Dipolar cycloadditions in solid-phase organic synthesis (SPOS)

[3 + 2] Cycloaddition and Cascade Radical Reactions for the Synthesis of Trifluoromethylated Tetrahydrobenzodiazepin-3-ones

A reaction sequence involving three-component [3 + 2] cycloaddition of azomethine ylides followed by CuI-catalyzed cascade trifluoromethyl radical addition and cyclization is developed for diastereoselective synthesis of fused-tetrahydrobenzodiazepin-3-ones.

One-Pot Synthesis of Triazolobenzodiazepines Through Decarboxylative [3 + 2] Cycloaddition of Nonstabilized Azomethine Ylides and Cu-Free Click Reactions

A one-pot synthesis of triazolobenzodiazepine-containing polycyclic compounds is introduced. The reaction process involves a decarboxylative three-component [3 + 2] cycloaddition of nonstabilized azomethine ylides, N-propargylation, and intramolecular click reactions.

One-pot high-throughput synthesis of a 160-membered library of methyl 3,5-diaryl-isoxazoline-5-carboxylate pharmacophores by a 2·2·2-component reaction

A simple and efficient methodology has been developed for the synthesis of methyl 3,5-diaryl-isoxazoline-5-carboxylates in a high-throughput fashion. This was accomplished in one-pot by a sequence of three 2-component reactions steps (2·2·2-CR), whereby compounds were obtained in overall 30-66% isolated yields. The functional group diversity was established by synthesizing a 160-membered library.

1,3-Dipolar cycloadditions of acetylenic sulfones in solution and on solid supports

Several representative acetylenic sulfones were immobilized on a polymer support derived from Merrifield resin by means of ester linkers that were used to couple free carboxylic acid groups on the solid support with benzylic hydroxyl functions on the arylsulfonyl moieties of the acetylenes. Several examples of reversed ester linkers, using Merrifield resin directly, were also successfully prepared. The 1,3-dipolar cycloadditions of the solid-supported acetylenic sulfones were investigated with a series of 1,3-dipoles, including benzyl azide, ethyl diazoacetate, diazomethane, as well as representative nitrile oxides, nitrile imines, nitrile ylides, nitrones, azomethine imines, azomethine ylides, munchnones, and sydnones. In general, analogous cycloadditions were also performed with acetylenic sulfones in solution phase for comparison. The cycloadditions typically afforded good to excellent yields of the desired products in both solution and solid phase, although the latter reactions sometimes required more vigorous conditions. Except in the case of benzyl azide and diazo compounds, where mixtures of regioisomers were obtained, the other 1,3-dipoles reacted with high regioselectivity and afforded essentially unique regioisomers. Cleavage of the products from the resin was smoothly effected by alkaline hydrolysis, while several attempts at reductive desulfonylation with sodium amalgam or samarium diiodide-HMPA resulted in N-O or C-O scission, in addition to cleavage from the polymer. The method provides access to a number of important classes of heterocycles, including variously substituted and functionalized triazoles, pyrazoles, 1,2-oxazoles, pyrroles, as well as their dihydro and bicyclic analogues. The success of the cycloadditions on polymer supports paves the way to future investigations of sequential transformations leading to libraries of useful heterocycles.

Fluorous mixture synthesis of two libraries with hydantoin-, and benzodiazepinedione-fused heterocyclic scaffolds

Diversity-oriented synthesis (DOS) and fluorous mixture synthesis (FMS) are two aspects of combinatorial chemistry. DOS generates library scaffolds with skeletal, substitution, and stereochemistry variations, whereas FMS is a highly efficient tool for library production. The combination of these two aspects in solution-phase synthesis of two novel heterocyclic compound libraries is presented in this paper. Mixtures of different fluorous amino acids undergo [3 + 2] cycloadditions followed by postcondensation reactions. The mixtures are then demixed by fluorous HPLC. Fluorous tags are removed by cyclization to afford hydantoin- and benzodiazepinedione-fused heterocyclic compounds as individual, pure, and structurally defined molecules. The application of MS-directed HPLC purification and parallel four-channel LC/MS analysis further increases the efficiency of FMS.

Recent advances in 1,3-dipolar cycloaddition reactions on solid supports

Solid-phase methods are of a great significance in organic synthesis. Recent developments of these methods are providing new ways to construct libraries of small organic molecules. Five-membered heterocyclic compounds, which can be utilized in a variety of applications, are formed in the 1,3-dipolar cycloaddition reaction between dipolarophiles and dipoles. This review deals with the solid-phase synthesis of heterocycles via [3+2] cycloaddition reaction. Cycloaddition reactions of polymer-bound dipoles and polymer-bound dipolarophiles and intramolecular solid-phase cycloadditions are discussed in separate sections. Reactions of dipolarophiles such as alkenes, alkynes, and imines with dipoles such as azomethine ylides, azomethine imines, nitrile imines, azides, nitrones, and nitrile oxides are described. The recent literature up to December 2003 is covered.

1,3-Dipolar cycloaddition on solid supports: nitrone approach towards isoxazolidines and isoxazolines and subsequent transformations

1,3-dipolar cycloaddition reactions of nitrones with alkenes and alkynes are well-studied reactions in solution-phase organic chemistry. However, the number of studies concerned with their application in solid-phase organic synthesis is rather low compared to other 1,3-dipoles, e.g. azides or nitrile oxides. This tutorial review aims to summarise the main approaches towards the application of nitrones in 1,3-dipolar cycloaddition reactions on solid supports in addition to subsequent transformations with polymer-bound isoxazolidines and reactions using polymer-bound catalysts.

Solid-phase synthesis of novel isoxazolocyclobutanones and isoxazolinocyclobutenones

[reaction: see text] The preparation of novel isoxazolocyclobutanone and isoxazolinocyclobutenone derivatives via a traceless solid-phase sulfone linker strategy is described. Key steps in the solid-phase protocol reported here include (i) sulfinate right arrow sulfone alkylation, (ii) four-member ring formation by sulfone dianion alkylation, (iii) heterocycle formation by nitrile oxide 1,3-dipolar cycloaddition, and (iv) traceless product release by cyclobutanol right arrow cyclobutanone oxidation with concomitant linker cleavage by sulfinate elimination.

Solid-phase development of a L-hydroxybenzotriazole linker for heterocycle synthesis using analytical constructs

The development of a 1-hydroxybenzotriazole linker for the synthesis of heterocyclic derivatives is described, utilizing analytical construct methodology to facilitate the analysis of resin samples. A UV-chromophore-containing analytical construct enabled the accurate determination of resin loading and the automated monitoring of key reactions using only small quantities of resin. The syntheses of an array of isoxazole derivatives are reported.

Stereoselective Solid-Phase Synthesis of a Triaza Tricyclic Ring System: A New Chemotype for Lead Discovery

Sequential pyrrolidine and hydantoin ring-forming reactions have been applied in the stereoselective solid-phase synthesis of a conformationally constrained, tricyclic triazacyclopenta[c]pentalene scaffold. These novel compounds share the structural complexity characteristic of certain alkaloid natural products and represent a source of chemical diversity that complements more traditional classes of heterocyclic compounds of interest as potential pharmaceutical agents. They are assembled in a 12-step reaction sequence from 4 variable building blocks by combining an intramolecular azomethine ylide cycloaddition reaction with a final cyclative cleavage from resin.

A metathetical cycloaddition-cycloreversion approach to the formation of furan scaffold libraries

A general cycloaddition-cycloreversion metathesis procedure for the selective formation of a furan-based template-directed scaffold is described. In addition, features relative to library construction, such as the chemoselective nature of dipole formation, are discussed. Through the investigation of the temperature sensitive cleavage step, the furan synthesis was found to be accelerated by aqueous medium at physiological temperature leading to pure product from the solid-phase under biologically relevant conditions. The chemoselective nature of the rhodium(II) mediated cycloaddition allowed the selective formation of a key dipole intermediate, in the presence of a number of carbeneactive functional groups, to facilitate the split-pool combinatorial synthesis of a small library of compounds.