The domino blocks: a simple solution for parallel solid-phase organic synthesis

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.

Application of Glaser-Hay Diyne Coupling To Constrain Nα-Amino Acid Amides via a N-N Bridge

We present the application of a Glaser-Hay diyne coupling for the synthesis of conformationally constrained N^α-amino acid amides with different diyne ring sizes. Twelve-membered rings were the smallest rings that could be prepared by this approach. We observed the formation of triethylammonium adducts in the cases of smaller (10- and 11-membered) rings. Calculation of the conformational barriers for the cyclization reactions of various ring sizes demonstrated that the formation of amino acid-derived smaller rings by this reaction is thermodynamically unfavorable.

Configuration-Dependent Medium-Sized Ring Formation: Chiral Molecular Framework with Three-Dimensional Architecture

This report describes a configuration-dependent [6 + 8 + 5] fused ring formation via a tandem cyclic N-acyliminium nucleophilic addition reaction. Cyclization of the acyclic precursor prepared on a solid phase using l-Ser and a racemic mixture of Fmoc- trans-2-aminocyclohexanecarboxylic acid predominantly yielded the cyclic diastereomer with the (1 R,2 R)-2-aminocyclohexane moiety rather than the tricyclic diastereomer from the (1 S,2 S)-enantiomer. In contrast, the model compound prepared with d-Ser predominantly cyclized with the (1 S,2 S)-2-aminocyclohexanecarboxylic acid substrate. The outcome of the cyclization was not influenced by the type of resin, the spacer, or the N-substituent. The analogous synthesis of the [6 + 7 + 5] fused ring system yielded inseparable diastereomers in a 1:0.6 ratio.

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 Trisubstituted Quinazolines

A traceless polymer-supported synthesis of 4-benzoylquinazolines was developed using the following commercially available building blocks: Fmoc-α-amino acids, 2-nitrobenzensulfonyl chlorides and α-bromoacetophenones. The acyclic intermediates underwent base-catalyzed rearrangement involving C-C and N-N bond formation followed by ring expansion and yielded resin-bound dihydroquinazoline-2-carboxylic acids. After they were released from the resin by treatment with trifluoroacetic acid, base-mediated decarboxylation produced the target quinazolines in moderate-to-high yields and purities.

Benzhydrylamines via base-mediated intramolecular sp(3) C-arylation of N-benzyl-2-nitrobenzenesulfonamides--advanced intermediates for the synthesis of nitrogenous heterocycles

N-Benzyl-2-nitrobenzenesulfonamides underwent base-mediated intramolecular arylation at the benzyl sp(3) carbon to yield benzhydrylamines. The presence of electron withdrawing groups on the aromatic ring of the benzyl group was required to facilitate the C-arylation. Unsymmetrically substituted benzhydrylamines are advanced intermediates toward nitrogenous heterocycles, as exemplified in the syntheses of indazole oxides and quinazolines.

Base-mediated intramolecular C- and N-arylation of N,N-disubstituted 2-nitrobenzenesulfonamides: advanced intermediates for the synthesis of diverse nitrogenous heterocycles

Structural and electronic features that facilitate and direct the intramolecular C- and N-arylation of 2-alkyl-2-{[N-(benzyl)-2-nitrophenyl]sulfonamido}acetic acid esters and amides were examined. The substitution pattern and amino acid carboxy-terminal functionality determined the arylation position. C/N-arylated products represent advanced intermediates for combinatorial synthesis of diverse nitrogenous heterocycles, including indazoles, quinazolinones, quinoxalinones, and 3-amino-2-oxindoles.

Solid-phase synthesis of trisubstituted 2,5-dihydrobenzo[f][1,2,5]thiadiazepine 1,1-dioxide derivatives

The solid-phase synthesis of trisubstituted 2,5-dihydrobenzo[f][1,2,5]thiadiazepine 1,1-dioxides is reported. Acyclic polymer-supported intermediates were prepared using commercially available building blocks: Fmoc-protected amino acids, 2-nitrobenzenesulfonyl chlorides, and bromoketones. The acyclic precursors underwent acid-mediated release from the resin and the cyclization was completed in solution.

Privileged structures as peptide backbone constraints: polymer-supported stereoselective synthesis of benzimidazolinopiperazinone peptides

A molecular scaffold comprising a privileged structure was designed and synthesized to serve as a peptide backbone conformational constraint. The synthesis of highly functionalized 2,3,10,10a-tetrahydrobenzo[4,5]imidazo[1,2-a]pyrazin-4(1H)-ones on a solid-phase support was performed via a tandem N-acyl-N-aryliminium ion cyclization-nucleophilic addition reaction. The synthesis proceeded with full stereocontrol of the newly formed stereogenic center. Conventional and microwave-assisted syntheses were compared with respect to efficiency and the optical integrity of the target compounds. Significant epimerization was observed during acylation with (S)- and (R)-2-bromopropionic acids under microwave conditions.

Polymer-supported stereoselective synthesis of tetrahydrobenzopyrazino-thiadiazinone dioxides via N-sulfonyl iminiums

The stereoselective synthesis of 1,2,11,11a-tetrahydrobenzo[e]pyrazino[1,2-b][1,2,4]thiadiazin-3(4H)-one 6,6-dioxides on a solid support via tandem N-sulfonyl iminium ion cyclization, followed by nucleophilic addition is reported. The synthesis proceeded with full control of stereoselectivity at the newly formed asymmetric carbon, under mild conditions, and using commercially available building blocks. The synthetic route provided high-purity crude products.

Synthesis of quinazolines from N-(2-nitrophenylsulfonyl)iminodiacetate and alpha-(2-nitrophenylsulfonyl)amino ketones via 2H-indazole 1-oxides

Base-catalyzed rearrangement of 2H-indazoles 1-oxides, prepared by tandem carbon-carbon followed by nitrogen-nitrogen bond formations from easily accessible N-alkyl-2-nitro-N-(2-oxo-2-aryl-ethyl)-benzenesulfonamides using glycine, 2-nitrobenzenesulfonyl chlorides, and bromo ketones/acetates, yielded high purity quinazolines.

Solid-phase synthesis and chemical properties of 2-(2-amino/hydroxyethyl)-1-aryl-3,4-dihydropyrazino[1, 2-b]indazol-2-iums

Amino alcohols, diamines, benzenesulfonyl chlorides, and bromoketones were used to prepare polymer-supported 2-(2-(2-(amino/hydroxyl)ethylamino)ethyl)-3-benzoyl-2H-indazoles. Acid-mediated release yielded 2-((amino/hydroxyl)ethyl)-1-aryl-3,4-dihydropyrazino[1,2-b]indazole-2-iums. In neutral pH, iminiums spontaneously cyclized to complex fused heterocycles 3,6,9,10-tetraazatetracyclo[7.7.0.0(2,6).0(11,16)]hexadeca-11,13,15-trienes, 3-oxa-6,9,10-triazatetracyclo[7.7.0.0(2,6).0(11,16)]hexadeca-11,13,15-trienes, and 3,7,10,11-tetraazatetracyclo[8.7.0.0(2,7).0(12,17)]heptadeca-12,14,16-trienes. Transformations were carried out under mild conditions and tolerated diverse substitution patterns.

Multiplicity of diverse heterocycles from polymer-supported alpha-acylamino ketones

Polymer-supported alpha-acylamino ketones were transformed to seven types of structurally unrelated heterocyclic compounds. Syntheses involved variety of chemical routes and comprised diverse chemistries (C-C, C=C, C-N, C=N, and C-O bond formations). Different sizes of heterocycles (4-, 5-, 6-, and 7-membered rings) were prepared, including dihydro-pyrrol-2-ones, pyrazin-2-ones, dihydro-triazepin-6-ones, morpholin-3-ones, imidazoles, beta-lactams, and isoquinolin-1-ones. Further elaboration to fused ring systems was also documented.

Polymer-supported alpha-acylamino ketones: preparation and application in syntheses of 1,2,4-trisubstituted-1H-imidazoles

Polymer-supported alpha-acylamino ketones were prepared from resin-bound amines, bromoketones, and carboxylic acids. Selective monoalkylation of amines by bromoketones was carried out via 4-nitrobenzenesulfonamides. There was a striking difference in the reaction outcome between 2-Nos and 4-Nos derivatives. Alpha-acylamino ketones were converted to imidazoles. The cyclization was performed on resin, allowing further polymer-supported elaboration of imidazoles including synthesis of bis-heterocyclic compounds. A small combinatorial array of imidazoles was synthesized. Target compounds were prepared under mild conditions using commercially available building blocks for the introduction of three points of diversity.

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.

Combinatorial libraries of bis-heterocyclic compounds with skeletal diversity

Combinatorial solid-phase synthesis of bis-heterocyclic compounds, characterized by the presence of two heterocyclic cores connected by a spacer of variable length/structure, provided structurally heterogeneous libraries with skeletal diversity. Both heterocyclic rings were assembled on resin in a combinatorial fashion.

Resin capsules: permeable containers for parallel/combinatorial solid-phase organic synthesis

A resin capsule is a permeable container for resin beads designed for multiple/combinatorial solid-phase organic synthesis. Resin capsules consist of a high density polyethylene ring sealed with peek mesh on both sides. The cylindrical shape of resin capsules enabled space-saving packing into plastic columnlike reaction vessels commonly used for solid-phase organic synthesis. Resin capsules have been evaluated for their use in combinatorial synthesis, and a set of model compounds with excellent purity was prepared.

Semi-automated high throughput combinatorial solid-phase organic synthesis

A semi-automated technique for massive parallel solid-phase organic synthesis based on a "split only" strategy is described. Two different types of purpose-oriented reaction vessels are used. The initial steps are performed in domino blocks, and the resin-bound intermediates then split into wells of a micro plate for the last combinatorial step. The domino block is a reaction block for manual and semi-automatic parallel solid-phase organic synthesis that simplifies liquid exchange and integrates common synthetic steps. The synthesis in micro plates does not use any filter for separation of resin beads from the supernatant liquid, and allows high throughput parallel synthesis on solid phase to be performed. This technique, documented on examples of diverse disubstituted benzenes, includes the use of gaseous cleavage in the last synthetic step and allows the synthesis of thousands of compounds per day in mg quantities.