaced by alternative rings see below, which gives each ligand a slightly different “bite angle”.  These compounds can be prepared in a modular fashion by reacting the appropriate “backbone” fragment with a silylated phosphine (the synthesis of the silylphosphine, which is stable and can be distilled, has been published – J. Org. Chem., 2003, 68, 1701)

The remainder of the presentation was about the use of these ligands in the asymmetric reduction of various substrates and the comparison with DuPHOS.  Moving from DuPHOS to the maleic anhydride based catASium® M ligand to the squaric acid based catASium® M ligand give a steadily increasing bite angle and steadily decreasing enantioselectivity  (in the hydrogenation of itaconates).  Different bisphospholane ligands show different pressure dependency in the reduction of β-acetamido-α,β-unsaturated esters. The maleic anhydride based ligand leads to better enantioselectivity for compounds with larger alkyl groups in the β-position.  In most cases the difference is small, but where the group is isopropyl the difference is significant, with DuPHOS giving very low enantioselectivity (<10% ee) compared to the catASium ® M ligand (65-70% ee).

Dr Wubbolts gave an overview of the chiral market in fine chemicals and commented on the advantages (e.g. high selectivity particularly for complex substrates) and drawbacks (the time taken to find and optimise a biocatalyst) of using biocatalysis.  DSM have developed the pluGbug® concept to try and shorten the time required to develop a biocatalytic process.  This aims to maximise research and production efficiency by limiting the number of microorganisms used then plugging the new gene in to the appropriate organism.  Three examples of biocatalytic C-C bond formation were presented, one example using hydroxynitrile lyase and two using aldolases.  The formation of the cyanohydrin, a pyrethroid insecticide intermediate, shown below has been scaled up to >200 Tpa.

The first aldolase example described was the use of DERA aldolase in the synthesis of the statin side chain intermediate STATON, while the second was the use of threonine aldolases (TA’s) to produce either enantiomer of α–amino-β-hydroxyamino acids.

The Challenge of Chirality in Process Research
Dr U.H. Dolling, Merck, USA

The presentation by Dr Dolling covered two process research projects carried out at Merck, most of which included a crystallisation driven enantioselective process.  The first example presented was the preparation of (D)-3-fluoroalanine, an intermediate in the synthesis of MK-642, a pro-drug of Cycloserine.  A high enantiomeric purity was required due to the high toxicity of the other isomer.  The racemic intermediate was prepared by reductive amination of 3-fluoropyruvic acid hydrate.  The N-Cbz protected derivative could be resolved with quinine in 25% yield (99% ee), but a simpler separation was achieved by exploiting the fact that the benzenesulphonate salt crystallised as a conglomerate.  A continuous crystallisation apparatus was developed in conjunction with the engineers.

The second example was the synthesis of Substance P, an NK-1 receptor antagonist.  The original synthetic route required 28 linear steps with the chirality being introduced via an early stage tartrate resolution of 3-hydroxy-2-phenylpiperidinol.  An alternative route was developed that included a double ring closing metathesis reaction to generate a key spirocyclic intermediate and a modified resolution using dibenzoyl tartrate that was significantly shorter.

The final example presented by Dr Dolling was the development of the manufacturing route for Aprepitant (MK-0869).  Aprepitant was expected to be a low dose compound and so with cost not being a big issue the medicinal chemistry route was scaled up.  In the first route the chirality was generated by a crystallisation induced transformation using bromocamphorsulphonic acid (BCSA) as a chiral auxiliary.  This was achieved by carrying out the crystallisation of the BCSA salt under acidic, racemising, conditions.  With the desired salt being much less soluble under the conditions used a 90% yield of 99% ee material could be achieved.

The next step was a reductive acylation with 3,5-bis(trifluoromethyl)benzoyl chloride in the presence of L-selectride to make the 3,5-bis(trifluoromethyl) benzoate, which was then subjected to a Petasis methylenation (see  Org. Proc. Res. Dev., 2004, 8, 256), followed by reduction and coupling with the triazinone fragment.  The overall yield for the synthesis was 53%, which had been demonstrated in the pilot plant and was ready to be transferred to manufacturing.  However the dosage increased 20-fold during clinical trials and precipitated a search for a much more efficient synthesis.

In the current synthesis the first chiral centre is introduced by asymmetric reduction of 3,5-bistrifluoromethylacectophenone, which is coupled with a racemic lactam.  The lactam is a 1:1 mixture of diastereoisomers, which can all be converted in to the desired isomer by a crystallisation driven epimerisation using the potassium salt of tetrahydrolinalool and linalool in heptane (>99%ee, 84% isolated yield from the starting lactam).

The final synthesis is only marginally better in yield terms, but it uses cheap reagents, requires minimal isolations, no low temperature reactions and is much less capital intensive than the original synthesis.

A Practical Asymmetric Synthesis of the Cardiovascular Agent UK-350,926 via a Dynamic Resolution Process
Dr S. Challenger, Pfizer, UK

UK-350,926 is an endothelin receptor antagonist and although there are and have been several endothelin receptor antagonists in development only Bosentan has reached the market so far.  In the original synthesis of UK-350,926 an early resolution of the diarylacetic acid intermediate, shown below, with (R)-α-methybenzylamine was used to generate the desired chiral centre, but optical purity tended to be eroded during subsequent coupling of the derived amide with 2-methoxy-4-methylbenzenesulphonyl chloride.

During the first process campaign minor modifications were made to the discovery route such as using a methyl ester in place of a benzyl ester (this had the advantage of giving crystalline intermediates) and changes to some of the reagents.  The early resolution was replaced by a late stage resolution, mainly because of the possibility of converting this in to a dynamic resolution.  Initially brucine was used as the resolving agent, but a dynamic resolution process using 2 equivalents of (S)- α-methybenzylamine was found to be more efficient although it needed “heavy” seeding to work.  The ee of the final product could be upgraded from 90% to >98% by crystallisation of the sodium salt.

In the final campaign in the pilot plant the synthesis of the diarylacetic acid was modified by coupling methylenedioxymandelic acid with the indole fragment in the presence of trifluoroacetic acid.  This change had two advantages.  It avoided having to handle the lachrymatory α–bromophenylacetic acid derivative and the diarylacetic acid product crystallised directly from the reaction mixture.  At this point the compound was withdrawn from development.

Dr Fotheringham described work carried out at Ingenza and the University of Edinburgh on the development of combined biocatalytic and chemical approaches to the deracemisation of amino acids – taking a racemic mixture and converting one enantiomer in to the other.  This can be achieved by using an amino acid oxidase that converts one enantiomer into an imine that is then reduced back to a racemic mixture with a non-stereoselective chemical reducing agent.  D-amino acid oxidases and L-amino acid oxidases have both been developed so that a racemic mixture of an amino acid can be converted in to either enantiomer.  Examples included conversion of piperazinecarboxylic acid to L-piperazinecarboxylic acid in 86% yield, >99% ee.  Improvements to the original system, where 500 equivalents of NaBH₄ were required, include using a transfer reduction system with Pd/C and formic acid.  Some examples are shown to demonstrate the broad applicability of the technique.

  This technique can also be applied to amines, but this requires novel activity as natural amine oxidases show little or no enantioselectivity.  Those enzymes which show low enantioselectivity were improved by directed evolution to give for example 77% yield, 95% ee in the deracemisation of α–methylbenzylamine (more examples can be found in Angew. Chem. Internat. Ed., 2003, 42, 4807).

The Synthesis of Omapatrilat: Issues and Challenges During Process Development
Dr P. Deshpande, Bristol-Myers Squibb, USA

This presentation concerned the development of a manufacturing route to Omapatrilat.  The fundamental synthetic strategy was to use D-phenylalanine, L-methionine and L-hydroxynorleucine as the main building blocks.  The early process route is shown below but there were a number of problems associated with this synthesis – the number of steps to make the various building blocks, the use of hydroxybenzotriazole mesylate, the use of trimethylsilyl iodide and the waste generation.

A modified long-term strategy was devised using L-normethionine dimer in place of L-methionine and the alternative C-6 building block.  The C-6 building block was prepared in 5 steps from tetrahydrofuran, but with only 1 isolated intermediate (ring opening of THF with HBr to produce 4-bromobutanol, which was directly oxidised with TEMPO and protected with ethylene glycol to generate the acetal of 4-bromobutanal.  This was followed by formation of the Grignard reagent, reaction with diethyl oxalate and finally hydrolysis to generate the lithium salt of the α-ketoacid, which was converted to the final intermediate by biocatalytic reductive amination).  The fully developed process is shown below (Vanlev being the trade name for Omapatrilat).

Comparing the final process with the early process route shows significant improvements with the number of process steps and isolations, the amount of organic and aqueous and organic waste and the kg of intermediates processed per kg of final product all being reduced.

Discovery and Applications of Enzymes for the Preparation of Enantiomerically Pure Non-Proteinogenic Amino Acids
Dr K. Holt, Dowpharma UK

Dr Holt gave an overview of the use of enzymes in the preparation of unnatural amino acids.  Several enzyme sources were covered including commercial sources, an in-house culture collection, external collaborations and selective enrichment.  Examples described included the resolution of (±)-N-acylpropargyl glycine and the use of integrating biocatalysis with conventional chemical technology to produce amino acids such as neopentyl glycine in high ee.  Some examples of the application of this technology were presented including the elaboration allyl glycine in to 4-hydroxpipecolates and the synthesis of bulgecinine, both of which are shown below.

Automated Oligosaccharide Synthesis as Platform for Drug Discovery
Professor P. Seeberger, ETH-Zürich, Switzerland

Professor Seeberger described the use of automated solid-phase techniques for saccharide synthesis and in the second half of his presentation he covered recent work on flow-through micro-reactors.  An example of an automated decasaccharide synthesis carried out on a solid support is shown below.  The total assembly time for this synthesis was 16 hours and an overall yield of 45% by hplc analysis (~33% isolated yield) was achieved.

A number of problems were solved during the development of this methodology such as how to carry out real-time monitoring of coupling yields.  This was achieved by incorporating UV active protecting groups.  In addition the linkage to the resin was modified to shorten the cleavage time.

This automated synthetic approach was used to create carbohydrate arrays to study carbohydrate – protein interactions and gain more information about biological systems such as viruses and to facilitate discovery of antibiotics that evade toxicity and resistance causing mechanisms.  In this latter area, work on developing an anti-toxin malaria vaccine was presented. 

The final part of the lecture concerned the use of flow-through micro-reactors to synthesize oligosaccharides with a view to providing a method which is more amenable to optimisation and scale up / numbering up.  By changing variables such as donor and acceptor concentration, reaction temperature and reaction time the formation of product can be optimised whilst minimising side product formation.  Perhaps surprisingly, formation of side products such as orthoester compound and the hydrolysed lactol (see below) is minimised by raising the reaction temperature.  Working above –20ºC can effectively eliminate orthoester formation, but small amounts of hydrolysed lactol (<5%) are still formed at 10ºC, which is the optimal temperature for maximum product yield.