Taming hazardous chemistry by continuous flow technology.

Over the last two decades, flow technologies have become increasingly popular in the field of organic chemistry, offering solutions for engineering and/or chemical problems. Flow reactors enhance the mass and heat transfer, resulting in rapid reaction mixing, and enable a precise control over the reaction parameters, increasing the overall process selectivity, efficiency and safety. These features allow chemists to tackle unexploited challenges in their work, with the ultimate objective making chemistry more accessible for laboratory and industrial applications, avoiding the need to store and handle toxic, reactive and explosive reagents. This review covers some of the latest and most relevant developments in the field of continuous flow chemistry with the focus on hazardous reactions.

The synthesis of neurotensin antagonist SR 48692 for prostate cancer research.

An improved synthesis of the molecule SR 48692 is presented and its use as a neurotensin antagonist biological probe for use in cancer research is described. The preparation includes an number of enhanced chemical conversions and strategies to overcome some of the limiting synthetic transformations in the original chemical route.

Solid supported reagents in multi-step flow synthesis.

The frequently overlooked benefits that considerably simplify and enrich our standard of living are most often hinged upon chemical synthesis. From the development of drugs in the ongoing fight against disease to the more aesthetic aspects of society with the preparation of perfumes and cosmetics, synthetic chemistry is the pivotally involved science. Furthermore, the quality and quantity of our food supply relies heavily upon synthesised products, as do almost all aspects of our modern society ranging from paints, pigments and dyestuffs to plastics, polymers and other man-made materials. However, the demands being made on chemists are changing at an unprecedented pace and synthesis, or molecular assembly, must continue to evolve in response to the new challenges and opportunities that arise. Responding to this need for improved productivity and efficiency chemists have started to explore new approaches to compound synthesis. Flow-based synthesis incorporating solid supported reagents and scavengers has emerged as a powerful way of manipulating chemical entities and is envisaged to become a core laboratory technology of the future.

Solid-supported reagents for multi-step organic synthesis: preparation and application.

Since the early days of combinatorial chemistry solid-phase organic synthesis has been the method of choice for the production of large libraries. Solution-phase synthesis is again gaining importance especially for the synthesis of parallel arrays of smaller, focussed libraries containing single compounds with high degrees of purity. In the field of solution-phase library generation, the use of solid-supported reagents, catalysts and scavengers is emerging as a leading strategy, combining the advantages of both solid-phase organic synthesis (e.g. allowing the employment of an excess of reagent without the need for additional purification steps) and solution-phase chemistry (e.g. the ease of monitoring the progress of the reactions by applying LC-MS, TLC or standard NMR techniques). An account of some of the most recent advances in this area of research will be presented.

Synthesis of trifluoromethyl ketones using polymer-supported reagents.

A two step synthesis of trifluoromethyl ketones from aldehydes is reported. A combination of polymer-supported reagents and sequestering agents were employed to effect the transformation without the need for chromatographic purification.

Highly diastereoselective lithium enolate aldol reactions of butane-2,3-diacetal desymmetrized glycolic acid and deprotection to enantiopure anti-2,3-dihydroxy esters.

[reaction--see text] The butane-2,3-diacetal (BDA) desymmetrized glycolic acid building block 1 undergoes efficient and highly diastereoselective lithium enolate aldol reactions with both aromatic and aliphatic aldehydes to afford, after an acidic methanolysis deprotection step, the enantiopure anti-2,3-dihydroxy esters in good yield.

Highly diastereoselective Michael addition reactions of butane-2,3-diacetal desymmetrized glycolic acid. Preparation of alpha-hydroxy-gamma-amino acid derivatives.

[reaction--see text] The butane-2,3-diacetal (BDA) desymmetrized glycolic acid building block undergoes efficient and highly diastereoselective lithium enolate Michael additions to alpha,beta-unsaturated ketones, lactones, and nitro olefins. Subsequent deprotection of these Michael adducts gives alpha-hydroxy acids in very high yield. Hydrogenation of the nitro group in some of the adducts leads to gamma-lactams, which can be easily converted into alpha-hydroxy-gamma-amino acid derivatives.

Solid-phase development of a L-hydroxybenzotriazole linker for heterocycle synthesis using analytical constructs.

The development of a 1-hydroxybenzotriazole linker for the synthesis of heterocyclic derivatives is described, utilizing analytical construct methodology to facilitate the analysis of resin samples. A UV-chromophore-containing analytical construct enabled the accurate determination of resin loading and the automated monitoring of key reactions using only small quantities of resin. The syntheses of an array of isoxazole derivatives are reported.

Development and application of a carbonyl-(13)C-enriched backbone amide linker for solid-phase reaction monitoring.

The synthesis and application of a carbonyl-(13)C backbone amide linker are described. The labeled unit is conveniently mixed with commercial resins, providing a rapid means of monitoring chemistry performed with this linker on solid support using conventional (13)C NMR methods.

Both mosquito-derived xanthurenic acid and a host blood-derived factor regulate gametogenesis of Plasmodium in the midgut of the mosquito.

Gametogenesis of Plasmodium in vitro can be induced by the combined stimulus of a 5 degrees C fall in temperature and the presence of xanthurenic acid (XA). In-vitro experiments showed that P. gallinaceum (EC(50)=80 nM) is much more sensitive to XA than P. berghei (9 microM), P. yoelii (8 microM), and P. falciparum (2 microM). However, in the mosquito vector, we do not know whether the temperature shift and XA are the only gametocyte-activating factors (GAF), nor do we know with certainty the true source(s) of XA in the mosquito blood meal. Previous studies indicate that XA is the only source of GAF in the mosquito. By defining, and then contrasting, the ability of an XA-deficient mutant of Aedes aegypti, with the wild-type mosquito to support exflagellation and ookinete formation in vivo, we determined the roles of parasite-, mosquito- and host blood-derived GAF in the regulation of gametogenesis of P. gallinaceum. Removal of both host and vector sources of GAF totally inhibited both exflagellation and ookinete production, whilst the lack of either single source resulted in only a partial reduction of exflagellation and ookinete formation in the mosquito gut. Both sources can be effectively replaced/substituted by synthetic XA. This suggests (1) both mosquito- and vertebrate-derived factors act as GAF in the mosquito gut in vivo; (2) the parasite itself is unable to produce any significant GAF activity. Studies are underway to determine whether vertebrate-derived GAF is XA. These data may form the basis of further studies of the development of new methods of interrupting malarial transmission.

Characterization of azadirachtin binding to Sf9 nuclei in vitro.

[22,23-(3)H(2)]dihydroazadirachtin was incorporated by Sf9 cells in culture and was bound specifically to the nuclear fraction. The observed association constant of the binding of the radioligand to a purified nuclear fraction was determined to be 0.037 +/- 0.008 min(-1) using a one-phase exponential association equation, and binding appeared to be to a single population of sites. The binding was essentially irreversible, and the dissociation constant was estimated to be 0.00065 +/- 0.00013 min(-1). An association rate constant of 7.3 x 10(6) M(-1) min(-1) was calculated from these data. Binding was saturable, and the receptor number and affinity were determined as B(max) = 23.87 +/- 1.15 pmol/mg protein, K(d) = 18.1 +/- 2.1 nM. The order of potency of semisynthetic azadirachtin analogues for competition for the binding site was as follows (IC(50) in parentheses): azadirachtin (1.55 x 10(-8) M) > dihydroazadirachtin (3.16 x 10(-8) M) > dansyl dihydroazadirachtin (7.40 x 10(-8) M) > DNP-azadirachtin (7.50 x 10(-8) M) > biotin dihydroazadirachtin (1.27 x 10(-7) M) >> 11-methoxy 22,23-dihydroazadirachtin (6.67 x 10(-7) M). [Originally published in Volume 34, Archives of Insect Biochemistry and Physiology, 34:461-473 (1997).]

Tetra-N-propylammonium perruthenate: a case study in catalyst recovery and re-use involving tetraalkylammonium salts.

The use of tetraalkylammonium salts or imidazolium ionic liquids in catalytic oxidations of alcohols with tetra-N-propylammonium perruthenate allows recovery and reuse of the oxidant; this concept may find application in the recovery of other homogenous catalysts.

A short stereoselective total synthesis of the fusarium toxin equisetin.

A short stereoselective synthesis of the fusarium toxin equisetin, an N-methylserine-derived acyl tetramic acid and potent inhibitor of HIV-1 integrase enzyme, is described using as the key step a stereoselective lithium perchlorate mediated intramolecular Diels-Alder reaction of a fully conjugated E,E,E-triene with a trisubstituted gamma,delta-unsaturated beta-ketothioester.

Chemoenzymatic synthesis of L-galactosylated dimeric sialyl Lewis X structures employing alpha-1,3-fucosyltransferase V.

L-Galactosylated dimeric sialyl Lewis X (SLeX) has been prepared employing a combination of chemical and enzymatic synthetic methods. GDP-L-galactose has been chemically synthesised. Enzymatic transfer of L-galactose onto the acceptor (Sia-alpha2,3-Gal-beta1,4-GlcNAc-beta1,3/6)2-Man-alpha1-OMe was achieved using the human alpha-1,3-fucosyltransferase V.

Polymer-supported reagents for multi-step organic synthesis: application to the synthesis of sildenafil.

Sildenafil 1 (Viagra), a well known and commercially important pharmaceutical drug, has been prepared using polymer-supported reagents in a multi-step, convergent process resulting in a clean and efficient preparation without the need for conventional purification methods.

Chemo-enzymatic synthesis of fluorinated sugar nucleotide: useful mechanistic probes for glycosyltransferases.

An effective procedure for the synthesis of 2-deoxy-2-fluoro-sugar nucleotides via Select fluor-mediated electrophilic fluorination of glycals with concurrent nucleophilic addition or chemo-enzymatic transformation has been developed, and the fluorinated sugar nucleotides have been used as probes for glycosyltransferases, including fucosyltransferase III, V, VI, and VII, and sialyl transferases. In general, these fluorinated sugar nucleotides act as competitive inhibitors versus sugar nucleotide substrates and form a tight complex with the glycosyltransferase.

Synthesis of high-mannose type neoglycolipids: active targeting of liposomes to macrophages in gene therapy.

The concise synthesis of five biantennary oligomannose neoglycolipids is presented. Employing a strategy based on the principles of reactivity tuning and orthogonal activation, the oligomannose moieties, isolated from the glycoprotein 63 of the parasite Leishmania mexicana amazonensis, were rapidly assembled taking advantage of common structural motifs found in these N-glycans. Deprotection of all structures was achieved in high yield by hydrogenolysis. The deprotected glycoconjugates were subsequently coupled to a cholesteroldiamine derivative using diethylsquarate as a linker. The resulting neoglycolipids will be used as additives to cationic liposome formulations in the active targeting of liposomes to macrophages.

1,2-Diacetals in synthesis: total synthesis of a glycosylphosphatidylinositol anchor of Trypanosoma brucei.

A full account on a total synthesis of GPI anchor 1 employing butanediacetal (BDA) groups and a chiral bis(dihydropyran) is presented. The reactivity of selenium and thio glycosides was tuned by the use of BDA groups. This allowed the assembly of an appropriately protected GPI anchor precursor 2 in just six steps from the six building blocks 5-10 including only one protecting group manipulation. myo-Inositol was desymmetrised with the bis(dihydropyran) derivative 15 and appropriately protected to give inositol acceptor 21 in nine steps and 17% overall yield. The use of common starting materials and BDA-protections give efficient access to building blocks 5, 6, 7 and 8. A new and improved synthesis of the glucosamine donor 28 is included. In summary, a highly convergent and efficient synthesis of GPI anchor 1, which is clearly adaptable to other GPI anchors, has been reported.