The stockholders of Fluorous Technologies approved a plan to permanently close the business and begin a liquidation process. While we had hoped for a better long term outcome, all of us at FTI agree this is the appropriate action at this time.

Thank you, one and all, for your tremendous support and interactions over the past 12 years.

We know many of you would like to continue to purchase fluorous products, and discussions are underway with other potential providers. In the meantime, please note that FTI is STILL TAKING ORDERS for materials currently in stock. We have updated our contact information to the following:

E-mail:    fluoroustech@gmail.com
Voicemail: 267-225-5384 (267-CALLFTI)

All knowledge-based assets of the company are available for purchase as part of the liquidation process, including trade secrets (production procedures, general fluorous know-how), intellectual property (FTI-assigned patents) and brand assets (trademarks, website, etc.). Please do reach out if you have an interest.

Finally, we’d like to take one last moment to highlight and thank our customers, the fluorous community, our partners and numerous dedicated employees.

Success with Fluorous

Hundreds of scientists from a broad spectrum of organizations purchased and successfully used FluoroFlash products.  All of our customers contributed to the validation and expansion of the technology.  Because of this extensive “in the lab” testing, we can confidently claim that fluorous separations deliver as promised in diverse research applications, from synthesis of small molecules to creation of microarrays for life science studies.

Notable Work

While we could not possibly list everyone who made contributions to the fluorous community over the past decade plus, the following customers and their coworkers went above and beyond to develop innovative new uses for fluorous technology.

Craig Lindsley provided one of the first external validations of the technology for small molecule synthesis in the paper, “Fluorous-tethered amine bases for organic and parallel synthesis: scope and limitations”  doi:10.1016/S0040-4039(02)01399-0

John Valliant developed “A New Strategy for Preparing Molecular Imaging and Therapy Agents Using Fluorine-Rich (Fluorous) Soluble Supports” doi:10.1021/ja0600375

The Curran Group employed fluorous mixture synthesis for natural product stereoisomer determination as exemplified in the paper, “A ‘Shortcut’ Mosher Ester Method To Assign Configurations of Stereocenters in Nearly Symmetric Environments. Fluorous Mixture Synthesis and Structure Assignment of Petrocortyne A” doi:10.1021/ja900849f

Eric Peters was the first to adapt fluorous separation techniques to proteomics applications in the paper, “Enrichment and analysis of peptide subsets using fluorous affinity tags and mass spectrometry” doi:10.1038/nbt1076

Nicola Pohl created “Fluorous-Based Carbohydrate Microarrays” as a first step towards a fluorous toolkit for glycomics.  doi:10.1021/ja054811k

Will Pearson developed methods for “Fluorous Affinity Purification of Oligonucleotides” doi:10.1021/jo050795y

Arturo Vegas at the Broad Institute created and validated “Fluorous-Based Small-Molecule Microarrays for the Discovery of Histone Deacetylase Inhibitors” doi:10.1002/anie.200703198

Ongoing Resources for Customer Support

The fluorous.com website will remain available as a resource to the fluorous community for the forseeable future, but that status may not last forever. To help ensure that all the valuable technical information we gathered, composed and organized over the past decade will live on, we have made arrangements for static copies of selected website pages, F-Blog, and Technical Newsletters to be freely available in our public ftp folder.

The Technology and Application pages from our website, as well as all Technical Application Notes, a 3 MB pdf file, can be downloaded at: ftp://fluorous.com/FLUOROUS_Technology_and_Applications.pdf

The F-Blog Archive, a 9 MB pdf file, can be downloaded at: ftp://fluorous.com/FBlog_Archive.pdf

The Fluorous Technical News Archive, a 4 MB pdf file, can be downloaded at: ftp://fluorous.com/FluorousNews_Archive.pdf

Take a moment to browse through and download the documents at ftp://fluorous.com as well as those found through our Presentations and Application Notes webpages.  There is a great deal of high quality work presented in these files.

Ongoing Product Distribution Channels

For immediate sale of our in-stock product inventory, please contact us at fluoroustech@gmail.com.  Additionally, you may want to contact the following distributors of fluorous products:

Sigma Aldrich continues to distribute select FluoroFlash products.  Visit their fluorous chemistry portal for more information.

Berry & Associates offers products for Fluorous Purification of Oligonucleotides.  Please see their website http://www.berryassoc.com/fluorous.asp for further details.

Evolution of Fluorous

The field of fluorous technology has a rich past to draw from and a bright future ahead.  As our operations draw to a close, we’d like to bring attention to the work done by the greater fluorous scientific community.

Publications

Since we started tracking the scientific literature, over ten thousand papers relating to fluorous chemistry have been published.  You can download our master reference database, a 4.3 MB EndNote file, at: ftp://fluorous.com/Fluorous_references.enl.  We reviewed many of these papers as they were published here on our scientific blog, F-blog.

The following publications are comprehensive and useful compilations of fluorous technology:

A Fluorous Symposium in Print was published by Tetrahedron in 2002.

The Handbook of Fluorous Chemistry was published in 2004, edited by John Gladysz, Dennis P. Curran, and Istvan Horvath.

A special issue of QSAR & Combinatorial Science dedicated to fluorous chemistry was published in 2006.

Conferences

Fluorous Technologies Inc. was a proud sponsor of (and participant in) the International Symposium on Fluorous Technology (ISoFT).  The next edition, ISoFT’13, will take place in Budapest, Hungary in June 2013.  Learn more about this important and thought-provoking conference at http://www.isoft13.mke.org.hu/

Those looking to keep abreast of fluorous technology as it moves forward should connect with ISoFT and its leaders.

ISoFT conference chairs:

Jean-Marc Vincent, Chair of the inaugural ISoFT held in 2005 in Bordeaux, France
Ilhyong Ryu, ISoFT’07 in Yokohama, Japan
Dennis P. Curran, ISoFT’09 in Jackson Hole, USA
István T. Horváth, ISoFT’11 in Hong Kong, China
József Rábai, ISoFT’13 to be held in Budapest, Hungary

International Advisory Board of ISoFTs:

Dennis P. Curran, University of Pittsburgh, USA
Richard H. Fish, LBNL Berkeley, USA
John A. Gladysz, Texas A&M University, USA
Kenichi Hatanaka, The University of Tokyo, Japan
Eric G. Hope, University of Leicester, United Kingdom
István T. Horváth, City University of Hong Kong, China
Pierangelo Metrangolo, Politecnico di Milano, Italy
Nicola L Pohl, Iowa State University, USA
Ilhyong Ryu, Osaka Prefecture University, Japan
Jean-Marc Vincent, University of Bordeaux, France (Permanent Secretary of ISoFTs)

The Future

The life science market clearly sees the utility of fluorous techniques and has expressed a defined interest in fluorous products. There is an enthusiastic and dedicated user base in place that would no doubt embrace a new source for fluorous materials. An opportunity exists, and I challenge someone or some entity to rise to the need in the marketplace.

Thanks to Fluorous Employees

The working atmosphere at Fluorous is mostly that: working — with chemists scribbling notes on the translucent fronts of the lab’s fume hoods and white lab coats representing the height of fashion. But the culture at the startup firm is young and fun, tinged with the satisfaction that comes with knowing their work could help millions of people.

- from a 2002 article about Fluorous Technologies Inc.

Thirty-two people worked at FTI over the years, creating and building a commercial enterprise from the ground up.

Through a collective effort, the company was able to secure and execute more than a dozen research grants, primarily through the SBIR process; produce hundreds of high quality batches in support of a 200+ product catalog; participate in numerous trade shows, economic development programs, and scientific conferences; publish dozens of technical application notes with details on product usage, tips and techniques; chronicle and promote the fluorous field in real-time through a scientific blog; and support our customers to the best of our ability.  Each employee of Fluorous Technologies made all of this possible through their creativity, passion, and dedication.

I would like to extend my sincere gratitude to our company founder Dennis Curran as well as all fluorous colleagues, past and present, with best wishes for future success.

Sincerely,

Philip E. Yeske
President and CEO
Fluorous Technologies Inc.

Dr. Christopher Blackburn at Millennium Parmaceuticals has just published a Technology Note in ACS Combinatorial Chemistry to address this issue. Essentially he made a tag, pictured above, which contains three principal components; a) a fluorous domain, b) an azo dye, and c) a reactive functional group, in this instance an aldehyde.  The concept for the molecule is quite similar to what Lo reported using fluorous rhenium complexes although the application is different.  He was then able to tag amines through a reductive amination and purify the tagged compound by FSPE.  Due to presence of the dye, he was also able to follow the FSPE process visually.    He then demonstrated the utility of the tag in two applications; one a solution phase peptide synthesis and the other in sulfonamide synthesis.  The tag was then easily cleaved using TFA to provide the desired products after FSPE.

At FTI, we actually have received more than one request for a visual fluorous tag such as this one, but we’ve never developed one.  One of the reasons is that to develop a whole suite of visible tags which would encompass all the possible ones that people would want was always considered a daunting task.  Another reason is that the FSPE is really quite a robust process so how much value would it really add?  A tough question to answer.  Finally, the azo tag does put some additional constraints on the fluorous tag.  Dr. Blackburn notes that unlike most fluorous tag, the added dye makes the compound insoluble in MeOH, CH2Cl2, and CH3CN.  It took some heating in THF to get it into solution.  The dye portion also adds some functionality, and therefore reactivity, that limits some of the chemical compatibility.  These issues certainly takes away from some of the appeal of fluorous chemistry.

Even with the caveats above, a visible fluorous tag could certainly have it’s place in certain applications and it’ll be interesting to see what the response to the Note is.

Besides PALs and other cross-linking strategies, another popular strategy in proteomics is isotope-coding in order to be able to directly compare different samples.  For example, if you wanted to compare protein levels in healthy cells vs. diseased cells, you could add the appropriate protein labeling agent to the healthy cells and the same protein labeling agent, except with some deuteriums of 13C’s in it, to the diseased cells.  Mix the two labeled samples together in equal amounts then compare the ratio of labeled proteins by MS.  Most of the time, you’d see a 1:1 ratio for the proteins, but every now and then you’d see a protein overexpressed or underexpressed in the disease cell state vs the healthy state.  This would then indicate that that protein may be important in the disease process.

In his group’s latest report, Prof. Zhang combines these two strategies by using a pair of isotope-encoded fluorous PALs.  As seen in the figure above, the difference in the two PALs is proton vs. deuterium substitution on the aromatic ring.  In practice, the authors used not the benzyl alcohol, but the NHS benzyl carbonate (not shown here) to first label a small peptide (RKRSRAE) through the side chain amine of the lysine. They took an equimolar ratio of the two in MeOH and irradiated the sample and found a near 1:1 ratio of the O-H bond insertion product of the carbene with MeOH.  This demonstrated that the isotope-encoded PAL’s behaved similarly and thus suitable for their intended use.

Next the authors looked at the effectiveness of the FSPE separation.  They took a 1:1 and a 2:1 mixture of the isotope-encoded insertion products from above and added them to a BSA tryptic digest.  As can be seen the amount of fluorous labeled peptides before FSPE is quite small in comparison to the BSA peptides.  The difference after FSPE, however, is quite dramatic and demonstrates the power of the FSPE in the sample enrichment.  They also found that the original isotope ratio was largely maintained, so no preferential enrichment of the deuterated over the protonated labels.

Next step:  a real mixture looking for some real answers.

But what do you do if your different substrates all lead to the same product?  How then do you differentiate between them? You use a different fluorous tag for each substrate, of course.  Now the mass differentiation comes not from the substrate, but from the fluorous tag which encodes for each substrate and that’s exactly what Northen et al have done in their latest publication in Rapid Communications in Mass Spectrometry.  They encoded three different disaccharides, maltose (S1), lactose (S2), and cellobiose (S3) with a unique, and mass differentiated, fluorous tag as seen below.  Note that the mass (427.2) of the disaccharide portion is the same for each substrate and that they only differ in stereochemistry, but that after tagging each has a unique mass.  Without the fluorous tag there would not be a simple way to differentiate between these species by MS.

The authors took their mixture of substrates and incubated it with three different enzymes and a mixture of all three.  An aliquot of the reaction was then spotted on a NIMS surface with the fluorous tagged compounds being retained.  As expected, each enzyme reacted stereospecifically with only one of the substrates; a substrate that was easily identified by its unique mass courtesy of the fluorous tag.  When using the mixture of the enzymes, all three were hydrolyzed to a uniquely fluorous tagged glucose.  The authors went on to demonstrate the use of cell lysates rather than purified enzymes and found essentially the same results.  The fluorous NIMS surface is critical in this since the fluorous tagged substrates and products and retained on the surface once spotted and an on-surface enrichment conducted.

The use of different fluorous tags for the encoding of substrates is the key element in fluorous mixture synthesis (FMS), where mixtures of substrates  can be prepared at once and later demixed using fluorous HPLC (F-HPLC) based on fluorous tag length.  In this work, the application is not synthetic, but analytical, and the compounds can be unequivocally indentified through mass, even when working with mixtures of substrates and enzymes.  As the authors note given the vast array of combinations of fluorous tags and linkers that could be made, each with distinct masses, this could be a general method by which to encode enzyme substrate libraries.

At FTI we have in the past done some cursory examination into the fluorous separation behavior of perfluoropolyethers as fluorous tags and compared them to perfluoroalkyl tags.  What we found, if I recall correctly, was that the oxygens basically functioned much like a CF2, so that a CF2OCF2CF2OCF3 tag had a similar F-HPLC retention time as a C6F13.  This was somewhat surprising since we always used fluorine count as a first approximation of fluorophilicity.  What the F-HPLC experiments told us, however, is that perhaps we should consider the size of the fluorous domain instead, in this case a 6 atom chain that is “rich in sp3 carbon-fluorine bonds”.

A just available paper from Kvicala et al at the Institute of Chemical Technology in Prague attempts to explain the fluorous behavior of perfluoropolyethers, at least as tags on NHC ligands in silver complexes.  The authors first prepared a series of substituted NHC ligands, either with traditional perfluoroalkyl tags or with perfluoropolyether tags.  They then formed the silver salts of these ligands.  Fluorous partition coefficients were then measured for the ligands and the complexes by partitioning between toluene and perfluoromethylcyclohexane.  The results were fairly striking as seen in the table below.  

Compounds 9a-h were all perfluoroalkyl tagged compounds which essentially had fairly equivalent partition coefficients, something in the single digits.  All of these compounds had a total of 12-16 CF2 or CF3 groups.  Compounds 12b, c, and f, however, were polyfluoroether tagged and possessed dramatically higher partition coefficients.  12b contains only 8 perfluorinated carbons, 12c has 12, and 12f has 10, so the overall fluorine content for compounds 12 is less than 9 yet they are more fluorophilic.  So what’s going on?  Well, if you consider the FTI results mentioned earlier and regard the oxygens in the perfluoropolyether as CF2′s than you get a pseudo-fluorous count for 12b, c, and f of 12, 18, and 15.  That’s now in the range of compounds 9, but still doesn’t account for the greater fluorous partition coefficient.

The authors then turned to computational chemistry.  DFT calculations were conducted on both the NHC ligands and the imidazolium salts.  What these calculations showed was that the most stable conformation of the perfluoroalkyl chain had the chain directed  straight out from the imidizole ring and beasically planar to it.  The perfluoropolyether chains on the other hand were twisted at the carbon-oxygen bond resulting in the chains being out of plane with the heterocyclic ring.  The authors hypothesize that fluorophobic ionic center of the imidizolium salts and the silver complexes are more shielded with the perfluoropolyether chains resulting in a significantly more fluorophilic compounds.  It’s certainly a reasonable hypothesis, particularly when one considers that fluorophilicity is based on solvophobicity more than on any attractive forces.

It’s an interesting paper which shows how structural elements can have profound effects on fluorophilicity.  Something we should certainly keep in mind more often.

In general, synthetic biopolymers are made either by solution phase chemistry of solid phase chemistry.  Solution phase chemistry has the advantage of better reactivity and the ability to monitor reactions and purify intermediates.  This means that you can use less amounts of monomer and reagents.  This also means, however, that syntheses can be quite labor intensive since purification is not always the easiest.  Solid phase methods, on the other hand, are highly labor efficient, but often times require large amounts of reagents and monomers due to the heterogeneous nature of the chemistry.  So the choice of solid phase or solution phase depends on a number of factors; accessibility to monomers, scale of synthesis, length of synthesis, etc.

Fluorous methods have been used in the synthesis of all three major classes of biopolymers.   Three major methods for the incorporation of fluorous techniques have emerged; fluorous capping, fluorous tagging, and fluorous supported synthesis.  The first two, capping and tagging, are used in conjunction with solid-phase methods.  The latter, fluorous supported synthesis, is a solution phase method where the solid phase is replaced by a fluorous tag.  The growing oligomer can then be purified each step along the way by either fluorous solid phase extraction (FSPE) or fluorous liquid-liquid extraction (FLLE).  Fluorous supported synthesis therefore tries to combine the main advantage of solution phase chemistry (lower stoichiometries) and the main advantage of solid phase chemistry (ease of purification).  This has been used to great effect in carbohydrate synthesis where just getting to the monomers can be difficult, so you don’t want to be using 5 equivalents of them.

A new ASAP paper in Organic Letters from researchers at Leiden University led by Codée and Van der Marel describe their use of fluorous supported synthesis in the preparation of fragments of teichoic acid, the best known of cell wall glycoproteins found in Gram-positive bacterial cell walls.  Not surprisingly these cell wall glycoproteins play an important, but not very well understood role in immunology and bacteriology.  Teichoic acid consists of repeating glycerol phosphate or ribitol phosphate units which are decorated with glycosyl and/or D-alanyl substituents.  Ready access to fragments of teichoic acid would allow further study of these interesting molecules to take place.  These same researchers have also reported traditional solution phase synthesis and solid phase synthesis of teichoic fragments and now extend that work to fluorous supported methods.

As the fluorous support they chose the F-Psc group as a phosphate protecting group.  The same authors developed the F-Psc group as a hydroxyl protecting group for carbohydrate synthesis a couple of years back and found it to be suitable as a phosphate protecting group.  They first then demonstrated it’s utility in techoic acid synthesis by producing glycerol phosphate oligomer shown below.  The synthesis was a 4 step cycle (phosphoramidite coupling, oxidation, DMT removal, and FSPE) that was repeated until the desired length was obtained after which the F-Psc group was removed by base and the secondary hydroxyls debenzylated by hydrogenation.  It proved to be quite successful and the FSPE was easily conducted for all of the compounds even as the oligomer grew longer.  Interestingly, the authors did note that as the oligomer grew longer that increasing amounts of phosphoramidite were required, so that once you got to an 18-mer you lost that advantage of solution phase chemistry.  Why that should be is unclear.

After demonstrating the value of the fluorous supported synthesis in the preparation of a unmodified teichoic acid, they then prepared a more complex version; a glycosylated hexamer 28 which also proved to be quite successful.  As a final demonstration the produced the fully glycosylated hexameric glycerol phosphate 39.  In each of these FSPE played a critical role as the last step of the 4 step coupling cycle.

One issue with FMS is the limited number of tag lengths that are practically available.  In theory any perfluorocarbon chain length should be usable from CF3 to CxF2x+1.  In practice, though, anything above C9F19 introduces solubility problems in organic solvents which defeats the purpose, since they won’t react in a similar fashion as the soluble components.  In addition, the odd numbered lengthed perfluorocarbons (C3,  C5, C7, and C9) are considerably more expensive than the even numbered chain lengths.  So now we’re down to just four lengths.  That then lowers the overall value of FMS since it limits the total number of reactions saved over parallel synthesis.

In response to that Prof. Curran has previously reported two methods to increase the number of reactions that can be conducted in a single pot and still be able to deconvolute the mixture.  One is to use fluorous tags in conjunction with oligoethylene glycol (OEG) tags of different length.  In this strategy, a two dimiensional separation strategy is used; a fluorous based separation and a RP-HPLC based separation.  Another is to use a double tagging strategy and separate the mixtures based on the cumulative fluorine count.  The synthesis of the cytostatins which we’ve covered in F-Blog before is an example.  This binary encoding works well, but is limited by additive redundancies.  In other words, the C6 and C4 tags in combination add up the same as the C2 and C8 tags in combination.  So even if you can easily separate the two by F-HPLC (not at all guarenteed), it’s impossible to predict which one is which.

A new paper from Dr. Curran’s labs in Nature Chemistry describes another double tagging FMS strategy which increases the number of molecules that can be made and overcomes the redundancy issue.  In this approach the researchers once again use two fluorous tags, but in this instance orthogonal tags rather than the same tags as in the cytostatin case.  The researchers produced a 16 member steroisomer library of macrosphelides (shown above) using a F-PMB and a F-TIPS tag as the orthogonal tags.  Each tag had four variants, C2, C4, C6, and C8, so 4 x 4 provides the 16 members.  By utilizing two tags that can be selectively removed, they were first able to partially deconvulute by FHPLC based on the cumulative fluorous content of both tags to provide 7 fractions.  These fractions were selectively deprotected of one tag and subjected to another FHPLC to completely deconvolute the mixture.  The figure below from the paper shows the redundancies and the fractions.

The macrosphelides are a series of natural product macrocycles containing five stereocenters.  The authors went on to prepare 16 of the 32 stereoisomers, leaving C3 stereochemistry fixed.  Each of the 16 stereoisomers had a distinct 1H and 13C NMR and they were able to match one of the isomers with previously characterized macrosphelide A and one with macrosphelide E (Note that the molecule they produced was enantiomeric macrosphelide E), thereby confirming the structure of each.

A problem was encountered, however, in that none of the spectra matched that of the reported structure of macrosphelide D.  After considering the possible structural isomers they hypothesized that macrosphelide D was actually a the ring contracted isomer shown below.  They then confirmed this through independent total synthesis.In conclusion the authors demonstrated that orthogonal binary fluorous tagging with double F-HPLC separation is a viable strategy for miaximizing FMS efficiency.  In the synthesis of the macrosphelides they required only six chemical reactions once the tagged intermediates were in place vs. 72 reactions if one were to do this in parallel synthesis mode.  In addition they once again demonstrated how important the synthesis of all possible isomers can be in natural products chemistry since it led to a definitive structural assignment of macrosphelide D.

The same authors now report a fluorous version which can provide enantiomer B directly with lower catalyst and substrate loading.   Instead of rendering the catalyst fluorous via the sulfonamide the fluorous tag is attached to the aromatic portion of the diamine catalyst.  So in this case the diamine is derived from L-tyrosine rather than L-phenylalanine to provide catalyst 4.  They then used 4 to catalyze the aldol reaction between cyclohexanone and p-nitrobenzaldehyde.  Compared to 2 they were able to use half the amount of catalyst and half the amount of cyclyhexanone(see entry 5) to achieve comparable results.  They then used fluorous solid phase extraction (FSPE) to recover and reuse the organocatalyst.  The authors went on to test a number of difference ketone and benzaldehyde combinations to test the generality of the reaction.  They found that while electron deficient benzldehydes are good substrates but that electron rich ones are not.  Other cyclic ketones were tried but none of them gave results that were as good as cyclohexanone.

The results themselves are good, but somewhat limited due to the structural requirements of the reaction.  From a fluorous perspective, however, there are some good takeaways here. First is that it demonstrates the flexibility of fluorous tagging.  The authors were able to change where the fluorous tag was on organocatalyst 2 quite easily in order to achieve a better catalyst in 4.  This was done without effecting the absolute stereochemistry.  Second is that the same general purification procedure, FSPE, was used regardless of where the fluorous tag was through a ether bond or a sulfonamide.  This then provides an opening for further refinements in the organocatalyst which will hopefully result in a more general reaction.

Just published in ACS Combinatorial Science is a paper from Park et al at Seoul National University describing their work at producing a diverse set of polyheterocyclic benzopyrans.  Within this paper they use six different reaction pathways to prepare 284 compounds.  Each reaction pathway produced a different core structure.  In five of the six pathways (paths II-VI) the authors used a fluorous tag while in the other pathway (path I) a solid phase support was used.  The structures are shown below with the authors starting from four different benzopyrans 3.

This paper is actually part II and in part I, published in 2010, the authors strictly used solid phase methods to produce 434 compounds.  In reading the current paper the advantages of fluorous methods become readily apparent.  First, the authors point out that they tried to use the solid phase strategy which worked reasonably well in the previous report, but found that the reactions were sluggish.  This resulted in either incomplete reaction or in high amounts of side products for pathways II-VI.  That is when they decided to turn to a fluorous strategy and use fluorous solid phase extraction (FSPE) as a purification method.  In this instance they used a fluorous silane to tag their four starting benzopyrans.  The solution phase nature of the chemistry clearly was needed in order to have a viable route to the desired compounds.  The average purity of the compounds derived from the fluorous pathways was ~87% without final HPLC purification.  That doesn’t seem too much higher than the 85% avg. purity reported from the previous paper using solid phase, but remember that the chemistry here was more demanding and that they couldn’t even get to the compounds in reasonable purity using solid phase methods.  The scheme below shows two of the pathways.  Note the use of FSPE for the purification of intermediates.

There are also some other more subtle aspects which were interesting.  For example, for pathway II the substituted triazolinediones were not all commercially available, so were prepared in-house by the authors.  If you have to make something, you don’t want to use more than you have to in any reaction.  Solid-phase techniques usually require large excesses of reagents which is not the case for fluorous methods.  There was also the manner in which the amount of compound is on the solid phase was determined by measuring the mass increase of the resin and then confirming that  after cleavage from the resin.  Let’s compare that to a fluorous tag which can be directly weighed and analyzed to NMR, LC/MS, or any other method to not only give you an amount, but also structural and purity information.

So once again we see how fluorous methods are applied to DOS and the value they provide.

Unlike DNA or peptide synthesis, the efficient automated synthesis of oligosaccharides has been difficult to achieve due to the increased complexity of oligosaccharides vs oligonucleotides and peptides.  There are more monomer units, stereochemistry in the coupling reaction, additional protecting groups, linear vs. branched structures, etc.  All of these factors increase the degree of difficulty, so even decades after the automated synthesis of peptides and DNA has been not only commercialized but commoditized, oligosaccharides remain unsolved.  The relative inaccessibility of oligosaccharides has undoubtedly held back the study of oligosaccharides in biological systems compared to DNA, RNA, and proteins.

A new review in Angewandte Chemie from Hsu et al describes the latest efforts in automated oligosaccharide synthesis.  The approaches include one-pot methods, chemoenzymatic syntheses, and phase supported methods.  Fluorous tags and separations, primarily in the form of fluorous solid phase extraction (FSPE), has been used in each of these synthetic strategies.  In addition to the actual synthesis, this new review also has sections on glycan microarrays and vaccine development using synthetic oligosaccharides.  Fluorous immobilization of course has been shown to be an effective method for the formation of glycan arrays.  This current paper then provides a good review of the field and places the contribution of fluorous methods in great context.

One of the little details that I found interesting is the authors’ characterization of the fluorous separation which they described as a “specific solvophobic interactions”.  This is in contrast to most reports which use terms such a “fluorous affinity” or “fluorous-fluorous interactions.”  Partitioning based on solvophobic effects is exactly the way that we like to describe fluorous separations, since there is no intermolecular forces between fluorous molecules that explain the partitioning observed.  As Prof. Craig Wilcox once rhetorically asked me, “Which compound has a lower boiling point, perfluorohexane or hexane?”  The answer is perfluorohexane which clearly indicates a lack of any intermolecular interactions or affinity which would explain the high partitioning observed.  So solvophobicity is a great term to describe fluorous partitioning, because it’s the incompatibility of fluorous domains with aqueous or organic phases that drives the the molecules into the fluorous phase rather than any attraction.  Push vs. pull, if you will.  The addition of the word specific is great.  It emphasizes that it’s not simply hydrophobic or lipophobic, but rather that the solvophobicity is beyond the usual phobicities that we’re accustomed to seeing.