Traceless Solid-phase Synthesis Research Articles

Traceless solid-phase synthesis and β-turn propensity of 1,3-thiazole-based peptidomimetics.

The design and solid-phase synthesis of 1,3-thiazole-based peptidomimetic molecules is described. The solid-phase synthesis was based on the utilization of a traceless linker strategy. The synthesis starts from the conversion of chloromethyl polystyrene resin to the resin with a sulfur linker unit. The key intermediate 4-amino-thiazole-5-carboxylic acid resin is prepared in three steps from Merrifield resin. The amide coupling proceeded at the C4 and C5 positions via an Fmoc solid-phase peptide synthesis strategy. After cleavage, the final compounds were obtained in moderate yields (average 9%, 11-step overall yields) with high purities (≥87%). Geometric measurements of Cα distances and dihedral angles along with an rmsd of 0.5434 for attachment with Cα of the β-turn template suggest type IV β-turn structural motifs. Additionally, the physicochemical properties of the molecules have been evaluated.

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.

Traceless Solid-Phase Synthesis of Ketones via Acid-Labile Enol Ethers: Application in the Synthesis of Natural Products and Derivatives.

In solid-phase organic synthesis, Wang resin is traditionally used for the immobilization of acids, alcohols, phenols, and amines. We report the use of Wang resin for the traceless synthesis of ketones via acid-labile enol ethers. We demonstrate the practicality of this synthetic strategy on the solid-phase synthesis of pyrrolidine-2,4-diones, which represent the core structure of several natural products, including tetramic acid. Base-triggered condensation of pyrrolidine-2,4-diones yielded 4-hydroxy-1,1′,2′,5-tetrahydro-2H,5′H-[3,3′-bipyrrole]-2,5′-diones.

Traceless Solid-Phase Synthesis of [6,7,8 + 5,6,7]-Fused Molecular Frameworks.

We report two synthetic strategies for traceless solid-phase synthesis of molecular scaffolds comprising 6- to 8-membered rings fused with 5- to 7-membered rings. Traceless synthesis facilitated preparation of target molecules without any trace of polymer-supported linkers. The cyclization proceeded via acid-mediated tandem N-acylium ion formation followed by the nucleophilic addition of O- and C-nucleophiles. The presented synthetic strategy enabled, through the use of simple building blocks without any conformational preferences, the evaluation of the predisposition of different combinations of ring sizes to form fused ring molecular scaffolds. Compounds with any combination of [6,7 + 5,6,7] ring sizes were accessible with excellent crude purity. The 8-membered cyclic iminium was successfully fused only with the 5-membered cycle and larger fused ring systems were not formed, probably due to their instability.

Traceless Solid-Phase Synthesis of Fused Chiral Macrocycles via Conformational Constraint-Assisted Cyclic Iminium Formation.

Natural products comprising chiral molecular scaffolds containing fused medium-sized cycles and macrocycles represent an important and relevant pharmacological target for the discovery and development of new drugs. Here, we describe traceless solid-phase synthesis of acyclic intermediates amenable to cyclization to medium (11) and large (12) fused rings. The key aspect of the synthetic strategy is incorporation of a specific conformation constraint that facilitates cyclization in favor of 11- and 12-membered rings rather than possible 7-membered ones. The role of constraints in preorganization required for cyclization is supported by computational analysis. The synthesis involves cyclic N-sulfonyliminium-nucleophilic addition chemistry as the key ring-forming reaction and proceeds with complete stereocontrol of the newly formed stereogenic center. We document the scope and limitations of this strategy in the synthesis of 11+5, 11+6, 11+7, and 12+6 fused rings representing molecular scaffolds with 3D architecture that mimic complex natural products.

Solid-Phase Synthesis of 1,3,7,8-Tetrasubstituted Xanthine Derivatives on Traceless Solid Support.

Traceless solid-phase synthesis of 1,3,7,8-tetrasubstituted xanthine (1,3,7,8-tetrasubstituted 1H-purine-2,6(3H,7H)-dione) derivatives has been developed. The solid-phase synthetic route began on a solid supported N'-cyano-N-substituted carbamimidothioate, which was prepared from cyanamide, isothiocyanate, and Merrifield resin. After N-alkylation of carbamimidothioate resin with ethyl 2-bromoacetate, an imidazole ring is introduced by Thorpe-Ziegler-type cyclization. The resulting imidazole resin is converted to 1,3,7-trisubstituted xanthine resin using sequential reactions, such as Lewis acid-catalyzed urea formation, pyrimidine ring cyclization, and N-alkylation. After oxidation of sulfides to sulfones, traceless cleavage with amine or thiol nucleophiles afforded the desired 1,3,7,8-tetrasubstituted xanthines in good purities and overall yields (eight-steps; 36 examples). This efficient solid-phase synthesis enables the incorporation of four diversity points into the preparation of the 1,3,7,8-tetrasubstituted xanthines.

Piperazine Amide Linker for Cyclative Cleavage from Solid Support: Traceless Synthesis of Dihydroquinoxalin-2-ones

A piperazine amide linker for cyclative cleavage from solid support and its use in the traceless solid-phase synthesis of dihydroquinoxalinones are described. Piperazine was attached to Wang resin via a carbamate linkage and acylated with Fmoc-amino acids. Following Fmoc group removal, resin-bound amines were reacted with 1-fluoro-2-nitrobenzenes. The nitro group of the resulting 2-nitroanilines was reduced, and acyclic precursors, in contrast to traditional ester-type linkage, remained attached to the resin. Target dihydroquinoxalinones were obtained either by acid- or microwave-mediated cyclative cleavage. The synthesis provided crude compounds of high purity and enabled the preparation of stable immobilized linear intermediates. The linker is suitable for combinatorial synthesis of compound libraries.

Solid-Phase Organic Synthesis of α-Iodo Ketones using Recyclable Resin-Bound Selenium Bromide

A novel facile method for the traceless solid-phase synthesis of α-iodo ketones using a recyclable resin-bound selenium bromide reagent is reported. Various ketones and β-dicarbonyl compounds can be converted to the corresponding α-iodo-substituted compounds in excellent yields and purities with simple workup procedure and mild conditions.

Traceless Solid‐Phase Synthesis of Trifluoromethylarenes

Save the best for last: In the first method for the trifluoromethylation of immobilized arenes, the aryl halides are attached through a dithioester linkage to the Merrifield resin and then functionalized by means of numerous reactions including transition-metal-catalyzed cross-couplings. In the final step the fluorinating cleavage reaction provides the trifluoromethylarenes in good yields and high purities. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Traceless solid-phase synthesis of N-substituted 3,5-bis(substituted-idene)piperidin-4-one derivatives

A series of N-substituted 3,5-bis(substituted-idene)piperidin-4-ones have been prepared using solid-phase organic synthesis. The synthesis starts with a Michael addition with piperidin-4-one serving as the donor and REM resin as the acceptor. Various aldehydes were then utilized through a Knoevenagel condensation to afford the 3,5-bis(substituted-idene)piperidin-4-ones on the solid support. The final products were removed from the support and a second diversity position was introduced through a Hofmann elimination using different alkyl bromides.

Solid-Phase Synthesis of Thiazolo[4,5-b]pyridine Derivatives Using Friedländer Reaction

Traceless solid-phase synthesis of 2,5,6,7-tetrasubstituted thiazolo[4,5-b]pyridine derivatives is described. Thorpe-Ziegler type cyclization of solid supported cyanocarbonimidodithioate with alpha-halo ketones afforded thiazole resin, which were converted to the desired thiazolopyridine resin by the Friedländer protocol under microwave irradiation conditions. After oxidation of sulfides to sulfones, nucleophilic desulfonative substitution with amines gave the target thiazolo[4,5-b]pyridine derivatives in good overall yields.

Traceless Solid-Phase Synthesis of 6-Amino- and 6-Hydroxyimino-1,3,5-triazine-2,4-diones and 1,3,5-Triazine-2,4,6-triones

A traceless solid-phase synthesis of 6-amino- and 6-hydroxyimino-1,3,5-triazine-2,4-diones and 1,3,5-triazine-2,4,6-triones has been developed. The strategy comprises of linking a preformed N-carbamothioylcarbamate to bromomethyl resin to give a S-linked isothiourea, which then undergoes cyclization with isocynates to yield the resin-bound 1,3,5-triazine-2,4-diones. Subsequent cleavage was accomplished by either direct substitution with a suitable amine or by oxidative activation of the thioether functionality followed by nucleophilic substitution. Using this synthetic strategy, we prepared a representative set of 31 6-amino-1,3,5-triazine-2,4-diones, 10 6-hydroxyimino-1,3,5-triazine-2,4-diones, and 8 1,3,5-triazine-2,4,6-triones.

Traceless Solid-Phase Synthesis of cis-Decalins

A traceless solid-phase synthesis of cis-decalins was reported. Thus, the solid-supported diene 3, prepared from 1 (7 steps), underwent the Diels-Alder reaction to give the cis-decalin derivatives. The 5-thiotetrazole linker group was cleaved with metal-assisted SN2′-type allylic substitution to give a variety of functionalized cis-decalines (8-12).

Traceless solid-phase synthesis of multiple sulfonamide-containing cyclic sulfides exploiting microwave irradiation

In this Letter, a new synthetic method of sulfonamide-containing cyclic sulfides using a microwave-assisted traceless solid-phase approach is described. Using this new method, many highly pure cyclic sulfides were efficiently synthesized based on intramolecular alkylation of the sulfides followed by elimination of the desired products from the generated sulfonium salts.

Traceless Solid-Phase Synthesis of Heteroannulated 1,3-Oxazin-6-ones

A microwave-assisted solid-phase synthesis of heteroannulated 1,3-oxazin-6-ones has been developed. Significant rate enhancement was observed for all steps carried out under microwave irradiation, and the overall reaction time was dramatically shortened when compared to the conventional procedures. A representative set of 20 bi- and tricyclic heteroannulated 1,3-oxazin-6-ones was prepared. Key steps in the synthesis are (i) five-member heterocycle formation, (ii) acylation of amine, and (iii) ring closure to give the heteroannulated 1,3-oxazin-6-one.

Efficient Traceless Solid-Phase Synthesis of 3,4-Dihydropyrazino[1,2-b]indazoles and Their 6-Oxides

A highly efficient novel traceless solid-phase synthesis of 3,4-dihydropyrazino[1,2-b]indazoles and their 6-oxides was developed by using commercially available building blocks, diamines, 2-nitrobenzenesulfonyl chlorides, and bromoketones/bromoacetates. Mild reaction conditions, diversely substituted building blocks, and high purity of crude products enabled effective combinatorial syntheses of libraries.

Traceless Solid-Phase Synthesis of 1,2,3-Thiadiazole Derivatives from Resin-Bound Acylhydrazine

A novel synthesis of 1,2,3-thiadiazole derivatives using a traceless solid-phase approach is described, in which many kinds of 1,2,3-thiadiazole derivatives were efficiently obtained in good yields and high purities via traceless cyclization cleavage of resin-bound acylhydrazones with thionyl chloride.

Traceless solid-phase synthesis of hydroxylated cyclopentenones

Intramolecular nitrone–alkene cycloadditions on solid phase can be performed using polymer-bound hydroxylamine. Condensation of this reagent with sugar derived 4-pentenals followed by N–O cleavage, quaternization of the amine thus produced, and finally oxidative elimination of the amino group detaches the chiral hydroxylated cyclopentenones from the polymer. The natural antibiotic pentenomycin I was prepared in this way.

Traceless Solid-Phase Synthesis of Mappicine Ketone Library via Multiple Chemoselective Palladium-Catalyzed Reactions on Benzenesulfonate Linker

We report here a solid-phase synthesis of a library of mappicine ketone, a leading antiviral compound with activity against herpes viruses and human cytomegalovirus. The synthesis is based on multiple chemoselective palladium-catalyzed reactions involving a regioselective intramolecular Heck reaction at C-2, substitution of Cl at C-7 by Suzuki-Miyaura cross-coupling reaction and Buchwald-Hartwig amination, and reductive cleavage of the benzenesulfonate linker at C-10.

A Traceless Solid-phase Synthesis of 1,4-Diazabicyclo[3,3,1]octan-3-ones

Abstract A novel synthesis of 1,4-diazabicyclo[3,3,1]octan-3-one derivatives using a traceless-solid-phase approach is described, in which many kinds of 1,4-diazabicyclo[3,3,1]octan-3-one derivatives have been efficiently obtained in high purity, based on an intramolecular alkylation of tertiary amines followed by an elimination of desired tertiary amines from the generated quarternary ammonium salts.