Facile synthesis of substituted alkynes by nano-palladium catalyzed oxidative cross-coupling reaction of arylboronic acids with terminal alkynes

The oxidative cross-coupling of terminal alkynes with arylboronic acids catalyzed by the porous palladium nanospheres was developed. In the presence of silver oxide, triphenylphosphine and cesium carbonate, the reaction provided the corresponding arylalkynes with good to excellent yields. There were also other obvious advantages such as broad applicability, high selectivity, simple experimental operation as well as the convenient preparation, high efficiency and reusability of catalyst. The possible mechanism of this transition-metal nanoparticles catalyzed oxidative cross-coupling reaction of two nucleophiles was proposed.     

Solid-phase synthesis of substituted 1,3,4-thiadiazoles

Two novel and facile syntheses strategies for the synthesis of substituted 1,3,4-thiadiazoles on solid support are described based on a resin-bound thiosemicarbazide: (a) treatment with aldehydes to form immobilised thiosemicarbazones, and oxidative cyclodehydration with iron(III) chloride forms resin-bound 1,3,4-thiadiazoles; and (b) treatment with di-(2-pyridyl)thionocarbonate affords immobilised 1,3,4-thiadiazole-2-thione which is selectively mono-S-alkylated to yield resin-bound 2-alkylthio-1,3,4-thiadiazoles. Acidic cleavage with trifluoroacetic acid yields the products from both studies in good yield and excellent purity.     

Solid-phase synthesis of substituted guanidines using a novel acid labile linker

A novel acid labile linker for solid-phase synthesis of substituted guanidines has been developed. Its synthetic utility is exemplified by high-yielding pyrazole displacement with structurally and electronically diverse sets of aliphatic and aromatic amines. The final cleavage is achieved by treatment with 95:5 trifluoroacetic acid/water for 1 h. The corresponding guanidines were obtained in high purity (80-95%) and good isolated yields (50-95%). The scope and limitations of this linker were further demonstrated by the solid-phase synthesis of an 880-member library of individual trisubstituted arylguanidines employing pyrazole displacement with a set of 11 anilines and two subsequent Mitsunobu N-alkylations with sets of 10 and 8 alcohols, respectively.     

Copper- or phosphine-catalyzed reaction of alkynes with isocyanides. Regioselective synthesis of substituted pyrroles controlled by the catalyst

The copper-catalyzed reaction of isocyanides (CNCH2EWG1) 1 with electron-deficient alkynes (RC[triple bond]CEWG2) 2 gave the 2,4-di-EWG-substituted pyrroles 3 selectively, whereas the phosphine-catalyzed reaction of 1 with 2 afforded the 2,3-di-EWG-subsituted pyrroles 4. Accordingly, regioselective synthesis of substituted pyrroles has been achieved by merely choosing the catalyst.     

Solid-phase synthesis of substituted 3-amino-3′-carboxy-tetrahydrocarbazoles

Two related solid-phase synthesis routes have been developed allowing the synthesis of 3-amino-3′-carboxy substituted tetrahydrocarbazole derivatives. Diversity can be introduced at the amino and carboxy functionalities and at the nitrogen and the aromatic ring of the tetrahydrocarbazole moiety. Both routes rely on Fmoc-protected 1-amino-4-oxocyclohexanone carboxylic acid as central core element. Derivatization of the carboxy function is achieved with amines, derivatization of the amino functionality is possible by reaction with alkyl halides, isocyanates, activated alcohols, sulfonic acid chlorides or carboxylic acids. The tetrahydrocarbazole scaffold is generated by Fischer indole cyclization with phenyl hydrazine derivatives, thereby introducing diversity in the aromatic moiety. N-Alkylation at the indole nitrogen with alkyl halides delivers N-substituted derivatives.     

The enantiospecific synthesis of chromanes and isochromanes using a variant of an intramolecular Nicholas reaction

The enantiospecific synthesis of chromanes and isochromanes obtained from an intramolecular Nicholas cyclisation reaction is discussed. During the course of this study we observed the formation of chroman-4-ones from a CAN deprotection step of a dioxolane and this is also discussed.     

Solid-phase synthesis of pyrroloisoquinolines via the intramolecular N-acyliminium Pictet-Spengler reaction

In the present investigation, solid-phase routes toward 1,5,6,10b-tetrahydro-2H-pyrrolo[2,1-a]isoquinolin-3-ones via the intramolecular N-acyliminium Pictet-Spengler reaction are established. Peptide aldehydes generated from their parent N-Boc 1,3-oxazines by acidic reaction conditions undergo intramolecular condensation reactions with the amide N of a solid-supported peptide backbone, thus forming a 1:1 epimeric mixture of a cyclic 5-hydroxylactam, which in turn is in equilibrium with the corresponding intermediate N-acyliminium ion. Provided appropriate acidic reaction conditions, a second ring may be appended via Pictet-Spengler cyclization from the aromatic ring of a neighboring phenylalanine derivative in the peptide sequence. The aromatic substitution pattern of the nucleophilic benzene ring of the phenylalanine derivative and the nature of the acidic reaction media are critically important for the course of the reaction, and both Lewis and Br?nsted acids may be employed to effect the cyclization process. This intramolecular reaction is under strict control of stereoselectivity, and only a single stereoisomer is detected in the crude products. A range of mono-, di-, and trisubstituted phenylalanines with diverse sets of electron-donating and electron-withdrawing substituents, pyrene, and naphthalenes have successfully been brought within the scope of the reaction, thus providing a unique scaffold for combinatorial library synthesis.     

Iron-catalyzed annulation reaction of arylindium reagents and alkynes to produce substituted naphthalenes

We report here an iron–bisphosphine complex-catalyzed annulation reaction of an arylindium reagent and two alkyne molecules that affords a substituted naphthalene derivative in moderate to good yield. The reaction represents a new example of iron-catalyzed C–C bond forming reactions via C–H bond functionalization.     

Solid-phase synthesis and structural characterization of highly substituted hydroxyproline-based 2,5-diketopiperazines

Two general solid-phase methods for the synthesis of a new class of 2,5-diketopiperazines (DKPs) containing the trans-4-hydroxy-L-proline amino acid residue (Hyp) have been developed. An N-protected hydroxyproline methyl ester was linked through the hydroxyl function to the Ellman resin. The synthesis procedures were conceived to enable a sequence of Hyp alkylation, Hyp N-acylation, cyclization, and amide bond alkylation. Up to three different centers of molecular diversity were introduced around the DKP scaffold. Highly functionalized bicyclic compounds were obtained in good yield and purity. The alkylation of hydroxyproline alpha CH was performed without control of the diastereoselectivity. During the final alkylation of the backbone, amide bond epimerization at the alpha-carbon atoms of the two amino acid residues was observed. The structures of representative DKPs were elucidated with multidimensional NMR experiments. The described reaction pathways can be applied to the identification of heterocyclic molecule inhibitors to diverse enzyme targets.     

Solid-phase synthesis of asymmetrically substituted "AB3-type" phthalocyanines

Synthesis of phthalocyanines with asymmetrical substitution on the periphery is often difficult due the problems in purification of the phthalocyanine mixtures obtained. Using a poly(ethylene glycol) (PEG)-based support with a Wang-type linker, we have developed the synthesis of monohydroxylated, oligoethylene glycol substituted phthalocyanines utilizing an amidine-base-promoted phthalonitrile tetramerization reaction. The use of a hydrophilic support allows symmetrical phthalocyanine product formed in solution to be readily and completely removed by washing while leaving the "AB3" product on the support. Acid cleavage with 10% trifluoroacetic acid provides the pure unsymmetrically substituted Pc. This method was applied to several metallo Pcs. Additionally, methods to avoid premature reactions on-resin that give A2B2 products are provided.     

Intramolecular Nicholas reaction: stereoselective synthesis of 5-alkynylproline derivatives

The intramolecular Nicholas reaction of propargylic alcohols derived from N, N-acyl-diprotected omega-semialdehydes obtained from glutamic acid provided stereoselectively 5-alkynylproline derivatives. The suitable choice of the N-protecting group (tosyl or benzoyl derivative) permitted control of the stereochemistry during the ring formation. Semiempirical calculations of the species involved in the cyclization support the observed stereochemistry.     

Application of the intramolecular PIFA-mediated amidation of alkynes to the synthesis of substituted indolizidinones

The construction of the title compounds has been achieved from properly substituted linear alkynylamides through the suitable combination of two key cyclization steps. First, an intramolecular PIFA-mediated alkyne amidation protocol leads to the creation of the pyrrolidinone nucleus, which under proper manipulation of the generated keto–carbonyl group permits the assembling of the indolizidinone skeleton by the introduction of a subsequent ring closing olefin metathesis step. Finally, its transformation into a series of substituted mono- and trihydroxylated indolizidinone derivatives is achieved by manipulation of the remaining unsaturated fragment under hydrogenation and dihydroxylation conditions.     

Solid-phase synthesis of modified oligopeptides via Passerini multicomponent reaction

A Passerini multicomponent reaction of N-protected-α-aminoaldehydes, carboxylic acids and solid-supported isocyanides has been successfully performed on Lantern™. The initially formed N-protected-β-amino-α-acyloxyamides gave, by piperidine-promoted deprotection and concomitant acyl migration, β-acylamino-α-hydroxyamides which, by OH-oxidation and cleavage off the solid support, provided β-acylamino-α-oxoamides. Formation and disappearance of the isocyano group has been followed using photoacoustic IR spectroscopy, a fast, reliable and non-disruptive technique that has been successfully applied for the first time to macroscopic solid supports.     

Novel formation and use of a Nicholas carbocation in the synthesis of highly substituted tetrahydrofurans

The first formation of a Nicholas carbocation through cleavage of a carbon-carbon sigma bond has allowed the preparation of highly substituted tetrahydrofurans in a formal dipolar cycloaddition reaction.     

Solid-phase synthesis of indolecarboxylates using palladium-catalyzed reactions

Polymer-supported, palladium-catalyzed cyclization reactions effectively synthesized indolecarboxylates. Palladium-catalyzed carbon-carbon bond-forming reactions of immobilized enaminoesters followed by transesterification yielded indole 2- or 3-carboxylates with various functional groups on the benzene ring. Indolecarboxylates were efficiently cyclized via an intramolecular palladium-catalyzed amination reaction of immobilized N-substituted dehydrohalophenylalanines, and immobilized N-acetyl-dehydroalanines were efficiently converted into indolecarboxylates via tandem Heck-amination reactions.     

Facile synthesis of fluorinated α-imino alkynes by the coupling reaction of imidoyl iodides with terminal alkynes

A series of N-arylbromodifluoroacetimidoyl iodides and 1-alkynes were converted into α-imino alkynes by using Pd(Ph₃)₂Cl₂/CuI as the catalyst under mild conditions. The reaction proceeded smoothly to give the coupling products in good to excellent yields.     

Solid-phase synthesis of isoquinolinones using Bischler-Napieralski cyclization

Isoquinolinone is a structural unit found in many natural products having various important biological activities. A traceless solid-phase synthetic approach has been developed to prepare isoquinolinone derivatives. This approach enables one to synthesize isoquinolinones having various moieties on benzene nuclei and also can produce derivatives with a proton on the amide nitrogen.     

Solid-phase synthesis of polymers using the ring-opening metathesis polymerization

We report a general method for the solid-phase synthesis of polymers via the ring-opening metathesis polymerization (ROMP). The method involves polymerization in solution to form a block copolymer, immobilization of the polymer via reaction of one block with a resin-bound functional group, modification of the other block, and liberation of the polymer from the resin. We demonstrated the utility of this approach by generating a block copolymer with an N-hydroxysuccinimidyl ester-substituted block (for on-resin functionalization) and a maleimide-substituted block (for conjugation to the resin). We showed that the Diels-Alder reaction can be employed to immobilize the polymers and that amines of diverse structure can be used to modify the resin-bound polymers. The reversibility of the furan-maleimide Diels-Alder adduct was exploited to liberate the polymer from the support. Specifically, treatment of the resin with cyclopentadiene resulted in complete polymer release. The resulting polymers are functional: they were as potent in assays with the lectin concanavalin A as polymers generated by traditional solution routes. We anticipate that this method can be used for the rapid synthesis of diverse polymers via ROMP.