Diastereoselective Oxidation Research Articles

Total Syntheses of Sparsomycin and Sparoxomycins A1 and A2 via Sulfenate-Anion-Mediated Iterative C-S Bond Formation.

The total synthesis of the pyrimidinylpropenamide antibiotics sparsomycin and sparoxomycins A1 and A2 has been achieved. The synthesis of sparsomycin relies on the iterative nucleophilic attack of sulfenate anions on alkyl halides to construct the dithioacetal monoxide chain with high diastereoselectivity. Subsequently, the reagent-controlled diastereoselective oxidation of the terminal sulfide moiety of sparsomycin directly provides sparoxomycins A1 and A2.

Selective Oxidations in the Synthesis of Complex Natural ent-Kauranes and ent-Beyeranes.

Syntheses of two natural products derived from the ent-kaurene kaurenoic acid are described for the first time using regio- and diastereoselective oxidations. Palladium- and manganese-mediated oxidations were used to accomplish the syntheses of two ent-beyerane metabolites. The use of the White-Gormisky-Zhao catalyst Mn(CF3-PDP) enabled the first application of a nondirected metal-catalyzed oxidation in an unactivated C-H bond in a total synthesis.

The First Highly Enantioselective Synthesis of 3-Sulfinyl-Substituted Isoindolinones Having Adjacent Carbon and Sulfur Stereocenters.

A highly stereoselective access to 3-sulfinyl-substituted isoindolinones has been achieved by a tandem organocatalytic addition/cyclization reaction of 2-carbobenzyloxy-N-tosylbenzylidenimine with thiols and succeeding diastereoselective oxidation with MCPBA. First, enantioenriched isoindolinone N,S-acetals have been obtained through a dynamic kinetic asymmetric transformation induced by a bifunctional chiral thiourea organocatalyst. In turn, the newly created carbon stereocenter enabled a high diastereocontrol in the subsequent sulfoxidation. Based on DFT calculations, a theoretical rationale for the stereoselectivity of the oxidation reaction is also provided.

Diastereoselective oxidation of menthyl arenesulfenates to sulfinates and access to enantioenriched aryl methyl sulfoxides

Three aryl methyl sulfoxides were prepared via the following sequence: synthesis of the menthyl arenesulfenate from the corresponding thiol, diastereoselective oxidation leading to the sulfinate, then reaction with a Grignard reagent. The aryl moiety was substituted with a methyl ester on the ortho position or a nitro group on the ortho or the para position. The sulfenate esters were obtained in fair to excellent yield (42–82%). Four different oxidizing agents were tested to obtain the corresponding sulfinate esters with diastereomeric excesses (de) ranging from 10 to 48%. After separation of the diastereomers and reaction with methyl Grignard reagent, two enantiopure sulfoxides and one enantioenriched sulfoxide (32% ee) were obtained.

A fully diastereoselective oxidation of a mesoionic dipole with triplet molecular oxygen.

Oxidations with molecular oxygen are ubiquitous processes in biological systems where cofactor-dependent enzymes activate either oxygen or hydrogen peroxide to induce multichannel pathways. In stark contrast, such slow atmospheric oxidations are seldom harnessed in chemical synthesis and analysis. The present study unveils an unusual aerobic oxidation of a mesoionic dipole leading easily to a more functionalized skeleton. Although the synthetic scope has not been explored, two key considerations emerge from this transformation, as it proceeds with complete diastereoselection and could be successfully extrapolated to structurally related mesoionic chirons without racemization. How this oxidation actually occurs proved to be puzzling from the onset and only high-level computation reveals a cascade transformation, whose results reconcile theory and experiment. Hopefully, the mechanistic insights should help us to understand better the autoxidative reactions of organic molecules.

Enantioselective Total Synthesis of Cymoside through a Bioinspired Oxidative Cyclization of a Strictosidine Derivative.

The first total synthesis of the caged monoterpene indole alkaloid cymoside is reported. This natural product displays a unique hexacyclic-fused skeleton whose biosynthesis implies an early oxidative cyclization of strictosidine. Our approach to the furo[3,2-b]indoline framework relied on an unprecedented biomimetic sequence which started by the diastereoselective oxidation of the indole ring into a hydroxyindolenine which triggered the addition of an enol ether and was followed by the trapping of an oxocarbenium intermediate.

Enantioselective Total Synthesis of Cymoside through a Bioinspired Oxidative Cyclization of a Strictosidine Derivative

AbstractThe first total synthesis of the caged monoterpene indole alkaloid cymoside is reported. This natural product displays a unique hexacyclic‐fused skeleton whose biosynthesis implies an early oxidative cyclization of strictosidine. Our approach to the furo[3,2‐b]indoline framework relied on an unprecedented biomimetic sequence which started by the diastereoselective oxidation of the indole ring into a hydroxyindolenine which triggered the addition of an enol ether and was followed by the trapping of an oxocarbenium intermediate.

Inherently Chiral Cavitand Curvature: Diastereoselective Oxidation of Tethered Allylsilanes

Syntheses of inwardly and outwardly directed allylsilanes those are tethered to new inherently chiral cavitands are described. Oxidized with mCPBA, these allylsilanes result in diastereomeric mixtures of epoxide molecules. Thus, it enables us to have comparative study of cavitand‐structure diastereoselectivity relationship, which revealed that an inward allylsilane group flanked by a dibenzo[f, h]quinoxaline and two bridged methylene groups have the best chemical yield and diastereoselection.

Development of a catalytic enantioselective synthesis of the guanacastepene and heptemerone tricyclic core

For nearly two decades, synthetic chemists have been fascinated by the structural complexity and synthetic challenges afforded by the guanacastepene and heptemerone diterpenoids. Numerous synthetic approaches to these compounds have been reported, but to date the application of enantioselective catalysis to this problem has not been realized. Herein we report an enantioselective synthesis of an advanced intermediate corresponding to the tricyclic core common to the guanacastepenes and heptemerones. Highlights of this work include sequential Pd-catalyzed decarboxylative allylic alkylation reactions to generate the two all carbon quaternary stereocenters, the use of ring-closing metathesis to close the A ring in the presence of a distal allyl sidechain, and a regio- and diastereoselective oxidation of a trienol ether to introduce oxygenation on the A ring.

Asymmetric synthesis of 7-aza-phomopsolide E and its C-4 epimer

A flexible, enantioselective route to highly functionalized α,β-unsaturated δ-lactones has been applied to the synthesis of 7-aza-phomopsolide E and its C-4 epimer. This approach relies on the application of the Noyori asymmetric hydrogenation of furyl ketone to produce the secondary furyl alcohol in high enantioexcess, which can be stereoselectively transformed into α,β-unsaturated-δ-lactones by a short, highly diastereoselective oxidation and reduction sequence. DCC and acid chloride couplings were used to introduce the side chains of the two natural product analogues.

Enzymatic Cascade Reactions for the Synthesis of Chiral Amino Alcohols from L-lysine

Amino alcohols are versatile compounds with a wide range of applications. For instance, they have been used as chiral scaffolds in organic synthesis. Their synthesis by conventional organic chemistry often requires tedious multi-step synthesis processes, with difficult control of the stereochemical outcome. We present a protocol to enzymatically synthetize amino alcohols starting from the readily available L-lysine in 48 h. This protocol combines two chemical reactions that are very difficult to conduct by conventional organic synthesis. In the first step, the regio- and diastereoselective oxidation of an unactivated C-H bond of the lysine side-chain is catalyzed by a dioxygenase; a second regio- and diastereoselective oxidation catalyzed by a regiodivergent dioxygenase can lead to the formation of the 1,2-diols. In the last step, the carboxylic group of the alpha amino acid is cleaved by a pyridoxal-phosphate (PLP) decarboxylase (DC). This decarboxylative step only affects the alpha carbon of the amino acid, retaining the hydroxy-substituted stereogenic center in a beta/gamma position. The resulting amino alcohols are therefore optically enriched. The protocol was successfully applied to the semipreparative-scale synthesis of four amino alcohols. Monitoring of the reactions was conducted by high performance liquid chromatography (HPLC) after derivatization by 1-fluoro-2,4-dinitrobenzene. Straightforward purification by solid-phase extraction (SPE) afforded the amino alcohols with excellent yields (93% to >95%).

Enzymatic Cascade Reactions for the Synthesis of Chiral Amino Alcohols from L-lysine.

Amino alcohols are versatile compounds with a wide range of applications. For instance, they have been used as chiral scaffolds in organic synthesis. Their synthesis by conventional organic chemistry often requires tedious multi-step synthesis processes, with difficult control of the stereochemical outcome. We present a protocol to enzymatically synthetize amino alcohols starting from the readily available L-lysine in 48 h. This protocol combines two chemical reactions that are very difficult to conduct by conventional organic synthesis. In the first step, the regio- and diastereoselective oxidation of an unactivated C-H bond of the lysine side-chain is catalyzed by a dioxygenase; a second regio- and diastereoselective oxidation catalyzed by a regiodivergent dioxygenase can lead to the formation of the 1,2-diols. In the last step, the carboxylic group of the alpha amino acid is cleaved by a pyridoxal-phosphate (PLP) decarboxylase (DC). This decarboxylative step only affects the alpha carbon of the amino acid, retaining the hydroxy-substituted stereogenic center in a beta/gamma position. The resulting amino alcohols are therefore optically enriched. The protocol was successfully applied to the semipreparative-scale synthesis of four amino alcohols. Monitoring of the reactions was conducted by high performance liquid chromatography (HPLC) after derivatization by 1-fluoro-2,4-dinitrobenzene. Straightforward purification by solid-phase extraction (SPE) afforded the amino alcohols with excellent yields (93% to >95%).

Stereocontrolled synthesis of polyhydroxylated bicyclic azetidines as a new class of iminosugars.

We report herein the development of a stereodivergent route towards polyhydroxylated bicyclic azetidine scaffolds, namely 6-azabicyclo[3.2.0]heptane derivatives. The strategy hinges on a common bicyclic β-lactam precursor, which is forged by way of a rare example of a cationic Dieckmann-type reaction, followed by IBX-mediated desaturation. Substrate-controlled diastereoselective oxidations then allow the divergent preparation of novel iminosugar mimics.

Asymmetric total synthesis of phomonol

Abstract An efficient asymmetric total synthesis of phomonol 1 is presented, starting from (S)-1,2-epoxypentane. The synthesis features Sharpless asymmetric dihydroxylation (AD), diastereoselective reductive etherification and Wacker oxidation as key steps.

Enantioselective Chemical Syntheses of the Furanosteroids (-)-Viridin and (-)-Viridiol.

Herein we describe concise enantioselective chemical syntheses of (-)-viridin and (-)-viridiol. Our convergent approach couples two achiral fragments of similar complexity and employs an enantioselective intramolecular Heck reaction to set the absolute stereochemical configuration of an all-carbon quaternary stereocenter. To complete the syntheses of these base- and nucleophile-sensitive natural products, we conduct carefully orchestrated site- and diastereoselective oxidations and other transformations. Our work is the first to generate these targets as single enantiomers.

Enantioselective synthesis of 1,2,3,4-tetrahydroquinoline-4-ols and 2,3-dihydroquinolin-4(1H)-ones via a sequential asymmetric hydroxylation/diastereoselective oxidation process using Rhodococcus equi ZMU-LK19.

A cascade biocatalysis system involving asymmetric hydroxylation and diastereoselective oxidation was developed using Rhodococcus equi ZMU-LK19, which gave chiral 2-substituted-1,2,3,4-tetrahydroquinoline-4-ols (2) (up to 57% isolated yield, 99 : 1 dr, and >99% ee) and chiral 2-substituted-2,3-dihydroquinolin-4(1H)-ones (3) (up to 25% isolated yield, and >99% ee) from (±)-2-substituted-tetrahydroquinolines (1). In addition, a possible mechanism for this cascade biocatalysis was tentatively proposed.

Asymmetric Oxidation Synthesis of Modafinil Acid by Use of a Recyclable Chiral‐at‐Metal Complex

AbstractThe enantioselective oxidation synthesis of chiral modafinil acid and its analogues with high enantiomeric excess has been developed by means of a chiral‐at‐metal strategy. Treatment of ruthenium complexes cis‐[Ru(bpy)2Cl2] or Δ/Λ‐[Ru(bpy)2(MeCN)2](PF6)2 (bpy is 2,2′‐bipyridine) with the appropriate prochiral thioether ligands afforded thioether complexes rac‐1, Δ/Λ‐1, rac‐2, Δ/Λ‐2, rac‐3, and Δ/Λ‐3. Diastereoselective oxidation of the thioether complexes in situ produced the corresponding sulfoxide complexes rac‐1a, Δ/Λ‐1a, rac‐2a, Δ/Λ‐2a, rac‐3a, and Δ/Λ‐3a. The configuration at the metal center in each case is stable during the coordination and oxidation reactions, and dictates the chirality of the sulfoxide ligand in the oxidation process. The chiral modafinil acids were obtained with ee values greater than 98 % upon their removal from the corresponding sulfoxide complexes in the presence of TFA/MeCN. Moreover, the chiral ruthenium precursors Δ/Λ‐[Ru(bpy)2(MeCN)2](PF6)2 are recyclable and reusable with complete retention of the configurations.

Efficient Preparation of a Versatile Chiral Synthon for 1,2-Diamines via the Fe(III)-Catalyzed Diastereoselective Oxidation of 2-Imidazolone and Its Application

Efficient Preparation of a Versatile Chiral Synthon for 1,2-Diamines via the Fe(III)-Catalyzed Diastereoselective Oxidation of 2-Imidazolone and Its Application

Studies on the diastereoselective oxidation of 1-thio-β-D-glucopyranosides: synthesis of the usually less favoured R(S) sulfoxide as a single diastereoisomer.

A detailed study on the diastereoselective oxidation of 1-thio-β-D-glucopyranosides is reported. It has been shown that the sense and the degree of stereochemical outcome of the oxidation are highly dependent on the substituent of the sulfur and on the protective group of the C2-OH. In the case of thioglycosides with a bulky aglycone, the mesylation of C2-OH has a significant effect on the stereochemical outcome of the oxidation, affording the usually less favoured RS sulfoxide as a single diastereoisomer. The absolute configuration of the final sulfinyl glycosides was ascertained by NMR analysis and corroborated by X-ray crystallography.

ChemInform Abstract: Enantio‐ and Diastereoselective Oxidation of N‐Alkylimines Using Chiral α‐Bromonitriles and Hydrogen Peroxide System.

AbstractThe system chiral α‐bromonitriles/hydrogen peroxide is examined for the enantio‐ and diastereoselective oxidation of N‐alkylimines in aqueous medium at room temperature yielding the corresponding oxaziridines.