Tetrahedron report number 103

Traceless Solid-Phase Organic Synthesis

Traceless solid-phase synthesis represents an ultimate sophisticated synthetic strategy on insoluble supports. Compounds synthesized on solid supports can be released without a trace of the linker that was used to tether the intermediates during the synthesis. Thus, the target products are composed only of the components (atoms, functional groups) inherent to the target core structure. A wide variety of synthetic strategies have been developed to prepare products in a traceless manner, and this review is dedicated to all aspects of traceless solid-phase organic synthesis. Importantly, the synthesis does not need to be carried out on a linker designed for traceless synthesis; most of the synthetic approaches described herein were developed using standard, commercially available linkers (originally devised for solid-phase peptide synthesis). The type of structure prepared in a traceless fashion is not restricted. The individual synthetic approaches are divided into eight sections, each devoted to a different methodology for traceless synthesis. Each section consists of a brief outline of the synthetic strategy followed by a description of individual reported syntheses.

Sodium Hypochlorite/Dowex 1X8-200: An Effective Oxidant for the Oxidation of Aromatic Amines to Quinones

Polymer supported hypochlorite ion is a useful oxidant for the oxidation of aromatic amines to the corresponding quinones.

Synthesis of Allylic Esters and Ethers Using Polymer-Supported Selenium Bromide

The treatment of adducts from polymer-supported selenium bromide on olefins with anhydrous carboxylate or alcohol and oxidation of the resulting products afford allylic esters and ethers in good yield and purity.

R–Cl/SnCl4/n-Bu4NCl-induced direct living cationic polymerization of naturally-derived unprotected 4-vinylphenol, 4-vinylguaiacol, and 4-vinylcatechol in CH3CN

A R–Cl/SnCl₄/n-Bu₄NCl-initiating system induced direct living cationic polymerization of naturally derived styrene monomers with phenol and catechol groups in CH₃CN.

Kinetics of resin-supported Mitsunobu esterification and etherification reactions

Solid-phase Mitsunobu reaction is very useful in organic and parallel synthesis. In this work, we optimize the solid-phase Mitsunobu esterification and etherification reactions and investigated their kinetics by single-bead FTIR microspectroscopy method. Thirteen solid-phase Mitsunobu esterification reactions proceeded at rates between 2.5 x 10(-3) and 19 x 10(-3) s(-1), while five etherification reactions at generally slower rates between 3.3 x 10(-3) and 8.9 x 10(-3) s(-1). We discovered that reaction rates, as in solution phase Mitsunobu reactions, linearly correlated to pK(a) values of acids and phenols used in the reaction. By studying side reactions and intermediates, we found that the solid-phase reaction mechanism also bears remarkable similarities to that of solution phase Mitsunobu reaction.

Inverse H-C ex situ HRMAS NMR experiments for solid-phase peptide synthesis

The growing importance of solid-phase peptide synthesis (SPPS) has necessitated the development of spectroscopic experiments that can be used to obtain structural and conformational information on resin-bound peptides. Despite the utility of two-dimensional high-resolution magic angle spinning (HRMAS) NMR experiments that provide homonuclear shift correlations, experiments that provide heteronuclear shift correlations are necessary for complex conformational and structural elucidatory problems. Here we report the optimization and implementation of non-gradient inverse NMR experiments for acquiring the 1H-13C shift correlations of resin-bound peptides. The use of non-gradient experiments is advantageous as many magic angle spinning (MAS) probes do not possess gradient coils. An HRMAS BIRD-HMQC experiment with a reduced 1JCH constant has proven very suitable for obtaining one-bond correlations. Long-range correlations can be interpolated by using a non-gradient HRMAS CT-HMBC-1 experiment where the resulting data is processed with forward linear prediction. It has been shown that removing the effects of 1H-1H J-modulation is crucial in order to view cross peaks that correspond to long-range correlations. Additionally, both experiments prove extremely useful over routine one-dimensional 13C HRMAS experiments for extracting carbon chemical shift data. The non-gradient HRMAS BIRD-HMQC and CT-HMBC-1 experiments can be used to assist in conformational analysis and to identify and deconvolute situations where accidental equivalence and seemingly correlated isochronous signals arise.

A Kinetic Study of Product Cleavage Reactions from the Solid Phase by a Biocompatible and Removable Cleaving Reagent, HCl

TFA has been widely used as a cleaving reagent in solid-phase organic synthesis. However, it is difficult to remove from the final product, and it is toxic to various cells. To search for an alternative, we studied the kinetics of HCl cleavage reactions of 18 resin-bound compounds on various linkers. HCl is very easy to remove completely from samples, and the residual HCl does not have a toxic effect in cell assays. Most compounds studied in this work can be easily cleaved using a low concentration of HCl (0.9-2.3%) and the minimal amount of time (60-90 min). Even in the most difficult case, a moderate 8% HCl and an extended time (10-15 h) are enough to cleave the product. Therefore, our kinetic studies establish HCl as a biocompatible, removable, and effective substitute for TFA when final compounds are used for biological screening and drug discovery.

Synthesis of 2-arylcycloalka-2,4-dienones using sulfone-based methodology

Addition of the anion derived from (phenylthiomethyl)phenyl sulfone to a selection of aryl omega-alkenyl ketones gives adducts that via a sequence of Lewis acid catalyzed rearrangement, alpha-allylation, and metathesis give rise to 2-thio-4-cycloalkenones. These in turn give cycloalkadienones upon oxidation and elimination. An attempt to develop a polymer-supported variant fails because of the reversibility of sulfone anion addition.

Use of a chiral surfactant for enantioselective reduction of a ketone

The influence of a chiral surfactant and a polymer-supported chiral additive on reduction of ketones using sodium borohydride will be described. Initial preparations involved methylation of (S)-leucinol to give (2S)-N , N-dimethyl-2-amino-4-methyl-1-pentanol (1) (67%). The chiral surfactant (2) was synthesized by reacting (1) with bromohexadecane (71%). The functionalized styrene for the polymer-supported chiral additive (5) was synthesized by reacting (1) with 4-vinylbenzyl chloride. Polymerization was carried out with 10% of the functionalized monomer (4), 5% cross-linking agent divinylbenzene, and 85% styrene with AIBN as the initiator. The activity of the chiral surfactant and polymeric additive were examined by using them as additives in a standard reduction of 2-pentanone with sodium borohydride to yield (R)- and (S)-2-pentanol (3) (20%). The resulting alcohol was analyzed by polarimetry (ee 9.5%) and also esterified with (2S)-methylbutyric acid prior to characterization by NMR. 13C NMR indicated an enantiomeric excess of 5.2% when the chiral surfactant was used, and 7% when the polymeric additive was used.

Hydroxamic acid-containing hydrogels for nonabsorbed iron chelation therapy: synthesis, characterization, and biological evaluation

Iron overload is a severe clinical condition and can be largely prevented by the use of iron-specific chelating agents. A successful iron chelator needs to be orally active, nontoxic, and selective. In this study, hydrogels containing pendant hydroxamic acid groups have been synthesized as potential nonabsorbed chelators for iron in the gastrointestinal tract. The synthetic method employed to introduce hydroxamic acid groups to polymer chains involved reaction of polymer gels based on N-acryloxysuccinimide, acryloyl chloride, and (2-hydroxyethyl)acrylate monomers with hydroxylamine. These hydroxamic acid-functionalized polymer gels swell favorably in water and effectively sequester iron. In vitro iron-binding properties of these hydrogels were evaluated from their binding isotherms by use of iron(II) alone and in the presence of other competing metal ions. These polymers bind iron over a broad pH range. The iron-binding properties of the polymers were found to depend on the concentration of hydroxamate groups on polymer chains. The in vivo iron-binding efficacy of the polymers was evaluated in rat as the animal model. The polymers prevented an increase in serum hemoglobin and hematocrit levels in the animals, thus suggesting the prevention of systemic absorption of dietary iron from the gastrointestinal tract. The animals also maintained normal body weight during the treatment period, indicating the absence of any apparent toxicity associated with these polymers.

Development of Polymer-supported π-Acid Catalysts

We reported that dicyanoketene acetals (DCKA), in which gem-dicyano groups of tetracyanoethylene (TCNE) are displaced with alkoxyl groups, function as pi-acid catalysts, and that a polymer (poly-DCKA-1) derived by copolymerization of a monomer, in which the alkoxyl group at the omega-position is modified with a styrene moiety, with ethyleneglycol dimethacrylate also shows high level of activities as a pi-acid. In this research, the effects of dicyanoketene ethylene acetal (DCKEA), polymers previously developed (poly-DCKA-1 and 2), new monomers, in which the alkoxyl group at the omega-position is modified with an ethereal moiety, and the corresponding polymers (poly-DCKA-3 and 4) were investigated on catalytic activities. Among the catalysts, the polymer (poly-DCKA-1) had the most efficient activity in monothioacetalization, cyanation, and Mukaiyama aldol reaction of acetals, two-component and three-component Mannich-type reaction, hydrolysis of acetals and silyl ethers, and two-component and three-component inverse electron demand Aza-Diels-Alder reaction. Remarkably, the polymer catalyst works more efficiently in water than in organic solvents and is recyclable.

Synthetic polymer systems in drug development

The use of synthetic polymers in drug discovery is reviewed, with particular reference to the increasing number of chemistries for therapeutic candidate preparation that employ linear polymers possessing differential solubilities in a variety of solvents. The uses of these polymers as primary components in liquid-phase organic synthesis, particularly applied to the generation of chemical compound libraries, are discussed, along with the challenges facing synthetic chemists and pharmaceutical scientists in preparing new and more efficient supports. The advantages inherent to the emerging field of liquid-phase combinatorial synthesis (LPCS) are considered and a number of key therapeutic targets prepared by newer polymer-supported routes are highlighted. Finally, the potential development issues for both liquid and solid-phase organic chemistries applied to parallel or combinatorial synthesis are discussed, taking into account future trends in the drug discovery and development process as a whole.

Solid-Phase Synthesis of an Isoxazolinopyrrole Library

A four-step solid-phase synthesis of isoxazolinopyrroles 8 that employs an acid-labile 2-(4-formyl-3-methoxyphenoxy)ethyl polystyrene HL resin 1 is reported. Resin-bound vinyl sulfone 5 is obtained by DIC coupling with acid 4, which was in turn synthesized in solution phase by a regioselective nitrile oxide 1,3-dipolar cycloaddition. The resin-bound pyrrole 7 was synthesized on solid phase by pyrrole annulation with various isocyano derivatives and potassium t-butoxide in which the sulfone alpha-anion generated by Michael addition gives the desired pyrrole through internal condensation followed by a sigmatropic [1,5]-hydrogen shift. The resulting isoxazolinopyrroles 8 were released from resin 7 by 10% TFA in moderate to excellent overall yields from 2-(4-formyl-3-methoxyphenoxy)ethyl polystyrene HL resin 1.

Preparation, physical properties, on-bead binding assay and spectroscopic reliability of 25 barcoded polystyrene-poly(ethylene glycol) graft copolymers

Here we describe the preparation of 25 beaded polystyrene-poly(ethylene glycol) graft copolymers from six spectroscopically active styrene monomers: styrene, 2,5-dimethylstyrene, 4-methylstyrene, 2,4-dimethylstyrene, 4-tert-butylstyrene, and 3-methylstyrene. These polymers were thoroughly characterized by Raman, infrared, and (1)H/(13)C NMR spectroscopies, and differential scanning calorimetry. Determination of the swelling properties, peptide synthesis, and on-bead streptavidin-alkaline phosphatase (SAP) binding assay further established that their physical and chemical properties where not significantly altered by the diversity of their encoded polystyrene core. Each of the 25 resins displayed a unique Raman and infrared vibrational fingerprint, which was converted into a "spectroscopic barcode". The position of each bar matches the peak wavenumber in the corresponding spectrum but is independent of its intensity. From this simplified representation similarity maps comparing 35 000 resin pairs were generated to establish the spectroscopic barcoding as a reliable encoding methodology. In effect, in 99% of the cases, the highest similarity coefficients were obtained for resin pairs prepared from the same styrene derivatives even after SAP binding assay. We have also shown that a small but unique combination of a resin's vibrations (30-40%) is sufficient for its identification. However, in rare cases where a resin's vibrational signature has been severely compromised, both the Raman and infrared barcodes were synergistically and reliably utilized to unequivocally identify its chemical make up.