Single Stereoisomer Research Articles

Cu-Catalyzed [3 + 3] Cycloaddition of Isocyanoacetates with Aziridines and Stereoselective Access to α,γ-Diamino Acids.

We report herein an efficient Cu-catalyzed formal [3 + 3] cycloaddition of isocyanoacetates with readily available aziridines of different substitution patterns, which provides a practical access to valuable 1,4,5,6-tetrahydropyrimidine derivatives. In particular, the use of enantiopure aziridines delivers disubstituted tetrahydropyrimidines bearing a 1,3-diamino unit in good yields as a single stereoisomer (>20:1 dr, > 99% ee). The heterocyclic products can also be easily converted to synthetically useful amino alcohol derivatives or α,γ-diamino acids.

Asymmetric Synthesis of Configurationally Stable Geländer-type p-Terphenyls via Rhodium-catalyzed Intramolecular Double [2+2+2] Cycloaddition

The asymmetric synthesis of configurationally stable oxygen-linked Gelander-type p-terphenyls has been achieved via the rhodium-catalyzed diastereoselective intramolecular double [2+2+2] cycloaddition of chiral propargylic alcohol-derived hexaynes. Centrochirality of the chiral hexaynes induced the axial chirality to give the Gelander-type p-terphenyls as single stereoisomers. Photophysical and chiroptical properties of the thus synthesized Gelander-type p-terphenyls were examined, which revealed that the di-n-butyl- and diphenyl-substituted one shows efficient ultraviolet emission and a relatively large absorption dissymmetry ratio.

Synthesis of a Chiral Auxiliary Family from Levoglucosenone and Evaluation in the Diels–Alder Reaction

A new family of chiral auxiliaries has been developed based on the lignocellulosic biomass pyrolysis product levoglucosenone. A promising single stereoisomer with an alcohol and π-stacking phenyl substituents was prepared in excellent yield in two steps from di­hydrolevoglucosenone without chromatography on >50 g scale. Acrylate esters prepared from the auxiliaries underwent diastereoselective Lewis acid promoted Diels–Alder reactions with cyclopentadiene (­endo/exo 98:2, endo d.r. up to 98:2), dimethylbutadiene (d.r. 93:7), and isoprene (d.r. > 98:2)

Synthesis of Enantiomerically Pure 6-Substituted-Piperazine-2-Acetic Acid Esters as Intermediates for Library Production.

The piperazine heterocycle is broadly exploited in FDA-approved drugs and biologically active compounds, but its chemical diversity is usually limited to ring nitrogen substitutions, leaving the four carbon atoms underutilized. Using an efficient four-step synthesis, chiral amino acids were transformed into 6-substituted piperazine-2-acetic acid esters as diastereomeric mixtures whose cis and trans products could be chromatographically separated. From six amino acids (both antipodes), a complete matrix of 24 monoprotected chiral 2,6-disubstituted piperazines was obtained, each as a single absolute stereoisomer in multigram quantities. These diverse and versatile piperazines can be functionalized on either nitrogen atom, allowing them to be used as scaffolds for parallel library synthesis or intermediates for the production of novel piperazine compounds.

Double rings made with heme

Biocatalysis Cyclic organic structures with adjacent three-carbon rings—bicyclobutanes—are useful starting materials for chemical and materials synthesis owing to their extreme ring strain. Constructing these molecules is a challenging task for organic chemists, especially if a single stereoisomer is desired. Chen et al. engineered a heme-containing enzyme to catalyze sequential carbene insertion reactions using an alkyne substrate. Starting with an enzyme that could only catalyze a single carbene insertion, a series of mutations led to variants that catalyzed efficient, stereoselective production of bicyclobutanes. By using a less reactive alkyne substrate and screening more variants with active site mutations, the authors found enzymes that stop at either enantiomer of the intermediate cyclopropene. Science , this issue p. [71][1] [1]: /lookup/doi/10.1126/science.aar4239

Enzymatic construction of highly strained carbocycles.

Small carbocycles are structurally rigid and possess high intrinsic energy due to their ring strain. These features lead to broad applications but also create challenges for their construction. We report the engineering of hemeproteins that catalyze the formation of chiral bicyclobutanes, one of the most strained four-membered systems, via successive carbene addition to unsaturated carbon-carbon bonds. Enzymes that produce cyclopropenes, putative intermediates to the bicyclobutanes, were also identified. These genetically encoded proteins are readily optimized by directed evolution, function in Escherichia coli, and act on structurally diverse substrates with high efficiency and selectivity, providing an effective route to many chiral strained structures. This biotransformation is easily performed at preparative scale, and the resulting strained carbocycles can be derivatized, opening myriad potential applications.

Steric factors on unsymmetrical O-hydroxyaryl N-Heterocyclic carbene ligands prevailing the stabilization of single stereoisomer of bis-ligated titanium complexes

Bis-ligated titanium (IV) metal complexes supported by bidentate unsymmetrical o-hydroxyaryl-substituted N-heterocyclic carbene ligands were synthesized and structurally identified. While the direct addition of the doubly deprotonated bulky imidazolidinium chloride salts [Dipp,4-RNHC-H]Cl (with Dipp = 2,6-diisopropylphenyl, R = H (2-hydroxyphenyl), and R = Me (2-hydroxy-4-methyl-phenyl)) with chloro-titanium precursor favors the formation of single stereoisomer corresponding to the bis-ligated titanium complexes trans-([κ2-C,O]-Dipp,4-RNHC)2TiCl2 (R = H (2-hydroxyphenyl) for 4aH, and R = Me (2-hydroxy-4-methyl-phenyl) for 4aMe), the reactivity with sterically less hindered imidazolidinium chloride salts [Mes,HNHC-H]Cl and [Dep,HNHC-H]Cl as protio-ligands (with Mes = 2,4,6-trimethylphenyl and Dep = 2,6-diethylphenyl) did not lead to single stereoisomer, but rearranged into a tetradentate salophen-like complexes cis-([κ4-O,N,N,O]-bis(imidazolidinylidene)TiCl2 as major isomer via presumably the NHCs dimerization from bis-ligated intermediates. These results combined with topographic steric maps as well as the buried volume descriptor (%Vbur) indicate that bidentate bulky N-Dipp-substituted NHC ligands offer some level of steric protection preventing the formation of other possible bis-ligated (C,O)-NHC-titanium stereoisomers and constitutional isomers.

Synthesis and Biophysical Characterization of the Chlorosulfolipids of Ochramonas danica

The freshwater algae Ochramonas danica produces a range of polychlorinated single-chain amphiphiles. Danicalipin A, a hexachlorosulfolipid makes up 90% of the polar lipid content of the flagellar membrane of O. danica. Its exotic structure presents both synthetic challenges and raises questions about its function within the membrane. To address these questions, we have combined total synthesis and membrane biophysics to investigate the effects of structural elements of chlorosulfolipids on their behavior in monolayers and bilayers. The discovery of a titanium-based catalytic, enantioselective dichlorination of allylic alcohols enabled the eight-step synthesis of (+)-Danicalipin A as a single stereoisomer in sufficient quantities for in vitro analysis. Nanoscale secondary ion mass spectrometry (NanoSIMS) confirmed that Danicalipin A is localized within plasma membrane of O. danica cells. Preliminary biophysical characterization of Danicalipin A has revealed that it alters the phase behavior and lateral organization in monolayers and bilayers of other lipids present in the membrane of O. danica. Danicalipin A incorporates into monolayers of phospho- and glycolipids at the air-water interface and increases the surface pressure at which the liquid-expanded to liquid-compact phase transition occurs, as well as increasing monolayer compressibility. Similarly, in giant unilamellar vesicles, Danicalipin A lowers the transition temperature of saturated phospho- and glycerolipids and causes phase separation. Natural and unnatural analogs to Danicalipin A are being synthesized to examine the effects of the stereochemistry, chlorination pattern, and sulfation on these biophysical properties. These results may reveal how the membrane of O. danica accommodates high concentrations of chlorosulfolipids, which are toxic to other organisms.

Vinyl Grignard-Mediated Stereoselective Carbocyclization of Lactone Acetals.

A novel Ferrier-type carbocyclization is reported. It involves a carbohydrate-derived lactone acetal synthesized from methyl α-d-glucopyranoside, which upon treatment with excess vinylmagnesium bromide provides a highly substituted carbocyclic product as a single stereoisomer. The yield is greatly increased when N,N,N',N'-tetramethylethylenediamine is added to the reaction mixture. Optimized reaction conditions have been applied to lactone acetals derived from other carbohydrates. Based on the obtained results, a possible reaction mechanism has been proposed. Furthermore, scalability of the reaction up to 15 g scale and derivatization of the carbocyclic product has been demonstrated, including the formation of a rare trans-bicyclo[4.3.0]nonene scaffold via a ring-closing metathesis. The structure of this and all carbocyclic products were confirmed by X-ray crystallographic analysis.

Catalytic Asymmetric Oxidative γ‐Coupling of α,β‐Unsaturated Aldehydes with Air as the Terminal Oxidant

A novel concept for catalytic asymmetric coupling reactions is presented. Merging organocatalysis with single-electron oxidation by using a catalytic amount of a copper(II) salt and air as the terminal oxidant, we have developed a highly stereoselective carbon-carbon oxidative coupling reaction of α,β-unsaturated aldehydes. The concept relies on the generation of a dienamine intermediate, which is oxidized to an open-shell activated species that undergoes highly selective γ-homo- and γ-heterocoupling reactions. In the majority of examples presented, only a single stereoisomer was formed.

Catalytic Asymmetric Oxidative γ‐Coupling of α,β‐Unsaturated Aldehydes with Air as the Terminal Oxidant

AbstractA novel concept for catalytic asymmetric coupling reactions is presented. Merging organocatalysis with single‐electron oxidation by using a catalytic amount of a copper(II) salt and air as the terminal oxidant, we have developed a highly stereoselective carbon–carbon oxidative coupling reaction of α,β‐unsaturated aldehydes. The concept relies on the generation of a dienamine intermediate, which is oxidized to an open‐shell activated species that undergoes highly selective γ‐homo‐ and γ‐heterocoupling reactions. In the majority of examples presented, only a single stereoisomer was formed.

FeCl3-Promoted [3 + 2] Annulations of γ-Butyrolactone Fused Cyclopropanes with Heterocumulenes.

The [3 + 2] annulations of γ-butyrolactone fused donor-acceptor (D-A) cyclopropanes with aromatic isothiocyanates and dialkyl carbodiimides promoted by FeCl3 is reported. A series of bicyclic/polycyclic γ-butyrolactone fused thioimidates and γ-butyrolactone fused amidines containing four contiguous stereogenic centers were obtained in excellent yields as single stereoisomers.

Serrulatic acid diastereomers identified from an antibacterial survey of Eremophila

In an age of growing antimicrobial resistance, new antibacterial agents are desperately needed. A rapid antibacterial and phytochemical survey was designed to screen for antibacterial leads in plants. The survey was applied to over 90 Australian native plants from the genus Eremophila, revealing Eremophila complanata and E. nivea×E. drummondii as active against Gram positive bacteria. Thin layer chromatography with bioautography, flash chromatography and nuclear magnetic resonance led to the isolation and identification of two diastereomeric serrulatic acids. A single stereoisomer of 7,8,16-trihydroxyserrulat-19-oic acid has been previously described as its methyl ester. This paper describes the NMR of both serrulatic acids epimeric at C15 and their methyl esters, and demonstrates their Gram positive antibacterial activity. It is the first time that stereoisomers of this serrulatic acid have been found together in some Eremophila species. Further characterization of E. complanata additionally found an abundance of α-selinene and β-selinene. The study validates a rapid survey approach to finding antibacterial phytochemicals.

Development of a Practical Synthesis of a Peripherally Selective Noradrenaline Reuptake Inhibitor Possessing a Chiral 6,7-trans-Disubstituted-1,4-oxazepane as a Scaffold

A practical synthesis of a peripherally selective noradrenaline reuptake inhibitor that has a chiral 6,7-trans-disubstituted-1,4-oxazepane as a new class of scaffold is described. The amino alcohol possessing the desired stereochemistry was obtained with excellent dr and ee, starting from a commercially available aldehyde via a Morita–Baylis–Hillman reaction, Michael addition, isolation as maleic acid salt, reduction, and diastereomeric salt formation with (+)-10-camphorsulfonic acid. The desired single stereoisomer obtained at an early stage of the synthesis was used for seven-membered ring formation in fully telescoped processes, providing the chiral 6,7-trans-disubstituted-1,4-oxazepane efficiently. In addition to controls of dr and ee of the chiral 1,4-oxazepane, and control of N,O-selectivity in SN2 reaction of the intermediate mesylate with a pyridone derivative, finding appropriate intermediates that were amenable to isolation and upgrade of purity enabled a practical chiral HPLC separation-free, c...

Synthesis of Chiral Tritylpyrrolidine Derivatives and Their Application to Asymmetric Benzoyloxylation.

An efficient synthesis of novel chiral tritylpyrrolidine derivatives has been developed. Single stereoisomers of various tritylpyrrolidine derivatives can be readily obtained through diastereomer separation by simple silica gel column chromatography. Representative compounds of this class have been shown to be efficient amine organocatalysts for asymmetric benzoyloxylation.

Chiral Silver Complex-Catalyzed Diastereoselective and Enantioselective Michael Addition of 1-Pyrroline-5-carboxylates to α-Enones.

A AgOAc/ThioClickFerrophos complex-catalyzed the highly diastereo- and enantioselective reaction between 1-pyrroline-5-carboxylates (1) and acyclic α-enones (2) in MeOH, in the presence of DBU, to give the single isomer Michael adducts (3) in high yields (up to 99%) with excellent enantioselectivies (up to 99% ee). Subsequent reduction of the Michael adducts with sodium cyanoborohydride successfully produced the fused pyrrolizidine ester as an almost pure single stereoisomer.

Enantioselective Synthesis and in Vivo Evaluation of Regioisomeric Analogues of the Antimalarial Arterolane.

We describe the first systematic study of antimalarial 1,2,4-trioxolanes bearing a substitution pattern regioisomeric to that of arterolane. Conformational analysis suggested that trans-3″-substituted trioxolanes would exhibit Fe(II) reactivity and antiparasitic activity similar to that achieved with canonical cis-4″ substitution. The chiral 3″ analogues were prepared as single stereoisomers and evaluated alongside their 4″ congeners against cultured malaria parasites and in a murine malaria model. As predicted, the trans-3″ analogues exhibited in vitro antiplasmodial activity remarkably similar to that of their cis-4″ comparators. In contrast, efficacy in the Plasmodium berghei mouse model differed dramatically for some of the congeneric pairs. The best of the novel 3″ analogues (e.g., 12i) outperformed arterolane itself, producing cures in mice after a single oral exposure. Overall, this study suggests new avenues for modulating Fe(II) reactivity and the pharmacokinetic and pharmacodynamic properties of 1,2,4-trioxolane antimalarials.

Synthesis of Spirocyclic Amines by Using Dipolar Cycloadditions of Nitrones.

Aliphatic ketones containing a chloride and alkene were heated with hydroxylamine to promote cascade, tandem condensation to oximes, cyclization to nitrones, and 1,3-dipolar cycloaddition to tricyclic isoxazolidines as single stereoisomers. Single regioisomers were obtained when three atoms linked the ketone and dipolarophile to give five-membered rings but mixtures resulted with four atoms in the tether unless a terminal ester was located on the alkene. The N-O bond in the products could be reduced to give spirocyclic amines and diamines.

Crystal-Packing-Driven Enrichment of Atropoisomers.

Crystal-packing forces can have a significant impact on the relative stabilities of different molecules and their conformations. The magnitude of such effects is, however, not yet well understood. Herein we show, that crystal packing can completely overrule the relative stabilities of different stereoisomers in solution. Heating of atropoisomers (i.e. "frozen-out" conformational isomers) in solution leads to complex mixtures. In contrast, solid-state heating selectively amplifies minor (<25 mole %) components of these solution-phase mixtures. We show that this heating strategy is successful for compounds with up to four rotationally hindered σ bonds, for which a single stereoisomer out of seven can be amplified selectively. Our results demonstrate that common supramolecular interactions-for example, [methyl⋅⋅⋅π] coordination and [C-H⋅⋅⋅O] hydrogen bonding-can readily invert the relative thermodynamic stabilities of different molecular conformations. These findings open up potential new avenues to control the folding of macromolecules.

Crystal‐Packing‐Driven Enrichment of Atropoisomers

AbstractCrystal‐packing forces can have a significant impact on the relative stabilities of different molecules and their conformations. The magnitude of such effects is, however, not yet well understood. Herein we show, that crystal packing can completely overrule the relative stabilities of different stereoisomers in solution. Heating of atropoisomers (i.e. “frozen‐out” conformational isomers) in solution leads to complex mixtures. In contrast, solid‐state heating selectively amplifies minor (&lt;25 mole %) components of these solution‐phase mixtures. We show that this heating strategy is successful for compounds with up to four rotationally hindered σ bonds, for which a single stereoisomer out of seven can be amplified selectively. Our results demonstrate that common supramolecular interactions—for example, [methyl⋅⋅⋅π] coordination and [C−H⋅⋅⋅O] hydrogen bonding—can readily invert the relative thermodynamic stabilities of different molecular conformations. These findings open up potential new avenues to control the folding of macromolecules.