Preparation, properties and synthetic potentials of fluorous boronates

A fluorous approach to the chemistry of boronic acids and its application in fluorous-phase techniques are described. Treatment of fluorous bromosilane 2 with allyl Grignard reagent followed by dihydroxylation provided fluorous diol 1. A series of boronic acids were attached to 1 by esterification. The formed fluorous boronates 4 were moisture sensitive and thus their synthetic potentials were limited. Thus a fluorous pinacol, 5, was designed and synthesized by treatment of fluorous bromosilane 2 with excess 2,3-dimethyl-2-butyenylmagnesium bromide 9 to afford fluorous tetramethyl ethene 8, and was dihydroxylated. Compound 5 was successfully used to prepare fluorous boronates in a one-pot process from organic bromides. We have demonstrated that olefin cross-metathesis can be carried out in a fluorous version. It is noteworthy that all of the fluorinated compounds reported in this paper were purified by simple liquid extraction.     

Peptide synthesis on fluorous support

New fluorous supports were synthesized and used to prepare a peptide having a C-terminal COOH based on fluorous chemistry. The hexakisfluorous chain-type support was suitable for the synthesis of a pentapeptide or a peptide derivative on a fluorous support whose fluorine content is over 40 w/w%. A bioactive peptide, Leu-enkephalin, was easily synthesized using an Fmoc-strategy based on fluorous chemistry.     

Tandem cycloisomerization/Suzuki coupling of arylethynyl MIDA boronates

A tandem gold-catalyzed cycloisomerization/Suzuki cross-coupling sequence involving arylethynyl-N-methyliminodiacetic acid boronates is described. Combining the mildness of homogeneous gold catalysis with the versatility of N-methyliminodiacetic acid (MIDA) boronates, this tandem two-step method enables the rapid assembly of various aryl-substituted heterocycles without having to isolate or purify any heterocyclic MIDA boronate intermediates. Another major advantage of this method is that a wide range of heterocycles bearing different aryl groups may be made from a single MIDA boronate alkyne precursor.     

A novel fluorous palladium catalyst for Suzuki reaction in fluorous media

Palladium(II) perfluorooctanesulfonate [Pd(OSO₂R_f8)₂] catalyses the highly efficient Suzuki reaction in the presence of a catalytic amount of perfluoroalkylated-pyridine as a ligand in a fluorous biphase system (FBS). The fluorous phase containing the active palladium species is easily separated and can be reused several times without a significant loss of catalytic activity.     

Syntheses of fluorous quaternary ammonium salts and their application as phase transfer catalysts for halide substitution reactions in extremely nonpolar fluorous solvents

Perfluoromethyldecalin solutions of the fluorous alkyl halides R_f8(CH₂)_mX (m=2, 3; X=Cl, I) are inert toward aqueous NaCl, KI, KCN, and NaOAc. However, substitution occurs at 100 °C in the presence of 10 mol % of the fluorous ammonium salts (R_f8(CH₂)₂)(R_f8(CH₂)₅)₃N⁺ I⁻ (1) or (R_f8(CH₂)₃)₄N⁺ Br⁻ (2) (10 mol %), which are fully or partially soluble in perfluoromethyldecalin under these conditions. Stoichiometric reactions of (a) 1 and R_f8(CH₂)₃Br, and (b) 2 and R_f8(CH₂)₂I are conducted in perfluoromethyldecalin at 100 °C, and yield the same R_f8(CH₂)_mI/R_f8(CH₂)_mBr equilibrium ratio (60-65:40-35). This shows that ionic displacements can take place in extremely nonpolar fluorous phases, and suggests a classical phase transfer mechanism for the catalyzed reactions. Interestingly, the non-fluorous ammonium salt mixture CH₃(CH₃(CH₂)_m)₃N⁺ Cl⁻ (3, Aliquat^® 336; m=2:1 7/9) also catalyzes halide substitutions, but under triphasic conditions with 3 suspended between the lower fluorous and upper aqueous layers. NMR experiments establish very low solubilities in both phases, suggesting interfacial catalysis.     

Total synthesis of cucurbitoside-like phenolic glycosides by double fluorous and acyl mixture synthesis

The first and simultaneous total syntheses of cucurbitosides A, B, G, and I, seguinosides C and D, and two unnatural analogs were achieved using the technique of fluorous mixture synthesis. The eight precursors of cucurbitoside-like phenolic glycosides were prepared by glycosylation of a mixture of two glucopyranosyl acceptors bearing different fluorous benzyl groups with a mixture of four apiofuranosyl donors bearing benzoyl, 3-methylbutyryl, 4-benzyloxybenzoyl, and 4-nitrobenzoyl groups, followed by a single run of HPLC with serially connected Fluophase^® RP and Inertsil^® ODS-3 columns. Finally, the individual pure disaccharide precursors were detagged to yield the eight cucurbitoside-like phenolic glycosides.     

Towards oligosaccharide library synthesis by fluorous mixture method

The synthesis of an oligosaccharide library by a fluorous tag method is reported here. Several acceptors and donors were mixed and glycosylated. The reaction mixture was purified by chromatography over fluorous HPLC to provide disaccharides in order of increasing fluorine content of the tag. This method could be applied to oligosaccharide libraries consisting of two sets of structural isomers.     

Ytterbium perfluorooctanesulfonates catalyzed synthesis of benzimidazole derivatives in fluorous solvents

Catalytic condensation of o-phenylenediamine and aldehydes was accomplished using rare earth(III)perfluorooctane sulfonates (RE(OPf)₃), RE = Sc, Y, La ∼ Lu) as catalysts in fluorous solvents. Ytterbium perfluorooctanesulfonates (Yb(OPf)₃) catalyzes the high-efficient synthesis of benzimidazole derivatives in fluorous solvents. By simple separation, fluorous phase containing only catalyst can be reused several times.     

Recyclable benzyl-type fluorous tags: Preparation and application in oligosaccharide synthesis

We herein described the design, synthesis and application of two recyclable benzyl-type fluorous tags with double fluorous chains. The benzyl-type fluorous tags were prepared in 3 steps from a commercially available fluorous alcohol. The glycosylation of the benzyl-type tags with imidate donors proceeded smoothly to provide the corresponding fluorous-tagged carbohydrates in good to excellent yields, which were readily purified by fluorous solid-phase extraction(FSPE). Efficient removal of the tags from tagtethered carbohydrates were conducted under the common catalytic hydrogenation condition and the initial benzyl-type fluorous tags could be regenerated [5_TD$IF]via a 2-step simple procedure in 69%–93% yields.The utility of the new benzyl-fluorous tag was demonstrated [7_TD$IF]via the FSPE-assisted synthesis of oligosaccharides Gb3.     

Total synthesis of macrocyclic glycosides, clemochinenosides A and B, and berchemolide, by fluorous mixture synthesis

The total synthesis of clemochinenoside A and the first total syntheses of clemochinenoside B and berchemolide were achieved simultaneously via macrocyclization of 4-O-(4-O-^F13benzyl-β-d-glucopyranosyl)syringic acid with 4-O-(4-O-^F17benzyl-β-d-glucopyranosyl)vanillic acid by a fluorous mixture synthetic method. The spectroscopic data of the synthetic products were identical with those of the natural products, although the optical rotation of clemochinenoside A differed from the published values in sign and magnitude.     

Recyclable hafnium(IV) bis(perfluorooctanesulfonyl)amide complex for catalytic Friedel-Crafts acylation and Prins reaction in fluorous biphase system

In fluorous biphase system, hafnium(IV) bis(perfluorooctanesulfonyl)amide complex (Hf[N(SO₂C₈F₁₇)₂]₄) was found to be a highly reactive and recyclable Lewis acid catalyst for Friedel-Crafts acylation and Prins reaction at significantly low catalyst loadings (≤1 mol%). In these reactions, Hf[N(SO₂C₈F₁₇)₂]₄ is selectively soluble in the lower fluorous phase and can be recovered simply by phase separation. Furthermore, the catalyst can be reused without decrease of activity.     

Highly efficient microwave-assisted fluorous Ugi and post-condensation reactions for benzimidazoles and quinoxalinones

The efficiency of an Ugi/de-Boc/cyclization strategy for construction of heterocyclic compounds has been improved through the incorporation of microwave and fluorous technologies. In the synthesis of substituted quinoxalinones and benzimidazoles, a fluorous-Boc protected diamine is employed for the Ugi reactions. Both the Ugi and the post-condensation reaction proceed rapidly under microwave irradiation and the reaction mixtures are purified by solid-phase extraction (SPE) over FluoroFlash^TM cartridges.     

Perfluoroalkylated-pyridine catalyzed Aldol condensations of aldehydes and ketones in a fluorous biphasic system without fluorous solvent

Aldol condensation of different ketones with various aromatic aldehydes proceeds efficiently in the presence of catalytic amount of perfluoroalkylated-pyridine in a fluorous biphasic system without fluorous solvent, which has prompted various concerns involving cost, solvent leaching, and environmental persistence. The catalyst can be recovered by simple cooling and precipitation and used again.     

Efficient synthesis of fluorinated biphenyl derivatives via Pd-catalyzed Suzuki coupling reactions in aqueous solvents at room temperature

A variety of fluorinated biphenyl derivatives were obtained in good yields in aqueous solvents at room temperature by Suzuki coupling reaction of aryl bromides and aryl boronic acid in the presence of high activity catalyst—some air-stable hemilabile PO coordinated cyclopalladated complexes. The structures of above catalysts were characterized by element analyses, IR, ¹H NMR, ¹³C NMR and ³¹P NMR.     

Metal bis(perfluorooctanesulfonyl)amides as highly efficient Lewis acid catalysts for fluorous biphase organic reactions

In fluorous biphase system, metal bis(perfluorooctanesulfonyl)amides are better Lewis acid catalysts than the analogous triflates toward either transesterifications, or direct esterifications, or Friedel-Crafts acylations or Baeyer-Villiger oxidations. These catalysts are selectively soluble in lower fluorous phase and can be recovered simply by phase separation. Furthermore, these catalysts can be reused without decrease of activity in most cases.     

p-Alkoxyphenyl-type heavy fluorous tag for the preparation of carbohydrate units

Carbohydrate glycosyl acceptor and donor moieties were synthesized efficiently by using the fluorous tag method. The p-alkoxyphenyl-type heavy fluorous tag was stable under all the reaction conditions used in the preparation of the various carbohydrate units. Each synthetic intermediate carrying the fluorous tag could be obtained in a simple straightforward manner by partition between fluorous and organic solvents.     

First synthesis of antimalarial Machaeriols A and B

A short and efficient synthesis of the naturally occurring antimalarial compounds, Machaeriols A and B, from commercially available citronellal via hetero-Diels-Alder cycloaddition and Suzuki coupling in 33% overall yield is described.     

Total synthesis of cucurbitoside A using a novel fluorous protecting group

The first total synthesis of cucurbitoside A was achieved using a new fluorous N-phenylcarbamoyl (^FCar) protecting group. The ^FCar group was introduced into carbohydrates in high yield and was selectively removed with Bu₄NNO₂ without damaging other acyl protecting groups. The synthetic intermediates were easily isolated by fluorous solid-phase extraction.     

An expeditious synthesis of bistratamide H using a new fluorous protecting group

A total synthesis of bistratamide H has been achieved using a new ‘highly’ fluorous amino protecting group, tris(perfluorodecyl)silylethoxylcarbonyl (^FTeoc) group. The synthetic intermediates were easily isolated by liquid-liquid extraction with fluorous solvent. The fluorous protecting group was demonstrated to be recycled.     

Towards functional fluorous surfactants. Synthesis of hydrophilic fluorous 1,2,3-triazolylmethyl ethers and di(1,2,3-triazolylmethyl) ethers

Copper(I)-accelerated Huisgen–Meldal dipolar cycloaddition reactions between polyfluoroalkyl azides and propargyl ethers of n-octanol and of triethyleneglycol monomethyl ether exhibited variation in yield of 1,2,3-triazol-4-ylmethyl ethers. Microwave acceleration, and in situ generation of the azides, provided improvements in yield and efficiency. In contrast, very good yields of equivalent fluorous triazoles were obtained from a range of n-alkyl azides with propargyl ethers of perfluorohexylethanol and of perfluoroheptylmethanol through conventional copper(I)-promoted reactions. Together, the resulting substances with systematic variations in polyfluoroalkyl and alkyl substituent length and position of substitution, and degree of oxygen content, make up small libraries of hybrid fluorous 1,2,3-triazol-4-ylmethyl ethers as candidates for study as hydrophilic fluorous surfactants. In addition, a pilot sample of di(1,2,3-triazol-4-ylmethyl) ethers with 1′-octyl-1-polyfluoroalkyl-substituents and 1′-nonyl-1-perfluorooctylethyl substituents were synthesised for the first time in an effort to develop more functional, fluorous surfactants.