COMMUNICATION

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A flow process for the multi-step synthesis of the alkaloid natural
product oxomaritidine: a new paradigm for molecular assembly
Ian R. Baxendale, Jon Deeley, Charlotte M. Griffiths-Jones, Steven V. Ley,* Steen Saaby andGeoffrey K. Tranmer
Received (in Cambridge, UK) 11th January 2006, Accepted 3rd February 2006
First published as an Advance Article on the web 15th February 2006
DOI: 10.1039/b600382f

A flow process for the multi-step synthesis of the alkaloid
natural product (¡)-oxomaritidine is described, mediatedthrough the use of microfluidic pumping systems that progressmaterial through various packed columns containing immobilized reagents, catalysts, scavengers or catch and release agents;
our route involves the combination of seven separate synthetic
steps linked into one continuous sequence utilizing flow
chemistry.
The often low availability of natural products from their sources oforigin, combined with the lack of structural analogues, creates aneed for laboratory synthesis. This is especially true if thesematerials are to be used as biological probes for drug discovery
programs. Consequently, new techniques are urgently required toovercome many of the bottlenecks associated with conventionalorganic synthesis. Also more use of automation is needed tofacilitate scale-up and to improve the current synthesis standards
in terms of reaction optimisation and product clean-up practices.
For some years, we have been evolving the concept of using
immobilised reagents, scavengers and catch and release techniques
to overcome many of these problems.1 It has always been a part ofour vision that these techniques would eventually be incorporated
into micro-2 and meso-flow3 reactors to greatly enhance current
processing capabilities, developing new strategies for how future
synthesis programmes may be conducted in a far more effective
fashion.
In recent years, the search for new synthetic methods has moved
beyond the traditional quest for new reactions and reagents. The
scope of the search has now broadened to incorporate newpractical preparative methods for increasingly complex organictransformations. One of the most exciting and potentiallysignificant developments is the innovative incorporation of flow
chemistry into lab based synthesis platforms. Flow chemistry
replaces traditional glassware with columns and cartridges that canbe pre-packed with immobilised reagents and catalysts, or throughthe use of precision manufactured reaction chips, that permit
controlled mixing and precise temperature control of reaction
sequences. This essentially transforms conventional reaction
methods into a flowing, dynamic system by either passing thestarting materials through an immobilized reagent or combining
the starting materials and reagents on a reaction chip. The productthen exits from the reactor into a chemically unreactive

environment that is beyond the reach of any further chemicaltransformation. These methods allow for the immediate collectionof a pure product upon elution from the reactor in an expedited
cost-conscious manner. An additional benefit to this mode ofchemistry is that it allows for the linking of individual reactions
into multi-step sequences, allowing for one reaction to flow
seamlessly into another, creating a rapid route to the desired more
complex product by combining multiple synthetic steps into a
continuous operation. In addition, the incorporation of automatedand real-time analysis facilitates rapid optimisation and excellent
quality control whilst also ensuring reproducibility. By varying
flow rates, solvents, temperatures and pressures, these informationrich sequences create opportunities for sensitive modification ofchemoselective processes and constitute a paradigm shift for theon-demand delivery of chemical substances. Flow chemistry
extends the opportunities available to organic chemists, providing
as it does, improved reactivity, better selectivity and simple in-line
analysis.
Here we report on the first multi-step flow through preparationof a natural product, utilising flow methods and techniques. Whatis significant in this work is that no product isolation atintermediate stages was necessary, no use was made of labour
intensive techniques such as product distillation or crystallisation,
nor was column chromatography required to furnish materials of
appropriate quality in order to progress to the next step in thesynthesis. The time saved by using these flow through methods
compared to conventional procedures is dramatic. For example,
even when supplied with an optimised procedure using standard
protocols, one might expect a typical synthesis using conventionalround-bottomed flasks of the Amaryllidaceae alkaloid natural
product oxomaritidine (1) (Fig. 1) to require some four days oflaboratory manipulation,4 involving, as it does, setting up of theapparatus, conducting and monitoring of the individual reactions,determination of the completion of the reactions, quenching(usually by water washing, solvent extraction and evaporation),and subsequently column chromatography to purify individual
products at all the intermediate stages.

Innovative Technology Centre, Department of Chemistry, University ofCambridge, Lensfield Road, Cambridge, UK CB2 1EW.
E-mail: svl1000@cam.ac.uk; Fax: +44 1223 336442;
Tel: +44 1223 336398

Fig. 1 The natural product (¡)-oxomaritidine (1).

2566 | Chem. Commun., 2006, 2566–2568

This journal is ß The Royal Society of Chemistry 2006

(¡)-Oxomaritidine (1) is a cytotoxic alkaloid of theAmaryllidaceae family of natural products.5 We have previously
reported a total synthesis of this biologically important compoundusing only polymer-supported reagents at every individual stage ofthe synthesis pathway.4 The significance of this report was thedevelopment of a route that was completed without the need forcolumn chromatography or the use of aqueous work-ups at anystage. Here we build upon this prior knowledge to devise a flow
through reaction sequence that produces the natural product in an
automated sequence from readily available starting materials in
less than a day (Scheme 1). This was accomplished by directly
coupling glass reaction columns6 in series, thereby providing
multiple synthetic transformations in one flow-through process.
In the first step of the sequence a commercially available
bromide, 4-(2-bromoethyl)phenol (2), was quantitatively converted
to the corresponding azide (3). This was accomplished by packinga glass column with an azide exchange resin7 and flowing asolution of the bromide through the supported reagent using aSyrris AFRICA1 system.8 The output reaction stream from thisstep containing the newly generated alkyl azide (3) was coupleddirectly into a second column containing a polymer-supported
phosphine,9 furnishing the corresponding aza-Wittig intermediate
trapped on the supported material. Simultaneously, within aseparate, but convergent channel, was prepared the aldehyde
coupling partner (5). A pre-packed column of tetraN-alkylammonium perruthenate (PSP)10 was used to facilitate

the oxidation from the commercially available, 3,4-dimethoxybenzyl alcohol (4). This product stream was then passed through thecolumn containing the immobilized aza-Wittig intermediate,producing the desired imine (6). The tetrahydrofuran (THF)
solution of imine (6) was then subjected to continuous flowhydrogenation utilizing a Thales H-Cube1 flow hydrogenator,11which contained a cartridge of 10% palladium on carbon as
catalyst. A more detailed report of this work has been given
previously by our group.12 The product of this reaction (7) was
then collected on-line and the THF solvent was removed using aVapourtec V-101 solvent evaporator13 and re-dissolved indichloromethane (DCM), an operation that can be readily
accomplished in less than ten minutes. It should be noted thatthe simple solvent-switch procedure following the flow hydrogenation step, from THF to DCM, represents the only producthandling operation provided by the user and serves as a usefulpoint of concentrating the product for the final transformations.
In the last series of coupled reactions, passage of the secondaryamine (7) onto a microfluidic reaction chip that combines anadditional stream of trifluoroacetic anhydride (TFAA) in DCM
resulted in the trifluoroacetylation of the amine to give the amide(8). The reaction stream was then passed through a shortscavenging column containing a silica-supported primary amine14
that removed any excess TFAA or residual trifluoroacetic acid
(TFA). The optimal conditions for this reaction were found to be 5
equivalents of the TFAA, a temperature of 80 uC, and a residence

Scheme 1 Synthesis in flow of (¡)-oxomaritidine.

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Chem. Commun., 2006, 2566–2568 | 2567

time of just over 3.5 minutes. It should also be noted that a backpressure regulator was essential in this procedure and wasconnected in-line with the exiting reaction stream, allowing heatingof DCM to 80 uC, well above its usual boiling point. This reaction
stream was then directed into a column containing polymersupported (ditrifluoroacetoxyiodo)benzene (PS-PIFA)15 which
performed the oxidative phenolic coupling, generating a sevenmembered tricyclic intermediate (9). The resulting product wasthen passed directly into a column that contained a polymersupported base16 which promoted cleavage of the amide bond,allowing for a 1,4-conjugate addition to spontaneously take place,generating the target compound (¡)-oxomaritidine (1). The
output from the completed synthesis in flow was evaporated
in vacuo and the reaction product was found to have in excess of
90% purity by 1H NMR spectroscopy. The purity assay is evenmore impressive when considering that the only impurity wasidentified as the ortho-coupled product derived from the phenolicoxidation and was easily separated from the desired product using
preparative HPLC, producing 20 mg of 1.
The reproducible yield over the entire sequence was found to beslightly greater than 40%. In an effort to identify the fate of theunrecovered material, the reaction sequence was dissected stepwise.
It was found that the phenolic oxidation gave only a moderate
50% yield, with the resulting impurities and unreacted starting
material being scavenged out by the polymer-supported baseduring the last step of the flow synthesis. The other six steps in thesequence occurred with quantitative to near quantitative conversion. This indicates that the only issue in our sequence of synthetictransformations was the inability of current supported reagents tooffer a high yielding synthetic alternative, a matter we plan toaddress and solve in the near future.
While in this work we describe just one multi-step preparationof a natural product using flow methods, we believe that such an
approach could be readily applied to the synthesis of othercompounds. The application of these techniques constitutes a newparadigm for molecular assembly and will certainly have a verysignificant impact on how synthesis programmes will be conductedin the future. The benefits attained are in terms of both the costand efficiency gained through the avoidance of extensive
purification and work-up procedures, the rapid optimisation andprecise control of reaction conditions, and a reduction in manual
handling. The swift adoption of these techniques and principles
will provide chemists with methods that allow for the rapid
production of chemical targets in a fashion far superior to that
permitted by conventional methods. With this ability to rapidly
access on demand targets it becomes possible to quickly
interrogate and probe biological systems, allowing for theimmediate integration of new screening and assay knowledge intothe next iteration of molecular design and, in turn, simplifying andexpediting the decision-making processes in the lead generationand optimisation stages of drug discovery programs.
We gratefully acknowledge financial support from the RS
Wolfson Fellowship (to IRB and SVL), the Natural Sciences andEngineering Research Council of Canada for a Postdoctoral
Fellowship (to GKT), GlaxoSmithKline (to CMGJ and JD), theCarlsberg Foundation (to SS) and the BP endowment and theNovartis Research Fellowship (to SVL).

2568 | Chem. Commun., 2006, 2566–2568

Notes and references
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S. V. Ley, Supported Reagents and Scavengers in Multi-Step Organic
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M. R. Buchmeiser), p. 53 (VCH, Berlin, 2003); (c) S. V. Ley,
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5 Z. Jin, Z. Li and R. Huang, Nat. Prod. Rep., 2002, 19, 454–476.
6 Commercially available Omnifit1 glass chromatography columns withadjustable height endpieces (plunger). Typically, polymer-supported
reagent is placed in an appropriately sized Omnifit column, usually
6.6 mm bore by 150 mm length, or shorter, and plungers are adjusted torelevant bed heights. Website: http://www.omnifit.com.
7 Azide exchange resin, azide on Amberlite1 IRA-400 (368342).
Commercially available from Aldrich. Website: http://www.sigmaaldrich.com.
8 Pumping of solvents through the reagent columns was performed using
the commercially available Syrris AFRICA system. Website: http://www.syrris.com.
9 Di(n-butyl)phenylphosphine polystyrene (01-64-0402). Commerciallyavailable from novabiochem. Website: http://www.novabiochem.com.
10 (a) B. Hinzen and S. V. Ley, J. Chem. Soc., Perkin Trans. 1, 1997,1907–1908; (b) R. Lenz and S. V. Ley, J. Chem. Soc., Perkin Trans. 1,
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11 Thales Nanotechnology, H-1031 Budapest, Záhony utca 7 (GraphisoftPark), Hungary. Website: http://www.thalesnano.com.
12 S. Saaby, K. R. Knudsen, M. Ladlow and S. V. Ley, Chem. Commun.,
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13 Vapourtec Ltd, Place Farm, Ingham, Suffolk, UK IP31 1NQ. Website:
http://www.vapourtec.com.
14 2-(4-(Ethylenediamine)benzyl)ethyl-functionalized silica gel, 200–400
mesh (552585). Commercially available from Aldrich. Website: http://www.sigmaaldrich.com.
15 (a) S. V. Ley, A. W. Thomas and H. Finch, J. Chem. Soc., Perkin Trans.
1, 1999, 669–672; (b) H. Toga, G. Nogami and M. Yokoyama, Synlett,1998, 534–536.
16 Ambersep 900-OH ion-exchange resin (301345000). Commerciallyavailable from Acros Organics. Website: http://www.acros.com.

This journal is ß The Royal Society of Chemistry 2006