Concise synthesis of rodgersinol and determination of the C-10 absolute configuration

Rodgersinol was synthesized via seven linear steps in 31% overall yield, and the absolute configuration of the C-10 stereogenic center was elucidated. The key feature of the synthesis involves the efficient Cu(II)-mediated coupling of two aromatic moieties for the diaryl ether intermediate and the enantioselective construction of the hydroxypropyl substituent by a regio- and stereoselective methyl addition to the chiral aryloxiranes in an inversion manner.     

Enantioselective Copper(I)‐Catalyzed Alkynylation of Oxocarbenium Ions to Set Diaryl Tetrasubstituted Stereocenters

An enantioselective, copper(I)‐catalyzed addition of terminal alkynes to isochroman ketals to set diaryl, tetrasubstituted stereocenters has been developed. The success of this reaction relies on identification of a Cu/PyBox catalyst capable of distinguishing the faces of the diaryl‐substituted oxocarbenium ion. This challenging transformation enables efficient conversion of readily available, racemic ketals into high‐value enantioenriched isochroman products with fully substituted stereogenic centers. High yields and enantiomeric excesses are observed for various isochroman ketals and an array of alkynes.     

First and second generation total synthesis of the teicoplanin aglycon.

Full details of studies leading to the total synthesis of the teicoplanin aglycon are provided. Key elements of the first generation approach (26 steps from constituent amino acids, 1% overall) include the coupling of an EFG tripeptide precursor to the common vancomycin/teicoplanin ABCD ring system and sequential DE macrocyclization of the 16-membered ring with formation of the diaryl ether via a phenoxide nucleophilic aromatic substitution of an o-fluoronitroaromatic (80%, 3:1 atropisomer diastereoselection) followed by 14-membered FG ring closure by macrolactamization (66%). Subsequent studies have provided a second generation total synthesis which is shorter, more convergent, and highly diastereoselective (22 steps, 2% overall). This was accomplished by altering the order of ring closures such that FG macrolactamization (95%) preceded coupling of the EFG tripeptide to the ABCD ring system and subsequent DE ring closure. Notably, DE macrocyclization via diaryl ether formation on substrate 57, the key intermediate in the latter approach incorporating the intact FG ring system, occurred with exceptional diastereoselection for formation of the natural atropisomer (>10:1, 76%) without problematic C(2)(3) epimerization provided the basicity of the reaction is minimized.     

Total synthesis and evaluation of [Psi[CH2NH]Tpg4]vancomycin aglycon: reengineering vancomycin for dual D-Ala-D-Ala and D-Ala-D-Lac binding

An effective synthesis of [Psi[CH(2)NH]Tpg(4)]vancomycin aglycon (5) is detailed in which the residue 4 amide carbonyl of vancomycin aglycon has been replaced with a methylene. This removal of a single atom was conducted to enhance binding to D-Ala-D-Lac, countering resistance endowed to bacteria that remodel their D-Ala-D-Ala peptidoglycan cell wall precursor by a similar single atom change (ester O for amide NH). Key elements of the approach include a synthesis of the modified vancomycin ABCD ring system featuring a reductive amination coupling of residues 4 and 5 for installation of the deep-seated amide modification, the first of two diaryl ether closures for formation of the modified CD ring system (76%, 2.5-3:1 kinetic atropodiastereoselectivity), a Suzuki coupling for installation of the hindered AB biaryl bond (90%) on which the atropisomer stereochemistry could be thermally adjusted, and a macrolactamization closure of the AB ring system (70%). Subsequent DE ring system introduction enlisted a room-temperature aromatic nucleophilic substitution reaction for formation of the remaining diaryl ether (86%, 6-7:1 kinetic atropodiastereoselectivity), completing the carbon skeleton of 5. Consistent with expectations and relative to the vancomycin aglycon, 5 exhibited a 40-fold increase in affinity for D-Ala-D-Lac (K(a) = 5.2 x 10(3) M(-1)) and a 35-fold reduction in affinity for D-Ala-D-Ala (K(a) = 4.8 x 10(3) M(-1)), providing a glycopeptide analogue with balanced, dual binding characteristics. Beautifully, 5 exhibited antimicrobial activity (MIC = 31 microg/mL) against a VanA-resistant organism that remodels its D-Ala-D-Ala cell wall precursor to d-Ala-d-Lac upon glycopeptide antibiotic challenge, displaying a potency that reflects these binding characteristics.     

Organocatalytic kinetic resolution cascade reactions: new mechanistic and stereochemical manifold in diphenyl prolinol silyl ether catalysis

A new cascade reaction involving an iminium-catalyzed intramolecular oxa-Michael addition followed by an enamine-catalyzed intermolecular Michael addition is reported herein. This cascade reaction generates enantiopure, highly functionalized tetrahydropyrans and tetrahydrofurans in a one-pot reaction and in up to 89?% combined yield and up to 99?% ee. This cascade reaction is catalyzed by diaryl prolinol silyl ethers, which are a privileged class of catalysts. The stereochemical outcome of these cascade reactions is unprecedented. Computational studies indicate that this stereochemical outcome arises from nonclassical hydrogen-bonding interactions between the electrophile and the substrate, and from entropic considerations of preorganization. The unprecedented configurations of the cascade products, combined with the computational models, reveal for the first time that asymmetric induction by diaryl prolinol silyl ether catalysts is not always exclusively reagent controlled. The stereochemical outcome also arises from a kinetic resolution or dynamic kinetic resolution of the β-stereocenter through an enamine-catalyzed intermolecular reaction. This unprecedented organocascade reaction mechanism may be adaptable to diaryl prolinol silyl ether-catalyzed cascade reactions, in which both the iminium- and enamine-catalyzed steps are intermolecular, an underdeveloped type of cascade reaction.     

Achieving conformational control over C-C, C-N and C-O bonds in biaryls, N,N'-diarylureas and diaryl ethers: advantages of a relay axis

The orientation of Ar-C, Ar-N and Ar-O bonds in biaryls, N,N'-diarylureas and diaryl ethers (whose conformers are distinguishable by NMR) may be controlled with a selectivity up to >95 : 5 by an adjacent stereogenic centre; the selectivity may be greater when a second stereogenic axis is inserted between the controlling centre and the slowly rotating bond.     

Synthesis of (S,S)-isodityrosine by Dötz benzannulation

[reaction: see text] A synthesis of (S,S)-isodityrosine 1, a naturally occurring, key structural subunit of numerous biologically active macromolecules, is described. A formal [3 + 2 + 1] cycloaddition (D?tz benzannulation) approach was utilized to simultaneously construct an aromatic ring and the diaryl ether linkage in one step. This key step was extended to the synthesis of (S,S)-isodityrosine in two separate convergent synthetic routes. This method demonstrates a novel and mild method for the synthesis of diaryl ethers.     

Asymmetric Synthesis of Biaryl Atropisomers Using an Organocatalyst-Mediated Domino Reaction as the Key Step

A three-pot synthetic method that features the use of an organocatalyst as the key step was developed for the preparation of biaryl atropisomers. The first reaction is an asymmetric domino Michael–Henry reaction catalyzed by diphenylprolinol silyl ether to afford the substituted nitrocyclohexanecarbaldehyde with four stereogenic centers and one defined configuration of a stereogenic axis with excellent enantioselectivity. Removal of the central chirality from the domino products afforded biaryl atropisomers having axial chirality with excellent enantioselectivity.     

Catalytic Asymmetric Synthesis of Axially Chiral Diaryl Ethers through Enantioselective Desymmetrization

Axially chiral diaryl ethers are a type of unique atropisomers bearing two potential axes, which have potential applications in a variety of research fields. However, the catalytic enantioselective synthesis of these diaryl ether atropisomers is largely underexplored when compared to the catalytic asymmetric synthesis of biaryl or other types of atropisomers. Herein, we report a highly efficient catalytic asymmetric synthesis of diaryl ether atropisomers through an organocatalyzed enantioselective desymmetrization protocol. The chiral phosphoric acid-catalyzed asymmetric electrophilic aromatic aminations of the symmetrical 1,3-benzenediamine type substrates afforded a series of diaryl ether atropisomers in excellent yields and enantioselectivities. The facile construction of heterocycles by the utilizations of the 1,2-benzenediamine moiety in the products provided access to a variety of structurally diverse and novel azaarene-containing diaryl ether atropisomers.     

Total synthesis of the ristocetin aglycon

The first total synthesis of the ristocetin aglycon is described employing a modular and highly convergent strategy. An effective 12-step (12% overall) synthesis of the ABCD ring system 3 from its amino acid subunits sequentially features an intramolecular aromatic nucleophilic substitution reaction for formation of the diaryl ether and closure of the 16-membered CD ring system (65%), a respectively diastereoselective (3:1, 86%) Suzuki coupling for installation of the AB biaryl linkage on which the atropisomer stereochemistry can be further thermally adjusted, and an effective macrolactamization (51%) for closure of the 12-membered AB ring system. A similarly effective 13-step (14% overall) synthesis of the 14-membered EFG ring system 4 was implemented employing a room-temperature intermolecular S(N)Ar reaction of an o-fluoronitroaromatic for formation of the FG diaryl ether (69%) and a key macrolactamization (92%) with formation of the amide linking residues 1 and 2. The two key fragments 3 and 4 were coupled, and the remaining 16-membered DE ring system was closed via diaryl ether formation to provide the ristocetin tetracyclic ring system (15 steps, 8% overall) enlisting an unusually facile (25 degrees C, 8 h, DMF, >/=95%) and diastereoselective (>/=15:1) aromatic nucleophilic substitution reaction that benefits from substrate preorganization.     

Total synthesis of aspercyclide C

The first total synthesis of (+)-aspercyclide C (1) is reported using a kinetically controlled RCM reaction to form the 11-membered, unsaturated lactone ring of this bioactive diaryl ether macrolide.     

A convenient synthesis of vinyl sulfides

Dithioketals are converted in good yields to vinyi sulfides by action of diethylzinc and methylene iodide. The reaction has been successfully applied to dialkyl and diaryl dithioketals, to spirocyclic analogs, and to a dithioacetal. A monothioketal produced a vinyl ether (enol ether) with the same reagent.     

Efficient novel 1,2-diphosphite ligands derived from d-mannitol in the Pd-catalyzed asymmetric allylic alkylation

A novel Pd/1,2-diphosphite catalyzed asymmetric allylic alkylation of 1,3-diarylpropenyl acetate with malonates was developed. Catalyst optimization via a variation in the protecting groups at the 1,2- and/or 5,6-positions of d-mannitol skeleton and in biaryl moieties of the ligands led to a ‘lead’ catalyst, which efficiently mediated the allylic alkylations. The activities and enantioselectivities of the reaction clearly showed that the stereogenic centers of the skeleton and the axially chiral diaryl moieties of the ligands had a synergic effect. The ligand 1,2:5,6-di-O-isopropylidene-3,4-bis[(S)-1,1′-binaphthyl-2,2′-diyl]phosphite-d-mannitol afforded excellent yields (up to 99%) and high levels of enantioselectivies (ee up to 98%) in 1,4-dioxane/CH₂Cl₂ mixture (v/v, 1:1) using [Pd(π-allyl)Cl]₂ as catalytic precursor and LiOAc as base. Dramatic changes in the sense and in the degree of the enantioselectivity depending on the configuration of the diaryl moieties of the ligands and reaction conditions were observed.     

Regioselective and enantiospecific rhodium-catalyzed allylic alkylation reactions using copper(I) enolates: synthesis of (-)-sugiresinol dimethyl ether

The transition metal-catalyzed allylic alkylation represents a fundamentally important cross-coupling reaction for the construction of ternary carbon stereogenic centers. We have developed a regioselective and enantiospecific rhodium-catalyzed allylic alkylation of acyclic unsymmetrical allylic alcohol derivatives using copper(I) enolates to prepare beta-substituted ketones. This protocol represents a convenient asymmetric Claisen rearrangement surrogate in which alpha-substituted enolates permit the introduction of an additional stereogenic center. The synthetic utility of this transformation was highlighted in the construction of a trans-1,2-disubstituted cyclohexene and the total synthesis of (-)-sugiresinol dimethyl ether. Finally, we anticipate that copper(I) enolates may prove useful nucleophiles in related metal-catalyzed reactions.     

Enantioselective total synthesis of (+)-obtusenyne

A total synthesis of the laurencia metabolite (+)-obtusenyne has been completed. The key steps include a Sharpless kinetic resolution and an asymmetric glycolate alkylation to establish the stereogenic centers adjacent to the ether linkage and a ring-closing metathesis reaction to construct the nine-membered ether without the aid of a cyclic conformational constraint. The synthesis was completed in 20 linear steps from commercially available 1,5-hexadiene-3-ol.     

Synthesis of Tetrahydroisoquinoline-Alkaloid Analogs with a Diphenyloxide Fragment

Chemistry of Natural Compounds - New synthetic benzylisoquinoline-alkaloid analogs based on the diaryl ether hernandial were prepared under Pictet–Spengler reaction conditions.     

A general method for the synthesis of bastaranes and isobastaranes: first total synthesis of bastadins 5, 10, 12, 16, 20, and 21

A general strategy for the synthesis of twenty naturally occurring bastadins (all but bastadin 3) is presented. A key retrosynthetic disconnection of the two amide bonds, common in all target molecules, bisects the macrocyclic core into two diaryl ether fragments, an alpha,omega-diamine (western part) and an alpha,omega-dicarboxylic acid (eastern part). Efficient preparation of the synthetically challenging o-mono or dibromo-substituted diaryl ether linkages was achieved employing the diaryl iodonium salt method. Regarding the western part, variations of the aliphatic chain were more efficiently secured by the preparation of two different alpha,omega-aminonitrile moieties. Cobalt boride mediated reduction of the nitrile functionality established the required diamines and, at the same time, provided the necessary variation of the aromatic-ring bromination pattern. Regarding the eastern part, two different dicarboxyl precursors had to be prepared in order to accommodate bromination-pattern variations. Coupling and subsequent macrolactamization of different combinations of these key intermediates may lead at will to any member of this family of marine natural products. Four bastaranes (bastadins 5, 10, 12 and 16) and two isobastaranes (bastadins 20 and 21) were synthesized as a demonstration of the flexibility and efficiency of the approach presented.     

Total synthesis of (+/-)-terpestacin and (+/-)-11-epi-terpestacin

The total synthesis of racemic terpestacin and 11-epi-terpestacin using the allene ether Nazarov reaction as the key step is described. All stereochemistry is derived from the stereogenic carbon atom that is formed during the Nazarov reaction.     

A General Method for the Synthesis of Bastaranes and Isobastaranes: First Total Synthesis of Bastadins 5, 10, 12, 16, 20, and 21

A general strategy for the synthesis of twenty naturally occurring bastadins (all but bastadin 3) is presented. A key retrosynthetic disconnection of the two amide bonds, common in all target molecules, bisects the macrocyclic core into two diaryl ether fragments, an α,ω‐diamine (western part) and an α,ω‐dicarboxylic acid (eastern part). Efficient preparation of the synthetically challenging o‐mono or dibromo‐substituted diaryl ether linkages was achieved employing the diaryl iodonium salt method. Regarding the western part, variations of the aliphatic chain were more efficiently secured by the preparation of two different α,ω‐aminonitrile moieties. Cobalt boride mediated reduction of the nitrile functionality established the required diamines and, at the same time, provided the necessary variation of the aromatic‐ring bromination pattern. Regarding the eastern part, two different dicarboxyl precursors had to be prepared in order to accommodate bromination‐pattern variations. Coupling and subsequent macrolactamization of different combinations of these key intermediates may lead at will to any member of this family of marine natural products. Four bastaranes (bastadins 5, 10, 12 and 16) and two isobastaranes (bastadins 20 and 21) were synthesized as a demonstration of the flexibility and efficiency of the approach presented.     

Development of the radical C–O coupling reaction of phenols toward the synthesis of natural products comprising a diaryl ether skeleton

Compounds comprising a diaryl ether skeleton exist among natural phenols. The diaryl ether skeleton is thought to be biosynthesized through the coupling of two or more phenols. It is an important structural feature in medicines and agrochemicals, and it is imperative to develop methods for constructing such skeletons in organic synthesis. However, by the synthesis method through the coupling of phenols, coupling occurs preferentially at the ortho-substituted carbon atom of phenols. In this study, various radical-generating reagents and conditions were investigated with the aim of developing a short-step construction method of the diaryl ether skeleton by the radical homo-coupling of two phenol molecules. In addition, cross-coupling reactions between radicals of 2,4,6-tri-tert-butylphenol and p-substituted phenol were conducted to synthesize eight C (ortho)–O coupling products. Based on the results, a computational chemical approach was employed to verify the cause of C (ortho)–O bond formation.