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PubMed2004

A modular synthesis of the lamellarins: total synthesis of lamellarin G trimethyl ether.

A modular synthesis of the lamellarin family of natural products has been developed that is based on the application of three iterative halogenation/cross-coupling reaction sequences. The ability to halogenate the pyrrole core in a regioselective fashion, even in the presence of highly electron-rich aryl substituents, has been established. The compatibility of Suzuki coupling conditions with free alcohols and phenols in the boronic acids has been employed to reduce the number of protection/deprotection steps. Indeed, the presence of a free phenol on boronic acid 3 has been determined to be critical for the successful final coupling in route to lamellarin G trimethyl ether, since protected versions fail to undergo coupling.

Biological Factors↗
PubMed2005

2-(aminomethyl)-oxazolines: highly modular scaffolds for the preparation of novel asymmetric ligands.

Highly modular chiral 2-(aminoalkyl)oxazolines have been prepared from alpha-amino acids and 1,2-amino alcohols. The amine-functionalized oxazolines were employed as scaffolds in the preparation of a number of different ligands with potential denticities varying from 2 to 5. The obtained ligands were employed and evaluated in the ruthenium-catalyzed asymmetric transfer-hydrogenation of acetophenone and in the titanium-catalyzed addition of diethylzinc to aldehydes. In the latter process, enantioselectivity up to 97% was obtained.

Amino Acids↗
PubMed2004

A modular and concise total synthesis of (+/-)-daurichromenic acid and analogues.

A modular and concise total synthesis of (+/-)-daurichromenic acid has been accomplished in four steps from ethyl acetoacetate, ethyl crotonate, and trans,trans-farnesal. A series of analogues of this natural product, which has potent anti-HIV activity, were also prepared from ethyl or methyl acetoacetate and a series of readily available alpha,beta-unsaturated esters and aldehydes.

Anti-HIV Agents↗
PubMed2005

A modular synthesis of unsymmetrical tetraarylazadipyrromethenes.

[reaction: see text] A stepwise route to unsymmetrical tetraarylazadipyrromethenes by a condensation of 2,4-diaryl-5-nitroso-pyrroles with 2,4-diarylpyrroles is described. This modular building-block approach allows for the introduction of up to four different aryl substituents on the azadipyrromethene and is tolerant of a varied substituent set. An efficient synthesis of the 2,4-diarylpyrroles building blocks from 1,3-diaryl-4-nitro-butan-1-ones by nitro hydrolysis to a keto-aldehyde and subsequent ammonia condensation reaction has been achieved. The facile conversion of 2,4-diarylpyrroles into their alpha-nitroso analogues by their reaction with sodium nitrite generated the second building block required for the synthesis.

Journal Article↗
PubMed2005

Modular total synthesis of lamellarin D.

[Chemical reaction: see text] A modular total synthesis of lamellarin D, a marine alkaloid with potent cytotoxic as well as topoisomerase I inhibition properties, has been accomplished. A sequential and regioselective bromination/Suzuki cross-coupling procedure was applied for the introduction of aryl groups at positions 1 and 2 of scaffold 1. Microwave-assisted 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ) oxidation to yield pyrroloisoquinoline 15, followed by phenol group deprotection and subsequent lactonization, gave lamellarin D (18% in eight steps from 1).

Alkaloids↗
PubMed2006

Modular approach for the development of supported, monofunctionalized, salen catalysts.

We report a modular approach toward polymer-supported, metalated, salen catalysts. This strategy is based on the synthesis of monofunctionalized Mn- and Co-salen complexes attached to a norbornene monomer via a stable phenylene-acetylene linker. The resulting functionalized monomers can be polymerized in a controlled fashion using ring-opening metathesis polymerization. This polymerization method allows for the synthesis of copolymers, resulting in an unprecedented control over the catalyst density and catalytic-site isolation. The obtained polymeric manganese and cobalt complexes were successfully used as supported catalysts for the asymmetric epoxidation of olefins and the hydrolytic kinetic resolution of epoxides. All polymeric catalysts showed outstanding catalytic activities and selectivities comparable to the original catalysts reported by Jacobsen. Moreover, the copolymer-supported catalysts are more active and selective than their homopolymer analogues, providing further proof that catalyst density and site isolation are key toward highly active and selective supported salen catalysts.

Journal Article↗
PubMed2006

New modular and efficient approach to 6-substituted pyridin-2-yl C-nucleosides.

A novel modular, efficient, and practical methodology of preparation of 6-substituted pyridin-2-yl C-nucleosides was developed. An addition of 2-lithio-6-bromopyridine 2b to TBDMS-protected 2-deoxyribonolactone 5 gave aduct 7 as an equilibrium mixture of anomeric hemiketals 1-(6-bromopyridin-2-yl)-1-hydroxynucleosides 7a,b and its open form 7c. Reduction of the adduct 7 with Et3SiH and BF3 x Et2O afforded the desired 6-bromonucleoside 8a as pure beta-anomer in a total yield of 32% over two steps from 5. Intermediate 8a was then subjected to a series of palladium catalyzed cross-coupling reactions and aminations to give a series of protected 1beta-(6-alkyl-, 6-aryl-, and 6-aminopyridin-2-yl)-2-deoxyribonucleosides 9. Catalytic hydrogenation of 8a gave an unsubstituted pyridine C-nucleoside, and diazotative oxodeamination of 6-aminopyridine nucleoside 9f by isopentyl nitrite in acetic acid gave 6-oxopyridine nucleoside 10i. Deprotection of silylated nucleosides 9 by Et3N.3HF gave a series of free C-nucleosides 10.

Hydrocarbons, Brominated↗
PubMed2006

Screening of a modular sugar-based phosphite ligand library in the asymmetric nickel-catalyzed trialkylaluminum addition to aldehydes.

We synthesized a modular sugar-based phosphite ligand library for the Ni-catalyzed trialkylaluminum addition to aldehydes. This library has been designed to rapidly screen the ligands to uncover their important structure features and determine the scope of the phosphite ligands in this catalytic reaction. After systematic variation of the sugar backbone, the substituents at the phosphite moieties, and the flexibility of the ligand backbone, the monophosphite ligand 1,2:5,6-di-O-isopropylidene-3-O-((3,3';5,5'-tetra-tert-butyl-1,1'-biphenyl-2,2'-diyl)phosphite)-alpha-D-glucofuranose 1c was found to be optimal, yielding high activities and enantioselectivities (ee's up to 94%) for several aryl aldehydes.

Journal Article↗
PubMed2006

Design, synthesis, and evaluation of a new generation of modular nucleophilic glycine equivalents for the efficient synthesis of sterically constrained alpha-amino acids.

A new generation of modular achiral glycine equivalents have been evaluated with respect to their synthetic utility for the production of tailor-made, sterically constrained alpha-amino acids, which proved to be the most efficient approach developed to date for the synthesis of symmetrical alpha,alpha-disubstituted-alpha-amino acids. Among the new series of achiral glycine equivalents, one was found to be a superior glycine derivative for the Michael additions with various (R)- or (S)-N-(E-enoyl)-4-phenyl-1,3-oxazolidin-2-ones representing a general and practical synthesis of sterically constrained beta-substituted pyroglutamic acids. In particular, the application of these complexes allowed for the preparation of several beta-substituted pyroglutamic acids which include electron-releasing and sterically demanding substituents in the structure thus increasing the synthetic efficiency and expanding the generality of these Michael addition reactions.

Amino Acids↗
PubMed2000

The utility of furan-, pyrrole-, and thiophene-based 2-silyloxy dienes as demonstrated by modular synthesis of annonaceous acetogenin core units and their pyrrolidine and thiolane analogues.

We report a modular strategy for obtaining the core units of annonaceous acetogenins and their nitrogen and sulfur analogues, which generates great structural diversity. This synthesis is based on the application of a reiterative vinylogous addition protocol involving a unique triad of silyloxy diene modules, 2-[(tert-butyldimethylsilyl)oxy]furan (TBSOF), N-(tert-butoxycarbonyl)-2-[(tert-butyldimethylsilyl)oxy]pyrrole (TBSOP), and 2-[(tert-butyldimethylsilyl)oxy]thiophene (TBSOT) and suitable heteroatom-stabilized carbenium ions. By combining TBSOF, TBSOP, and TBSOT nucleophilic synthons with certain tetrahydrofuran, pyrrolidine, and thiolane acceptors, the construction of varied, adjacently linked oligo-heterocyclic motifs related to the core segments of the annonaceous acetogenins is assured. At first, the reliability of the pivotal coupling maneuver was certified, by assembling a collection of 18 model constructs, covering all oxygen, nitrogen, and sulfur heteroatom combinations (i.e., compounds 7-9, 13-15, and 19-21). This uniformed protocol was then suited to forge advanced bis-tetrahydrofuran, bis-pyrrolidine, and bis-thiolane scaffolds encompassing the heterocyclic core portion of various binuclear annonaceous acetogenins and relatives. The utility of this synthesis was demonstrated by the preparation of a repertoire of eight isomeric bis-tetrahydrofuran units, 41-48, two bis-pyrrolidine units, 62 and 63, and four bis-thiolane units, 78-81.

Furans↗
PubMed2005

Modular approach toward supramolecular functional assemblies: characterization of Donor-spacer-acceptor ternary complexes.

It is shown that the noncovalent donor-spacer-acceptor (DSA) motif is useful in constructing an electron-transfer assembly. As a representative example, the equilibrium and structure of one of the DSA assemblies, consisting of Zn-tetraphenylporphyrin, a spacer unit bearing pyridine and amidinium moieties, and 3,4-dinitrobenzoic acid, were studied in detail by the extensive use of UV-vis titration, fluorescence spectroscopy, and 1H NMR, with the help of a three-component equilibrium model. Complex formation and fluorescence quenching in 20 different DSA complexes constructed from a library of five donors, two spacers, and two acceptors were investigated. It has been experimentally shown that supramolecular modular approach is useful for a systematic and quick search for a functionally optimized assembly.

Journal Article↗
PubMed2000

Modular construction of dendritic carbosilanes. Organization Of dendrimer connectivity around bifunctional precursors that are adapted for sequential convergent and divergent propagative steps

Regiospecific hydrosilylation of 1-bromo-4-(prop-2-enyl)benzene offers an efficient route to molecular building block precursors that can accommodate sequential divergent and convergent steps for dendritic extension, establishing a modular methodology for assembly and organization of connectivity used for synthesis of modified carbosilane dendrimers including 14.

Journal Article↗
PubMed2000

A modular synthetic approach toward exhaustively stereodiversified ligand libraries.

[structure] This report describes a modular approach to the synthesis of stereodiversified natural product-like libraries. Monomers 2 and 3 were coupled in parallel by silyl-tethered olefin metathesis to generate all 16 stereoisomers of cis-enediols 1. All 16 stereoisomers were incorporated into chimerae having flanking peptidic segments. These chimerae exhibited a broad range of hydrophobicities, raising the possibility that stereochemical variation might be used to tune the pharmacologic properties of small molecules.

Alkenes↗
PubMed2002

Modular evolution of a chiral auxiliary for the 1,3-dipolar cycloaddition of isomünchnones with vinyl ethers.

[reaction: see text]. The 1,3-dipolar cycloaddition reaction has long been recognized as a powerful methodology in organic synthesis. More recently, this reaction has become a popular manifold for the construction of chemical diversity. Herein, we report the development of a chiral template for the facially selective cycloaddition of isomünchnones, a common class of 1,3-dipoles. The modular format of the asymmetric unit allowed a systematic optimization of selectivity. In addition, the chiral auxiliary was removed through an unusually facile ester aminolysis.

Amino Acids↗
PubMed2003

New set of orthogonal protecting groups for the modular synthesis of heparan sulfate fragments.

Six strategically chosen monosaccharide building blocks, which are protected by a novel set of four orthogonal protecting groups (Lev, Fmoc, TBDPS, and All), can be employed for the efficient synthesis of the 20 disaccharide moieties found in heparan sulfate. The properly protected disaccharide building blocks can be converted into glycosyl donors and acceptors, which can be used for the modular synthesis of a wide range of well-defined oligosaccharides that differ in sulfation pattern. [structure: see text]

Carbohydrate Sequence↗
PubMed2005

Modular synthesis of cyclic peptidomimetics inspired by gamma-turns.

[reaction: see text] A series of peptidomimetics based on a gamma-turn motif were synthesized using a modular approach, in which N-protected piperidones were reacted with a selection of 2-hydroxyalkyl azides derived from common l-amino acids. Hydrolysis of the initially formed iminium ethers afforded the targeted series of substituted 1,4-diazepin-5-ones.

Amino Acids↗
PubMed2004

Modular synthesis of fluorous trialkylphosphines.

A modular synthetic protocol was developed for the preparation of fluorous trialkylphosphines with a different number of methylene spacers and various lengths of the fluorous ponytails P[(CH(2))(A)R(FX)][(CH(2))(B)R(FY)][(CH(2))(C)R(FZ)] (A, B, C = 3 and 4; X, Y, Z = 4, 6, and 8). [reaction: see text]

Journal Article↗