Mining the Chemical Space: Application of 2/4-Nitrobenzenesulfonamides in Solid-Phase Synthesis

Synthesis of Imidazo[2,1-a]isoindolones via Rearrangement and Tandem Cyclization of Amino-Acid-Based N-Phenacyl-2-cyano-4-nitrobenzensulfonamides

Esters of amino acids were reacted with 2-cyano-4-nitrobenzenesulfonyl chloride or 2-cyano-4-(trifluoromethyl)benzenesulfonyl chloride and alkylated with various α-haloketones. The corresponding sulfonamides were heated in a solution of ammonium acetate, which yielded imidazo[2,1-a]isoindolones in one step. The key reaction was based on intramolecular C-arylation followed by spontaneous cycloaddition and cyclocondensation. Two approaches have been developed: (i) solid-phase synthesis starting from amino acids immobilized on Wang resin, which allows the rapid preparation of target compounds using the cyclative cleavage strategy, and (ii) traditional solution-phase synthesis using amino acid methyl esters as the starting materials. The advantages and drawbacks of both alternatives are compared.

Peptide-Triazolobenzodiazepine Hybrids: A Catalyst-Free on-Resin Strategy to Build Complex Therapeutic Motifs Into Peptides

The merger of peptide and heterocyclic chemistry presents significant opportunities for enhancing the structural and functional diversity of therapeutic agents. In this study, we introduce a streamlined on-resin method to modify peptides at the N-terminus amines and the side chain amines of Lys/Orn amino acids by incorporating the biologically active triazolobenzodiazepine scaffold. In the presented solid-phase peptide synthesis (SPPS), the triazolobenzodiazepine core is formed on-resin through a combination of N-alkylation, amide bond formation, and an alkyne-azide 1,3-dipolar cycloaddition, of which the latter two happen as a single-step one-pot reaction- proceeding under mild conditions without metal catalysts. This protocol tolerates a wide variety of amino acids and functional groups, providing a versatile method for synthesizing peptide-triazolobenzodiazepine hybrids. Using orthogonal protection group strategies, we further demonstrate the method's adaptability for two site modifications in peptides involving both the N-terminus and Lys side chain amines. These modifications enhance the scope of "peptide medicinal chemistry" by creating multifunctional peptides with potential therapeutic applications. The method's compatibility with SPPS, room temperature conditions, and elimination of metal catalysis make it an efficient and powerful tool for peptide modification.

Integrating 3,4-Dihydro-2H-1,4-oxazine into Peptides as a Modification: Silver Triflate-Catalyzed Cyclization of N-Propargyl N-Sulfonyl Amino Alcohols for SPPS Applications

We present a methodology yielding 3,4-dihydro-2H-1,4-oxazine by cyclization of N-propargyl N-sulfonyl amino alcohols using silver triflate as a catalyst at ambient temperature. Additionally, we showcase the applicability of this methodology in solid phase peptide synthesis (SPPS) to introduce the oxazine heterocyclic ring into short peptides containing serine and threonine. Notably, Rink amide resin supported the on-resin formation of 3,4-dihydro-2H-1,4-oxazine, while 2-CTC resin facilitated the oxazine formation in a one-pot process involving peptide cleavage, deprotection, and subsequent C-O ring formation, thus offering a versatile method for the late-stage modification of peptides.

Efficient synthesis of pentasubstituted pyrroles via intramolecular C-arylation

Immobilized L-aspartic acid beta-methyl ester (Fmoc-Asp(OMe)-OH) was reacted with 4-nitrobenzenesulfonyl chloride, followed by alkylation with various α-haloketones. The resulting intermediates were treated with potassium trimethylsilanolate, which yielded tetrasubstituted pyrroles after a one-step transformation consisting of sequential C-arylation, aldol condensation and spontaneous aromatization. The discovered synthetic strategy enables fast and simple access to pentasubstituted and functionalized pyrroles from a number of readily available starting materials.

Synthesis of Polycyclic Tetrahydroisoquinolines and Tetrahydrobenzo[d]azepines from Polymer-Supported Allylglycine

Herein, we report a multistep synthesis of polycyclic tetrahydroisoquinolines and tetrahydrobenzo[d]azepines starting from Wang resin-immobilized allylglycine. After sulfonylation with 2/4-nitrobenzenesulfonyl chlorides, Mitsunobu alkylation with various phenylalkynols yielded the corresponding (phenylprop-2-yn-1-yl)-sulfonamides. "Interior" ring-closure enyne metathesis (RCEM) using a Grubbs catalyst second generation (Ru2) yielded functionalized tetrahydroisoquinoline/tetrahydrobenzo[d]azepine intermediates. "East-side" [4 + 2] cycloaddition with representative dienophiles was followed by the "west-side" construction of different heterocycles using various electrophiles to finally furnish a set of novel molecular frameworks bearing fused [6 + 6] or [6 + 7] rings. The developed methodology enables the facile parallel synthesis of novel, pharmacologically promising compounds derived from privileged scaffolds.

Synthesis of 2-Alkylsulfonyl-imidazoles with Three Diversity Positions from Immobilized α-Acylamino Ketones

The synthesis of novel imidazole derivatives via immobilized α-acylamino ketones is reported in this article. The key intermediates were prepared from the Wang-piperazine resin-supported Fmoc-amino acids. After their sulfonylation with 4-nitrobenzenesulfonyl chloride (4-Nos-Cl), followed by alkylation with α-bromoketones and cleavage of Nos group, the resulting α-acylamino ketones were reacted with Fmoc-isothiocyanate. The corresponding Fmoc-thioureas were subjected to the Fmoc-cleavage and spontaneous ring-closure to imidazole scaffold. The resulting imidazole-thiones were alkylated with alkyl halides and oxidized using meta-chloroperbenzoic acid ( mCPBA). Trifluoroacetic acid (TFA)-mediated cleavage yielded the corresponding trisubstituted 2-alkylsulfonyl imidazoles in good crude purity and acceptable overall yields. In the case of sulfides, prepared from alkyl bromides, the unexpected products brominated at the C⁴ position of the imidazole were obtained.

Convenient Synthesis of Thiohydantoins, Imidazole-2-thiones and Imidazo[2,1-b]thiazol-4-iums from Polymer-Supported α-Acylamino Ketones

The preparation of 5-methylene-thiohydantoins using solid-phase synthesis is reported in this paper. After sulfonylation of immobilized Ser (t-Bu)-OH with 4-nitrobenzenesulfonyl chloride followed by alkylation with various bromoketones, the 4-Nos group was removed and the resulting polymer-supported α-acylamino ketones reacted with Fmoc-isothiocyanate. Cleavage of the Fmoc protecting group was followed by the spontaneous cyclative cleavage releasing the 5-methylene-thiohydantoin derivatives from the polymer support. Reduction with triethylsilane (TES) yielded the corresponding 5-methyl-thiohydantoins. When Fmoc-isothiocyanate was replaced with alkyl isothiocyanates, the trifluoroacetic acid (TFA) mediated cleavage from the polymer support, which was followed by the cyclization reaction and the imidazo[2,1-b]thiazol-4-iums were obtained. Their conversion in deuterated dimethylsulfoxide led to imidazole-2-thiones.

Polymer-Supported Stereoselective Synthesis of Benzoxazino[4,3-b][1,2,5]thiadiazepinone 6,6-dioxides

Herein, we report the stereoselective synthesis of trisubstituted benzoxazino[4,3-b][1,2,5]thiadiazepinone 6,6-dioxides from polymer-supported Fmoc-Ser(tBu)-OH and Fmoc-Thr(tBu)-OH. After the solid-phase synthesis of N-alkylated-N-sulfonylated intermediates using various 2-nitrobenzenesulfonyl chlorides and bromoketones, the target compounds were obtained via trifluoroacetic acid (TFA)-mediated cleavage from the resin, followed by cyclization of the diazepinone scaffold. Except for the threonine-based intermediates, the inclusion of triethylsilane (TES) in the cleavage cocktail yielded a specific configuration of the newly formed C³ chiral center. The final cyclization resulted in minor or no inversion of the C^12a stereocenter configuration.

Stereoselective Polymer-Supported Synthesis of Morpholine- and Thiomorpholine-3-carboxylic Acid Derivatives

Herein we report the polymer-supported synthesis of 3,4-dihydro-2H-1,4-oxazine-3-carboxylic acid derivatives using immobilized Fmoc-Ser(tBu)-OH and Fmoc-Thr(tBu)-OH as the starting materials. After the solid-phase-synthesis of N-alkyl-N-sulfonyl/acyl intermediates, the target dihydrooxazines were obtained using trifluoroacetic acid-mediated cleavage from the resin. This approach was also studied for the preparation of dihydrothiazines from immobilized Fmoc-Cys(Trt)-OH. Inclusion of triethylsilane in the cleavage cocktail resulted in the stereoselective formation of the corresponding morpholine/thiomorpholine-3-carboxylic acids. Stereochemical studies revealed the specific configuration of the newly formed stereocenter and also the formation of stable N-acylmorpholine rotamers.

N-Sulfanylethylaminooxybutyramide (SEAoxy): A Crypto-Thioester Compatible with Fmoc Solid-Phase Peptide Synthesis

An N-sulfanylethylaminooxybutyramide (SEAoxy) has been developed as a novel thioester equivalent for native chemical ligation. SEAoxy peptide was straightforwardly synthesized by conventional Fmoc solid-phase peptide synthesis without a problem. Moreover, SEAoxy peptide could be directly applied to native chemical ligation owing to the intramolecular N-to-S acyl shift that releases the peptide-thioester in situ. This methodology was successfully applied to the synthesis of two bioactive peptides.

Fused Ring Molecular Scaffold with 3D Architecture for Constrained Peptidomimetics: Polymer-Supported Stereoselective Synthesis of Tetrahydrobenzo[e]pyrazino[2,1-c][1,2,4]thiadiazinone 6,6-dioxide via N-Acyl Iminiums

3,4,4a,5-Tetrahydrobenzo[e]pyrazino[2,1-c][1,2,4]thiadiazin-1(2H)-one 6,6-dioxides, molecular scaffolds with 3D architecture, were synthesized on solid supports via tandem N-acyl iminium ion cyclization followed by nucleophilic addition. The modular synthesis proceeded under mild conditions using commercially available building blocks and provided crude products with respectable purity. The synthesized compounds are applicable as fused nitrogenous heterocyclic compounds in drug discovery and as constrained peptidomimetics incorporated into a peptide backbone.

Solid-Phase Synthesis of 2,3-Dihydrobenzo[f][1,2,5]thiadiazepin-4(5H)-one 1,1-Dioxides with Three Diversity Positions

Synthesis of 2,3-dihydrobenzo[f][1,2,5]thiadiazepin-4(5H)-one 1,1-dioxides from polymer-supported α-amino acids is described herein. Different α-amino acids immobilized on Wang resin were sulfonylated with various 2-nitrobenzenesulfonyl chlorides. The resulting 2-nitrobenzenesulfonamides were alkylated with alcohols according to the Fukuyama-Mitsunobu procedure. After reduction of the nitro group and cleavage from the polymer support, the final intermediates were reacted with thionyl chloride, and target compounds of good crude purity and acceptable overall yields were obtained. The chiral HPLC studies revealed the impact of the cyclization step on the resulting stereochemistry. The developed strategy allows for simple production of desired compounds with the application of parallel/combinatorial solid-phase synthesis using commercially available building blocks.